Jian-Yang Li1, R. M. Nelson1,2, J. C. Castillo-Rogez3, M. D. Boryta2, K. S. Manatt3, C. L. Vides2,4, J. Quinones2,5, C. T. Russell6, C. A. Ray- mond3

1Planetary Science Institute (jyli@psi.edu), 2Mt. San Antonio College, 3California Institute of Technology, Jet Propulsion Laboratory, 4California Polytechnic State University at Pomona, 5California State University at Los Angeles, 6University of California at Los Angeles

Background: One of the most enigmatic areas, and the brightest spot on Ceres, the Cerealia Facula inside theOccator crater, shows the spectroscopic signature of natrite (Na2CO3) (De Sanctis et al., 2016). Dawn performedobservations very close to the opposition geometry of this area with a minimum phase angle of <1 deg (Schröderet al., 2017). In order to help interpret the low-phase observational data and to better understand thephotometric behavior of Cerealia Facula, we measured the phase function of natrite of various grain sizes inlaboratory with goniometer, and analyzed the photometric behavior of the samples, in particular the dependenceof the opposition surge with respect to grain size.

Summary: The phase functions of natrite of seven grain sizebins from 20-75 μm were measured with a laboratorygoniometer from 0.05 deg to 15 deg phase angle. The phasefunctions all show two linear segments with two slightlydifferent linear slopes outside the opposition surge, and awide exponential component starting from phase angle 2.5deg and a narrow exponential component within phaseangle 0.5 deg. The strength of opposition monotonicallydecreases with grain size. These data and the results will beused to characterize the phase function of the CerealiaFacula near the opposition collected by Dawn camera.

Figure 2. The measured phase curves of natrite samplesthat were sieved into seven size bins as in the legend (inunit of µm). Measurements were performed at phaseangles a from 0.056 deg to 15 deg with a laser ofwavelength 0.635 µm to illuminate the samples. Foreach size bin, the data are averaged over five runs,binned into the phase angle bins and normalized to 5deg and shifted vertically for clarity as plotted. Grain-size dependence for the phase curves is evident.

Figure 6. The decreasing trend of OE area andOE height based on both models. The OEarea is the area between the phase curve andits linear extrapolation from outside of OEdown to opposition. The OE amplitude is thedifference of the phase curve and the linearextrapolation at opposition. This behavior isconsistent with high albedo samples Al2O3

(Nelson et al., 2018) when grain size is muchlarger than wavelength.

P33A-2866 UnderstandingRegolithPhysicalPropertiesofAtmosphereless SolarSystemBodiesbasedonRemoteSensingPhotopolarimetricObservations:EvidenceforEuropa’sPorousSurfaceRobertM.Nelson(1),MarkD.Boryta(2),BruceW.Hapke(3),KennethS.Manatt,(4)Yuriy Shkuratov(5),VladimirPsarev (5)KurtVandervoort(6),DesireeOliviaKroner(7),Adaze Nebedum (2),ChristinaVides(2,6),JohnQuinones(2,8)

1.PlanetaryScienceInstitute,PasadenaCA2.Mt.SanAntonioCollege,WalnutCA,3.UniversityofPittsburgh,PittsburghPA, 4.NASAJPLCA,5.Karazin University,Kharkiv,Ukraine,6.CaliforniaStatePolytechnicUniversity,Pomona.,7.UniversityofCalifornia,LosAngeles,LosAngeles,CA,8.CaliforniaStateUniversity,LosAngeles,LosAngelesCa,USA

