UnlockingtheClimateRecordStoredwithinMars'PolarLayeredDeposits

FinalReport

KeckInstituteforSpaceStudies

IsaacB.Smith PaulO.Hayne ShaneByrne PatBecerra MelindaKahreWendyCalvin Chris9neHvidbergSarahMilkovichPeterBuhler MargaretLandisBrionyHorganArminKleinböhlMaMhewPerryRachelObbardJenniferStern SylvainPiqueuxNickThomas KrisZacnyLynnCarter LaurenEdgar JeremyEmmeMThomasNavarroJenniferHanleyMichelleKoutnikNathanielPutzigBryanaL.HendersonJohnW.Holt BethanyEhlmannSergioParra DanielLalich CandyHansenMichaelHechtDonBanfield KenHerkenhoffDavidA.PaigeMarkSkidmoreRobertL.StaehleMaMhewSiegler

YorkUniversityandPlanetaryScienceIns9tuteUniversityofColorado-BoulderUniversityofArizonaUniversityBernNASAAmesUniversityofNevada-RenoUniversityofCopenhagenJetPropulsionLaboratoryJetPropulsionLaboratoryPlanetaryScienceIns9tutePurdueUniversityJetPropulsionLaboratoryPlanetaryScienceIns9tuteDartmouthUniversityGoddardSpaceFlightCenterJetPropulsionLaboratoryUniversityBernHoneybeeRobo9csUniversityofArizonaUnitedStatesGeologicalSurveyNewMexicoStateUniversityUniversityofCalifornia,LosAngelesLowellObservatoryUniversityofWashingtonPlanetaryScienceIns9tuteJetPropulsionLaboratoryUniversityofArizonaCaliforniaIns9tuteofTechnologyGeorgiaIns9tuteofTechnologyCornellUniversityPlanetaryScienceIns9tuteHaystackObservatoryCornellUniversityUnitedStatesGeologicalSurveyUniversityofCalifornia,LosAngelesMontanaStateUniversityJetPropulsionLaboratoryPlanetaryScienceIns9tute

0.Execu)veSummary1.Background 1.AMarsPolarScienceOverviewandStateoftheArt

1.A.1Polaricedepositsoverviewandpresentstate 1.A.2PolarLayeredDepositsforma)onandlayers 1.A.3ClimatemodelsandPLDforma)on

1.A.4TerrestrialClimateStudiesUsingIce 1.BHistoryofMarsPolarInves)ga)ons 1.B.1Orbiters 1.B.2Landers 1.B.3PreviousConceptStudies

2.StudyObjec)vesandOverview 2.AAimsandObjec)vesofthisStudy 2.BFirstWorkshop 2.CSecondWorkshop3.StudyResults 3.AMajorScienceQues)onsandObjec)ves

3.A.1Fluxes 3.A.1aObserva)ons 3.A.1bModeling 3.A.2Forcings 3.A.3LayerProcesses

3.A.4Record 3.A.5AgeDa)ngtheNPLD 3.BKeyProper)esandMeasurements 3.B.1 RequirementsforMeasurementsofFluxesinthePolarRegions 3.B.2 Requirements for Measurements of Layer Proper)es in the PolarRegions 3.B.3 Requirements for Measurements of Composi)on in the PolarRegions 3.CMissionConceptsandApproachestoMeasurements 3.C.1Orbiter 3.C.2Sta)cLander 3.C.3Small-SatNetwork 3.C.4MobilePlaXorm

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0. Execu)veSummary

T tw eo rrestrialplanetsinoursolarsystemhaveclimatesdrivenbymanycomplexfactorsincluding their orbits, geology, and vola9les. Mars offers a unique opportunity to study theeffectsoforbitalchangesinrela9veisola9on.Therecent(Myrto10sofMyr)orbitofMarshasundergonelargevaria9onsrela9vetochangesonEarthorVenus.Thesevaria9onshavedrivenclimate change in the absence of other complica9ng factors such as geological ac9vity, largeimpactsorvola9le loss fromtheplanet.AlthoughMilankovitchcyclesalsooperateonEarth’sclimate,theireffectsarecomplicatedbytheoceans,life,ac9vevolcanism,and,more-recently,anthropogenicforcing.Inthenear-future,thousandsofterrestrialexoplanetswillbediscoveredandknowledgeoftheirorbitsandatmosphericcomposi9onswillbeobtainable.Marspresentsourbestopportunitytostudytheeffectsoforbitalchangeonbodiesboth inwithinoursolarsystemandbeyond.

Mars possesses a record of its recent climate in icy layers analogous to terrestrial icesheets. The North and South Polar Layered Deposits (NPLD & SPLD) of Mars each containthousandsofobservedlayersinstackedsheetsexceedingthicknessesgreaterthan2km.HiRISEimagery of layer exposures on shallow slopes allow layers as thin as a few decimeters to beresolved,althoughpervasivesublima9onlagdepositsandcameraresolu9onmayhidethinnerlayers.Eachoftheselayerscontainsinforma9onontheclima9chistoryduringitsdeposi9on.Indecreasingabundance,thePLDarecomposedofwaterice,dust,salts,geochemicalweatheringproducts, trapped gasses, and other materials including cosmogenic nuclides. Materials fromstochas9cprocesses,suchasimpactejectaandvolcanicashfall,mayalsobeincludedinsomelayers. With detailed measurements of layer composi9on, it may be possible to extract age,accumula9on rates and likely atmospheric condi9ons and surface processes at the 9me ofdeposi9on. During a two-part workshop of more than 35 Mars scien9sts, engineers, andtechnologists, hosted at the Keck Ins9tute for SpaceStudies, we focused on determiningthe measurements needed to extract the climate record contained in the polar layereddepositsofMars.Thegroupconvergedonfourfundamentalques9onsthatmustbeansweredin order to extract the most informa9on possible from the record and themeasurements required toanswertheseques9ons.

1. Whatarepresentandpastfluxesofvola5les,dust,andothermaterialsintoandoutofthepolarregions?

2. Howdoorbitalforcingandexchangewithotherreservoirs,affectthosefluxes?3. Whatchemicalandphysicalprocessesformandmodifylayers?4. Whatisthe5mespan,completeness,andtemporalresolu5onrecordedinthePLD?

Eachques9onrelatestoan importantpartof formingapolar ice layer.Addressingthefirstques9on is necessary todeterminewhatmaterials are available for layer forma9on. Thesecond ques9on focuses on external drivers on the planetary climate and availability ofmaterials that can be trapped. The third ques9on directly relates to layer forma9on, fromatmospheric deposi9on to modifica9on aeer emplacement. The final ques9on asks whatinforma9onisavailabletoassigndatestothelayersandunlocktheclimaterecord. Answeringoneques9oninisola9onisinsufficient,andaseriesofmissionsaMemp9ngto

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extracttheclimaterecordmustaddressallfourques9onstoaccomplishthegoalofdeterminingtheclimaterecordonMars.Furtherworkrequiresacombina9onofmeasurementsfromorbitandthesurface,modeling,andlaboratory inves9ga9ons.Wehaveiden9fiedmissionconceptsthatcouldbestaddresstheseques9ons. In order to answer Ques9on 1, a polar orbiter capable of measuring wind speeds,ver9callyresolvedatmosphericcomposi9on(especiallywatervaporabundances),temperature,andsurface frost is required.Theorbitermustoperate forat leastoneMarsyear inorder todetermine the seasonal cycle of available materials. Instruments required to answer thisques9on were iden9fied as a sub-mm sounder, thermal-infrared sounder, and a wide-angleimagingsystem. Ques9on2dealswiththeavailabilityofsurfacewaterandcarbondioxidereservoirsatdifferent9mes.Toaddressthisques9on,anorbitalmissionshouldhaveaninstrumentcapableofmeasuringdepthto,concentra9onof,andthicknessofanysubsurfaceiceonMars.Ourstudydetermined that a radar sounder at much higher frequencies than currently available is themost capable instrument to achieve these measurements. A second, polarized Synthe9cApertureRadar(SAR)modeisrequiredtodirectlydetectthepresenceofwatericewithinafewwavelengthsofthesurfaceandprovidemapsofsuch. Ques9on3requiresmeasurementstobetakenatthesurfacethatrecordatmosphere-surfaceinterac9onsandac9velayerforma9onontheresidualicecapontopofthePLDifitisoccurring today. Addi9onally, a subsurface component should assess post-deposi9onmodifica9on, and a drill will be required to access and deliver this material to properinstrumenta9on. ForQues9on4, both landedandorbital assets are required.Anorbital radar sounderwould be adequate to match the ver9cal resolu9on of our highest-resolu9on cameras. Thiswouldestablish lateralcon9nuitybetweenexposuresandprovideaccumula9onhistoryacrosseachPLD. In a separatemission, thesemeasurements canbe "ground truthed"by amethodthat canaccessmanyver9cal layersofmaterial, comprising thousandsormillionsof yearsofhistory.Twosuchideasaretotakeadrillcapableofboring>100mver9cally,oracapableroverthat can drive down the low-inclina9on spiral troughs and sample material every ~0.5 mver9cally. We propose the following Mars Polar Research Program: one orbiter to makeatmosphericandsubsurfacemeasurements;asetofsmall-satlanderscapableofmeasuringthelocal environment, either atmospheric or surface/subsurface; a small Discovery or NewFron9ers-class lander with atmospheric sensors and meter-class drilling capabili9es formeasuringthelocalenvironmentandassessingwhatmaterialsareavailableforageda9ng;andfinally a flagship-class mobile mission capable of sampling hundreds of meters of ver9calstra9graphy.

1.Background

1.AMarsPolarScienceOverviewandStateoftheArt

1.A.1Polaricedepositsoverviewandpresentstate

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ThepolesofMarshostapproximatelyonemillioncubickilometers(Smithetal.2001)oflayeredicedeposits.DiscoveredbyMariner9imaging(Murrayetal.1972),thesePolarLayeredDeposits (PLD) have long been thought to record mar9an climate in an analogous way toterrestrialpolar icesheets.BothPLDarecomposedprimarilyofwater ice (Grimaetal.2009)butcontaindustuptoafewpercentoftheirtotalmass,aswellasmaterialspoten9allyrelatedtospecificgeologicevents,suchasvolcanicashandimpactejecta.Thever9calandhorizontaldistribu9on of these materials are thought to record atmospheric condi9ons includingtemperature,rela9vehumidity,andaerosoldustcontent. This iconic PLD layering is visible through radar sounding and visible imaging of bedexposures introughsandscarps.AlthoughthesurfacesofthePLDareamongtheflaMestandsmoothest surfacesonMars, theyareboundedby steep scarps that typically exposeup to akilometerofver9callayering.Addi9onally,intheinteriorofthePLD,arcuatetroughsexposeafew hundred meters of bedding, which orbital imagers can observe. The bounding scarpsappear to be erosional in nature, with ac9ve erosion observed at the steepest north polarexamples (Russell et al. 2008). However, geologic structureswithin the PLD indicate that theinternaltroughsareconstruc9onalfeatures,andthattheyexperienceerosionontheirsteeperequatorwardfacingwalls,leadingthemtohavemigrated10sofkilometerspolewardasthePLDaccumulated(Smithetal.2010). Varia9ons in the orbital configura9on of Mars lead to clima9c varia9ons, which arethoughttoberecordedinthePLD.ModeledTheorbitalparameterswhoseoscilla9onsprimarilydrive these clima9c changes are the planet’s obliquity, orbital eccentricity, and argument ofperihelion. The orbital solu9on goes back 20Myr (Laskar et al. 2004). Obliquity cycles havecharacteris9c9mescalesof~120kyrand~1Myr,eccentricityvarieson9mescalesof~1.2Myr,and theargumentofperihelionhasaprecession cycleof~51kyr.Over the last20Myrs, theobliquityofMarshas variedbetween15and45degrees (Laskaret al., 2004).High valuesofobliquitymeanthatthepolesreceivemoresunlightonaveragethanthemid-la9tudes,whichleadstoabla9onofpolarice.Conversely,lowobliquitypromotesaccumula9onofpolarice.Thepresentobliquityisaround25°,whichislowenoughthattheaverageinsola9onatthepolesislowerthanatthemid-la9tudes,buthighenoughthatthenetmassbalancethat isdifficulttodis9nguish from zero (Bapst et al. 2018). Recent obliquity varia9ons between ~15 and ~35°straddlethisvalueandmayhaveledtolargevaria9onsintherateofpolaraccumula9on.Therearemul9pleangularunconformi9esinthestra9graphicrecord(Tanaka2005)thatshowperiodsofnetabla9onhaveoccurred,andextremelydustylayersmaybesublima9onlagdepositsthatrepresentdisconformi9esintherecord(Fishbaughetal.2010).Orbitalsolu9ons(Laskaretal.,2004)alsoshowthatthemeanobliquitybetween5and20Mawas~35°–substan9allyhigherthanthe~25°from4Matothepresent.Uniquesolu9onspriorto20Macannotcurrentlybederived,butsta9s9calargumentsshowthatahighmeanobliquityiscommonandthatthemostprobable obliquity over all of mar9an history is ~42 degrees (Laskar et al., 2002). Thus, thecurrentthickpolardepositsmaybeatypicalcomparedtomostofmar9anhistory. TheageofthePLDhasbeenes9matedtobebetweenafewmillionyearsinthenorthtoup to 100millions of years in the south, on the basis crater coun9ng andmodeling. ImpactcratersontheuppermostsurfaceofthePLDcanyieldaloweragelimit.TheNPLDcraterrecordindicates that crater infill with ice is ongoing and is fast enough to erase craters 100 m indiameterover9mescalesofkyrto10sofkyr(Banksetal.2010;Landisetal.2016).TheSPLDcraterrecordisconsistentwithanon-accumula9ngsurfacethatisbetween30Maand100Main age (Koutnik et al. 2002). Model simula9ons of polar ice stability provide another age

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constraint.Severalstudies,describedinsec9onsbelow,arguethatthehighobliqui9espriorto4-5Mamake iceaccumula9onat thenorthpole impossibleprior to that9me.Thishasbeeninterpreted as an upper limit for the age of theNPLD. However, the insula9ng effects of lagdepositsareincompletelyaccountedforinthesestudiesandtheobviousoldercraterreten9onageoftheSPLDmustalsobereconciledwiththeseconclusions. Inaddi9ontotheicybeddingthatmakeupmostoftheirstructure,thePLDarepartlycoveredwithbrightresidualicecapsthatinteractstronglywiththecurrentclimate.Thenorthpolar residual icecap isexposedat theendof thespringaeerseasonalCO2andwater frostssublimate.Stereoimageryshowsthatithaslessthanameterofrelief,andatextureoflightanddarkpatches(orridges insomeloca9ons) inrepea9ngpaMernswithahorizontalscaleontheorderofdecameters. Italsoexhibitsanevolu9onof icegrainsizeandalbedothroughout theyear(Langevinetal.2005;Brownetal.2016).Fine-grainedseasonalfrostthataccumulatesintheFall/Wintersublimatesinthespringandsummerandolderlarge-grainediceisexposedfora por9on of the summer, sugges9ng that net abla9on is currently occurring (Langevin et al.2005).However,thislarge-grainediceisdust-free,soabla9onhasnotbeensignificantenoughtoproducea lagdeposit and ice is clearly currently accumula9ngwithinpolar impact craters(Landisetal.2016).Atthesouthpole,theSPLDpossessapartlyburiedreservoirofCO2ice(upto1kmthick)thatiscomparableinmasstothecurrentCO2atmosphereoftheplanet(Phillipsetal.2011;Biersonetal.2016;Putzigetal.,2018).ThisCO2iceiscappedbyawatericelayerontopofwhich amuch smaller (~1%of the current atmosphere) surfaceCO2 ice deposit exists(Bibringetal.2003;ByrneandIngersoll,2003;Titusetal.2003).Thisdeposit isknownastheSouth Polar Residual Cap (SPRC), and it hosts a wide variety of sublima9on-driven erosionalfeaturesthathavebeenobservedtogrowandchangeeveryMarsyear.TheserapidchangesinthemorphologyandappearanceoftheSPRCareevidenceof itssensi9veinterac9onwiththecurrent climate. Not only do these sublima9on features grow by by severalmeters per year(Thomas et al. 2009; 2016; Buhler et a., 2017), but summer dust storm ac9vity has beenobservedtoleadtodis9nc9vebrighthalosontheedgesofthesefeatures(Becerraetal.2015),whichmaybeassociatedwithfurtherwinter9mecondensa9onofCO2ice,andinturnallowthisicecaptosurvive for longerperiodsof9me.This interac9on,whichweobserveyear toyear,maybedirectevidencefornewPLDstructuresformingcurrently,althoughverydifferentfromthewatericebedsthatcomprisemostoftheSPLDvolume

1.A.2PolarLayeredDepositsforma)onandlayers

Duringobliquityvaria9ons,thetransferof ice,aerosols,anddustfromlowla9tudestothe poles causes the forma9on of alterna9ng polar bedswith variable ice puri9es. The bulkcontentoftheNPLDis~95%purewaterice(Grimaetal.2009).However,individualbedsmaybeupto50%dust(Lalichetal.,2017).Thedust-richbedsmayhaveformedduringperiodsofhighdustaccumula9onrela9veto ice,and/orduringperiodsof ice loss.Dust-poorbedsmusthaveformedduring9meswhenicedeposi9onwashighrela9vetodustaccumula9on(Hvidbergetal.,2012).Itisthereforenecessarytostudythestra9graphyofthePLDinordertointerpretthe9ming of deposi9onof each layer and build a climate record that 9es bed sequences topar9cularinsola9oncyclesin9me.Thiswillallowustoreconstructtheclima9chistoryofMars.

Observingthestra5graphicrecord

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Accumula9onatthePLDisnotuniforminspace(horizontally)or9me(ver9cally).BothPLDsarethickestnearthenorthpoleandthinneratthemargins(Smithetal.,2001);howeverthethickestSPLDisslightlyoffsetfromthepole. IntheNPLD,thereareclearpaMernsof localand regional accumula9on that were affected by exis9ng basal topography (Brothers et al.,2015)orotherlargestructures(e.g.aburiedchasmathathasbeenfilledin(Holtetal.,2010)).Addi9onally, unconformi9es, caused by deposi9on following erosion by sublima9on or windabla9on,arenumerous(TanakaandFortezzo2012;Smithetal.,2016),andmustbeconsideredinthecontextofthestra9graphiccolumn. The differences in rela9ve accumula9on with geographic loca9on, as well as post-deposi9onalmodifica9onprocesses,affectthefinalstateofthesedimentaryrecordinthePLDthatwe can observe. Therefore, in order to detect the periodic signals corresponding to theastronomically-forced insola9on, we need to be able to observe and describe intrinsicproper9esofthebedsthatmakeuptherecord,i.e.,proper9esthatrelatemostcloselytotheenvironmentalcondi9onsatthe9meofaccumula9on.Theseproper9esmay includesinteringrate,dust/icera9o,orconcentra9onsofdifferentisotopesinabed.OnEarth,theseproper9escan be extracted directly from ice or sediment core samples, but without such samples ofmar9anice,wemustrelyonremotesensing.

Remotesensingobserva5onsofexposedbedsequences

Forthepurposesofstudyingtheinternalstra9graphyofthePLD,weuseorbitalimagingatvariouswavelengthswithinthevisibleandinfraredspectrumtoextractinforma9onfromthebeddingoutcropsexposedat thecharacteris9cspiral troughsof thePLD (CuMs,1973;SchenkandMoore,2000;SmithandHolt,2010;2015).Theinforma9onwecanextractfromthesedataare:brightnessoflayers,topographyoftheoutcropwithstereoimaging,composi9ones9matesfromspectrometersandcolor-ra9os. Thanks to the High Resolu9on Imaging Science Experiment (HiRISE) (McEwen et al.,2007)onboardNASA’sMarsReconnaissanceOrbiter(MRO),weareabletoextractreflectanceand topographic informa9on at the scale of the thinnest exposed beds (~ 1–2meters). It isreasonable to assume that these proper9es are more closely related to the exposure of apar9cular layer than its intrinsic proper9es.However, the no9ceable varia9on in topographicexpressionaswellasbrightnesschangeswithdepthinalloftheseoutcrops,indicatethattheseproper9esmusthavesomeindirectrela9onshiptothecomposi9onoftheinternalbedding.Forexample,adarkerexposedlayermayberelatedtoahigherdustcontent inthe internal layer,andamoreprotrudinglayermayalsobeassociatedwithahighdustcontentthatinsulatesandprotectsthatpar9cularbedfromerosioncomparedtoneighboringbeds. The main advantage of visible data from the outcrops is the meter–scale resolu9on.Nevertheless, theseobserva9onsare limited to the top400–800mof thePLDs, i.e., to themaximumdepthsofexposures.Addi9onally,theoutcropsarescaMeredthroughouttheextentof the PLD, and therefore, correla9ons must be made between a sequence exposed in oneloca9on, and one exposed in a different one,which is not a trivial endeavor (Fishbaugh andHvidberg,2006;MilkovichandPlaut,2008;Becerraetal.,2016).

Observa5onsofthesubsurfacestructurewithradar

Radar instruments like the Mars Advanced Radar for Subsurface and Ionosphere

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Sounding (MARSIS; (Picardietal.,2004))onboard theEuropeanSpaceAgency's (ESA's)MarsExpress, and the Shallow Radar (SHARAD; (Seu et al., 2007)) on MRO, are sensi9ve to thedielectricproper9esofthetargetmaterial.Iceisnearlytransparenttoradarwaves,allowingtheradiowavetopenetratetothedeepestpor9onsofthePLDs,asdeepas3.5km,andpermixngviewsoftheinternalstructure. Differences in silicic content (primarily dust, but possibly ash or ejecta) between icylayerswill cause changes indielectricpermixvity that result ina changeof the speedof theradarwave,causingareflec9onthatisrecordedbytheinstrument(NunesandPhillips,2006).Itisthankstothesedatathatweknowthatthebulkcomposi9onofthedepositsisrela9velypureice,with5to10%dustconcentra9onsfortheSPLDandNPLD,respec9vely(Plautetal.,2007;Grimaetal.,2009).The rela9onshipbetweenpermixvityanddustcontent isnotcompletelyunderstood.Forexample,aseriesofthin,dust-richbedsmayreflecttheradarwaveinasimilarmanner toa single, thickdust-rich layer (Lalichetal.,2017).Nevertheless, it is likely that theradarresponseisdirectlyrelatedtothedustcontentofthePLDbedsatdepth. Onedifficulty inmatching internal stra9graphy asmeasuredwith SHARAD to exposedoutcrops observed with orbital imagers is that the ver9cal resolu9on of current radarobserva9onsismuchlowerthanthatofthelaMer.Wherecomparisonshavebeenmade,radarreflectorsmimic the exposed layers in spectral frequency and geometry,most likely becauseboth types of observa9ons observe variable concentra9ons of dust (Chris9an et al., 2013;Becerra et al., 2017a). Yet, with the current radar assets, a bed-to-bed correla9on betweendatasetsisnotpossible,althoughaMemptsarebeingmadetocorrelategroupsofvisiblebedstoradarreflectors(Becerraetal.,2018).

Large-scalegeologicframeworkofthePLD Establishing broad-scale rela9onships between layer packets and geologic units in thePLDprovidesthegeologicframeworkwithinwhichthefiner-scalestra9graphymustbeanalyzedand interpreted. Through a survey of vast amounts of imaging data from the Mars OrbiterCamera(MOC),theThermalEmissionImagingSystem(THEMIS),MRO’sContextCamera(CTX),and HiRISE, coupled with topographic data from the Mars Orbiter Laser Al9meter (MOLA),Tanaka et al. (2007, 2008, 2012) searched for common characteris9cs and unconformi9es todevelopaPLD-scalestra9graphiccolumnandgeologicmapforbothPLD.Theyderivedtheagesof theunits fromcratersta9s9csandvisualstra9graphyprinciples.FromTanakaetal., (2007;2008; Tanaka and Fortezzo 2012), the units most relevant to PLD stra9graphy that we caniden9fyatthisscaleinclude: a)IntheSouth:• Aa4b: South Polar Residual Cap (SPRC), composed en9rely ofCO2 up to 8 m thick that

interactswiththeatmosphereofMars.• Aa4a:SPRClowerlayercomposedof~2mofwaterice.• Aa3:CO2unitupto1kmthickthatismostlycutofffromatmosphericinterac9ons(Phillips

etal.,2011;Putzigetal.,2018)es9matedtobe~300kyrold(Biersonetal.,2016).• Aa2:Highwatericelayereddepositswithabulkdustcontentofupto10%.Thisunitmakes

up the bulk of the SPLD. Cratering record es9mates for the age of this unit rangebetween10(HerkenhoffandPlaut,2000)and100Myrs(Koutniketal.,2002).

• Aa1:Oldestknownwatericeunitinthesouth,similartoAa2incomposi9on.

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b)IntheNorth:• Abb4:NorthPolarResidualCap (NPRC).Roughly1mdeposit composedofwater ice,and

thoughttobecurrentlyaccumula9ng(Brownetal.,2016).• ABb3:PlanumBoreum3Unit.Awater iceunit80-300mthickthatcapsmost low-sloping

surfaces(SmithandHolt2015;Smithetal.,2016).• ABb1:PlanumBoreum1Unit.Awater iceunitthatmakesupthebulkoftheNPLD(upto

2000m)primarilyexposedatspiraltroughoutcrops(Becerraetal.,2016;Byrne,2009;Fortezzo,2012;SmithandHolt,2010).Fromstra9graphicanalysisand

• accumula9onmodels,itisthoughttobeabout4–5Myrsold(Levrardetal.,2007;Hvidbergetal.,2012;Becerraetal.,2017b).

• Abbc = Planum Boreum Cavi unit. Uppermost unit of the dusty "basal unit" beneath theNPLD. The age of this unit is highly uncertain,with es9mates ranging fromMiddle toLateAmazonian(Fortezzo,2012;Tanakaetal.,2008).Recently,ithasbeenfoundtobetransgressivewiththe lowermostNPLDdeposits(EwingandKocurik2015;NerozziandHolt,2017).

Image-basedcorrela5onofexposedsequences

Images of bed exposures can be used to define and classify discrete layer sequencesbased on their morphological proper9es. From there, con9nuous depth profiles can beextracted and directly compared to synthe9c stra9graphies built with models of ice andsedimentaccumula9on. StudiesbyFishbaughandHvidberg(2006)andbyMilkovichandPlaut(2008)correlatedspa9allydis9nctsequencesusingthehighestresolu9onimagesavailableatthe9metoproposethefirstrela9vestra9graphiccolumnsfortheNPLDandSPLDrespec9vely.Morerecently,nearcomplete imaging coverageof thePLDwithCTXhasallowed for individualbeds tobe tracedhundredsofkilometersalongthesametrough,aidingtheconstruc9onofstra9graphiccolumnsthatarevalidforlargeareasofthePLDwithahighdegreeofconfidence(Becerraetal.,2016).Naturally,itisimpossibletotracelayersacrossdifferenttroughsrelyingsolelyonimagesoftheexposures, in which cases the method of morphologically correla9ng bed sequences ispreferred.Correla9ngdifferentoutcrops in thisway involves an immenseamountofdetailedmappingworkthatcanresultincoherentstra9graphicrela9onshipsvalidforlargeareasofthestudiedregion.However,becauseoftheperiodicandrepe99venatureofthePLDlayering,bedsequencesatdifferent stra9graphicdepths canappearvery similar, addinga largeamountofuncertaintytocolumnsbuiltsolelywiththismethod. With layer-scale imagesandtopography,con9nuousprofilesofvaria9ons inbrightnessortopographicexpressionwithdepthcanbeextractedforpar9cularexposures.Theseprofilescan then be directly compared to climate proxies such as insola9on or temperature changeswith9me(Laskaretal.,2002),oranalyzedforperiodiccyclesthatmatchthoseoftheclimatesignals(MilkovichandHead,2005;PerronandHuybers,2009;Limayeetal.,2012;Becerraetal.,2017b).Proper9essuchasbedprotrusion,localslope,andbrightnesscanbeextractedfromaDigitalTerrainModel(DTM)ofoneloca9onontheNPLD.ProtrusionwasdefinedbyBecerraetal.(2016)asthedifferencebetweentheactualtopographyintheHiRISEDTMandalinearfittotheslopeof the troughwall inapar9cular segmentof theprofile.Thisquan9ty representsaproxyfortheresistancetoerosionofthevariousbedsandisrelatedtolocalslope,whichisjust

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thederiva9veoftheeleva9onprofile.

Radar-basedstra5graphy

The ability to directly probe the internal structure of the PLD, made possible bysubsurface sounding radar, signified a substan9al step forward in the study of Mars PolarScience.Thedatareturnedbytheseinstrumentsconsistsoftwo-dimensionalradargramprofilesthatdisplaythepowerreturnedatthedetectoralongthetrackoftheradarvs.the9medelaybetweentransmissionoftheradarsignalandapar9cularsurfaceorsubsurfacereturn.The9medelay is roughly analogous to depth, but distor9ons occur due to surface topography andgeometry of the observa9on, as well as the change in speed of the radar wave in differentmedia.Therefore,9medelaycanbeconvertedtodepthusingasimple9me,velocity(modifiedbythematerialproper9es),anddistancerela9onship. If the depth or eleva9on difference between two points is known, then the dielectricvalueofthematerialcanbees9mated.Grimaetal.(2009)followedthisapproachfortheNPLDusingMOLAmeasurementsof theeleva9ondifferencebetween the topof theNPLDand thesurrounding plains and SHARAD radargrams with returns from surface and the base of theNPLD. They calculated an average value of ε' ~ 3.15, typical of nearly pure water ice undermar9ancondi9ons.Plautetal.(2008)calculatedasimilarvaluefortheSPLDusingMARSISdata. When radar data coverage is dense enough, various radar units can be mappedthroughout the whole PLD, resul9ng in thickness measurements of prominent units and, inessence,a radar-basedstra9graphy (Putzigetal.,2009;SmithandHolt,2015).Addi9onally,athree-dimensional radarvolumecanbeconstructedformappingstructures ingeometriesnotpermiMedwithonlytwo-dimensionalprofiles(Fossetal.,2017;Putzigetal.,2018).

Time-seriesanalysisandcyclostra5graphy

Afirststepintheprocessofunderstandingpaleoclimateistosearchforperiodici9esinthedatathatmatchthoseoftheclima9cforcingmechanismsthatarehypothesizedtohaveledto the PLD accumula9on. Assuming that 9me/depth-dependent func9ons like those in thestra9graphicprofilesareforcedbyhighlyperiodicfunc9onslikeMars’insola9onhistory(Laskaret al., 2004), 9me-series analysismethods decompose the data func9ons into their periodiccomponents. The twomost prominent peaks in power correspond to the 51 kyr precessioncycleofMars’argumentofperihelion,andtothe120kyroscilla9onoftheobliquity.Thetwocycleshavebeencompared to those in thestra9graphicdata (Becerraetal.,2017b),and thespa9alra9owasfoundtomatchfavorablywiththemodeledperiodici9es.

Detailedstra5graphyoftheNPLD

Stra9graphic studies of the NPLD are extensive, and data-based analyses have beengenerally accompanied by models of accumula9on based on orbitally-driven climate cycles.Fishbaughetal.(2010b)werethefirsttoconstructastra9graphiccolumnbasedontopographicandmorphologicconsidera9onsfromasingleHiRISEDTM.Theydefinedtwoprincipaltypesofbeds or bedding packets: Marker Beds (MBs, so-called because of Malin et al. (2001)’siden9fica9onof the“original”MarkerBed invarioussitesontheNPLDwithMOC),whicharethick, dark and have a characteris9c hummocky texture in the DTM they analyzed; and Thin

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LayerSets (TLS),whichare setsof resistantbeds,each~1–2m in thickness. In the spacingbetweenMBs,theauthorsreportedfindingthe30mperiodicitypreviouslyobservedbyLaskaretal.(2002)andMilkovichandHead(2005);andinthespacingbetweenthinlayersinTLS,theyobserved the 1.6 m wavelength of Perron and Huybers, (2009). Later, Limaye et al. (2012)measured bed thicknesses and performed FFT analysis on brightness and slope profiles of 3DTMs in theNPLD. They found a low variance in bed thicknesses,with themajority of bedsmeasuredtobejustafewmetersthick.Theirspectralanalysisconfirmedthe1.6mwavelength,but observed no 30 m wavelength. In addi9on, they observed a similar spacing betweenpaMerns inbrightnessprofilesandpaMerns in localslope,showingthatbrightnessmayinfactcorrelatewithtopographyinsomeloca9ons. Withcon9nuedobserva9onsof theNPLDbyHiRISE, thedatasetofDTMsof theNPLDgrew enough that correla9ons of layer sequences based on topographic, morphologic andreflec9veproper9es couldbeaMempted.Becerraet al. (2016)used linearprotrusionprofilestakenfromHiRISEDTMs,aswellasanalysisoforthorec9fiedimagestoclassifybedsequencesin16DTMsacrossthePLD,extendingtheMBandTLSclassifica9onofFishbaughetal.(2010b).Inaddi9on,theyusedacombina9onofsignal-matchingofprotrusionprofilesandlayertracingacross thebrightanddarkbedsseen inCTX images tocorrelateabedsequencepresent in6DTMs,resul9nginastra9graphiccolumnofprominentMBs.Becerraetal.,(2017b)performedwaveletanalysisonbrightness,slope,andprotrusionprofilesfromthe16DTMs.Theyiden9fiedtwo dominant stra9graphic wavelengths in all profiles: a common ra9o of wavelengths of1.98+0.15inthestra9graphicdata,systema9callylowerthanthe2.35ra9ooftheorbitalsignalsinthe2Mainsola9on.Thisra9omatchesthatobservedinsynthe9cstra9graphiesgeneratedbytheaccumula9onmodelofHvidbergetal. (2012), lending furthercredibility to thatmodel. Ifone assumes that the surface is young or currently accumula9ng, then this results in meanaccumula9onratesof0.54mm/yrforthetop500moftheNPLD.

Radar

OneofthefirstmajordiscoveriesbySHARADwastheconfirma9onthattheNPLDbedsare laterally con9nuous throughout almost the en9re extent of the dome, over 1000 km(Phillipsetal.,2008).Thecon9nuousradarreflec9onsobservedtypicallyconsistoffourpacketsoffinelyspacedreflectorsseparatedbyhomogeneousinterpacketregionsofmaterialwithfewornodielectric interfaces. Phillips et al. (2008) explained thepacket/interpacket structurebyrela9ng it to approximately million-year periodici9es in Mars’ obliquity and/or orbitaleccentricity. Periods of high obliquity and eccentricity would have resulted in high dustaccumula9onfromenhancedsublima9onandthereforeformedreflectorpacketsduetovaryingsiliciccontent.Periodsoflowobliquityamplitudeswouldhavemeanthighiceaccumula9onandlowduststormac9vity,resul9ngintheinterpacketzonesthathavefewerreflectors.Putzigetal.(2009)extendedthepreviouswork(seeFigs.9and10inPutzigetal.(2009))tosuggestuniformdeposi9onanderosionpaMernswerecommonthroughoutmostofNPLDhistory. Studieswith theSHARADdatasetalsoallowedresearchers toexplain the forma9onofChasmaBoreale(Holtetal.,2010)andtheonset,migra9on,andmorphologicaldiversityoftheiconicspiraltroughs(SmithandHolt,2010;2015). Finally, Smith et al. (2016) used SHARAD data to iden9fy and map a cap-wideunconformity in the NPLD. Thematerial that accumulated post-unconformity was called theWidespread Recent Accumula9on Package (WRAP). This package represents a change in

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SHARADreflectorproper9esinthetop<320moftheNPLD.ThemodelofLevrardetal.(2007)predicted that theupper~300mof theNPLD shouldhaveaccumulatedduring the last~400kyrs,correspondingtoasharpdropinobliquityandaverageinsola9onamplitude,whichwouldhave signified large amounts of ice being transported frommid-la9tudes to the north polarregion,andtheendofamar9an“IceAge”.Smithetal.(2016)foundgoodagreementbetweenpredicted andmeasured thickness andbetween a predicted andmeasured volume since thelastmar9an"iceage"andpresentday(Headetal.,2003).TheyascribetheWRAPunconformitytoapproximatelythispointinNPLDhistoryandes9matetheageoftheWRAPlayertobe~370kyrs, implying a maximum accumula9on rate locally of 0.86 mm/yr during that enhancedaccumula9onperiodandanaveragevaluecloserto0.32mm/yrfortheen9redeposit.

1.A.3ClimatemodelsandPLDforma)on

Introduc5onandMethodology

Modelsarecri9cal tools for inves9ga9ngthepastclimateofMars. Inpar9cular,globalclimatemodels (GCMs) have been used to study aspects of the Amazonian climate and theforma9onofthePLDs.SincetheAmazonianperiodischaracterizedbyasolarluminositysimilartothecurrentsolarluminosityandanatmosphericmasscomparabletowhatitistoday,wecanstudy the climate by considering how changes to the orbital parameters would affect thepresent-dayMarsclimate.Thus,thegeneralmethodologyforthesestudiesistoexecuteaGCMthat does a reasonable job of reproducing the current Mars climate with modified orbitparameters(obliquity,eccentricity,argumentofperihelion).

ModelingtheCurrentClimate

ThecyclesofCO2,dust,andwateraretheclimateofMars.SignificanteffortwithintheGCMcommunityhasbeeninvestedoverthepastfewdecadesinimprovinghowGCMshandleandpredictthesecycles.Thisrequirestheimplementa9onofawiderangeofphysicalprocessesthatgovernhowdust,CO2,andwatercycleinto,through,andoutoftheatmosphere.

Of the three climate cycles, the CO2 cycle is the most straigh�orward to simulate inGCMs. Surface energy balancemethods are used to compute surface CO2 condensa9on andsublima9onasCO2cycleintoandoutoftheseasonalCO2polarcaps.Atmosphericcondensa9onof CO2 is usually handled with a simple scheme in lieu of represen9ng the more complexmicrophysicalprocessesofcloudforma9on(Forgetetal.,1999;Haberleetal.,2008;Guoetal.,2009), although recent work has been done on improving how CO2 clouds are simulated inGCMs(Listowskietal.,2013;Dequaireetal.,2014).

Significantefforthasbeen invested in improvingthehandlingofwatercyclephysics inGCMs.TheNPRCandNPLDoutliersaregenerallyconsideredtobethesolesourceofwatertotheatmosphere(Navarroetal.,2014),butsomeinves9ga9onshaveincludedaregolithsource(BöMger et al., 2005). Cloudmicrophysical schemes have grown in sophis9ca9on to explicitlyinclude the physics of nuclea9on, growth, and size-dependent gravita9onal sedimenta9on(Montmessin et al., 2002/2004). The inclusion of cloud radia9ve effects has proven quite

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challenging due to the many complex feedbacks involved, but significant progress has beenmadeinrealis9callyrepresen9ngtheseasonalcyclesofwatervaporandcloud(Navarroetal.,2014;Haberleetal.,2018).

Thedustcycleremainsthemostchallengingofthethreeclimatecyclestosimulatefully.Inves9ga9onsthatincludethephysicsofdustlieingbasedonresolvedsurfacewindstressanddustdevils (and/orunresolvedsmall-scale lieing)areabletocapturegeneralbehaviorsoftheobserveddustcyclebutarethusfarunabletorealis9callysimulateothers(Kahreetal.,2006;Basu et al., 2004; Newman et al., 2002). In par9cular, capturing the observed interannualvariability of global dust storms remains elusive. In lieu of using fully interac9ve methods,studies that focus on other aspects of themar9an climate generally use prescribed or semi-prescribeddustmethodsbased. Inthesestudies,thehorizontaland/orver9caldistribu9onofdust is constrained by observa9ons (e.g., Mars Global Surveyor's Thermal EmissionSpectrometer (TES) and Mars Reconnaissance Orbiter's Mars Climate Sounder (MCS)(Montabone et al., 2015).Wemust be cau9ouswhenweuse the dust observa9ons for pastclimatestudiesbecauseitisunlikelythattheseasonalpaMernsofatmosphericdustremainthesameasorbitparameterschange.

CurrentUnderstandingofAmazonianClimate

Modeling theAmazonian climate involves runningGCMs that aregenerally capableofcapturing the main components of the current Mars climate for different orbit parameters.BecauseGCMsarecomplexandrequiresignificantcomputa9onalresources,itisnotpossibletoexplicitly simulate changing orbit parameters. Instead, combina9ons of obliquity, eccentricity,andseasonofperihelionarechosentomapouttrendsandbranchpointsinthebehavioroftheclimate.Whendesigningthesesimula9ons,thetotalinventoriesandavailablesurfacereservoirsofCO2,dust,andwatermustbetakenintoaccount.Theeffectsofincreasingordecreasingtheobliquityaregenerallymoresubstan9althanchangingtheeccentricityorseasonofperihelion.

Obliquity varia9ons have important consequences for the CO2 cycle. As obliquityincreases, theannualmean insola9onat thepoles increases.Forobliqui9esgreater than54°,the poles receive more insola9on than the equator. This drives more extreme seasonalvaria9ons in surface temperature and surface CO2 ice. Overall, the global average surfacepressuredecreaseswithincreasingobliquitybecausemoreCO2cyclesintoandoutofthepolarice caps seasonally (Mischna et al. 2003; Haberle et al. 2003; Newman et al. 2005). At lowobliqui9es(<~20°),permanentCO2icecapsformandtheatmospherecollapses(Haberleetal.2003;Newmanetal.2005;Manningetal.,2006;2019). Inacollapsedstate, theequilibratedatmosphericmasscouldbequitelow(~30Pa).

Obliquity varia9ons largely control where water ice is stable on the surface. At lowobliqui9es (<30°) when the polar insola9on is low, water ice is stable at the pole and theatmosphere is rela9vedry (Mischna et al, 2003; Forget et al. 2006; Levrard et al. 2007). Thehemispherewhere thepolarwater ice resides is likelycontrolledby theseasonofperihelion,with the north favored when the perihelion occurs near southern summer sols9ce and thesouthfavoredwhenperihelionoccursnearnorthernsummersols9ce(Montmessinetal.,2007).At moderate obliquity (30-40°), water becomes stable in the middle la9tudes and theatmosphere is considerably weMer (Madeleine et al. 2009). At high obliquity, water ice is

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destabilizedfromthepolesandbecomesstableatlowla9tudes(MischnaandRichardson2005Forgetetal.2006;Levrardetal.2007).Animportantfinalpointhereisthattheradia9veeffectsofwater icecloudshavesignificanteffects,par9cularlyatmoderate tohighobliquity.Modelspredict op9cally thick clouds that formuphigh,which allows them to significantlywarm thesurface,enhancethemeancircula9on,andproducesignificantsnowfall(Madeleineetal2014;Haberleetal.2012;Kahreetal.,2018).

Increasing obliquity significantly enhances predicted dust lieing and atmospheric dustloading.Asobliquityincreases,theHadleycellisenhancedduetoanincreasedequator-to-poletemperature gradient. The stronger return flow from the overturning circula9on increasessurfacestressandthuswind-stressdustlieing(Haberleetal.,2003).Oncedustislieed,posi9veradia9ve/dynamic feedbacks furtherenhance theHadley cell anddust lieing (Newmanetal.,2005). While this predicted behavior is robust, there are poten9al caveats that must beconsidered. The first is the incorrect assump9on that an infinite amount of surface dust isavailable for lieing everywhere on the planet. The second caveat is that early dust cyclesimula9ons at high obliquity did not includewater ice clouds. Clouds can scavenge dust andprovideaddi9onalradia9ve/dynamicfeedbacksthatneedtobefullyunderstood.

ModelingthePolarLayerDeposits

The PLDs contain a record of the mar9an climate over 9me, so it makes sense thatrealis9callymodeling this recordwill require theuseofmodels that are capableofmodelingthoseevolvingclimatestates.WhiletherehavebeensomeaMemptstouseresults(orgeneralbehaviors) from GCMs to self-consistently model the PLDs, this process has proven verychallengingduetothecomplexi9esoftheprocessesinvolved.

ThemostcomprehensivepublishedaMempttodothissofarisbyLevrardetal.(2007).Inthisstudy,GCM-predictedpolaricedeposi9onandremovalratesoverarangeofobliqui9eswereusedincombina9onwiththecomputedobliquityhistoryfromLaskar(2004)overthepast10millionyearstoquan9ta9velytracktheevolu9onofpolarsurface icereservoirs.Thestudyfoundthatthenorthcapcouldbegingrowingabout4millionyearsago.Theauthorsiden9fiedaparadox in their results,wherebyonly~30 layerscouldhavebeengeneratedby thechangingobliquityduring thepast4millionyears,which is inconsistentwith thevisible layeringof theNPLDbutsimilarinordertothenumberofradarreflectorsobserved.Thisislikelybecausetheamount of dust deposited in the polar regions (forming layers) varies on shorter 9mescales.ThatstudyLevrardstudywasbasedonanearlyGCMthatcontainedawatercyclebutnotaninterac9vedustcycle.Interac9vedustcyclestudiessuggestthattheamountofdustdepositedinthepolarregionscouldvarysignificantlywithvaryingorbitalconfigura9ons(e.g.,Newmanetal.,2005).Fullycoupleddustandwatercyclesimula9ons,likethosepresentedinapreliminaryforminEmmeMetal.(2018),willfurtherourunderstandingofhowthePLDshaveformedduetoorbit-drivenvaria9onsinthemar9anclimate.

1.A.4TerrestrialClimateStudiesUsingIce

Terrestrial glaciers and ice sheets provide accessible field sites that can connect our

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understandoflocalprocessestothoseonMars.TheyrepresentakeycomponentoftheEarthclimate system and interact dynamically with climate through several different processes.Duringglacialcycles,growthandretreatofglaciers inthenorthernhemisphereenhancestheeffects of climate changes through the ice-albedo feedback. In the presentwarming climate,surfacemel9ngandmasslossisfurtherenhancedduringsummerduetothedarkeningeffectofmeltwater and dust at the surface of retrea9ng glaciers and ice sheets. Increased dischargefrommarinetermina9ngglaciersduetowarmoceantemperaturesmaysubsequentlyinfluenceocean circula9onwith global effects. Three key themes related to terrestrial glaciers and icesheetsstandoutinmodernterrestrialclimatesstudies:understandingtheglacialcyclesofthePleistocene, deriving the paleo-clima9c history from the ice core archive, and es9ma9ng themasslossfromglaciersandicesheetsandtheircontribu9ontosealevelincrease.