rnelson@psi.edu

Introduction

Wepresentangularreflectanceandpolarizationphasecurvesfor13well-sortedparticlesizefractionsofαAluminumOxide(A2O3)measuredinthelaboratorywithagoniometricphotopolarimeter(GPP)ofnoveldesign.Theparticlesizeswerelargerthan,approximatelyequalto,andsmallerthanthewavelengthoftheincidentradiation(λ=635nm).Thephaseangle(αα)observedwas0o.056<α<15o.TheseA2O3 particulateshaveveryhighnormalreflectance(>~95%),therefore,incidentradiationhasaveryhighprobabilityofbeingmultiplyscatteredbeforebeingeitherabsorbedorelsebackscatteredtowardtheincidentdirection.Thereflectancephasecurvesforalltheparticlesizefractionsexhibitapronouncedincreaseinreflectancewithdecreasingphaseangleatα~<3o,consistentwithourpreviousreports(Nelsonetal.,1998;2000;2002;Shkuratov etal,2003;Piatek etal.,2004;Nelsonetal.,2016).Thelinearandcircularpolarizationratiomeasurementsinourearlierstudiessuggestthatthecauseofthisnon-linearincreaseinreflectanceasphaseangleapproaches0o isradiationthatismultiplyscatteredinthemediumbythecoherentbackscattering(CB)process(Shkuratov,1988,Muinonen,1989).Thepolarizationphasecurvesforparticlesizefractionswithsizeparameterx=2πr/λ<~1,showthatthelinearpolarizationrapidlydecreaseswithincreasingphaseanglefrom0o andreachesaminimumnearα=~2oforphaseangles0<α<~2o.Longwardof~2o,thepolarizationincreasesasphaseangleincreaseswithacrossoverpointbetween12o <α<~20o,dependingonparticlesize.Forsizeparameters>~1wedonotdetectpolarization.Thispolarizationvs.phasebehaviorisdistinctfromthepolarizationphasecurvesobservedinmoreabsorbingparticulatematerialsobservedinmaterialsinthelaboratoryandinlowalbedosolarsystemobjectssuchastheMoonandasteroids(Umov,1905,Lyot,1929,Dollfus,1961,1985,1996,Zellneretal.,1977,Harrisetal.,1989,Lietal.,2015,Belskaya etal.,2015).ThenegativepolarizationobservedinthesehighlyreflectivemediaisaconsequenceofthehigherprobabilitythatbackscatteredphotonsresultfrominteractionwithpairsorensemblesofparticlesasaconsequenceoftheCBprocess.TheseresultsarehighlyrelevanttounderstandingtheunusualnegativepolarizationbehaviorobservednearsmallphaseanglesthathasbeenreportedoverseveraldecadesonthethreehighlyreflectiveGalileansatellitesIo,EuropaandGanymede(Rosenbush etal,1997,2015).ThesurfacesofthehighalbedoGalileansatellitesmayeffectivelyscatterelectromagneticradiationasiftheywereextremelyfinegrainedwithvoidspace>~95%,andhavinggrainsizesoftheorder<=λ.TheGPPusedinthisstudywasmodifiedfromourpreviousdesign.Thesampleispresentedwithlightthatisalternatinglypolarizedperpendiculartoandparalleltothescatteringplane.Therearenoanalyzersbeforethedetector.TheHelmholtzReciprocityPrinciple(HRP)statesthisopticalarrangementproducesaphysicallyidenticalresulttothetraditionallaboratoryreflectancepolarizationmeasurementsinwhichtheincidentlightisunpolarizedandtheanalyzersareplacedbeforethedetector.WebelievethatthisisthefirstexperimentalverificationoftheHRPreportedbyHelmholtzfirstin1856.OurresultssuggestthatEuropa’sregolithhasahighvoidspace-perhapsexceeding90%!ThishasimportanceinthedesignofproposedEuropasurfaceinstrumentsonfuturemissions.