ThegeneralflowpaTernoficesheets

On Earth, ice sheets form when climate condi9ons allow snow to accumulate at thesurfaceover thousandsofyears,andthey ini9ateandgrowfrommountainousareas thatarecolderthanlowereleva9ons. Inthelargeterrestrial icesheets, iceaccumulates intheinteriorbysnowfall,iceflowsslowlytowardsthemarginastheicesheetsspreadduetogravity,andiceis lost along themarginsby surfacemel9ngand runofforbydischarge into theocean frommarine termina9ng glaciers. Although ice sheets respond to climate changes on longer andshorter 9mescales, they may gradually reach a steady state where snow accumula9on isapproximately balanced by discharge and runoff. The large ice sheets of Greenland andAntarc9cawere es9mated to be close to steady state in the year 2000, at which point theybegan losingmass. In the high-eleva9on interior of these vast ice sheets, the ice sheets aremorethan3kmthickandbuiltupbylayersofpastsnowfall.Thelayershavebeencompressedintoglaciericeandthinnedastheygraduallysankintotheicesheet,andtheywerestretchedasthe icemoved slowly towards the coast. Climate proxies from ice core drilled through theselayersneedtotakeintoaccountthethinningandstretchingofthelayersduetoflow.

Transforma5onofsnowtoice

Freshly deposited snow in the interior of theGreenland or Antarc9c ice sheets has adensityof300-400kmm-3.Densityvaria9onsinthetoptensofmetersofthesnowpack(firn)reveal an annual stra9graphy due to temperature and impurity content aswell as individualevents, e.g. wind crusts formed by packing ac9on of wind on previously deposited snow orsummermeltlayers.Withinthetop60-120m,thefirngraduallytransformsintoglaciericewitha density of 830 km m-3. Flow and compac9on of air bubbles below the firn-ice transi9on,increase thedensity further to920kmm-3,which remainsapproximately constantat greaterdepths.Thedepthof thefirn-ice transi9ondependsmainlyon themeanannual temperatureandtherateofsnowfall.IntheinterioroftheAntarc9cicesheetwheretheaccumula9onrateis2-5cmofice/yrandmeanannualtemperaturesbelow-50°C,thefirn-icetransi9onoccursinadepthof>100m.IntheinterioroftheGreenlandicesheet,theaccumula9onrateis15-30cmof ice/yr,theannualmeantemperatureisaround-30°C,andthefirn-icetransi9onoccurs inadepth of 70-80m.Glacier ice contains air bubbleswith samples of the atmosphere from the9me of the bubble close off and are used to document varia9ons in past atmosphericcomposi9on(CuffeyandPaterson,2010).

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Composi5on,stra5graphyand5mescales

Terrestrial ice sheets generally consist of nearly pure water ice with a small impuritycontent. Impuri9es originates from aerosols or par9cles deposited with the snow. Impurityrecords have an annual varia9on related to the atmospheric circula9on and transport or arerelatedtospecificevents,e.g.volcanicerup9onsorforestfires.Thestra9graphyinicecoresisobserved by the electrical conduc9vity measurement (ECM), by con9nuous high-resolu9onprofiles of concentra9ons of chemical impuri9es using the con9nuous flow apparatus (CFA)method,orbyconcentra9onofdustpar9cles.ECMisameasureof theacidityof the iceandusedtoiden9fyvolcanicreferencehorizonsduetotheirhighconcentra9onofsulfuricacid,andclimate transi9ons due to the shies in impurity concentra9ons and thereby changes inconduc9vity. CFA provides mul9ple con9nuous records of impuri9es and thereby allowiden9fica9onofannuallayers.InGreenland,theannualaccumula9onrateissufficientlyhightopreserve an annual signal, and it is possible to detect the seasonal cycle in several impurityconcentra9on records, e.g. insoluble dust, Ca2+, NO3

-, ECM. Insoluble dust par9cles fromcon9nentsarecarriedbywindsthroughthetroposphereanddepositedovertheicesheets.Thepar9clestypicallyhaveasizeoforder0.1-2μmandabulkdustconcentra9onof50-200μgperkg of ice in the interior of Antarc9ca andGreenland, respec9vely. During glacial periods, theconcentra9onof con9nentaldust increasesbya factorof10-100 (Greenland)anda factorof20-50(Antarc9ca).InGreenland,thedustconcentra9oninglacialiceishighenoughtoinfluencethecrystalsize,andlayersofhighdustconcentra9onareassociatedwithsmallcrystals.Crystalsizeisgenerallyintheorderof0.1-1cm2andincreaseswithdepthbutdropsattheHolocene-Pleistocenetransi9onwithafactorof2.Althoughthedustisnotvisiblebyitself intheglacialice,dust-richlayerscanbeiden9fiedinthevisiblestra9graphyoficecoresascloudybands.Thevisiblestra9graphyinGreenlandicicecoreshasrevealedannuallayersintheglacialicedowntoaresolu9onof1cm(CuffeyandPaterson,2010) InGreenland,icecoreshavebeendatedaccuratelybylayercoun9ngbackto60kyragousingacombina9onofvisiblestra9graphy,concentra9onofdustandchemicalimpuri9es.IntheinteriorofEastAntarc9ca, theannualaccumula9on isonly fewcen9metersandannual layerscannot be iden9fied. Ice cores in East Antarc9ca are dated from a combina9on of referencehorizonswithdatestransferredfromGreenlandic icecoresorotherpaleoclima9crecordsandice flow modeling, taking into account thinning of layers due to flow and using consistentrela9onsbetweenclimateandsnowaccumula9onrate.

Radarstra5graphy

Radio-echo sounding is used to map the stra9graphy of terrestrial ice sheets. Thestra9graphyinthetoptensofmetersismappedwithsnowradarsystems,employedfromthesurfacewithgroundpenetra9ngradar(GPR)systemsorfromairplanescarryingradarsystems.ThesnowstructureofthedrysnowzoneinGreenlandhasforexamplebeenmappedwithanairborneKu-bandsynthe9capertureradar(SAR)systemandlinkedtoin-situobserva9onsfromshallowicecoresandpitstudies.Theobserva9onsrevealtheannuallayerstructureinthetop15-20meterrelatedtodensityvaria9onsinthefirn,whichformsduetoseasonalvaria9onsintemperature,snowfallandimpurityconcentra9ons.Repeatedsurveysoverseveralyearsshowhowlayerssinkintotheicesheetandnewlayersformatthetopandallowdetailedmappingof

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spa9alandtemporalvaria9onsoftheannualsnowaccumula9on(Simonsenetal.2013). In general, the radar stra9graphy of ice sheets arises from transi9ons in the bulkelectrical conduc9vity of themedia due to density varia9ons, volcanic reference horizons, orclima9ctransi9onsassociatedwithchangesinimpurityconcentra9onoftheice.Deepinternalradarlayershavebeenlinkedtoicecoresanddatedusingicecore9mescales(e.g.MacGregoret al 2015). InGreenland, they are associatedwith abrupt climate transi9ons during the lastglacial, known as Dansgaard-Oeschger cycles, or with glacial-interglacial transi9ons. In bothGreenland andAntarc9ca, echo-free zones occur. InGreenland, ice deposited during the lastglacialmaximumbetweenapproximately15and30kyrBPcontainnodis9nctradar-echolayers,possibly due to a generally high level of con9nental dust (CaCO3 and Ca2+ ions), whichneutralizesanyvolcanicaciditypeaksduringthatperiod.TheoldesticeinbothGreenlandandAntarc9caareechofreeorhaveafewblurredlayers,andhereithasbeensuggestedthatfloweffects in thesedeep andold layers couldhave smearedout the transi9ons, e.g. by thinninglayersor shearingand folding layersofdifferent rheology. The internal radar layers show thepastsurfacesoftheicesheetastheysinkdownandaresubjecttoflow,includingthinningandstretching, local varia9onsofbasalmel9ng, and large-scaleevolu9onof the ice sheetand itsflowpaMern.Largestructuresofupto50%oftheicethicknesshavebeenobservedwithradarin bothGreenland andAntarc9ca and proposed to originate from large-scale folding or fromrefrozenmeltwater plumes through a process similar to the forma9onof permafrostwedges(Belletal2011),buttheoriginofthesestructuresiss9llnotknown.

Theclimatearchiveinicecores

Paleo-clima9c records from terrestrial ice coreshave revolu9onized theunderstandingof theEarthclimatehistoryand, togetherwithoceansedimentcores,providedknowledgeofglacialcyclesandbeyond.Thekeycontribu9onsoficecorestothepaleoclima9ccommunityaretheir accurate and independent 9mescales as well as the unique informa9on of pasttemperaturesfromoxygenisotopes(e.g.δ18OandδD)andpastatmosphericcomposi9onfromair trapped within the ice (e.g. CO2, CH4) (Cuffey and Paterson, 2010). The ability to detectannual varia9ons inmany different parameters, such as oxygen isotopes, insoluble dust, andchemical impuri9es— and thereby iden9fy and count each year—has contributed tounderstandingthe9mingandphasesofclimatechangesinthepast.Terrestrialicecorescontainan undisturbed stra9graphic record of ~100 kyr (Greenland) and ~1 Myr (Antarc9ca). Olderlayers are disturbed by flow or removed by basalmelt. The climate archive of terrestrial icecores is con9nuously expanded with new climate proxies and new techniques thatmakes itpossible to detectmore parameters and reduce sample size. One example is the con9nuousflowapparatus(CFA)technique.Iceiscon9nuouslymeltedalongtheicecoreandanalyzedinasemi-automa9cwet-chemistry lab toprovide recordsof isotopes, ions, ca9ons, insolubledustpar9cles, conduc9vity, black carbon, etc., that contain informa9on about atmospheric andoceanic circula9on paMerns, sea ice condi9ons, forest fires, far-field humidity, etc. Theinterpreta9onoftheseproxyrecordsisdoneincombina9onwithclimatemodelingandpaleo-clima9crecordsfromotherarchives.

Selec5onofthedrillsiteandrecommenda5onsforMars

IcecorerecordsfromtheinteriorGreenlandandAntarc9caarewidelyusedtoinferthe

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climate history of Earth because the 9mescale is accurate and reliable and the records havebeen interpreted to provide the hemispheric or global climate history. The ice core recordscontain con9nuously deposited layers and allow age determina9on from measurements ofcons9tuentsknowntovaryonannual9mescalescombinedwithreferencehorizonsknownfromotherstra9graphicrecords.ComparisonsbetweenicecoresorpitsspacedovertensofmetersininteriorGreenlandshowsignificantlocalvaria9onsinsnowaccumula9onrateduetosurfaceroughnessandwindeffects,wheresnowisredistributedatthesurfaceorremovedduetowindscouring, butdecade-averaged records are similar. Radar stra9graphy from the topmetersofthefirntothedeep(2-3km)layersarecon9nuousandsmoothlyvaryingandshowsimilarlythatthestra9graphyofthelargeicesheetsrepresentslarge-scaleclimatevaria9onsinthepast,notlocalsnowaccumula9onpaMerns. Priortoanicecoredrillingprogram,dataiscollectedfromtheareaandusedforthedrillsite selec9on and planning of the campaign. Surface campaigns and radar surveys provideinforma9on on annual snow accumula9on rate, ice thickness, ice flow velocity, surfacecondi9onsandlocalweather,aswellas internal layerstructure. Ingeneral,sitesarepreferredover smooth bedrock, with smooth, unfolded internal layers, no signs of basal mel9ng orcomplicated flow over complex underlying topography (e.g.mountains), only liMle horizontalmovement, with cold surface condi9ons, and nomel9ng at the surface or at the base. Theop9malsitehasundisturbedlayersandnocomplica9ngupstreamcondi9ons. OnMars, thepolarcapsarenotasdynamicas theterrestrial icesheets.Althoughthelayers in the mar9an polar caps are not significantly influenced by flow, and no mel9ng isexpectedtooccuratthesurfaceorthebase,similarconsidera9onsarerelevantforstudyingthestra9graphyor selec9ngapoten9aldrill siteas for terrestrial studies.Radar surveys showaninternal layer structures with con9nuous layers across the polar caps, sugges9ng that theselayersrepresentlarge-scaleclimatevaria9onsinthepast,similartothelayersobservedintheterrestrialpolar icesheets (Phillipsetal.,2008).Whilethecurrentannualsnowaccumula9onon the polar caps of Mars is es9mated from atmospheric observa9ons of humidity incombina9onwith climatemodels, it is notwell knownhowor if the current annual rates ofdeposi9on and sublima9on relates to the layer sequence. The deposited layers may not berelated to the annual cycles, but to climate cycles on centennial, millennium or orbital9mescalesmuch longer thanyears.Theresolu9onof the layeringalsodependsonhowpost-deposi9onalprocessesmodifythesurfaceofthelayereddeposits(Smithetal.,2013).

1.BHistoryofMarsPolarInves)ga)ons

1.B.1Orbiters

Spacecraeobserva9onsofthepolarregionsofMarsbeganwiththeMariner7flybyin1969.Theseasonalcapswereassumedtobewatericeun9lthisevent.Predic9onthattheCO2atmosphere should condense in the winter (Leighton and Murray, 1966) was confirmed byMariner 7 infrared spectrometer observa9ons that recorded the appearance of forbiddentransi9onsof CO2 ice (Herr andPimentel, 1969). Imageryof thepolar regionsbegan in 1971withMariner9andcon9nuedwiththecomprehensivecoveragebyVikingorbiters(1976-80).Inthemodernera,imageryateverincreasingspa9alresolu9onhasbeenobtainedfromtheMars

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GlobalSurveyor(MGS),Odyssey,MarsExpressandMarsReconnaissanceOrbiter(MRO)orbitersusingtheHighResolu9onStereoCamera(HRSC),MOC,THEMIS,CTX,andHiRISE instruments.Infrared observa9ons to observe surface proper9es began with mul9-channel instruments(Infrared Thermal Mapper (IRTM) on Viking) and advanced to full spectroscopy andcomposi9onalmeasurementswiththeMGSthermalemissionspectrometer(TES)andcon9nueswithMarsExpress’ObservatoirepourlaMinéralogie,l'Eau,lesGlacesetl'Ac9vité(OMEGA),andMRO’sCompactReconnaissanceImagingSpectrometerforMars(CRISM).Infraredspectroscopyfocused on atmospheric cons9tuents and processes began with the Infrared InterferometerSpectrometer (IRIS) instrument on Mariner 9 and con9nued with MGS’ TES, Mars Express’Planetary Fourier Spectrometer (PFS), and MRO’s Mars Climate Sounder (MCS) (e.g. Smith,2008).GammarayandneutronspectroscopyonOdysseyhasbeenusedtodetectbothburiedicedepositsandthecolumndensityoftheseasonalice.Laseral9metryfromtheMarsOrbiterLaserAl9meter(MOLA)hasbeenusedtodeterminethethicknessoftheseasonalicedeposits.Radar measurements to probe the subsurface has been performed from bothMars ExpressusingtheMarsAdvancedRadarforSubsurfaceandIonosphereSounding(MARSIS)instrumentandMROusingtheShallowRadar(SHARAD)sounder.Table1summarizesthesepastspacecraeandinstruments. The history of Mars polar cap observa9ons through the Viking era is summarized inMars,theUniversityofArizonaPressbook,inchaptersbyJamesetal.,(1992)andJakoskyandHaberle (1992). Approximately one decade ago, Titus et al. (2008) provided an update andreviewofnewinforma9onemerginguptothatpoint,summarizingmar9anpolarprocessesfortheCambridgemonograph“Themar9anSurface”. Op$cal and infrared observa$ons have focused on annual processes, inter-annualvariability,composi9onandphysicalstate.Thisincludesseasonalcapadvanceandretreat,masswas9ng, mobility of high and low albedo deposits, geomorphology of features in the southresidualCO2ice,thepresenceofH2O,CO2andnon-icematerials,andtheirgrainsizesorop9calpathlengths.Stra9graphyisalsoamajorcomponentofimagescience. Seasonal Processes – Themost recent analyses of the seasonal cap cycle fromMarsColor Imager (MARCI), TES, andMCS establish amar9andecadeof observa9ons (Mars Years(MY)23-32)(Calvinetal.,2015,Calvinetal.,2017)andallowcrea9onofaclimatologicalmodelaverage over many years (Piqueux et al., 2015). Spectrometer systems have studied thecomposi9onoftheretrea9ngcap,mappingwaterinbothanannularringatthecapedgeandasalagontheretrea9ngnorthseasonalcap(Appéréetal.,2011;Brownetal.,2012).Inthesouth,theproper9esofthe“cryp9cterrain”intheretrea9ngseasonalcap,andtransientbrighthalossurroundingpitsintheresidualCO2icehavebeenstudied(Langevinetal.,2007;Schmidtetal.,2009;Becerraetal.,2015).The latestatmospheric soundinghasdemonstratedsnowfall isanimportantprocessinformingseasonalcaps(Hayneetal.,2014). Residual Ices and Long-term Geomorphic Change – Both OMEGA and CRISM haveobserved the proper9es of the residual ice caps, no9ng grain size changes, layer proper9es,shiesbetweendeposi9onandsublima9on(Langevinetal.2005;Calvinetal.2009;Brownetal.2016). In the south, water ice observed at the margins of the south residual cap has beenmodeled (Douté et al. 2007). To date liMlework has been done tomodel the composi9onalproper9esof thecomplexgeomorphologyof thesouthresidualCO2 ice (Thomasetal.2009).However long termanddetailedanalysisusingCTXandHiRISEhasclassified this surface intoseventeen morphological units with varying thickness and inferred deposi9onal 9meframes,anditissuggestedthatwellover80%ofthesouthernresidualcaphasbeenresurfacedinsome

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Spacecra@ InstrumentsforPolarStudies Proper$es

Mariner7Flyby-1969

TVCameraInfraredSpectrometer

RGBFilters19to14.4μm~200x100km

Mariner9Orbiter1971-1972

ImagingSystem WideandNarrowAngle

InfraredInterferometerSpectrometer(IRIS)

5-50μm~110kmspot

Viking1&2Orbiters1976-1980

ImagingSystem ~40mto300m

InrafredThermalMapper(IRTM) 5thermalbandsand1solar(albedo)channel30-170kmspot

MarsGlobalSurveyor(MGS)1996-2006

MarsOrbiterCamera(MOC) NA-B/W-2-12m/pixWA-RB-240m/pix

ThermalEmissionSpectrometer(TES)

5-50μm6kmspot

MarsOrbiterLaserAl9meter(MOLA)

180mspotsize,0.2mver9calaccuracy

Odyssey2001-Present

ThermalEmissionImagingSystem(THEMIS)

VIR-5channels20m/pixTIR-10channels100m/pix

GammaRaySpectrometer(GRS) 600kmresolu9onoftheupper1m

MarsExpress2004-Present

HighResolu9onStereoColorImager(HRSC)

4colorandpan,stereo,10m/pixSuperresB/W2.3m/pix

OMEGASpectrometer 0.36to5.2μm,~200mto1km/pix

PFSFourierSpectrometer 1to45μm,~10kmspot

MARSISRadarSounder ~100mver9calres/>2kmpenetra9ondepth

MarsReconniassanceOrbiter2006-Present

MARCI 5color,~1km/pix

CTX B/W,~5m/pix

HiRISE B/Ww/colorstrip25cm/pix

CRISM 0.36-3.92μm,18to200m/pix

MCS 8channel,4to6kmspots

SHARAD 8-15mver9calresolu9on/upto2.5kmpenetra9on

Table1:ListofNASAandESAmissionsthatcollecteddataatMars.

fashioninthelast40MarsYears(Thomasetal.2016). Non-ice material composi$on and proper$es – Several studies have examined thedistribu9on of hydrated materials, altered glasses and basal9c materials, and the origins ofgypsum on the north polar dunes (Horgan et al. 2009; Horgan and Bell, 2012;Massé et al.,2012). Thermal proper9es have demonstrated that the north polar dune material overliesshallowgroundiceorice-cementedsands(Putzigetal.2014). Recentradarobserva$onsbyMARSISandSHARADradarsoundinghavedemonstratedthatthetransparencyoftheSPLDicetoradarsignalsequatestodustcontamina9onoforder10to15%(Seuetal.,2007;Plautetal.,2007;Zuberetal.2007).Observa9onsbySHARADseealarge “reflec9on free zone” in the interior that is interpreted as a large deposit of CO2 ice(Phillipsetal.2011).Expandedcoverageandanalysisshowsthisunithasseverallayersthatareinferredtobestabilizedbyinterbeddedandoverlyinglayersofwaterice(Biersonetal.2016).Detailedthree-dimensionalmodelinghassuggestedthelong-termaccumula9onhistoryoftheNPLDandiden9fiesburiedstructures(Fossetal.,2017;Putzigetal.2018).

1.B.2Landers

The polar regions of Mars have been a high-priority target for landed missions fordecades. Since their discovery in Mariner 9 images (Murray et al., 1972), the polar layereddeposits were suggested to contain a climate record, which could be accessed in at least alimitednumberof exposures (CuMs, 1973;Howardet al., 1982).High-resolu9on images fromtheMarsGlobalSurveyor’sMarsOrbiterCamera(MOC)revealedmuchmoreextensivelayereddepositsthanhadbeenpreviouslyrecognized,especiallyinthesouthpolarregion(Malinetal.,1998).

Mars Polar Lander. Landing site selec9on for NASA’s Mars ’98 mission was guidedtowardsthesouthpolardeposits,whereremotesensingdatarevealedavastvolumeoficejustbeneathalayerofdustorregolith.ThismissionbecameMarsPolarLander,whichhadprimaryscienceobjec9vestodig intothesubsurfacetosearchforwater, tomeasuretheatmosphericcomposi9on, and to survey the south polar layered deposits to beMer understand theirforma9onmechanisms. Its landingsitewas locatedinaregion73–78°Sand170–230°W.Inaddi9on to themain lander, theMarsPolar Lander spacecraealso contained twopenetratorprobes,calledDeepSpace2(DS-2)designedtosampletheatmosphereduringdescent,andthesubsurfacesoilandicelayersfollowingimpact.Unfortunately,amalfunc9onintheMarsPolarLander’s descent stage caused the catastrophic loss of both the primary payload and DS-2(Albeeetal.,2000).Subsequenteffortstolocatethelanderusinghighresolu9onimaginghavenotsucceeded. Phoenix. Resurrected as the Phoenix mission, the Mars Polar Lander spacecrae wasrebuilt and repurposed to a high la9tude loca9on. Launched in 2007, Phoenix landed at aposi9onof68.22°N,125.7°WonMay25,2008.Itsprimarypurposewastostudythegeologichistoryandhabitabilityof subsurfacewaterby samplingmaterial at thishigh-la9tude landingsite.Although its loca9onwasover700kmfromtheNPLD,thePhoenixmission’s landingsitewas on top of a region with well-established ground ice, where remote sensing and modelcalcula9onshadshownicelikelytobepresentintheupperfewcen9meters(Fanaleetal.,1986;MellonandJakosky,1993;Feldmanetal.,1993;AharonsonandSchorghofer,2006;PutzigandMellon,2007). Indeed,therobo9carmandscoopstruckicebeneatha layerof loose, ice-freesoil~5cmthick(Mellonetal.,2009).Furthermore,Phoenixdetectedapparent liquiddroplets

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onitslanderlegs,whichcouldbeduetocondensa9onormel9ngondeliquescentsalts(Rennóetal.,2009).Significantexchangeofwatervaporbetweenthesurfaceandatmospherewasalsodetectedbythelander’sthermalandelectricalconduc9vityprobe(TECP;Zentetal.,2010).

1.B.3PreviousConceptStudies

Inthepast15years,mul9plemissionstothenorthernpolardepositshavebeenproposedaspart of either concept studies or spacecrae proposals to NASA. None passed beyond theproposal stage. They are summarized here to represent past polar community thinking onobserva9onalstrategies.Onecanseethatwhilethetechnologyandexplora9onpla�ormshavevaried,thecoreobserva9onalstrategiesdesiredbythepolarcommunityhasnot.

DecadalSurveyconceptstudiesInamissionconceptstudyassociatedwiththeSS2012PlanetaryScienceDecadalSurvey,Calvinet al (2010) iden9fied how to use the following basic architectures for near-future polarmissions: Discovery-class orbiters, New Fron9ers-class orbiter, Discovery-class lander, NewFron9ers-classlander,andNewFron9ers-classrover.

Discovery-classorbiterThismissionconceptissuitedtotwofundamentalanalysesatlong9mescales(>1Marsyear):inves9ga9on of the current climate and seasonal cap proper9es, or inves9ga9on of surfaceenergybalanceandcomposi9on.Thefirst inves9ga9onwould focusonweathercamerasandatmospheric sounding techniques, while the second would u9lize mineralogy/spectroscopymeasurements and an ac9ve sounder for observa9ons during the polar night. Bothinves9ga9onswouldberegional(cap-wide).

NewFron5ers-classorbiterThis mission concept is similar to the Discovery-class orbiter, but could support bothinves9ga9onsandassociatedinstrumentsuitesoutlinedabovewithasinglepla�orm.

Discovery-classlanderThis mission concept is suited to detailed (cm-scale) stra9graphic analysis of the layereddepositsandthecurrentsurface-atmosphereinterac9onsonashort(<1Marsyear)9mescale.Itwouldlandasta9onarypla�ormatthebaseofastackofexposedpolarlayereddepositsandexaminethelayersop9callywithremotesensingtechniquessuchasimagingandspectroscopy.Ameteorologicalpackagewouldbeincluded.Thisanalysiswouldbefairlylocal;layersexposedalong the troughwall couldbeobserved inmul9pleplaces froma sta9onarypla�orm for anassessmentofmeters-scalevaria9ons.

Discovery/NewFron5ers-classlanderwithsubsurfaceaccessThis mission concept is suited to very detailed (mm-to-cm scale) chemical and stra9graphicanalysisofthelayereddepositsandthecurrentsurface-atmosphereinterac9onsonashort(<1Marsyear)toalonger(>1Marsyearifusingaradioisotopicpowersource)9mescale.Itwouldlandasta9onarypla�ormontopofastackofpolarlayereddepositsandexaminethelayersviaadrillandinsitumeasurementtechniques.Therearemanyvaria9onsofthisconcept,includingthermaldrillvsmechanicaldrill technologyandtakingmeasurementsdown-boreholevs from

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thesurfacepla�orm.Thisanalysiswouldbeextremelylocal–asingledrillloca9on.

TwoproposalsforDiscovery-class(MarsScout)landerswithsubsurfaceaccessaredescribedinmoredetailbelow.Neitherwereselected.

NewFron5ers-classroverThis mission concept is suited to very detailed (mm-to-cm scale) chemical and stra9graphicanalysisofthelayereddepositsandthecurrentsurface-atmosphereinterac9onsonashort(<1Marsyear)toalonger(>1Marsyearifusingaradioisotopicpowersource)9mescale.Itwouldlandarovercarryingarockcorerordrill,andcollectsamplesforanalysisthroughoutitstraverseacross exposures of layered deposits. This analysis could be semi-regional (on the scale ofmul9plekm),dependingonthedistancetraversedbytherover.

AsimilarroverconceptwaspartofaNASAVisionstudyandisdescribedinmorebelow.

Thermaldrill-basedlanderconceptsThree detailed mission concepts from the last 15 years have focused on accessing thestra9graphywithinthenorthernpolardepositsusingathermaldrill,whichmeltsaholeintheicethroughtheproduc9onofheatwithinthedrill itselfanddescendsonatetherastheholedeepens.Thus,thedrilldescendsintotheholebytheactofcrea9ngthehole.Analysisoftheinterior of the polar depositsmust be done on thewalls of hole as the drill descends usinginstrumenta9on fit within the thermal drill itself, or upon the resul9ng meltwater pumpedthroughthetethertoa instrumentedsurfacepla�orm.Thethreemissionsdescribedhereareeachbuiltaroundadifferentversionofthethermaldrilltechnology.Notethatwedescribethebig-pictureconceptsofeachmissionbutdonotdescribedetailsoftheinstrumenta9onanddrilltechnologiesinvolved.

Cryoscout,MarsScoutProposalCryoscoutwasproposed in2002byPrincipal Inves9gatorFrankCarseyand teamto theMarsScoutProgram, fora2007 launchopportunity. Theoverallmission conceptwas to repurposethe2001MarsSurveyorlanderpla�ormforananalysisofthenorthernpolarstra9graphyviaathermaldrillthatwoulddescendupto80mintothePLD(Zimmermanetal,2002).

MissionoverviewCryoscoutwouldhavelandedonthesurfaceoftheNPLDinlatenorthernspringof2008.Asolarpowered mission, it would have used a gimbaled solar array to take advantage of thecon9nuouspolarsunlighttoconductdrillopera9onsfor90days(Zimmermanetal,2002;HechtandSaunders2003).

The Cryoscout thermal drill (or “cryobot”) was an ac9ve water-jexng system that wasan9cipatedtodescendatanaveragerateof4cm/hour(HechtandSaunders,2003).Thefrontof the cryobot contained mul9ple heaters, water jet nozzles, and a pump bay. The cryobotwouldbeginbypassivelymel9ng the ice around its nose; asmeltwaterbuilt up, the cryobotwouldpumpmeltwaterintoitsinteriorandpressurizeittousethefrontwaterjetstoremovepar9culatesawayfromthefrontheaters.Intheinteriorofthecryobotwasalsotheinstrumentbay,with awindow for imaging instruments and small chambers formeltwater analysis. The

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tether for thecryobotwascarried insidetheaebayof theprobe itselfandunspooledas theprobedescended;thefixedendofthetetherwasatthesurfacepla�orm.Powerforthecryobotanddatafromthecryobotwerecarriedalongthetether(Zimmermanetal,2002).

The thermal drillwould have descended in a 1mm thick “melt-water jacket” and the tetherfixedinplaceastheholefrozeshut~metersbehindthecryobot(Zimmermanetal,2002).Thismeantthatmeasurementoffine-scalevaria9onsinlayerstra9graphyandchemistrywithdepthwouldhavebeencomplicatedbymixingofthemeltwaterandconcentra9onofpar9culatesasthedrilldescended(HechtandSaunders,2003).

InstrumentsuiteTheinstrumentsuiteforCryoscoutwasdividedintoapackageinthecryobotprobecasing,asetof instruments incorporated into the tether itself, and a package on the lander pla�orm (allsummarizedfromHechtandSaunders,2003).

• Withinthethermaldrill:• Imagingnephelometertorecordthevisiblestra9graphy

• 1-mmver9calresolu9oninnephelometermode• full-colorstereoimagesat10micronsperpixel

• A suite of electrochemical sensors to determine salt composi9on andabundanceinmeltwater

• Isotopiclaserhygrometertomeasurevaria9onsinrela9veHandOisotopicabundanceinmeltwater

• Withinthetether:• Distributed fiber thermometer incorporated into the tether to collect the

9me dependent ice temperature profile, including the thermal wavepenetra9oninthetop~20mandthegeothermalheatfluxbelowthat.

• Onthesurfacepla�orm:• Stereoscopic imager to study the dynamics of polar surface, including

atmosphericopacityandsurfacealbedo• Surfaceversionoftheisotopiclaserhygrometertorecordthemovementof

water vapor, provide a baseline measurement of isotopic ra9os, andmonitorbasicmeteorology.

Chronos,MarsScoutProposalChronoswasproposed in2006byPrincipal Inves9gatorMichaelHechtand teamto theMarsScoutProgram, fora2013 launchopportunity.Theoverallmissionconceptwasanupdate totheCryoscoutproposalthatincludedtwothermaldrills,oneshallow(10m)andonedeep(75m)(Hecht,2006).

MissionoverviewChronoshadasimilarmissionprofiletothatofCryoscout:usingthe2001MarsSurveyor/2007Mars Pheonix pla�orm to land in late northern spring for a solar-powered, 90-day mission.However, the thermal drill technology had evolved considerably, and the associatedobserva9onalstrategywasupdatedtomatch.

TheChronos thermaldrillwasapassive,dry-holesystemthatpumpedmeltwaterup through

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thetetherasthedrilldescended(Smithetal,2006);itdidnotincludetheac9vemeltwaterjetsoftheCryoscoutthermaldrill (Zimmermanetal,2002).SimilartoCryoscout,thethermaldrillwasseparatedintomul9plebaysincludingapumpbayandaninstrumentbay,andthetetherservedasapathwayforpoweringthedrillandforreturningdatafromthedrill.Thetetherdidnot unspool from the thermal drill but from the surface pla�orm (Smith et al, 2006). Itwasan9cipatedtodescendatratesof20-45cm/hour(Hecht2006).

A significant new feature of the Chronos drill systemwas bringingmeltwater to the surfacethroughtheheatedtether.Thismeantaddi9onalmeasurementsofthemeltwaterwerepossibledue to not being required to fit instrumenta9on into the drill itself. Furthermore, the spa9alscale of the mixing of meltwater in the tether was much smaller compared to that of theCryoscout borehole, which translates to finer spa9al resolu9on of meltwater-basedmeasurementsofvaria9onsinlayerstra9graphyandchemistry(Hecht2006).

Chronoswastocarrytwoiden9caldrills,onewitha10mtetherandonewitha75mtether.The shallow drill was to be deployed first, allowing for a rapid assessment of near-surfacestra9graphy.Opera9onal lessons learned from the shallow drill could then be applied to thedeploymentofthedeepdrill(Hecht2006).

InstrumentsuiteThe instrumentsuite forChronoswasdivided intoapackage inthethermaldrillcasingandapackageonthelanderpla�orm(allsummarizedfromHecht,2006).

• Withinthethermaldrill:• Imagingnephelometertorecordthevisiblestra9graphy

• 1-mmver9calresolu9oninnephelometermode• full-colorstereoimagesat10micronsperpixel

• Temperature sensor to determine the ice temperature profile during drillsleepperiods(deeperdrill)

• Miniature seismometer that freezes into the ice tomeasure fundamentalproper9esoftheinteriorstructureofMars(shorterdrill)

• Onthesurfacepla�orm:• Stereoscopic imager to study the dynamics of polar surface, including

atmosphericopacityandsurfacealbedo• A suite of electrochemical sensors to determine salt composi9on and

abundanceinmeltwater• Isotopic laser spectrometer to measure varia9ons in rela9ve isotopic

abundanceofHandOinthemeltwater• Meteorology package to characterize the surface fluxes of the polar

deposits

PalmerQuest,NASAVisionStudyPalmer Quest was a NASA Vision Study under the direc9on of Frank Carsey in 2005, todeterminehowtheapplica9onofkeyNASAstrategictechnologydevelopments(e.g.,advancednuclear systems forplanetary explora9on) couldbeused to furtherNASA science goals (e.g.,assessing the presence of life and evalua9ng the habitability at the base of the north polardeposits)(Carseyetal,2005a).Thismakesitaverydifferentsortofcreaturethanthemission

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proposals such as Chronos or Cryoscout,whichwere designed for par9cular launchwindowsandbudgetenvelopes.

MissionoverviewPalmerQuestencompassedamul9-pla�ormexplora9onofthenorthpolardeposits,includingalanderwithathermaldrilltoexplorethelayereddepositsinsitu,arovertolaterallyexplorethesurfaceexposureofthelayereddeposits,andasurfacesta9onthatwoulddeploysomedistancefromthedrillpla�ormtomeasuresurfacefluxesandmeteorological condi9ons (Carseyetal,2005b).

Lander/drillplaXormconceptandinstrumenta5onThelanderforPalmerQuesthadmanypurposesoverthecourseofthemission.Itfirstwastoserveas thedescentvehicle forall thecomponents.Oncethesurfacesta9onandroverweredeployed, it then served as the drill pla�orm support for the thermal drill as well ascommunica9onsrelayforthedrillandthesurfacesta9on.

Thethermaldrillwasavaria9onontheac9vewaterjetsystemofCryoscout,butwithanuclearreactor insidethecryobot.Thereactorwouldbothcreatetheheat fordrillingandpowerthecryobot,thedrillpla�orm,andthesurfacesta9onviacablinginthetether(Carseyetal,2005a).Thecryobotwouldmeltdownthroughseveralkmof ice to thecontactwithwhatwasat the9mecalledtheBasalUnit,whereitwouldmonitorthemeltwaterandlookforastrobiologicallypromisingchangesinchemistry(Carseyetal,2005b).

All scien9fic instrumenta9onwouldbecontainedwithin the thermaldrill, includingadescentimager,organicandinorganicsensors,ramanspectroscopy,andamassspectrometer(Carseyetal,2005a).

Surfacesta5onconceptandinstrumenta5onThesurfacesta9oncomponentofPalmerQuestwasintendedtoobservetheforma9onofthetopsurfaceof thepolardepositsbycharacterizingthepolarsurface/atmosphere interac9ons,and thushad tominimize radia9ve/conduc9ve/convec9ve interac9onsof thesta9onwith thesurface and the atmosphere. It consisted of a deployable tetrahedral base structure severalmetersinheight.Thebasestructuresupportedasuspendedpla�ormtomonitor9me-varyingatmospheric condi9ons, and a scanning pla�orm in close proximity to the top surface of thepolarcap(Carseyetal,2005a).

The surface sta9onwouldbe connected to thedrill pla�ormbya cableprovidingpoweranddata transferbetween the twocomponents. Itwouldbedeployed100maway fromthedrillpla�orm inorder tominimize the thermaleffectof thedrillpla�ormon itsmeasurements. Itwouldoperateforatleast2.5Marsyears(Carseyetal,2005a).

Theinstrumentsuiteonthesurfacesta9onincludedtemperatureandpressuresensorsonthebaseframe,alaseral9meterandcameratomonitorchangesinthepolarsurfacewith9me,andothermeteorologicalinstrumenta9on(Carseyetal,2005a).

RoverplaXormconceptandinstrumenta5on

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The Far Roving Arc9c Mission (FRAM) rover component of Palmer Quest was intended toprovide regional context for the subsurface measurements and to link the subsurfacemeasurements to orbital spacecrae observa9ons. It would operate for 100 days. It had aradioisotope power source, a four-wheel drive system with inflatable wheels, and a 10 cmultrasonic percussive drill. FRAMwas to have deployed from the lander, dragged the surfacesta9on100mawayfromthelander,andproceededtodriveacrossmul9pletroughs.Atregularspa9al intervals along its traverse, the rover would autonomously stop to perform scien9ficobserva9ons including post-drill science; the roverwould not have been commandedwith acustomizedsetofobserva9onseveryday(Carseyetal,2005a).

Theinstrumentsuiteoftheroverwouldbedividedbetweenthoseontheroverdeckandthoseon an instrument deployment arm. Post-drilling, the instrument deployment arm wouldposi9onitsinstrumentsuitefordown-holemeasurements(Carseyetal,2005a).

• Ontheroverdeck:• Scienceimagerformorphologicalassessmentofthesurface• Engineeringcamerasfornaviga9onalpurposes

• Ontheinstrumentdeploymentarm:• Tunablediodelaserspectrometer• Microscopicimager• Ramanspectrometer• Chemicalsensors

While none of themission concepts outlined abovemade it past (or inmany cases, to) theproposal stage, common themes of explora9on are clear: accessing the local subsurfacestra9graphy,placing it in theregionalcontextof thebroaderpolardeposits,andrela9ng it totheseasonalevolu9onofthepolarsurface.

2.StudyObjec)vesandOverview

2.AAimsandObjec)vesofthisStudy

GoalsofthisStudyProgram:Inves9gateandinterprettheclimatesignalsrecordedinthePLD,eitherfromremotesensingorinsituanalysis.AlthoughsignificantprogresshasbeenmadeusingdatafrommissionssuchasMROcombinedwithclimatemodelingstudies,effortstolinkPLDproper9estoclimateprocessesareintheirearlystages,andthereiss9llmuchambiguity.We therefore propose a study program to iden9fy the key measurements needed tofundamentallyadvanceMarsclimatescience. To accomplish the study goals, we brought together experts to: 1) Establish the “bigpicture”ques9onsinMarsclimatesciencethatcouldbeaddressedbyinves9ga9ngthePLD;2)Iden9fyandpriori9zemeasurementsthatcouldaddresstheseques9ons;3)Defineinstrumentsormissionstoperformthehighestprioritymeasurements;4)Iden9fykeytechnologyneedsfornear-termdevelopmentandalong-termpathwayforproposingamissiontothePLD. Althoughthisstudyprogramopenedwithan invita9ontothinkbroadlyandcrea9vely,weenvisionedafocusonlandedmissionscapableofchemicalandisotopicanalysis.Thistypeofinves9ga9onwould fundamentally change thewayMars climate science isdone,becausewe

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would have “ground truth” measurements that could be 9ed to exis9ng and future orbital/remotesensingobserva9ons.Detailedknowledgeofthedeposi9onal,erosional,andstructuralprocessesac9ngonthePLDthroughouttheirhistorywouldresolveuncertain9esandbringtherecentclimateofMarsintosharperfocus. We seek a path to performing the first paleoclimate study of another planet withdetailedanalysisoflayersinthepolaricecapsofMars.Tofindandmapoutthispath,ourstudyprogramwillbringtogetherexpertsonMarspolarenvironments;Earth’sclimate,asrecordedinicecores;andspace instrumenta9onandmissiondesign.Theseexpertswill comeupwithanapproach to revolu9onizeunderstandingofMars’ climatehistory through inves9ga9onof thePLDusingrecentlydevelopedtechnologiesandcrea9veapproaches. WhereasthemajorfocusofthecurrentMarsprogramisontheancienthabitabilityandpossible present-day liquid water ac9vity, we see an opportunity to look at the geologicallyrecent(20Myr)habitabilityandclimatehistorythatisstoredinthePLD.SamplingthePLDcouldprovidearecordofthepastseveralmillionyearsofclimatethatwouldbeunprecedentedandincomparableinitsdetail.Specifically,linkingthePLDproper9estoastronomicalclimate-forcingmechanisms (obliquity, eccentricity, etc.) has the poten9al to impact the whole field ofplanetaryclimatology. ThePLDhavebeenanimportanttargetofinves9ga9onsincethe1970s,whentheywerediscovered to have layers thought to contain a record of accumula9on and erosion of ice(Murrayetal.,1972).S9ll,thepolarsciencecommunityonlymakesupasmall frac9onofthelarger Mars research community. In the current Mars Explora9on Program Analysis Group(MEPAG) goals (ofwhich there are 4) polar science appears directly under both geology andclimate. Explora9onof thepoles could also advanceMEPAG’s technology goal, and a climaterecord informs us of habitability and thus also advancesMEPAG’s life-science goal. AlthoughpolarscienceisimportantenoughtoaddresseveryMEPAGgoal,itisnotastandalonegoalandthus it is difficult for polar science to compete with dedicated geology- or climate-drivenmissionsandinstruments. Our approach is to make the PLD a primary target for further research. Recentdiscoveries(seebelow)haveshownthatthereisabundantrecordofMars’historywithinthesedepositsand thatadvancements in thisfield,especiallymajoradvances inextrac9ngdetailedclimate records, will benefit the en9re research community, including atmospheric scien9stsandgeologists. Fromascien9ficpointofview,thisistheopportune9metosetthedirec9onoffutureexplora9on of the PLD. TheMars Polar Science community held a conference (Smith et al.,2018)in2016thatdis9lledandsummarizedmanyyearsofresearchperformedonthedatasetsoftheMROmissionandothers.MROrepresentsthelastinalineofpolar-orbi9ngmissionsthathave enabled seminal advances inMars polar science. However, sinceMRO launched,Marsmissions have centered on large rovers that inves9gate equatorial sites represen9ng ancientMars.Thus,weareata turningpoint in theexplora9onof thePLDwhereexis9ngdatahavebeenwell-studiedandnewdataaremanyyearsaway. Technological developments have also occurred in the past decade thatmake a polarmissionmore technically feasible and affordable than in thepast. Entry, descent and landingtechnologyhasevolvedtoincludenewmethodsofplacinglargepayloadsonthesurfacewithgreater loca9onaccuracy(suchasairbagcushionedrollsusedontheMarsExplora9onRoversandtheskycraneusedfortheCuriosityroverandplannedagainforthe2020rover). Muchprogresshasbeenmade in construc9ng theore9calmodels that relatedifferent

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climatestoPLDaccumula9onandabla9on(Levrardetal.2007;Hvidbergetal.2012).SeveralrecentstudieshavecomparedMROobserva9onsofthelayerstothesemodelsandtheyshowbroad agreementon the recent accumula9on rateof thenorthernPLD (Becerra et al., 2016;Landisetal.,2016;Smithetal.,2016).Thus,wearereadytoplacelayerproper9es(andthe clima9c informa9on they contain) measured in a future landed inves9ga9on into achronologicalandclimatologicalframework.

2.BFirstWorkshop

Significant new progress and breakthroughs using data from missions such as MROcombinedwithclimatemodelingstudiesbecomelesslikelywith9me.WethereforeconvenedastudyprogramattheKeckIns9tuteofSpaceSciencetoiden9fythekeymeasurementsneededto fundamentallyadvanceMarsclimatescience.Theobjec9vesof thefirstworkshopwere toestablishthemosteffec9vemethodsofinves9ga9ngandinterpre9ngthelayersoftheNPLDinsearchofclearclimatesignals. Researchers using op9cal and radar instruments have made many advancements inrecent years and have answered long-held ques9ons about the forma9onof theNPLD.Withtheseadvancements comenewques9onsandhypotheses to test. For example, in2016, twopapersaddressedspecificques9onsabouttheclimaterecord.ThefirstusedradarsoundinginanaMempttoputanexactdateonanunconformitytrackedacrosstheen9reNPLD(Smithetal., 2016). This paper compared the volume of ice accumulated since that unconformity topublishedes9matesoficetransfertothepolesduringthelastmajorretreatofmid-la9tudeice,ca370,000years.Thosevolumeswerelessthanafactoroftwoapart,lendingcredencetotheinterpreta9onthatthisunconformityrepresentedthelastmaximuminmid-la9tudeiceextent.Onthat9mescale,thetotalamountoficeonMarsdoesn’tvarybymuch,butthesurfaceareacoverage decreases as the PLD gainmaterial through transporta9on from themid- and low-la9tudes. The second study compared overlapping periodici9es in NPLD stra9graphy topredictedclimate forcingsand foundthat therewasagoodagreement (Becerraetal.,2016).Theseresultslentobserva9onalsupporttoapreviouslypublishedmodel(Hvidbergetal.2012)that related depth in the PLD to age. Thus, recent studies havemoved forward to the pointwhere ages can be assigned to layers with and specific hypothesis can be tested by newobserva9ons. Webroughttogetherexpertstodeterminewhatmajorscienceques9onsremaintobeansweredonthistopic.Aeerdefiningthetop-levelscienceques9ons,workshopaMendeeswereasked to iden9fy and priori9ze measurements that could address these ques9ons, defineinstrumentsormissionconceptstoperformthehighestprioritymeasurements,andiden9fykeytechnologyneedsfornear-termdevelopmentandmissionproposal.