TheExperimentThedataweretakenonthelongarmGPPnowlocatedattheDepartmentofEarthSciencesandAstronomy,MountSanAntonioCollege,WalnutCA.Aninstrumentofthistypeisgenerallyclassifiedasa“polarization-sensitivewell-collimatedradiometer,”andtheAl2O3samplesareclassifiedas“discreterandommedia”(Mishchenko,2015).AschematicdrawingoftheinstrumentisshowninFig1.Wemeasuredthereflectanceandpolarizationphasecurvesof13well-sortedparticlesizefractionsofAluminumOxide(Al2O3)industrialopticalabrasiveswithdiameters0.1≤D≤30µm.Thesesamplesareidenticaltothosestudiedinourpreviousexperimentswhichwereintendedtoaddresstheamountofmultiplescatteringinthematerialsbymeasuringthephaseangledependenceofthecircularpolarizationratio(Nelsonetal.,2000,2002).TheAl2O3powdersaresuppliedbythemanufacturerinwell-sortedsizefractionsthatarelargerthan,comparableto,andsmallerthanthewavelengthofourmonochromaticincidentlight(λ=0.635µm).Theyarechemicallystable,permittingrepeatedinvestigationoverlongperiodsoftimebydifferentinvestigationteams.Sinceourinitialinvestigation,similarangularscatteringinvestigationshavebeenperformedonthesesameAl2O3 particulateswithGPPinstrumentsatotherlaboratoriesaroundtheworld,permittingustounderstanddifferencesininstrumentalperformancebymeasuringidenticalmaterialsondifferentinstruments(Shkuratov etal.,2002;Kaasalianen,2003;Piatek etal.,2004;Gundersonetal.,2008).Theseparticulatematerialsareappropriateplanetaryregolithanaloguesforsolarsystemobjectsofhighgeometricalbedo,particularlyEuropawherewatericeisubiquitous.Watericeexhibitsaplate-likecrystallineformasdoesAl2O3.(FurukawaandWettlaufer,2007).Theparticulatesampleswerehorizontallyandlaterallyhomogeneous,andwereilluminatedwithlinearlypolarizedmonochromaticlight.Thehepolarizationoftheincidentlightalternatedbackandforth‘in’and‘perpendicularto’thescatteringplane.InastronomicalobservationsandpreviouslaboratoryGPPinvestigations,theanalyzersarecommonlyplacedjustbeforethedetector.Inourlaboratoryconfiguration,thepolarizationoftheincidentbeamischanged.Inthistechnique,polarizedlightispresentedtothesample,andtheintensityofthereflectedcomponentismeasured.ApplyingtheReciprocityPrincipledevelopedbyHelmholtzin1856(seeMinnaert,1941;Hapke,2012,pages264-265),ourlaboratoryobservationsarephotometricallyequivalenttotheastronomicalobservationsandtopreviouslaboratoryinvestigations.Thesamplesweregentlypouredintosamplecupsandthecupswerelightlyshakentoallowforsettling.Noattemptwasmadetopackthesamples.Thus,thefirstsurfaceencounteredbythelaserbeamwastheresultofa“natural”settlingprocessinordertobestsimulateapowderedsurfaceofaplanetaryregolith.Thisresultedinthesampleshavingveryhighvoidspaceascalculatedfromthemassofthepowderinthecupcomparedtothevolumeofthecup.ThevoidspacecalculatedforeachparticlesizeistabulatedinTable1.AtypicalsetofreflectancephasecurvesforAl2O3isshowninFigure2.ThetypicalsetofpolarizationphasecurvesisshowninFigure3.

Fig1.ReconfiguredGoniometricPhotopolarimeter.TheGPPusedinpreviousexperimentswasmodifiedsuchthatthesampleisnowpresentedwithlinearlypolarizedlight.Thepolarizationstateisvariedbyrotatingahalf-waveplateplacedintheincidentlightpath.Thedetectormeasuresthevariationintheintensityofthereflectedradiationasafunctionofphaseangle,α.Thisisanexactreplicationoftheastronomicalmeasurements,assumingtheHelmholtzReciprocityPrincipleapplies.

Table1.ParticlesizesofAluminumOxideparticulatesamplesusedinthisstudy.