2.CSecondWorkshop

Thesecondworkshop,held inNovember2017,focusedondesigningmissionconceptsthat could address the major science ques9ons within the context of the current DecadalSurvey, "Visions and Voyages" (Council, 2011). This document did not advocate for a NewFron9ers or Flagship mission to the poles, but it did stress the importance of Mars polarinves9ga9onsrelatedtoiceandclimateandspelledouttheneedforanewpolarorbiter.

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In that context, we focused on designing missions that could fit within the NASADiscovery and Small Innova9ve Missions for Planetary Explora9on (SIMPLEX) programs i.e.programsthatwerelargeenoughtodevelopsignificantpayloadsthatcouldreachMarssurfaceororbit,andwithinthescopeoutlinedbyVisionsandVoyages.Thepar9cipantsacknowledgedthatsendingaFlagship-classrovertobeginthese inves9ga9onswouldbeexpensiveandhaveuncertainscien9ficpayoffandsodevelopedacampaignofmissionsthatbeganwithpathfindingexplora9onandorbitalscience. Wediscussedthreetypesofmissions:anorbiter,"small-sat"landers,andasta9clander.Theorbiterandsta9clanderwouldfitwithintheDiscoveryProgram,andthesmall-satscouldfitwithin the SIMPLEXprogram.Wedetail the results of the secondworkshop in sec9ons 3.C.1through3.C.3

3.StudyResults

3.AMajorScienceQues)onsandObjec)ves

Weu9lizedseveralbreakoutsessionstodeterminewhataretherelevantques9onsthatneedtobeanswered inorderto fullydescribetheclimatesystemand itspolarrecord.Manyques9onsweresimilarorrelated,anditbecameclearthatfourtopicswerethemostimportant,high-levelaspectsoftheclimaterecordstoredinthePLD. Intheirshortforms,itwasnecessarytodescribewhatmaterialsgottothepolarregions,the sources of those materials and external forcing required to transport them, how thosematerials were included in the polar ices, and finally, how thosematerials were distributed.Herewepresentthefourmajorques9ons:

1.Whatarepresentandpastfluxesofvola5les,dust,andotherrefractorymaterialsinto andoutofthepolarregions?

2.Howdoorbitalforcingandexchangewithotherreservoirs,affectthosefluxes?

3.Whatchemicalandphysicalprocessesformandmodifylayers?

4.Whatisthe5mespan,completeness,andtemporalresolu5onrecordedinthePLD?

In shorthand, we refer to these ques9ons as, “Fluxes,” “Forcings,” “Layer Processes,” and“Record.”

3.A.1Fluxes

3.A.1aObserva)ons

Measuringthemodern-dayfluxofmaterialbetweenthepolarcapsandtheatmosphereis a key first step to interpre9ng the climate record stored in the PLD. It provides theopportunitytolinkobservableclima9cprocessesandtheirtranscrip9onintothepolargeologicrecord.Weexpectthatvaria9onin(i)theisotopicandchemicalcomposi9onand(ii)totalmass

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ofmaterialexchangingbetweentheatmosphereandpolardepositswillyieldobservablesignalsin the PLD. Therefore, we focus our aMen9on on measuring these quan99es and theatmosphericprocesses(relatedtoglobalclimateandlocalweather)thatgoverntheirflux.Todothis, we propose (i) measuring global and regional (km-scale and larger) fluxes; (ii) in situmeasurement of fluxing material in the near-surface atmosphere; (iii) in situ surfaceaccumula9on; and (iv) in situmeasurement of the top few cen9meters of ice, which shouldrecord informa9on that can be related to the past few decades of mar9an atmosphericobserva9on. Fourcategoriesofmaterialexchangebetweentheatmosphereandthepolardeposits: •CO2 •H2O •Refractorymaterials(e.g.dust,salts,volcanicdebris,hydrocarbons) •Tracevola9lespecies

ThelargestcomponentofthegrossannualfluxisCO2,whichincondensedformreachesseveralhundredkgm-2overthePLD.However,orbitalobserva9onsshowthatannually,thereisnonetfluxofCO2ontothenorthpolarcapandthesignofnetCO2fluxontothesouthpolarcapis unknown, although the magnitude is probably, on average, <1 kg m-2 per mar9an year(Thomasetal.,2016).Nevertheless,measuringtherela9vepropor9onofwinterCO2laiddownthroughdirectdeposi9onvs.snowfallisimportantbecausethesedifferentmodesleadtovastlydifferentthermal,radia9ve,andstructuralproper9es(e.g.Colapreteetal.,2005).Thiswillaffect(i) the incorpora9onof co-depositedmaterial into the polar caps and (ii) the preserva9onofmassive buried CO2 deposits beneath the south polar cap (Phillips et al., 2011). Addi9onally,measuring seasonal isotopic varia9on (e.g. Villanueva et al., 2015) during deposi9on andsublima9on of CO2 will aid in the interpreta9on of the isotopic abundances of trapped gaswithinthePLDandfromthedeepmar9angeologicrecord(e.g.inmar9anmeteorites). ThegrossandnetannualexchangeofH2Oontoeitherpolarcapisunknown,butthenetfluxisthoughttoaverage0.5kgm-2overthelastfewMyr,butislesstodayorperhapsnega9ve(e.g.Brownetal.,2014;Smithetal.,2016).H2OformsthebulkofthePLD,withavariable,butlow, dust frac9on (Grimaet al. 2009). ThefluxofH2O rela9ve to that of refractorymaterialsundermodernclima9ccondi9onsisakeymeasurementfordecipheringtheclima9cmeaningofthesignalofvariabledustfrac9onstoredinthePLD. Inaddi9ontotherela9vemassfluxesofH2Oandrefractorymaterial,thechemicalandisotopiccomposi9onofthismaterialwillyieldimportantinforma9on.Thechemicalcomposi9onof the refractory material will provide informa9on about its provenance(s), while poten9alseasonal isotopic variability in the H2O flux (e.g. from mass-dependent temperaturefrac9ona9on or changing source reservoirs) may be used to interpret H2O isotopic varia9onrecordedinthePLD.Importantly,thisisotopicvaria9onmaybeannual(asisseenonEarth;e.g.Werneretal.,2000)andmayprovidethesmallestresolvablecyclicsignalinthePLD.Finally,thefluxof trace vola9les incorporated into thepolar cap (e.g. trapped in gasbubblesduringfirnsintering, if present) compared to the ambient atmospheric trace vola9le composi9on willprovideinsightintodecipheringtrappedatmosphericgasbubblesinthePLD. Wenowpresentalistofobservablesthatwillallowustodeducethemodernmaterialfluxatthepolarcap-atmosphereinterface.ThisfluxmaMersregardlessofwhetherthereisnetaccumula9onorremovaloficefromthecapsurfacebecausebothofthesestatesarerecordedinthePLDrecord(i.e.,aseitherthebuildupoficeorthedevelopmentofanon-vola9lelag).

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BasicKeyObserva9ons: • Wind speed profile near the surface boundary layer. This observa9on is importantbecausethenear-surfacewindspeedisdeterminedbynonlinearprocessesthatareimpossibleto model a priori. When observed concurrently with the number density of atmosphericrefractorymaterialandH2O,thisisabasicinputforcalcula9ngregionalflux. • 4-dimensional (al9tude, la9tude, longitude, and 9me) number density map ofatmosphericdustandH2O.Theidealminimumresolu9onforthesemeasurementsisa10-pointver9cal grid within the first half-scale height (scale hight is ~11km) and half-scale heightresolu9on up to 80 km, at a 4x diurnal cadence, resolved across 12 longitudinal bands, allobservedoveronefullmar9anyear.IsotopicmeasurementsofH2O(e.g.D/H,δ18O,δ17O)isalsoimportant for tracking its provenance. Combined with wind speed, the number density ofmaterialisabasicinputforcalcula9ngregionalflux. • In situ surface mass flux of H2O, refractory material, and CO2. It is most cri9cal toobtain this measurement at one well-selected loca9on; however, addi9onal measurementloca9onswouldallowthedetermina9onofregionalvariability,whichcanbestrikingevenover~10m-scalesonEarth(cf.C.Hvidbergpresenta9onatKISSMarsPolarworkshop).Characterizingthechemicalandisotopicmakeupofthesurfacematerialisalsodesirablefordeterminingtheprovenance of the fluxing material and connec9ng in situ measurements to globalmeasurements. •Obtaining thesemeasurements concurrently is important for accuratelydeterminingfluxesandconnec9nglocaldeposi9ontoglobalclima9cprocesses.

Addi9onal desirable observa9ons will improve our understanding of the processesgoverningdeposi9on,diagenesis,andincorpora9onofmaterialintothePLDaswellasabla9onprocessesandresidence9meofmaterialthatisonlytransientlyincorporated.

Vola9lematerial: •Clouddensityandpar9clesize/shape •H2Ocloudvs.vapordistribu9on(inal9tude,la9tude,longitude,and9me) •Cloudcondensa9onnucleicomposi9on,shape,andsize •Snowfallvs.frostaccumula9onrates •Iceandsnowcrystalstructure •Surfacesublima9onrate •Icegrainsize,sinteringrateandairbubblecontent •Iceisotopicabundance •Pressureandtemperatureduringcondensa9on •Icethermal,op9cal,materialproper9es(e.g.albedo,emissivity,conduc9vity,density, strength)

Refractorymaterial •Par9clesizeandshapedistribu9on •Sublima9onlagthickness,permeabilitytovaportransport,andstrength •Dustloeingrate •Dustelectriccharge

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3.A.1bModeling

Therearetwoimportantconsidera9onsregardinghowatmosphericmodelscanandwillbe used in conjunc9on with observa9ons to quan9fy the fluxes of non-vola9le and vola9lematerialintoandoutofthepolarcapsonMars.Thefirstisthatmodelswillbeinvaluabletoolsforinterpre9ngandexpandingtheobserva9ons(thusenablingthecompletefulfillmentofthesciencegoal)andthesecondisthatmodelsrequireobserva9onsforvalida9onpurposes.

Interpre5ng/ExpandingObserva5ons:

Weexpect significant spa9aland temporal varia9ons in the surfacefluxesofdustandwater over the en9re cap region due to regional and local-scale circula9ons and spa9alvaria9ons in surfaceproper9es.While itwouldbeop9mal tohaveanetworkof a significantnumberofhighlycapablemeteorologicalsta9onsplacedstrategicallyaroundthepolarregionstomeasurethespa9alvaria9onsandtemporalevolu9onofdustandvola9lessurfacefluxesandtransport, it isunlikelytobefeasiblefromacostperspec9ve.Modelswillthereforebecri9caltools for extrapola9ng informa9on from a small number of sta9ons (possibly just one) to acomprehensiveunderstandingofwhat isoccurringover theen9reregion.Addi9onally,unlessthereareobserva9onsfromorbitoftheglobaltransportofdustandvola9les,concurrentwithin situmeasurements,global-scaleclimatemodelswill likelybeneeded toprovide thisglobalperspec9ve.

ModelValida5on:

Beforemodelscanbereliablyusedtoextrapolateinforma9ongainedfromoneorafewloca9onstotheen9repolarcapregion,theymustbevalidatedwithobserva9ons.Windsandturbulence in the lower atmosphere control the exchange of vola9les and dust between thesurface and atmosphere and transport thosematerials (in the polar regions and elsewhere).However,todate,theseprocesseshavenotbeencomprehensiblyorreliablymeasuredonMars.Near surface wind measurements have been acquired by the Viking Landers, Pathfinder,Phoenix,andCuriosity,butthesedatasetssufferfromcalibra9onissuesandotherproblems.

Windsabove~1.5-2.0mhaveneverbeendirectlymeasuredonMarsapartfromafewentry, descent and landing profiles. Instead, our understanding of the winds throughout thebulk of the atmosphere have come from deriving thermally balanced winds from observedthermal structures and atmospheric models (Navarro et al, 2017). Models require windobserva9ons for valida9on, par9cularly near the surface where the thermal wind balanceapproxima9oncannotbeused.Carefullyconsideredandwell-calibratedmeasurementsofnear-surfaceswindsorwindprofiles from the surfacewill provide this cri9calmodel valida9on. Inaddi9ontowindobserva9ons,directobserva9onsoftheratesofexchangeofdustandvola9lesbetweenthesurfaceandtheatmosphereinthepolarregionswillprovidecri9calconstraintsformodels.Current state-of-the-artglobal- and regional-scalemodels include thephysicsofdustlieingandremoval,andwaterandCO2icesublima9on,deposi9on,andsnowfall.Intheabsenceofmeasurementsofthefluxesfromeachprocess,however,itisdifficulttoknowifthemodelsarehandlingtheseprocessescorrectly.

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3.A.2Forcings

Theclima9cstateofMarsisdrivenbychangesintheorbitalstateoftheplanetandbythepresenceand longevityofreservoirs.Principally,obliquityvaria9onson~105yr9mescalesdrivethemovementofwatericefrommid-la9tudestothepoleorequator,andtheen9rewaterbudgetoftheplanetispoten9allyaffected.Addi9onally,smallereffectsonthepaMernofglobalinsola9onoccurdueprecessionoftheargumentofperihelionandvaria9onsintheeccentricityofMars'orbit (Laskaretal., 2004).Combined, these threeorbital cyclesdrivemuchofMars'climate. ExternalforcingsofMars’climateincludesolarwinds,cosmicrays,impactors,etc.,thatact to strip material away from the upper atmosphere or, on smaller scales, implant newmaterialsintotheatmosphere.SecularmasslossdrivenbythesolarwindhasputMarsintoitscurrent, low-pressure state. However, on 9mescales relevant to the Amazonian period,especiallywithinthelast~300Myr,atmosphericstrippingisnotsuspectedtoaffecttheclimatesignalinthePLD. Therearealso internalclimateforcings.Themajor internalclimatedriver is thepoorlyunderstooddustcycle.Severallocalandregionalduststormsoccurrepeatedlyonannual9mescales.Thebiggestperturba9onoftheatmospherebydustisfromplanet-encirclingdusteventsthatcoveralllongitudesandaresoop9callythickthatvisibleimagerscannotdetectthesurface.In theseevent,atmospheric temperatures increaseglobally,affec9ngwatervapordistribu9onand transport.Thecauseand9mingofplanetencirclingdusteventsarenotunderstood,butobserva9onsrecordthemeveryfewmar9anyears. Besides the dust cycle, volcanic outgassing must play a role in the variability ofatmosphericcomposi9onandmaybepressure.Themostrecentvolcanicerup9onsaredatedatonly~10Myr,sotheyarerelevanttoAmazonianclimate,andevidenceoferup9onsmayexistwithinthePLD.

Reservoirs

Itisimportanttoiden9fythereservoirsofvola9lesandrefractorymaterialsthatcanbetransported.Watericeisthevola9leingreatestquan9tyatandnearthesurfaceofMars.Thetwo PLD contain more than 2/3 of the known water budget of the planet (although deepaquifers containing comparable amounts of water are suspected to exist). Addi9onally, mid-la9tude glaciers and ice sheetsmake up another large frac9on of thewater budget. Smallerknown reservoirs include the regolith, through water-binding to minerals or salts, theatmosphere,andporefilling ice.Watermaybe injected intotheatmosphereviavolcanoesorcomets. Carbondioxide is theprimary cons9tuent of themar9an atmosphere,with amass of~2.5x1016kg(JamesandNorth,1982).Thisfluctuatesbyapproximately1/3eachseasonastheatmospherefreezestothewinterpole.Addi9onalmassisfoundatthesouthpolarresidualcap(SPRC),athindepositofice(<~10m)thathasminorvaria9onsinsurfacetextureandmargins(Thomas et al., 2016) and may have been present in some state for several hundred tothousands of years. Thismakes up only a small por9on of the CO2 budget. The othermajorreservoirofCO2iceisburiedbeneaththesurfaceoftheSPLD,spa9allycorrelatedtotheSPRC.The volumeof this ice is es9mated to be asmuch as 16,500 km3 (Putzig et al., 2018) and if

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releasedtotheatmospherewouldmorethandoublethesurfacepressureeverywhere.Thisunithassequencesofdeposi9on(Biersonetal.,2016),sugges9veofclimatesignals(Manningetal.,2018)goingbackasfarasmany105years.SmallerCO2reservoirsexistthatmayplayimportantrolesinclimate:regolith,minerals,andpoten9allyclathrates. Finally, dust reservoirs are found acrossmuch of the planet. The PLD are nearly purewatericebutmaycontain~5-15%atmosphericdustbasedondielectricmeasurements(Grimaet al., 2009) and gravity data (Zuber et al., 2007). Other loca9ons on the planet, such aspedestalcraters,containstra9fiediceanddustaswell.Atmosphericdustisvisiblefromground-basedimagery,andmanyloca9onsonMarshaveafinelayerofsurfacedustreducingcontrastinvisibleandIRbands.

ObservablesinthePLD

Inordertodeterminetheclimatecyclesandul9matelytheforcingsthatcreatethem,insitu measurements of the layers of the PLD must be made. Here we list some knownobservablesandsomethatarehypothesizedtoexist. From orbit, the thickness, albedo, spa9al frequency, and dielectric proper9es of thelayershavebeenmeasureddownto~1mresolu9onforop9callyobservedsurfaceexposuresanddownto~8mforsubsurfacedetec9onofradarreflectors.Weknowconfidentlythatsomelayershavehigheralbedoandlowerdustcontentandthatsomelayershavehigherdustcontentdecreasing their albedo. However, shadows and the presence of seasonal frost affect thosemeasurements,crea9nguncertaintyastotheirtruenature. Topographicmeasurementsfromstereoimageryprovideanotherwayofmeasuringthelayer thickness and spacing, and this technique does a good job of connec9ng layers at oneexposure to layersatanother (Becerra2016).Thesemeasurementscanbedonebecause thelocalresistancetoerosionpermitssomelayerstoerodemoreslowly,crea9ngstepsatresistantlayers.Wesuspectthatvariabledusttoicera9oisthecause;however,wearepresentlyunabletoconfidentlydeterminethesourceofthiserosionresistance. On Earth, layers that were deposited at different 9mes record some frac9on of theatmosphericcons9tuentsatthe9meofdeposi9on,includingisotopicandchemicalsignatures.Thosematerialsarebeneficialfortrackingatmospherictemperatureandpressurethrough9me.MarsPLDsaresuspectedtohavevariabilityintheirchemicalcomposi9ondependingonwheneach layer was deposited. If bubbles exist in the PLDs, they may contain samples of pastatmosphericgasses. Dust grain size variability and chemical nature may be measurable. Atmosphericpressure has varied through 9me (perhaps being double a few 100 ka; (Phillips et al. 2011;Biersonetal.2016)),andthedustcarryingcapacityofMarsatmospheremusthavevariedaswell,e.g.higherpressuresshouldsustainlargerdustgrainsastheytransporttowardsthepoles.Thus, as dust lieing varies, so too may the dust reservoirs accessible for lieing. If thosereservoirs are not iden9cal in chemical composi9on, then the source of the dust may beiden9fiable.Alongwithtrappingarecordofvariabilityinpressureanddust,eachlayerwillhaveuniquecons9tuentsthataffectthecompac9onrateandpossiblythefinaldensity.

ClimateSignal

Bymeasuringtheknownandpoten9alobservablesfromthesurface,wecantrackthe

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variabilityateachlayer,goingbackwardsin9me.Ifthosevariablesarecyclical innature,thenperhaps we can 9e those signals to the periodic nature of seasonal or annual cycles, duststorms,solarcycles,andlongerperiodcyclesdrivenbyorbitalchanges.TyingtheforcingstothemeasurementsistheprimarymethodtounderstandingMars'climateintheperiodrecordedbythePLD.

3.A.3LayerProcesses

Wefirstdiscussedfivemajorareasofobservables: composi9on, layer stra9graphy, icemicrostructure, atmospheric processes, and surface to subsurface processes. These weresubsequentlyconsolidatedintothreecategories:composi9on,stra9graphy,andcurrentsurfaceprocessesandmeteorology. Composi9onwasiden9fiedasthetopprioritybecauseit istheprimarywaytodeduceprocessesthatformandmodifylayers,iden9fysourcesofmaterialsthatmakethelayers,infersurface-atmosphereinterac9ons,andpreserveclima9cevents.Therela9veabundancesofthemostabundantcons9tuents(waterice,carbondioxideice,salts,anddust)willbekeyforstudyrelated to these topics. Iden9fying trace quan99es of other materials (organics, isotopes,perchlorate, clathrate) and iden9fying composi9on at mul9ple spots (if landed) or beMerver9cal resolu9on than currently available (if in orbit) are also important. The physicalcharacteris9csofmaterials in the layers (e.g. grain sizedistribu9on,density)will alsoprovideinforma9on about their origin and diagenesis. These observa9onswill require an instrumentcapable of iden9fying ices, dust and trace cons9tuents. Image acquisi9on at small-scaleexcava9onsorexis9ngexposurescanprobenear-surfacestra9graphyandcomposi9on. Stra9graphy places composi9on in context and links the historical record to currentprocessesandevolu9onover9me.Weneedtocharacterize thestra9graphyatascalebelowthe resolu9on available from orbit (1-10m), but also relate it to the broader context of thestra9graphy iden9fiable from orbit. Imaging from a surface perspec9ve as well as soundingtechniques (e.g.ground-penetra9ng radar)aredesirable.Elucida9ng theevolu9onof thePLDrequiresunderstandingchanges indeposi9onratesbyobservingbeddinggeometry, thicknessandunconformi9estounderstandhowdeposi9onhaschangedthrough9me. The study of current surface processes and meteorology illuminates how layers areformingor eroding todayand links currentprocesses tohistorical processespreserved in thestra9graphy.Thisrequires insituobserva9onoftheabla9onand/oraccumula9onofdustandice, salta9on and surface transport of materials, and measurements of wind, insola9on,humidity,andalbedo. Layer processes link directly to four of the five key Mars polar science ques9onsiden9fied at the 6th interna9onal conference on Mars polar science and explora9on, assummarized by Smith et al. (2018). In par9cular, they relate to ques9ons about the polaratmosphere (ques9on 1), Perennial surface ices (ques9on 2), the polar record of the pastclimate(ques9on3),andthepresent-daypolarsurfaceac9vity(ques9on5). The three priority areas we iden9fy (composi9on, stra9graphy, and current surfaceprocesses) are directly linked to many observa9ons that could be made by the Next MarsOrbiter, whose instrument complement was recently studied by MEPAG (NEX-SAG Report,2015).Thatreportiden9fiednewsciencethatcouldbedonebyanorbiterto:

1.“Mapandquan5fyshallowgroundicedepositsacrossMarstogetherwithshallowlayeringof

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water and CO2 ices at the poles to beTer understand the global water inventory andatmospheric exchange today, and how ground ice records climate change on geologicallyyoungerMars(e.g.,overobliquityvaria5oncycles).”

2. “Measure winds and characterize transport and other dynamic processes to understandcurrentclimate,water,anddustcycles,withextrapola5ontopastclimates”.

Included in the NEX-SAG report were high-level, proof-of-concept measurement techniquesmatureenoughfordevelopmentofanorbiterforlaunchin2022,including:

• VisibleimagingofHiRISE-class(30cm/pixel)orbeMer(~10-15cm/pixel)• Polarimetric radar imaging with penetra9on depth of a few (<10) meters and spa9al

resolu9on of ~15 m/pixel to detect ices; a radar sounding mode would aidcharacteriza9onoftheoverburdenmantlingsubsurfaceicelayers

• Short-wave IR mapping with a spa9al resolu9on of ~6 m/pixel with sufficient spectralresolu9ontodetectkeyprimaryandsecondaryminerals,salts,andices

• Long-waveatmosphericsoundingforwind,temperature,&water-vaporprofiles• ThermalIRsoundingforaerosolprofiles• Mul9-bandthermal IRmappingof thermophysicalsurfaceproper9es (e.g., iceoverburden

andthermaliner9a)andsurfacecomposi9on• Global,km-scale,wide-angleimagingtomonitorweather,duststorms,andsurfacefrosts

LandedmissionconceptsandinstrumentsthatcanaddressthesepriorityareaswereconsideredaspartoftheMarsPolarClimateconceptstudythatwaspreparedaspartofthe2013DecadalSurvey (NASA Mission Concept Study, 2010). Measurement technologies that can addresscomposi9on, stra9graphy, and current processes from landed or rover pla�orms includeconceptsthathavealreadyflownonlandedmissionsorareinprepara9on,suchas:

• SceneandDistanttargetso MastImagers(stra9graphy,unconformity,beddinggeometry)o Mast Spectroscopy (LIBS or Infrared) (composi9on of ice and non-ice

cons9tuents)o Environmental monitoring (pressure, temperature, wind vectors, humidity,

ultravioletflux)o Atmosphericsounding(FTIR,Lidar,clouds,dust,surface-atmosphereexchange)o Surfaceheatflow(thermalmetamorphism)o Groundpenetra9ngradar(layerproper9esoftheupper10sofmeters)o Neutronorgammaraydetectors(near-surfacecomposi9on)

• Samplescaletargetso Drill,RATorcorers(accesstosubsurfacestra9graphy)o DrillImagers(down-holeimagingoflayerproper9es)o High-resolu9onImagery(imagingofextractedsamples)o InfraredorRamanSpectroscopy(down-holeorsamplecomposi9on)o Tunable laser ormass spectroscopy (sample composi9on of trace components

andisotopesD/H,C12,13,14)

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3.A.4Record

In order to address the science ques9on “What is the 9mespan, completeness, andtemporalresolu9onrecordedinthePLD?”weiden9fiedthree9me(andsize)scalesthatneedtobeaddressed. The largest physical scale (and by extension temporal scale) is characterizing thestra9graphy of the PLDs as a whole. The geologic rela9onships between the four packets ofradarlayersandthebasalunitcharacterizedby(Tanakaetal.,2008;Phillipsetal.,2008;Tanakaetal.,2012)wouldbebeMerconstrainedbyradiometricagedatesand/orexposureagedatesoflithicscontained inthecaviandbasalunits.Addi9onally,SHARADdatahasbeencombinedtomakea3dimensionalmapof theNPLD (Fossetal.,2017) includingcirculardepressions thathavebeen interpretedtobecraters (Putzigetal.,2018).Usingthesize-frequencydistribu9on(SFD) of these circular depressionswould give addi9onal informa9on about the expected ~5MyrhistoryoftheNPLDfromicestabilityandclimatemodeling(Jakoskyetal.,1995;Laskaretal., 2002; Levrard et al., 2007).Whereas this 3Dmodelinghasbeen completed for the SPLD,internal volumetric backscaMer or "fog" inhibits comparing the climate record that could bepreservedatbothPLDs(WhiMenandCampbell,2018).Theseconstraintswouldcharacterizethe9mespanandthelargesttemporalresolu9oncontainedwithinthePLDs. Theintermediatephysicalscaleweiden9fiedwasthetroughexposurescale,orontheorderof~100sofmetersver9calscale.Iden9fyingunconformi9esisapriorityforunderstandingthe completeness of the record contained in the PLDs because hiatuses in accumula9on orperiodsofabla9oncouldalterorfailtopreservesomeoftherecord.Onewayofdoingthisislooking for layers with high concentra9ons lithic material in the stra9graphy exposed withintroughs,whichmaybesublima9onlags.ThebulkdustcontentoftheNPLDis~5%(Grimaetal.,2009)andtheSPLD~15%(Zuberetal.,2007)sosublima9on-lag-dominated layers, ifpresent,do not dominate the contents of the PLDs and are likely quite thin. However, visible/near-infraredspectral inves9ga9onsofthe layerssuggestthat lithicsandsaltsmaybepresent,andvariability in their concentra9onmay help understand the rela9ve impacts of hiatuses in therecord. Thus, being able to fully characterize the physical (e.g. ice crystal size, air bubblepresence and/or concentra9on, density) and chemical (e.g., lithic mineralogy, dustconcentra9on,saltconcentra9on,gas/iceisotopiccomposi9on,gas/iceelementalcomposi9on)proper9esof these layerswill enable the search forpaMernswithin them that could link thelargerstructuretopossibleannuallayers(orpacketsofannuallayers)nearthesurface. Finally, it is importanttostudythecurrentsurfaceanduppermeterofthePLDsatthesub-materscale.Understandingthefine-scalestructuresinthenear-surfaceofthePLDiskeytointerpre9ng the deeper record. Understanding what cons9tutes a layer and linking layers toobserved near-surface processes is cri9cal for iden9fying the finest spa9al (and thereforetemporal)resolu9onoflayersrecordedinthePLD,andul9matelyassigningabsulateagestothePLDclimaterecord. Whether an annual layer is preserved in the uppermost region (decimeters tometersscale)ofthePLDsisnotknown.Addi9onally,basedonapparentlyconflic9nglinesofevidenceaboutthecurrentmassbalanceoftheNPLD(Kieffer,1990;Langevinetal.,2005;ChamberlainandBoynton,2007),itisuncleariftheNPLDiscurrentlyaddingannuallayersorlosingpartofapast climate record. Addi9onally, liMle is known about how the PLD material densifies withdepthandwhetheranyairbubbles frompreviousmar9an climates couldbe capturedwithinthe ice, similar to terrestrial examples (e.g. Svennson et al., 2005). A beMer model for ice

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evolu9on willallowustoconstrainthepastcondi9onsthatnear-surface icehasexperienced(e.g.isthecurrentsurfacebeingexhumedorundergoingpreserva9onbycompressingafluffierfrostlayer?).Addi9onally,physicalandchemicalmeasurementsofthenearsurfaceice(e.g.dustcontent,op9calproper9es,electricalconduc9vity,lithic/gas/icecomposi9on)couldleadtothedetec9onandevalua9onofrecurring,andpossiblyannual,paMernsthatcouldbelinkedtothe9mescaleofthelarger,trough-scaleexposures. Weiden9fiedseveralkeymeasurementsthatwouldaddresstheaboveareasofneededknowledgeandrankedthemaccordingtopriority. •Ourtopprioritywastoobtainanabsoluteagethroughcosmogenic/radiogenicnucleiabundance forany loca9onwithin thePLD.Thiswillallowresearchers toputan independentageconstraintonatleastoneofthelayers,andwithlessuncertaintythanabsolutecraterages(e.g.HerkenhoffandPlaut,2000;Koutniketal.,2002;Banksetal.,2010;Landisetal.,2016),asradiometric/cosmogenicda9ngisindependentofcraterproduc9onmodels.Thereareavarietyof radiosotopic systems andmethods available for age determina9on on the Earth (e.g. 14C,10Be,K-Ar, 36Cl, 3He, 36Ar,Ne).Op9cally s9mulated luminescence techniquesareanaddi9onaltechnique available for age determina9on. These methods require the presence andiden9fica9onofadatablelithiclayer(e.g.,impactorvolcanicsediments).ThetransferabilityofthesemethodstoMarsneedsfurtherstudy. • Intermediate prioritywas given to the search for paMerns in ver9cal distribu9on ofimpurity frac9on and chemistry (iron, silica, aluminum, sulfur and chlorine) tomatch orbitalhistoryonthe100sofmdepth/troughscale.Thestudyofimpuri9escouldbecarriedoutusingop9calmicroscopy and context imaging combinedwith a composi9onal technique like alphapar9cle x-ray spectroscopy (APXS), laser-induced breakdown spectroscopy (LIBS), x-rayfluorescence(XRF),and/orvisible/near-infraredspectroscopy.Annuallayersarees9matedtobeonthemicronsto~mmscale(e.g.Laskaretal.,2002;Hvidbergetal.,2012)andobtainingthatver9calresolu9onwouldassistindeterminingthesmallest9meresolu9onrecordedinthePLDlayers. •Intermediateprioritywasalsogiventothemeasurementofthecratersize-frequencydistribu9on (SFD)withdepth todetermine rela9veages fromver9cal craterdistribu9on. Thever9cal crater SFD would require higher resolu9on, and therefore higher power, radar thanSHARADtodetectcratersbelowthe7kmdetectedinini9alsurveys(Putzigetal.,2018).Whileabsoluteageresultswoulds9llbecraterproduc9on-func9ondependent,quan9fyingthecratersize-frequencydistribu9onwithdepthplacesrela9veageconstraintsonthePLD.Addi9onally,tying the 9mescales presented in the ver9cal crater distribu9on to a fixed radiometric orcosmogenic age from our top measurement priority would significantly improve theinterpreta9onofthisresult. •We also iden9fied three suppor9ngmeasurements that were key in answering ourcentralscienceques9on.ThefirstisdeterminingthecompletenessofthePLDwithacatalogofunconformi9es throughout the stack. To map unconformi9es, we considered again higherpower(andthereforehigherresolu9on)radaraswellasanetworkofseismicsta9onsthatcouldmapthedistribu9onof layerpacketsandgaps.Thesecond ismeasuringcurrentatmosphericcondi9ons and surface effects, ideally through a combina9on of surface microscopy and anetworkofmeteorologicalsta9ons.Thirdisgrainsizemeasurementwithdepthinthefirstfewmetersofthesurface.Thiswouldrequiremicroscopicandspectroscopictoolstodropdownaboreholeafewmetersintothesurface.

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3.A.5AgeDa)ngtheNPLD

Inorderto9ephysicallayersseeninradartoobliquityandclimatecyclesonMars,ageconstraintsmustbeplacedonlayeredmaterials.Forma9onandexposureagesareobtainedonEarth (and Mars, e.g. Farley et al. 2014) using radiogenic and cosmogenic nuclides. Briefly,cosmogenic nuclides are produced through any interac9on of primary or secondary cosmicradia9onwithmaMercontainingarangeoftargetelements.Stablenuclidesbuildupover9measasurfaceisexposedtocosmicrays.Radiogenicnuclidesaredaughterproductsofradioac9vedecayandalsobuilduponsurfacestothepointofsatura9on,whichreferstothestatewhentherateofproduc9onequalstherateofdecay.Whenasurfaceisburied,theaccumula9onofcosmogenic and radiogenic nuclides slows down or stops en9rely, depending on the nuclide(Figure 1.) The cosmogenic stable nuclides remain, while the radiogenic nuclides decay intostable daughters. The produc9on rate of cosmogenic nuclides on Mars is es9mated frommeteorites,andcanbeused,alongwithameasurementoftheabundanceofthesenuclidesinalayer,todeterminehowlongthematerialinthelayersatonthesurface.Theburialageofthematerial can then be determined by the difference between the abundance of cosmogenicnuclidesincurrentlyexposedsurfacematerialsvs.buriedmaterials.

Cosmogenic and radiogenic da9ngtechniques are widely used in studies of icecores and glacial geomorphology on Earth,with 10Be (half-life ~1.5 My), 26Al (half-life~700 ky), and 14C (half-life ~5700 y) as themost frequently used da9ng systems (e.g.,Fabel et al, 2002; Bond et al., 1993;Rinterknechtet al., 2006). For themostpart,age da9ng is done on lithic material andvolcanic ash entrained in ice, although insome cases trapped gases are analyzed (e.g.,Buizert et al., 2014) with state-of-the-artnoblegasmassspectrometers.However, forthistechnique,verylargemassesofice(100sof kg) are required to obtain sufficientmaterial to analyze with high sensi9vity,precludedoingthisonMars.

Cosmogenic and radiometric da9ng onMars presents significant analy9cal challengesdue to the amount of material necessary for measurement and the necessity (in mosttechniques)tohavearobustmasses9mateofmaterial.Asuccessfulapplica9onofradiometric(K-Ar) and exposure age da9ng on Mars using cosmogenic 21Ne, 3He, and 38Ar have beenperformed onGale Cratermudstone (Farley et al., 2014) using the SAM instrument onMSL.However, this work was done on mar9an regolith, where rocks with high concentra9ons oftarget elements (e.g., Mg, Ca, Al) ensure the presence of measurable abundances ofcosmogenicnuclides.

Because the NPLD are mostly ice, achieving meaningful ages for the NPLD materialhingeson thepresenceofdatable lithicmaterial in the formofdust, volcanic ash,or impactejecta.TheyoungageoftheNPLD(~5x106yearsold)alsodrivestherequirementforextremely

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Figure1Produc5onof34Ar,3He,and22NewithdepthonMars,fromFarleyetal.,2014

high precision measurements in order to achieve meaningful ages, with minimum ageresolu9on on the order of 104-105 years. For comparison, the exposure age measurementsachievedbySAMwere78±30millionyears(Farleyetal.,2014),sugges9nganimprovementinprecisionofatleast2ordersofmagnitudewouldberequiredformeaningfulmeasurementsoftheNPLD.

A mission to age-date the NPLD would require a highly selec9ve and sensi9ve massspectrometersimilarinsize,mass,andpowertotheSAMinstrumentonMSL.Arangeofmassspectrometry techniques to op9mize exposure age da9ng using cosmogenic nuclides arecurrently in development to address the need for absolute age da9ng in planetary science.These techniques represent improvements in sensi9vity, resolu9on, and selec9vityovermassspectrometers currently used in planetary explora9on. One promising technique indevelopmentisresonanceioniza9onmassspectrometry(RIMS),whichusesalaserorionbeamto removeneutral atoms from solids, then uses tuned lasers to excite electrons of a specificelement and ionize it for measurement by mass spectrometer. This technique has beendevelopedfortheRb-Srgeochronometer(Andersonetal.2012;2015)butcouldbeappliedtocosmogenic nuclides aswell. In addi9on, recent developments using isotopedilu9on removethe need for obtaining amass es9mate, which should significantly reduce themagnitude oferror (Farley et al., 2013). A precursor mission to establish the presence and abundance ofdatablelithicmaterialinthetopoftheNPLDcouldinformeffortstorefinetheseandotherageda9ngtechniques(e.g.,Cohenetal.,2014)forapplica9ontotheNPLD.

3.BKeyProper)esandMeasurements

Based on the discussions within the breakout sessions, our group converged on anagenda for measurements that can be made with exis9ng and foreseeable technology toaddressourprimaryandsecondaryques9ons.Thissec9ondescribesthosemeasurementsandtherequirementsassociatedwiththem.

3.B.1RequirementsforMeasurementsofFluxesinthePolarRegions

Anunderstandingofthelayeringofthepolardepositsanditsrela9ontopastclimatesrequiresanunderstandingof theprocesses that control theaccumula9onanderosionof thepolarcapsinthecurrentclimate.Forthisitisnecessarytoquan9fythedeposi9onofdustandvola9lesonthepolarcapaswellastheinflowandou�lowofdust,waterice,andwatervaportoandfromthepolarregion.Thisissummarizedasfluxmeasurements.Fluxmeasurementswillhavetobeperformednearthesurfaceinordertoquan9fyfluxesthataredirectlyrelevanttodeposi9onandabla9onprocesses.Thesemeasurementswill likelyhavetobeperformedfromsurfacesta9onsandwouldhavetobesupplementedbyorbitalmeasurements(global,ver9calprofiles of dust, water vapor, and water ice clouds) in order to connect the surfacemeasurementswiththeprocessesthatgovernthecurrentglobalclimate.

Measurementrequirements

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To assess polar fluxes, atmospheric measurements at and near the surface shouldinclude,windspeedanddirec9on,temperature,pressure,andhumidity.Ifthesemeasurementsare performed with a sufficient accuracy and repe99on rate, they will be able to resolveatmospheric fluxes relevant to the current climate. Of cri9cal importance is to measure thetransport by turbulent eddies in combina9on with humidity and par9cle measurements. Toachieve this, the requirementonawindmeasurementwouldbeaprecisionofa fewcm/s inthree direc9ons at a temporal resolu9on of ~20 Hz at least once per hour. Surface pressuremeasurementsshouldprovidelong-termstabilityof0.1mbarwithaprecisionof0.02mbartobe frui�ul. The temperature measurements should fulfill a 0.5 K accuracy requirement. Thecombina9onofthesemeasurementswouldallowthecharacteriza9onofheatandmomentumfluxesinthelowermostpolaratmosphere. Humidity will provide informa9on on water vapor content and transport in thelowermostatmosphere.Theaccuracyrequirementforthismeasurementisoforderafewpartsper million (ppm). Humidity measurements with a high temporal resolu9on (~20 Hz) incombina9onwithwindmeasurementsofthesameresolu9onwouldallowthedetermina9onofwatervaportransportthroughturbulenteddies.Alterna9vely,watervaportransportcouldbederived from similar frequency measurements of the water vapor gradient at 2-3 heightsmeasuredfromatalltower,theheightofwhichwouldhavetobedetermined. In addi9on to wind and water vapor, fluxes of dust and condensates will have to becharacterized.CO2condensatesarehardtomeasurein-situandareonlypermanentonpartsofthesouthernpolarcap,sotheconsidera9onsherearerestrictedtowater icecondensates. Inorder to obtain fluxes in the atmosphere, dust and water ice par9cles should be countedindividually. Instrumenta9on should be sensi9ve to micron-sized par9cles and able todiscriminatebetweendustandwaterice.Togetherwithhourlywindmeasurements,par9culatemeasurementswouldenablethedetermina9onofdustandwatericepar9cletransportthroughturbulenteddies.Anaddi9onaloralterna9vemeasurementwouldbethedirectaccumula9onofdustandwater iceon thesurface (or robo9cdevice).Thiswouldprovidemassflux inonedirec9on,perhapsasgreata0.5to1mmperyear.Microgramsensi9vityisrequired.Inaddi9on,it should be possible to discriminate between dust and water ice accumula9on, perhaps bysor9ngorsublima9ngtheice.Furthermore,measurementsofop9calsurfaceproper9escouldsupplement the characteriza9on of environmental condi9ons and the interpreta9on ofdeposi9onanderosionmeasurements. Inordertogiveacomprehensivepictureoffluxesinthepolarregion,5-6sta9onswiththeaforemen9onedmeasurementcapabili9eswouldbeneeded.Fluxvaria9onsareexpectedtoprimarily varywith la9tudeandeleva9on so that sta9onsplacedon thepolar cap alongonelongitude at various la9tudes would be able to capture the dependence on la9tude andeleva9on.

Poten5altechnologies

Measurements of wind speed and direc9on would be best acquired by a sonicanemometer. It can be set up to measure in three dimensions and would fulfill themeasurement precision and temporal resolu9on requirements (Banfield and Dissly, 2005). Asonic anemometer could alsomeasure temperature to an accuracy of ~0.2 K. An alterna9vetechnology couldbeawind lidar,which couldprovide ver9calprofilesofwindanddirec9on.Whiletheyarefrequentlyusedinground-basedapplica9onsonEarth,nowindlidarhasbeen

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deployedonMarsyet.AhotwireorhotfilmanemometerasdeployedonMarsPathfinderorMarsScienceLaboratoryhasameasurementsensi9vityoforder1m/sat1Hz.Whiles9lluseful,itwouldnotbeabletoresolveturbulenteddies.Surfacepressurecanbemeasuredbymagne9creluctance diaphragm sensors (Hess et al., 1977) or capaci9ve sensors (Gómez-Elvira et al.,2012),whichhaveextensiveheritagefromViking,MarsPathfinder,PhoenixandMarsScienceLaboratory. For humidity measurements a laser hygrometer based on Tunable Diode Lasertechnology(TDL)provideshighsensi9vityathighrepe99onrates(Websteretal.,2004).SuchaninstrumenthasbeendeployedontheMarsScienceLaboratory. Measurements of dust and water ice mass flux could be achieved by aerosol op9caldetectors or nephelometers. Nephelometers measure par9cle densi9es that pass through alight beam and a light detector (e.g. a laser emiMer and a photocell receiver) either intransmission or scaMering geometry. They could also measure par9cle size distribu9ons andop9calproper9es.Deposi9onmeasurementscouldbemadebyamicrobalancedevice.Thesedevices work with thermally stabilized piezoelectric transducers, whose frequency ispropor9onaltothemassdepositedonthesensor.TheywereproposedfortheMEDUSAsurfaceinstrumenta9onpackage(Venturaetal.,2011).Fortheapplica9onofwatericemeasurementsit would have to be possible to heat microbalance in order to sublime deposited vola9les,yieldingthedifferencebetweenwatericeanddustdeposi9on.Measurementrequirementsandpoten9altechnologiesforin-situmeasurementsaresummarizedinTable3.1.

Table3.1:Measurementrequirementsandpoten5altechnologiesforin-situfluxmeasurements.

Measurementsfromorbitalassetscouldsupplementorreplacesomeoftheaforemen9onedin-situmeasurements.Globalmeasurementsofver9calprofilesofdust,watervapor,andwatericeclouds would be necessary to connect the surface measurements with the processes thatgovern the current global climate. These kinds of measurements have been provided, forexample,bytheMarsClimateSounder(Kleinböhletal.,2009,2017,McCleeseetal.2010)onMarsReconnaissanceOrbiter.Contribu9onstodeposi9onanderosionprocessescouldpossiblybe made using lidar or interferometric synthe9c aperture radar (INSAR). However, it is not

Measurement Sensi$vityrequirement

Measurementfrequency

Numberof

sta$ons

Poten$altechnology

Wind(nearsurface)

fewcm/s 20Hz 5-6 Sonicanemometer

Watervapor/humidity

fewppm 20Hz 5-6 TunableLaserSpectrometer

Dust/watericeflux

individualpar9cles,μm

radius

<1Hz 5-6 Nephelometer

Dust/watericeaccumula9on

<1μg daily 5-6 Micro-balance(evapora9oncapabili9es)

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certain that the accuracy of such instrumenta9on from orbit would be sufficient to directlymeasure deposi9on or erosion rates over short 9me scales. Finally, it may be possible tosupportdeposi9onanderosionstudiesbybringingoutanar9ficialsurfaceoradyeonadefinedareaofthepolarcap.Measuringchangesintheop9calproper9esofthedyedsurfaceovertheseasonfromasurfacecameraorfromorbitcouldprovideinforma9onondustoricedeposi9onaswellaserosionprocesses.Caremustbetakenwiththeinterpreta9onofsuchmeasurementsas the dyed surface will likely change the surface roughness and radia9ve proper9es incomparisontoanunmodifiedsurface.