ParticleSize(µm) voidspace(%) Particlespercm3

0.1 97.68 4.43E+130.5 96.30 5.66E+111.0 95.50 8.59E+101.2 94.94 5.60E+101.5 95.81 2.37E+102.1 84.60 3.18E+103.2 83.10 9.85E+094.0 82.21 5.31E+095.75 81.74 1.83E+097.1 76.83 1.24E+09

12.14 75.41 2.62E+0822.75 70.70 4.75E+0730.09 65.85 2.39E+07

Figure 2. Phase curves of the Al2O3 30.09 µm particulate materials (bottom) is normalized at 5 deg. Decreasing particle sizes are incremented 0.05 in normalized 5 deg. reflectance. They are (proceeding upward) 22.75, 12.14, 7.1, 5.75, 4.0, 3.2, 2.1, 1.5, 1.2, 1.0, 0.5, 0.1 µm respectively. The pronounced change in the character of the phase curves of particles <=1.5 and those >= 2.1 µm is indicates a change in particle shape. The smaller particles are equant and the larger ones are platlet shaped.

TheReflectancePhaseCurvesIngeneral,thereflectancephasecurvesofthethirteendifferentparticlesizesarecharacterizedby(startingfrom0.05o andproceedingincreasingphaseangle):Averysharp,highreflectance,oppositionpeakwithreflectancerapidlydecreasingfrom0.05o<α~<3otransitioningto,Agradual,linearlydecreasingreflectancefrom~5– 10o,andAnon-lineardecreasebeyond~10o.Thereflectancephasecurvesforthe13Al2O3 particulatesamples,normalizedat5o,areshowninFigure6.Theyaredisplacedverticallyby0.1foreachsize.Beginningfromthebottomandproceedingupwardthephasecurvesareshownforparticlesizes30.09,22.75,12.14,7.1,5.75,4.0,3.2,2.1,1.5,1.2,1.0,0.5,and0.1µmrespectively.SimilartoourearlierworkwherewefollowedtheapproachofAkimov (1980)andalsoShkuratov (1983,1997,Shkuratov etal.,2011)inusinganexponentialfunctiontoapproximatetheshapeofthereflectancephasecurve.ThisheuristicapproachhasobviousvalueinapplyingtheseresultstoastronomicalobservationsofASSBs.WefittedthedatatotheexpressiontothePSIMTSAC1function)

ThisheuristicapproachhasobviousvalueinapplyingtheseresultstoastronomicalobservationsofASSBs.

Polarization Phase CurvesPolarization phase curve measurements were made by presenting the sample with linearly polarized light and

measuring the intensity of the reflected component at phase angles 0.05<α<15o. In 2002, we measured the polarization phase curves of these same Al2O3 materials using a GPP at the Karazin University in Kharkiv, Ukraine5. The Karazin University GPP had been previously calibrated to the JPL GPP and the reflectance results from both instruments were consistent when measuring like materials. The Al2O3 samples are highly reflective. Therefore, the reflected photons are expected to be multiply scattered in

the medium. Therefore the polarization of reflected light would be expected to be randomized. This appears to be the case for particle sizes 3.2 µm and larger; no polarization effect is observed. However, for smaller particle sizes significant negative polarization is observed, particularly as phase angle decreases shortward of a few degrees. For the smaller, equant particles of size <= 1.5 µm the polarization decreases as phase angle decreases. Thepolarizationphasecurvesforsizefractionsofradiuslessthanorcomparabletothewavelengthoftheincidentlightarecharacterizedby(startingfromα=0.05o andproceedingtoincreasingphaseangle):Averystrongdecreaseinpolarizationbeginningatα=0.05o withcontinueddecreaseto~1o,dependingonparticlesize.Apolarizationminimumbetween1o and2o dependingonparticlesize.Thesmallestparticlesizesexhibitthelowestpolarizationminimum.Aftertheminimumisreachedthereisadecreaseinnegativepolarizationreachingacrossoverpointbetween12oand20o (estimated),dependingonparticlesize.ThepolarizationphasecurvesforthefivesmallestparticlesizesareshowninFigure3Theyaredisplacedverticallyby1%withthesmallestparticlesizeatthetopofthefigure.Thepolarizationphasecurvesthatwemeasurecaneasilybedescribedbyafunctionoftheform:

(5)where‘y’isthepolarization,‘α’isthephaseangle(indegrees),and‘a’,‘b’,and‘c’arethebestlinearleastsquarefitcoefficientstoeachsetofdataforeachparticlesize.Equation5,PSIMTSAC2,mayproveusefultoremotesensingobserversineffortstocomparetheselaboratorymeasurementstoastronomicaldata.Onceagain,aheuristicapproachmayprovetobeofvaluetoastronomicalobserverswhoareassessingthenatureofASSBs,andparticularlythosewhoinvestigatethepossibleimpacthazardsofsuchobjectstoplanetEarth.

Figure 4. Comparison of laboratory polarization phase curves of fine grained particulate materials (Figure 3) with the polarization phase curves of Europa reported by groundbased astronomical observers. In the laboratory measurements the polarization decreases as particle size becomes smaller, and the polarization minimum is observed as smaller phase angles. This suggests that sunlight is scattered from Europa by a surface that is very fine grained and extremely porous.

Understanding the Regolith of the Highly Reflective Galilean Satellites

In 1997 Rosenbush et al, reported polarization phase curves of Europa that were unlike those reported for many other solar system objects. In subsequent reports they find that Europa, Ganymede and Io, the high albedo Galilean satellites, the polarization phase curves have a pronounced asymmetric minimum at about 0.5o These are shown in Figure 4.. We assume the Aluminum Oxide particulates simulates the scattering properties of a particulate water ice surface of Europa. We are able to approximately replicate the polarization phase curve of Europa assuming an extremely fine grained and highly porous materials with void space exceeding 90% (see table 1).

ConclusionsThereflectanceandpolarizationphasecurvesofhighlyreflectiveparticulatematerialsareconsistentwiththereflectanceandpolarizationpropertiesofselectedhighalbedosolarsystemobjectssuchasIo,Europa,Ganymede,Saturn’sringsandtheasteroids44Nysa,64Angelina(Belskaya etal.,2017fig4)andcouldberelevant,ifproperlyinterpreted,tounderstandinghighalbedoregionssuchasSputnikPlanumonPluto(Burattietal,2017,Howardetal,2017),theOccator regiononCeres(DeSanctis,etal.2017,Schröder etal.,2017),Uranian satelliteUmbriel(Sori etal.,2017,Saturn’sMoonEnceladus(Scipioni etal,2017),The~0-2o polarizationminimummaybeassociatedwith)coherentbackscattering.Importantfeaturesinthepolarizationphasecurveoftheseparticles(suchastheslopelongwardofthepolarizationminimumcorrelatestronglywithparticlesize.ShadowhidingmayplayaroleinloweralbedoparticlesandproduceapolarizationphasecurvebyaprocessdifferentfromtheprocessthatcreatesthepolarizationphasebehaviorinhighalbedoparticlesofsmallparticlesizeThesciencevalueoffuturedeepspacemissionstoEuropamissionswouldbestronglyenhancedifthespacecraftwereabletocouldtakepolarizationphasedata,particularlygivenourlaboratoryresultsandtheastronomicalobservationsofVeraRosenbush andcolleagues.Obviously,ahighporosityregolithwouldbehazardtospacecraftthatlandedonEuropa’ssurface,althoughwenotethatsimilarconcernswereraisedaboutthelunarregolithbeforethe1959landingoftheLuna2roboticspacecraftandthesubsequenthumanlandingsbyApolloastronauts.Laboratoryexperimentalistswithinterestinundertakingbi-directionalreflectanceandpolarizationmeasurementmightwellconsiderusingaGPPdesignbasedontheHRPandtherebyreducethenumberofopticalsurfacesthatthelightpathmustencounterandhenceaccruegreatersignaltonoiseintheirfinaldataproducts.AfterthoughtWenotethatCBandtheHRPareremarkablysimilarconstructs,eachpositingraysoflighttravelingthesamepathbutinoppositedirections(Wiersmaetal.,1995,p1742).WithgreatrespectintendedtothosewhopioneeredunderstandingofCBwesuggestthatCBmightperhapslogicallyfollowfromtheinitialworkofHelmholtzhimselfin 1856