3.B.2RequirementsforMeasurementsofLayerProper)esinthePolarRegions

The polar layered deposits (PLD) are expected to consist meteoric ice. Forma9on ofthesedepositsbeginswiththeaccumula9onanddensifica9onofatmosphericiceassnowintofirnordirectdeposi9onthateventuallyfillsporesbycompac9onanddiffusion.

On Earth, in the top 10-20 meters of firn, the gas in pores exchanges with theatmospherethroughconvec9on.Belowthisdepth,porositydecreasesandgastransportoccursviadiffusion.Atadepthofapproximately50–100m,sinteringof icecrystalstrapsthegasinbubblesandthefirn isno longerpermeable(Sowers,1992;Gow,1969).Thespecificclose-offdepth(~75monEarth)dependsonaccumula9onrateandgravity.Meteoriciceischaracterizedby the impuri9es present in the environment during its forma9on. Varia9ons in theconcentra9on of isotopes and ions with depth are used as a da9ngmechanism and for thestudyofpaleoclimate.

OnMars, it is unknown what frac9on of annual accumula9on occurs via snowfall vsdirectsurfacedeposi9on.Observa9onsbythenear-polarPhoenixlandershowedbothwatericecrystals and surface frosts (Whiteway et al. 2009).Models of PLD accumula9on and thermaliner9asuggestthatthesurfacelayerisdense,implyingthatdensifica9onmayberapidandthefirnonlyafewcmthick(Arthernetal2000);however,directobserva9onsarelacking.

Weobservediscrete layersofpar9culate impuri9es (Fishbaugh2010a;2010b;Becerra2016)(e.g.dust,andpossiblyvolcanicash)withinanicematrixthatcharacterizespecificlayers.These will reflect periodic deposi9onal events or may be lag deposits from episodes of iceabla9on, associated with climate drivers. Deposi9onal events of par9culates may reflectstochas9cprocessessuchasglobalduststormsorashfallfromvolcanicac9vity.Thethicknessandstra9graphyof the thesealterna9ng layerscan informusaboutmar9anclimateover theperiodsampledbydrilling,andperhapsallowustoinfercyclicaleventsinpastclimate.

Inthecontextoflayerthickness,frequency,stra9graphy,andstructuralrela9onshipsofthemar9anpolarlayereddepositsitisparamounttogathermeasurementsatvariousloca9onsand depths. Since the size of the smallest layer (or for thatmaMer, what defines a layer) isunknown, high-resolu9on measurements are needed. We envision a combina9on of twomethodstodoso.Bothwouldofferanunprecedentedlookatthestructure,stra9graphy,andlayer thickness and proper9es. The first would involve sampling of layers using mul9plemethodsalongthegentlyslopingtroughwallsatvariousloca9ons.Thiscouldbegatheredviaamobilepla�ormthatcoversmanykmoftransectandaccesseshundredsoflayers.

ThesecondmethodisusingboreholesdrilledwithinthePLDs.A1-mdrill(e.g.HoneybeeRobo9csIcebreakerDrill)wouldofferrock/icecompetency(inMPa)anddensitymeasurementswithin 20% error. Ideally, these measurements would be taken con9nuously but discrete

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measurements at 10 cm intervals would be sufficient. Once a borehole is drilled, an op9calborehole imager (e.g. Mount Sopris Instruments QL40-OBI-2G Op9cal Televiewer) could beloweredintotheboreholeinordertoiden9fyandcountlayers,andiden9fydepthsofinterestfor further inves9ga9on.Anop9cal loggercaptures360° imagesof theboreholewallandcandetectdiscretelayers(i.e.ofpar9culatesinice),layerthicknessanddip.Ver9calresolu9onisafunc9onofthedeviceandtheloggingspeed.Weexpecttoneed<1mmresolu9oninordertodiscern the thinnest layers within the ice associated with approximately annual deposi9on.Onceareasofinteresthavebeeniden9fied,amicroscopicimagerwitharesolu9onofabout30μm/pixelwouldrecordhigh-resolu9onimagery.Resis9vityprobes loweredwithinaboreholecouldbeusedtohelpcorrelateground-penetra9ngradar (GPR)datawithvisible layers in thesubsurface.Mul9ple boreholes also offer the chance for cross-borehole tomography (seismicimagingmethod).

Surface-basednon-destruc9vegeophysicaltechniquescouldbedeployedaswell,albeitwith coarser resolu9on. The top layers of the PLDs are not resolved by current soundinginstrumentsSHARADorMARSIS.Inordertoresolvetheseshallowfeatures,ahigher-frequencyGPR, either in orbit or on a rover/lander,would be useful especially one that offers >100mpenetra9on depthwith 10 cm range resolu9on. Similarly, an advanced ac9ve source seismicsystem could resolve the layers and offer varying depth penetra9on depending on thesepara9onofsourceandreceiver,however,thismethodisrestrictedtoarover/landersystem.

InEarth’sPolarRegions,researchconcerningthediscretelayersofpar9culateimpuri9esis done by collec9ng ice cores, returning them to a laboratory, and subjec9ng samples to avarietyofanalyses,suchas: thinsec9onstereology (for icegrainsizeandcrystalorienta9on);electrical conduc9vitymeasurement (ECM) for iden9fying varia9ons in acidity (that on Earthreflect forest fires, a seasonal marker in some loca9ons); meltwater analysis for dustconcentra9on,ionandisotopechemistry;andX-raymicrocomputedtomography(microCT).

Drillingandwithdrawingcores isstraigh�orwardonEarth,wherehumans interveneateverystate.HandlingandsamplingthemonaMarsrobo9clanderwouldbesignificantlymoredifficult. Besides bringing a core to a lander deck, it is foreseeable that instruments cold bedesignedtofitinaborehole,suchasanECMoranop9calboreholelogger.Eachhaslimita9ons

MicroCT can examine layering by par9cle size and shape, and poten9ally by rela9veatomic weight. MicroCT provides nondestruc9ve three-dimensional visualiza9on andcharacteriza9onoftheinternalfeaturesofmul9phasematerialswithspa9alresolu9ondowntoseveral microns. It has been used extensively in the study of deposi9onal processes insedimentaryrock (e.g.FalvardandParis,2017)andmorerecently in ice (Obbardetal.,2009;Iversonetal.,2017). Inice,microCTcanmeasuremicron-scalelayerstra9graphy,par9clesize,shape,volumeconcentra9onwith respect todepth,pore size, shapeanddistribu9onwith respect todepth,andpoten9allydiscrimina9onof salts fromFe-rich sediments.TheuseofmicroCTanalysis todifferen9ateashlayersfromspecificerup9onshasbeendemonstratedintheWestAntarc9cIceSheet (WAIS) core (Iversonetal.,2017, in review). Samplehandlingwouldbedifficult, soweseekamicroCTsystemthatcouldanalyzeafreestandingcoreleebyacoringdrill. TheexpectedpolycrystallinenatureofPLDicemaycontainusefulinforma9on.OnEarth,crystalorienta9on(fabric)andsizeandshape(texture)provide informa9onaboutstress,flowand temperature in glaciers and are cri9cal for understanding its mechanical behavior andmodelingitsmovement. Crystals,alsocalledgrains,are ini9ally small,butgrowwith9me (grainarea increases

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linearlywithage),aneffectofthetendencyofthesystemtoreducefreeenergybyminimizinggrain boundary area. Terrestrial models of this growth process and how it is limited byimpuri9escomparewellwithobserva9ons(e.g.Durandetal.,2006).Pastmodelsofthemar9anPLD (Kieffer, 1990) can relate ice grain size vs depth in the uppermost meters to surfaceaccumula9onrate.Thusameasurementofgrain sizeasa func9onofdepthhasu9lityat thePLD.Grainsizemeasurementtechniquesneedtobeeffec9vefrom10to1000micronsinordertoconductthisinves9ga9on.Thestandardapproachtothismeasurementwouldbetoobservethinsec9ons from icecoresunderpolarized light.Withinaboreholehowever, thisquan9ty isdifficult tomeasure. Sublima9on occurs preferen9ally along grain boundaries (Obbard et al.,2006),sore-imagingtheboreholeathighresolu9ons10sofdaysaeerit iscreatedmayallowgrainboundariestobevisible.Alterna9vely, if the iceremainsgranularandporouswithintheuppermostmetersthennear-IRspectroscopycanbeusedtoinfergrainsizes.Itisunlikelythatthisinves9ga9oncouldbeconductedbytraversingatroughofexposedlayersbecausetheyareallexposedinthecurrentepochandexperiencethesametemperaturevaria9ons. The ice phase expected under mar9an surface condi9ons is the same as terrestrialsurfacei.e.iceIh.Thishexagonalarrangementofwatermoleculesinparallelplanesmeansthatplas9cdeforma9onismucheasierperpendiculartothec-axisthanotherdirec9ons.Icegrainsinhigh-strainenvironmentstendtorotateun9ltheirc-axispointsperpendiculartothedirec9onof strainaeerwhich they canbe readilydeformed.Unstrainedmeteoric ice shouldnot showpreferred alignment of ice crystal basal planes. Thus an inves9ga9on into basal planeorienta9on could provide a constraint on past ice flowwithin the PLD. Such an inves9ga9onwouldhavealowchanceofsuccess.Thelowtemperaturesandgravitymeanthatthesenear-surface layersareunlikely tohavebeenaffectedbyflow.Stra9graphic studies (Karlssonetal.2011)haveshownthaticeflowhaslikelybeenminimalovermostofthePLDalthoughmodelspredict high flow rates over rare loca9ons where slopes are very high (Sori et al. 2016).Searching for preferred grain alignments in traverses of trough walls is poten9ally morepromisingasmoderatesurfaceslopesarenearbyandolder ice layershavebeenmoredeeplyburied and so have experienced higher stresses and temperatures. However, the technicallimita9onsdescribed in thegrain size inves9ga9onaboveareevenmoresevere in iden9fyingpreferredgrainalignments.Although itwouldbeausefulmetric tomeasure,grainalignmentseemstechnicallydifficultatthis9me.

3.B.3RequirementsforMeasurementsofComposi)oninthePolarRegions

The clima9c record of the polar layered deposits is stored as variability in both thephysicalandchemicalnatureoftheiceandentrainedsediments.Inthissec9on,wediscussthemeasurementsthatcouldbemadetoconstrainthecomposi9onalandelectricalproper9esofthePLDandoutlinepossibleinstrumentpayloadsolu9onsandinves9ga9onstrategies.

RequiredMeasurements

Thevola9lecomponentofthepolarlayereddepositsisinferredtobecomposedlargelyofwaterice,withotheratmosphericgasestrappedduringdeposi9onintheformofmicro-scalebubbles.ItisunknownwhetherornotCO2iceispresentwithinthePLD,asclathrates,butithasnotbeenruledoutasapossibility.Thecomposi9onoftheicescanbeconstrainedbothbased

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ontheirbulkabundance(e.g.,H2Ovs.CO2frac9onsat1wt%orbeMer)andbasedonisotopicra9os.Determiningthe isotopiccomposi9onofnew ice formingat thepolesandthe isotopicrecord within the PLD would place important constraints on source reservoirs and modernfrac9ona9onprocesses. Isotopicmeasurements should includeD/H to a precision of 100/milovertherangeof100-9000mil,18O/16Oataprecisionof3/mil,and13C/12Cataprecisionof5/mil.Thesemeasurementscouldbedirectlycomparedtoisotopicmeasurementsofthemodernupper atmosphere by MAVEN or ancient hydrated minerals in Gale crater sediments andmar9anmeteorites.Thesecaninformmodelsoflongtermatmosphericescape. Like in ice cores on Earth,we hypothesize that the PLD contains relict gases, trappedduringdeposi9onasbubbles, andpossiblyas clathrates. Thesegasesprovidea recordof thecomposi9on of the atmosphere under recent clima9c condi9ons, which could be assessedbasedonabundancesofmajor(CO2,N2,Ar,O2,COataprecisionof0.1%orgreater)andminor(H2O,NO,Ne,HDO,Kr,Xeataprecisionof10-100ppb)gasesmeasuredbyViking,aswellasother importantvola9le species likeS,Cl, andCH4.Bubble sizeanddensitywouldalsobeanimportantindicatoroficecompac9onandmaturity. Mul9ple types of non-ice solids have been iden9fiedwithin the PLD based on orbitalspectroscopy, including mar9an surface/atmospheric dust (composed of an assemblage ofphases including nanophase ferric oxides, primaryminerals, and S/Cl-rich salts;Morris et al.,2004;Yenetal.,Hamiltonetal.,2005;Yenetal.,2005),perchlorateandsulfatesalts(Horganetal., 2009; Calvin et al., 2009; Masse et al., 2010, 2012), and primary mafic sediments (e.g.,pyroxeneandglass;Horgan&Bell,2012;Horganetal.,2014).OrganicsmayalsobepresentinthePLDfrommeteori9cinputs.Thever9caldistribu9onofthese“impuri9es”withinthePLDisnotwell constrained, but they are probably present both in dis9nct sedimentary layers (e.g.,aeolian layers, past sublima9on lags, ash layers, and impact ejecta) and as a volumetriccomponent incorporated during seasonal deposi9on. The composi9on, grain size, anddistribu9onofthesenon-icesolidsareallimportantmeasurementsforconstrainingthehistoryofthePLDandlinkstoatmosphericprocesses. Determiningthemineralogy(precision0.1wt%)andelementalcomposi9on(e.g.,Si,Fe,Cl, S at 0.1 wt%) of soluble impuri9es like salts would help constrain modern and recentatmospheric chemistry and the degree of rock/water interac9ons on the surface. Similarly,determiningthecomposi9onandabundance(sensi9vityofppmtoppb)oforganicswouldplaceimportant constraints on modern meteori9c inputs on Mars. Determining the mineralogy(precision 1wt%) and grain size (for ~1 μm par9cles and larger) of insoluble impuri9es likeprimarymineralsandglasseswouldhelpconstrainejectaandvolcanic inputs to thePLD,andwould help to determinewhether to not thesematerials are in sufficient quan9ty that theycouldbeusedtodeterminetheageof individual layers. Inthecaseofvolcanicashor impactejectawithameltcomponent,crystalliza9onagewouldconstraintheageofthat layerofthePLD. In principle, the deposi9on age for a broader range of sediments could be determinedbasedon9mesinceexposuretocosmicraysorvisiblelightasdiscussedinSec9on3.A.5. Determining bulk proper9es of the PLD including conduc9vity and permixvity wouldalso assist in constraining thedistribu9onof ices, bubbles, and lithics through the stack, andwouldprovidekeyinputsformoreaccurateanalysesofexis9ngRADARdata.

MeasurementStrategies

Composi9onisprimarilyusefulasaproxyforpastclimateandothergeologicinfluences

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onthePLD,andthus ismostusefulwhenextractedasasemi-con9nuousstra9graphicrecordthatcanbecorrelatedwithclimatemodels.To9evariabilityincomposi9ontoclima9ccycles,adetailed ver9cal sampling of the composi9on of the PLDwould be required,most likely at aver9cal resolu9onon theorder of 0.5mm. Landing site selec9on should account for regionswith significant dust cover and ongoing modifica9on of the ice surface (e.g., Smith & Holt,2010). To interrogate a sufficiently long temporal record, access to hundreds of meters ofver9calstra9graphymayberequired. OnEarth,icestra9graphiesaretypicallyanalyzedasintactcoresaeertheyarebroughttothesurface.OnMars,thistypeofanalysisisprobablyinfeasible,soanalysiscouldtakeplace(1)withintheborehole,viadirectsampling, remotesensing,orcontactmeasurementsof theboreholewall,or (2)on thesurfacepla�ormondrill cuxngs (e.g. crushedand/ormelted iceextracted at intervals by the drill). Remote sensing and/or contactmeasurementswithin theboreholewouldrequirecommunica9onwithinstrumentsondeckorinstrumentsthatfitintheborehole. Direct sampling of ice for detailed chemical analyses could also occur along theboreholewalls,eitheraspartofthedrill itselforviaaseparateprobeinsertedaeerdrillingiscomplete. Care must be taken in sample prepara9on, as crushing or mel9ng may releasetrappedgasesandmel9ngmayplacetheminsolu9on.Mel9ngmayalsobeproblema9cinthepresenceofsediments.

PossiblePayloadElements

Ice composi9onand icegrain sizeaswell as themineralogyof impuri9es couldall beassessed rapidly via short-wave infrared reflectance spectroscopy (SWIR; 1.0-2.6 microns),whichhasbeenshownviaorbitalremotesensingtobehighlysensi9vetokeyphasesinthePLD(references above) and has been successfully miniaturized for a variety of spaceborneapplica9ons.Alterna9vely,laser-inducedscaMeringviaRamanspectroscopycanalsodeterminemineralogyandicecomposi9on,andRamaninstrumentsliketheMars2020ScanningHabitableEnvironmentswithRaman&LuminescenceforOrganics&Chemicals(SHERLOC)andSuperCamspectrometerswill be flownonmul9ple in situmissions toMars over the next few years. Inprinciple, either reflectanceorRaman spectroscopy couldbe implementedwithin aboreholeusingfiberop9ccables. Mul9plemissions have successfully interrogated the bulk chemistry of the surface ofMars,usingoneofseveralcommontypesof instruments.Bulkchemistryofboreholewallsorother solid samples could be assessed via X-ray fluorescence (XRF), as implemented in thePlanetaryInstrumentforX-rayLithochemistry(PIXL)instrumentonMars2020,viaalphapar9cleX-ray spectroscopy (APXS), as implemented on mul9ple Mars rovers, or via laser-inducedbreakdown spectroscopy (LIBS), as implemented in the MSL ChemCam and Mars 2020SuperCam instruments. LIBShasbeen implementedwith longfiberop9cs cablesandmaybemoreapplicabletosmalldiameterboreholes. Determining the composi9on of trapped gases requires another set of techniques.Tunable laser spectroscopy (TLS), as implemented on the Phoenix lander, could provideinforma9on on the bulk and isotopic composi9on of major gases. Any addi9onal analysisbeyondthiswouldrequireamassspectrometer,perhapssimilartoNeutralGasand IonMassSpectrometer (NGIMS) onMAVEN. Amass spectrometer that also included pyrolysis or laserabla9on, such as SAM on MSL, could be further applied to non-ice solids to detect andcharacterize organics, or poten9ally u9lized for exposure age da9ng of sediments via

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cosmogenicnuclides.Allof theseapproacheswould require transportofmeltwaters, trappedgases, and poten9ally trapped sediments up through the borehole to the surface pla�orm.Alterna9vely,burialagescouldpoten9allybeextractedwithintheboreholeviaamuchsimplertechnique like op9cally s9mulated luminescence (OSL), although this technique has lowheritageforplanetarymissions.

3.CMissionConceptsandApproachestoMeasurements

OurteamscameupwithastrategytoaddressthethemeofunlockingtheclimaterecordintheNPLD.Thestrategyismul9facetedandcannotbecompletedinasinglemission. Thefirstpartofthestrategyistolearnaboutthefluxesofmaterialsintoandoutofthepolarregions.ThesearethematerialsthatcomprisethebulkofwhatisstoredinthePLD,anditis of utmost importance to their availability on daily, seasonal, and inter-annual 9mescales.Por9onsof this topicmustbeaddressed fromorbitbecausepointdata from the surfacewillonlysupplyalimitedknowledgeofpolarfluxes.However,surfacedataisalsoarequirementinorder togethigh resolu9onat the lowest scaleheight in theatmosphere (ground truthingoforbitaldata)andatthesurface-atmosphereboundary.Anorbitalcomponentisalsonecessarytosurveyallofthevola9lereservoirsonMarsthatinteractwiththeatmosphere. Surface-based observa9ons are necessary to determine the chemical and physicalnatureofthelayersthemselves.Ourteamhascomeupwithaseriesofmissions,ofincreasingcomplexity, that will move towards determining the climate record. This series was decidedbecause some ini9al reconnaissance of availablematerialsmust be done before sending thefinestprecision,mobileinstruments.Forexample,wedon'tknowtheisotopesavailableinthepolarice,sosendinginstrumentsthatmeasureisotopera9osathighprecisionisnotwarrantedun9lthoseisotopeshavebeenshowntoexistinsufficientquan99esforaccuratemeasurement. Theseriesofmissionsthatwedetermined include1)DiscoveryorNewFron9ersClasssta9c landercapableofmeasuring surfaceandsubsurfaceproper9esusingameterclassdrillandatmosphericproper9es;2)severalsmall landersthathave instrumentstoeithermeasuretheatmosphericorsurfaceproper9es;and3)alarge,FlagshipClassmobilepla�ormcapableofaccessingthemanylayersexposedinaspiraltroughoratapolarscarp.Theinstrumentsforthemobile pla�orm may be baselined now, but the final selec9on should be based on whatmaterialshavebeenmeasuredinpreviousmissions.

3.C.1Orbiter

Anorbitermissiondedicatedtothestudyofthemar9anpolarregionshasthepoten9altogreatlyadvanceourunderstandingofthepresentdayforcingsandfluxes inandoutofthehigh la9tudes, and also help characterize fundamental proper9es of the PLD themselves. Anorbiter would have the advantage of providing data over the en9re planet, possibly acrossmul9plemar9anyears,includingduringwinterseasons.Inaddi9on,wenotealandedmissionmakingsimilarmeasurementswouldcomplementthissuitebyproviding"groundtruth"data.

Completecoverageoftheplanetandatalllocal9meswouldbeideal,butwithasingleorbitertradesmustbemade.A90°inclina9onwouldreducedatagapsatthepoles(asopposedto~87/93°as forMGS,ODY,andMRO).However, this reducescoverageatotherpartsof theplanets. In addi9on, an orbital configura9on allowing for a range of local 9mes would be

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desirable(asopposedtothefixedAM/PMorbitsofMGS,ODY,MRO)inordertogainaccesstodiurnal processes on the ground and in the atmosphere, and further enhance the scien9ficvalueofexis9ngdatasets.However,accesstovariouslocal9mesoeencomesattheexpenseoforbitalcoverageatthepoles.Ontheotherhand,increasingthebaselineofcurrentatmosphericobserva9onswithMarsClimateSounder(MCS)andMARCIisalsodesirable.

Anellip9calorbitalconfigura9onwouldallowforalowperiapsis(~150km)overthepoletobestudied.Theuseofsolar-electricpropulsionandorbitchangingmaneuverscouldenabletheorbiter toalternatebetweenperiapsis at theNorthand theSouth caps indifferentMarsyearsorstrategicallyselectedseasons.

Thesciencepayloadcouldbecomposedofinstrumentsselectedtofitwithinoneoftwobroad themes: 1) Fluxes and Forcings, and 2) PLD Physical Proper9es.We note that severalinstrumentshaveaverystrongflightheritageandcouldpoten9allybeproposedwithmodesttechnologicalinvestments.SeeTable2foralistofinstrumenttypeandkeymeasurementgoals.

FluxesandForcings

Of highest priority is tomeasure global wind speeds atMars. Thismeasurement hasnever been made and represents one of the largest strategic knowledge gaps in ourunderstandingofpresentmar9anclimate.Weforeseetwoinstrumentssuppor9ngthispriority,amicrowavesounderoraLIDAR.Besidesmeasuringwindspeeds,eithercouldprovideprofilesof temperature,water vapor andother tracers in theatmosphere. Thiswouldprovideanewbenchmark for the performance of General Circula9on Models. Together with a sounding

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Figure3.1:Orbiterschema5cshowingaspacecrabwithlargeradarreflectorandextendedsolarpanelstosupportapowerfulsuiteofinstruments(googlesearch).

instrument, a wind profiler with coverage in the lowest scale height would providecomplementary capabili9es to link transport of atmospheric cons9tuents simultaneously towindspeeds. -AnIRsounderinspiredbyMCSanddedicatedtocharacterizingthepolaratmospherewouldprovidetemperature,watervapor,dust,CO2andH2Oicecloudprofilesaswellassurfacetemperatureandemissivity.Improvementinordertoobtainhighver9calresolu9onandbeMercoverageinthelowestscaleheight(closetothesurface)areimportant. PLDPhysicalProper5es

-AsoundingmoderadarP-orL-Bandfrequenciescouldresolvefinelayeringsequenceswithin the top 100s ofmeters of the PLDwith a high ver9cal resolu9on of ~50 cm. Such aninstrumentwouldbecomplementarytoMARSISandSHARADandcouldhelpexplorethemostrecentfewhundredsofthousandsofyearsworthofrecordinthePLDs. - Very high spa9al resolu9on imagers (mul9-wavelength visible, similar toHiRISE, andhyperspectral visible /near IR spectrometers, similar to CRISM) would have the poten9al toresolvefiner layer structureswithin the stra9graphic record, andconstrain their composi9on.We note that to generate fundamentally new science informa9on, the spa9al resolu9on ofthese instruments would need to be at least an order ofmagnitude beMer than HiRISE andCRISM. -AnSynthe9cApertureRadar(SAR)instrumentwithinterferometriccapabili9es(InSAR)

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Table2:OriberStraw-manPayload

ontheorderof1cmcoulddeterminever9calchangesofsurfacetopographyatthepoles(masswas9ngorseasonalaccumula9on)givingthepoten9altocharacterizethecurrentimbalanceinthe energy budget of the caps. This instrument, while being highly valuable for polarobserva9ons, would also provide a new dataset for the en9re planet, changing the wayscien9stsstudymar9angeology. - Finally, a mul9purpose LIDAR instrument could provide valuable surface andatmosphere science, with high-resolu9on topography (ver9cal resolu9on beMer than 1 cmdesirable todetectsurfacechangesandhelpcharacterizefluxes towardsoroutof thePLDatthe seasonal and inter-annual 9me scales). Mul9ple wavelengths could allow thecharacteriza9onofsurfaceCO2andwatericeop9calproper9es.Itcouldalsobeusedtodetectandcharacterizecloudswithinthepolarnight. Hybridorbital/surfacemissionscouldalsobeconsidered.Surfaceassetsmaybeabletoprovidegroundtruthtocalibrateregionaldatasets(forexampleforthestudyofvola9lesordustfluxes at the surface) and be able to deploy surface calibra9on targets, reflectors, or radiobeacons to improve theaccuracyandprecision formonitoringof changes. Impactormissionscouldgatherdataduringdescentandprovideextremelyhighspa9alresolu9onofsurfaceandnear surface proper9es,well beyondwhat can be achieved fromorbit. Finally, inspired fromterrestrial glaciology, an instrument could disperse a dye on the surface. The changes (color,albedo,etc.)ascharacterizedfromorbitcouldproveameanstodeterminethemassbalanceatmul9pleloca9ons.

3.C.2Sta)cLander

Observa9onsfromthesurfaceare invaluablefor insitucomparisons,providinggroundtruthandhigherresolu9onthanmeasurementscapable fromorbit.Atthesurface,numerousexperiments can characterize atmospheric, surface, and subsurfaceproper9es andprocesses.Thisisabsolutelyrequiredtocompletethestatedpurposeofthisstudy. Anetworkofsimilarpla�ormstomakethesemeasurements fromthesurface is ideal;however, this type ofmissionmay be larger than even a Flagship class, and sowe discuss astraw-man payload for a single sta9c lander, keeping in mine the desire to repeat thesemeasurementsatseveralsitessimultaneously. Landingsitechoicewillbeinfluencedby1)safetytothemissionand2)comple9ngourscience requirements. If all sites aredetermined tobeequally safe, then theop9mal landingsitewillbeonethatmaximizessciencereturns.Becausethemissionistounderstandsubsurfacelayering, atmospheric processes, and surface-atmosphere interac9on, loca9ons with greatersurface-atmosphereac9vity (e.g.highmassbalanceandwind speeds)arepreferred.Anotherimportant considera9on is subsurface layering. It isundesirable to choosea loca9on thathasuniqueorlocalprocessesthatdonotrepresentthewiderPLDorgeneralbehavior.Thisexcludesloca9onsnear theboMomof thespiral troughs,wheredustand iceaccumulatemorequicklythaninotherregionsdotolocalatmosphericeffects(Smithetal.,2013). Wean9cipatethatasta9clanderisbestsuitedasareconnaissancemissiondesignedtocharacterizetheenvironmentattheNPLD.ThisfirstdeliverytotheNPLDsurfaceshouldincludeinstrumenta9onthatiscapableofaddressingstrategicknowledgegaps,includingmeasuringallrelevant atmospheric parameters and processes, observe surface changes and proper9es atvariablescales,andaccessthenearsubsurface,orthetop100cm.ThismissionshouldbeoftheNASA Discovery or New Fron9ers class and provide the groundwork for future a Flagship

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mission(seesec9on3.C.4)designedadvancetheknowledgegained. A payload that characterizes the uppermost 100 cm of icewill require access to thatdepth.Poten9almethodstodosoincludethermalormechanicaldrillingandanalysisinsidetheboreholeoranalysisofmaterialbroughttoinstrumentsonthelanderdeck.Someinstruments

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Figure 3.3a: Honeybee’sIcebreakerdrillsuccessfullyp ene t r a ted i c e w i t hperchlorate under marspressure and temperaturecondi5ons.

Figure3.3b Icebreakermissionwould land intheNorthernPolarRegionsanddeploy1mclassdrill.

! !

Figure 3.2: CAD rendi5on of a lander asset using theMER EDL architecture. A) A drill armextendspast the landinghardwareandmechanicallydrills~50cm intothesubsurfaceusingseveralsmallsteps.B)Pneuma5chardwaremovesthedrillcugngstothelanderdeckwheretheycanbeanalyzedbyonboardinstruments.GraphiccourtesyofHoneybeeRobo5cs

! !

are too large to fit in a borehole, making in situ analysis infeasible, so a combina9on oftechniquesisconsidered.Herewe concentrate on an approach that includes amechanical drill, as opposed to amelt-probe.Themaindriverforusingamechanicaldrillisthesignificantpowersavings.Cryogeniciceis 3-4x more conduc9ve than warm ice, and as such, the mel9ng approach is extremelyinefficient(>80%ofheatislostintosurroundingice).Thiswouldwarrantdevelopmentoffissionreactortosupplypower.Addi9onally,meltprobeswouldalterthematerial(andinturnreduceitssciencevalue)andwouldnotbeabletopenetratematerialwithsignificantdustfrac9on. Modifying the drill string to include instruments would add significantly more effort,cost,and risk. Insteadweassumethat thedrill string (with some limitedsensingcapabili9es)wouldbepulledoutoftheholetoallowaloggingtoolwithanaly9calsuitetobeloweredbackin. The cuxngs generated during the drilling processwould be returned to and analyzed byeven more capable lander-based instruments. This all implies, there are three possibleapproaches to the inves9ga9on: 1. Sensing while drilling, 2. Logging the borehole and 3.Analyzingdrilledsamples,a3-9erapproachthatop9mizessciencereturnandreducesrisk. Sensingwhiledrillingisthefirstapproach.Drill-integratedinstrumentsthatwouldhelpwith thedrillingprocess, reducemechanical risks, andprovide sciencedata. The instrumentsanddatainclude:

• Materialstrengthanddensityfromdrillingtelemetry• Densityfromultrasonicvelocity• Subsurfacetemperaturefromtemperaturesensor• Bulkelectricalresis9vity(requiredtodetect“slush”forma9onbutalsoindica9veofsaltcontent)fromresis9vitysensor

Addi9onal instruments could also be packaged inside a drill string, but the extent ofwhatcanbeincludedlargelydependsonthediameter,androbustnesstoshock,vibra9onsandtemperature. Examples of drill integrated instruments include microscopic imager, LIBS anddeppUV/Raman. Someof these systems reachedhighTRLs.Other sample acquisi9onop9onsincludeacquiringasampledownholeandvola9lizingitinsideadrillbit.AcarriergascouldthencarryevolvedortrappedgassesdirectlyintoaGC/MSonthesurface.Instrumentscouldalsobeplacedclosetotheboreholeitselfandsniffforanyvola9lescomingoffthepileofcuxngs.TheResource Prospector rover has successfully implemented Near Infrared Spectrometer thatlookedatcuxngsasthesewerebeingbroughtuptothesurfacebythedrill. For instruments lowered into the bore hole aeer drill and cuxng extrac9on, weenvisionedthelogging(insituanalysis)packagetocontain:

• op9calcamerasystem• microscope• fine-scaleelectricalresis9vityprobe• temperaturesensor

Theop9calcamerasystemshouldprovideawidefieldofview(analogoustoafish-eyelens)andtogetherwithatemperaturesensorshouldaddressmostoftheques9onsassociatedwith“boreholelogging”.Themicroscopemayhaveashortworkingdistanceandthusbeabletoreach sub-micron resolu9ons. Both theop9cal camera and themicroscopewill require someformofillumina9onsystem(e.g.mul9colorLEDs).Inaddi9on,weforeseetheinclusionoflaserin theboreholeasasource forperformingLaser InducedBreakdownSpectroscopy (LIBS)anddeepUV/Raman.Thespectrometerwouldresideonthedeckofthelanderandbefedviafiber-op9ccable,whilelaserwouldbeintegratedwithadrill.Placingalaserinaloggingtoolitselfwill

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reduceasignificantriskoffiberdamageduetohighpowerlaser. A further element would be the inclusion of a small side-drill designed to penetratespecificlayersiden9fiedbytheop9calcamera.This“boreholesampler”wouldcaptureasmallvolumeofmaterialfromaspecificloca9onintheborehole,iden9fiedtobeofinterestbyotherinstruments(e.g.LIBSorRaman).Thissamplewouldthenbeanalyzedwith,forexample,amassspectrometeronthelanderdeck. Finally, we note that significant science can be achieved by analyzing samples ininstrumentsthataretoodifficultorimpossibletopackageintoalongandslimtube(i.e.tofitinsideadrilloraloggingtool).Manyoftheseinstrumentsorsciencepackagesexist–thebestexampleistheSampleAnalysisatMars(SAM)instrumentonboardofMSLrover. We also note a strong interest in using themicro-CT technique to study the internalstructureof the ice in anon-destruc9vemanner. This couldbeachievedbyproducinga coreratherthananemptyborehole.Aminiaturizedmicro-CTwouldthenbeplacedaroundthecoreor the core drill to make the measurement. It is clear that this idea has several technical

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Table 3: Lander Instrumentation. Drill (D); Borehole Logger (BL); Lander Deck (LD)

Typeofmeasurement

Instrument Heritage Location Comment

Optical Pan-cam(multi-spec) Many D,BL,LD

Optical Microscope Many D,BL

Environmental Groundpenetratingradar LD

Environmental Metstation Many LD

Environmental Iceanddustaccumulationsystem

LD Quartz-crystalmicrobalance?

Environmental LIDAR Phoenix LD

Analytical Tunablelaserspectrometer MSL LD

Analytical Massspectrometer Phoenix LD Connectedviaavacuumpumpandtubingtothemouthoftheborehole

Analytical DeepUV,RamanorVNIR(withfiberopticlead)

Mars2020 D,BL,LD Fibre-fedfromtheborehole

Elemental LIBS MSL,Mars2020

D,BL,LD Fibre-fedfromtheborehole

Ice structure microCT ExamineCore

challenges to overcome (e.g. maintenance of the integrity of the core during drilling anddevelopment of the instrument itself) but the scien9fic return appears to be sufficient towarrantfurtherstudy.Someformofnepholometermightalsobeconsidered,oralterna9velyamicro-CTcouldbeperformedonlargeicechipsthatwillbeproducedduringdrilling.

BoreholeDrilling Drilling in cryogenicenvironments is difficult andnot yetproven. Therearenumeroustechnicalchallengestoovercome:

a. Samplingresolu5onLayeringofthePLDmaybeasfineas0.5mm,andthereforeinves9ga9onsshouldapproachthisresolu9on.However,dataacquisi9onwith this levelof spa9al resolu9onwouldbedifficult toimpossible with robo9c sampling because of data volumes are much higher and the 9merequired toperformtheexperiment is too long.Hence,weenvision selec9ngposi9onsdowntheborehole forhigh resolu9on inves9ga9on.Elsewhere, lowerver9cal resolu9onacquisi9onalong the columnwouldbeperformed. This implies that a robust opera9ons conceptwill benecessarywithgroundinterac9oninthelooptofinalizetheexperimentplan.

b. Instrumentintegra5onandresourcesInstrument capable of performing the proposed tasks do not yet exist. Therefore,miniaturiza9ontechnologyisprobablyneeded,andaccommoda9onandfurthertrade-offsandde-scopesmayberequired,dependingonthesizeofthelandingpla�ormandbudget.

c. RemovalofdrilledmaterialThesmallestusefuldiameterof theborehole is likely tobearound5cm.Thus,at least7,500cm3ofmaterialwillneedtoberemovedfromtheborehole.Thismaterialneedstobedumpedclose to the landeror transportedby somemeans to the landerdeck. Thedumpedmaterialmay alter beforemeasurements aremade. For examples icemay sublimate, leaving fines or

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Figure 3.4. Honeybee/JPL Autogopher1 drill penetrated to 3 m depth in hard gypsum andcaptured cores. Corebreakoffand reten9onwasproven tobedifficult becauseoeen coreswouldbreakupalongweaklayers.

! !

dustunbound.Thismaybecomemobile(blownawaybywind)therebyaffec9ngmeasurements.d. Boreholeintegrity

Materialmayfallintotheborehole,affec9ngcon9nueddrillingandsampleknowledge.e. Re-condensa9onwithintheborehole

Thedrillingprocessgeneratesheatthatcanleadtothesublima9onofvola9lesclosetothedrillbit.Thegasproducedcanthenre-condenseonthecolderwallsoftheboreholeinterferingwithmeasurements.

f. SedimentlayerThemixingra9oofnon-vola9lematerialinindividuallayersisuncertain.Thismayaffectdrillingopera9ons.Newlyexposeddustmaybemobilized,affec9ngmeasurements.

g. Diffusivelossoftrappedgasesh. Othericeevolu9on

Assessmentofhowtheiceevolvesaeeritbecomesexposed(intheboreholeorthecuxngsonthesurface)isrequired.Thisinvolvesvola9liza9onorrecrystalliza9on.

Jus5fica5onforrejec5onofspecificalterna5ves

a. Coringandcoreextrac9onThe technical challenges in implemen9ng an automated coring system and maintaining theintegrity of the core throughout extrac9on and analysis are substan9al. Core integrity maysufferfromfracturingandlossofcohesionifsec9onsareloadedwithun-cementedmaterial.

b. AnalysisofchipsfromthedrillingprocessIcesplintersorchipswillbeproducedbythedrillingprocess.Thesechipscanbecollectedandbrought to the deck instruments for analysis. This opera9on, for example, is being done onCuriosity and numerous missions baseline chips for analysis by the science instruments;however,exactknowledgeoftheformerburialdepthwillbedifficulttorecord.

3.C.3Small-SatNetwork

Besidesanorbiterandsta9clander,numerousop9onsareavailableforsmall-sat,short-livedmissions that canperform rapidmeasurementswhilebeing scaMeredover theNPLD. Inpar9cular, the MarsDrop pla�orm is of interest (Staehle, et al., 2015). These units areapproximatelythesizeofa6Ucubesatandshapedlikeanice-creamcone.ThedomecarriesaparachutebutnootherEDLcomponents.Landingonatargetmorespecificthanthe1000kmdiameter polar cap is not required. Once on the surface, several solar panels would deploy,revealingthepayload. Because of their small size, MarsDrop spacecrae must contain targeted inves9ga9ons.Mul9plemissionsofthisscalecouldbesentsimultaneouslytomeasureatmosphericproper9es,atmospheric cons9tuents, or surface and subsurface proper9es. For the atmosphere, a sonicanemometer coupled with temperature, pressure, and humidity instruments would be adesirableaddi9ontothemeteorologicalnetworkscaMeredacrossthepolarlandscape.SendingaTLStomeasuretheatmosphericcomponentscouldalsobeofhighvalue.Finally,apackagethat is able to measure ice proper9es would significantly contribute to themission science.Examples include electro-conduc9vity or ground penetra9ng radar that could fit within theMarsDroppackage.

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3.C.4MobilePlaXorm Once a sta9c lander mission is complete, and we have more informa9on about thecomposi9onofthenon-icematerialsinthePLDandtheirabundances,thenextstepwouldbeto deploy a mission capable of sampling significantly deeper than ~1 m. Two op9ons areavailableforaccessing>100mofice.Bothwouldcarryupdatedinstrumenta9onbasedontheknowledgegainedfromtheprevious,sta9clander.Thefirstop9onincludesa100mclassdrill.Thesecondincludesamobilepla�ormwitha1mdrill.The100mclassdrillwouldbemountedon a sta9c lander and penetrate to the target depth, while providing samples to the landermounted instruments and deploying downhole instruments. The mobile pla�orm wouldtraverse the gently slopingoutcropsof spiral troughs to samplemany layers.Given sufficientdura9on,thismissioncouldrepeatmul9pletransects,enhancingourknowledgeofthelayers. The mission would likely fall into the Flagship class and carry numerous scien9ficinstruments forahighlycapablepayload.Therovercould leveragehighheritagefromseveralcomponentsoftheCuriosityrover,reducingcostandimprovingreliability.Aninstrumentsuiteon a flagship rover would likely carry several instruments: a stereo imager; meteorologicalsta9on,anda1-meterclassdrill.Thestereoimagerwouldassesslayerthicknessandfrequency,assesssurfacecondi9ons,andhelpwithterrainnaviga9on.Themeteorologicalpackage,similarto the met sta9on on the sta9onary lander, would con9nue atmospheric science at a newloca9ons.The1-meterclassdrillwouldbeusedtoaccessmaterialbeneaththesurface,whichislikelycoveredbyalagseveraltotensofcmthick. Addi9onal instrumentswouldbe selectedaeer results from theprevious sta9c landerwereinterpreted.Westressthisstrategybecausesendinginstrumentsonanexpensivemissionwillonlybevaluableifthematerialstobemeasuredareknowntobeofsufficientabundancetomakethecostworthwhile.Withthatsaid,webelievethatsomeorallofthefollowingsuitewillprovidenecessarymeasurementstohelpunlocktheclimaterecordstoredintheNPLD.Onthe

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Tunable laser spectrometer (D/H, ™18O, ™17O)

MET sensors (temperature, pressure, humidity)

• One or more “micro-landers” deliver ~1 kg payload to surface of PLD

• Determine isotopic composition of ice layers, atmospheric fluxes

Sampling system (heater/gas inlet, photodiodes)

Sonic anemometer (wind velocity)

Figure3.5:Deploymentofanetworkofsmall-satMarsDropspacecraZcarryingeitheratmosphericorsurfacemeasurementinstrumenta)on.

roverdeck,weenvisioncarryingagroundpenetra9ngradarandchemistrysuiteencompassingwet geochemistry and amass spectrometer. For down-boreholemeasurements, we envisioncarrying an op9cal imager (for grain size and layer thickness), MI (for grain size and layerthickness), thermal and electrical conduc9vity, tuning laser spectrometer, nephelometer (forpar9clesizedistribu9onofimpuri9esintheice),Micro-spectroscopy,andmicroCT. Measuring the composi9on of each layer, including the ra9o of datable isotopes is ofutmost priority. Besides da9ng the layers, somemeasurements thatmay be possible include10Be,D/H,18O/16O.HavingD/Hthrough9memaybeoneofthegreatestmeasurementswecanmake. Itwouldalsobevaluabletodetermine ifother isotopescanprovide informa9onaboutclimate processes. Some of the technological issues that will need to be overcome includedrilling (from a rover or a lander), mobility in a polar environment (in the case of a rover

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Figure 3.6a: Honeybee’s Icebreakerpenetrated2m in2hours inAntarc$ca. Icechipsaslargeas0.25inchwerecaptured.

Figure3.6b:Honeybee/JPLAutoGopher2drillreached7.5mdepthin40MPagypsum

!

!

pla�orm),andsurvivingapolarnight.

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Figure 3.8a. Honeybee’s Planetary Deep Drillpenetrated 13.5 m and 10.5 m in hard gypsumover a 5week dura$on. The drill took boreholepictureswitha0.5micron/pixelmicroscopeandtwocolorLEDs(UVandwhite).

H o n e y b e e / J P L WAT S O N d r i l lincorporates Mars2020 SHERLOCKinstrument(DeepUV/Raman)

! !

! ! !

Honeybee LITA drill on CMUZoerover

Honeybee ARADS drillon NASA Ames KREX2rover.

Honeybee TRIDENT drill on NASAJSCResourceProspectorrover.

! ! !

Figure3.7.Severaldrillsweredeployedonvariousrovers.

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Figure3.9.Deepdrillcouldpoten$allybeplacedonarovertoallowforbecersiteselec$on.

! !

ListofAcronymsAPXS AlphaParticleX-raySpectroscopyCFA ContinuousFlowAparatusCRISM CompactReconnaissanceImagingSpectrometerforMarsCTX ContextCameraDTM DigitalTerrainModelECM ElectricalConductivityMeasurementFRAM FarRovingArcticMissionFTIR FouriertransforminfraredGCM GlobalClimateModelGPR GroundPenetratingRadarGRS GammaRaySpectrometerHiRISE HighResolutionImagingScienceExperimentHRSC HighResolutionStereoColorImagerIRIS InfraredInterferometerSpectrometerIRTM InrafredThermalMapperKISS KeckInstituteforSpaceStudiesLIBS Laser-inducedBreakdownSpectroscopyMARCI MarsColorImagerMARSIS MarsAdvancedRadarforSubsurfaceandIonosphereSoundingMB MarkerBedMCS MarsClimateSounderMGS MarsGlobalSurveyorMOLA MarsOrbiterLaserAltimeterMOC MarsOrbiterCameraMRO MarsReconnaissanceOrbiterMSL MarsScienceLaboratoryNPLD NorthPolarLayeredDepositOMEGA ObservatoirepourlaMinéralogie,l'Eau,lesGlacesetl'ActivitéPFS PlanetaryFourierSpectrometerPLD PolarLayeredDepositRAT RockAbrasionToolRIMS ResonanceIonizedMassSpectrometrySAM SampleAnalysisatMarsSAR SyntheticApertureRadarSHARAD ShallowRadarsounderSFD Size-frequencyDistributionSPLD SouthPolarLayeredDepositTES ThermalEmissionSpectrometerTLS ThinLayerSetsTHEMIS ThermalEmissionImagingSystemWRAP WidespreadAccumulationPackageXRF X-rayFluorescence

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