ReferencesFurukawa,Y.andJ.S.Wettlaufer.2007.PhysicsToday,December2007,pp70-71.Hapke,B.W.1993.TheoryofReflectanceandEmittanceSpectroscopy.CambridgeUniversityPress,Cambridge,NewYork,Melbourne.Hapke,B.W.2012.TheoryofReflectanceandEmissionSpectroscopy.SecondEdition.CambridgeUniversityPress,Cambridge.ISBN978-0-521-88349-8Hapke,B.W.,R.M.Nelson,andW.D.Smythe.1993.Science,260,509-511.Hapke,B.W.,R.M.Nelson,andW.D.Smythe.1998.Icarus,133,89-97.Kolokolova,L.,F.LaForgia,M.Lazzarin,A.Magrin.2014.B.A.A.S,46,5,120,304.01.Minnaert,M.1941.Astrophys.J.,93,403-410.Mishchenko,M.I.2015.‘Measurementandmodelingofelectromagneticscatteringbyparticlesandparticlegroups.’inPolarimetryofStarsandPlanetarySystems. CambridgeUniversityPress,CambridgeISBN978-1-107-04390-9,L.Kolokolova,J.Hough,andA.Levasseur-Regourd eds.pp13-34.Muinonen,K.1990.LightScatteringbyInhomogeneousMedia:BackwardEnhancementandReversalofPolarization.Ph.D.thesis,UniversityofHelsinki.Muinonen,K.1994.CoherentbackscatteringbySolarSystemdustparticles.InAsteroids,CometsandMeteors1993(A.Milani,M.DiMartino,andA.Cellino,Eds.),pp.271–296.KluwerAcademic,Dordrecht,Netherlands.Muinonen,K,APenttila,O.Wilkman,G.Videen.2014.B.A.A.S,46,5,172,415.12.Nelson,R.M.,B.W.Hapke,W.D.Smythe,andL.J.Horn.1998.Icarus,131,223-230.Nelson,R.M.,B.W.Hapke,A.S.Hale,J.L.Piatek,W.D.Smythe.2002.ElectromagneticandLightScatteringbyNonspherical Particles,B.S.Gustafson,L.Kolokolova,G.Videen eds,ISBN0-9702316-2-8,235-238.Nelson,R.M.,B.W.Hapke,W.D.Smythe,andL.J.Spilker.2000.Icarus,147,545-558.Nelson,R.M.,W.D.Smythe,B.W.Hapke,A.S.Hale.2002.PlanetaryandSpaceScience,50,849-856.Rosenbush etal.2014.in“PolarizationofStarsandPlanetarySystems”(Eds.Kolokolova,Hough,Levasseur-Regourd),CambridgeUniversityPress,2015.Rosenbush V.,N.Kiselev,andV.Afanasiev.2015.Icymoonsoftheouterplanets.InPolarimetryofStarsandPlanetarySystems.CambridgeUniversityPress,CambridgeISBN978-1-107-04390-9,L.Kolokolova,J.Hough,andA.Levasseur-Regourdeds.pp340-359.Shkuratov,Yu.1983.Sov.Astron.27,581–583.Shkuratov Yu.G.1985.Astron.Circular,SternbergStateAstronomicalInstituteMoscow,No.1400,pp.3–6.Shkuratov,Yu.G.1988.Kinem.Fiz.Nebes.Tel.4,33–39.Shkuratov,Yu.G.1989.Anewmechanismofformationofnegativepolarizationoflightscatteredbythesolidsurfacesofcosmicbodies.AstronomicalObservatoryShkuratov,Yu.,A.Ovcharenko,E.Zubko,O.Miloslavskaya,K.Muinonen,J.Piironen,R.Nelson,W.Smythe,V.Rosenbush,P.Helfenstein.2002.Icarus159,396–416.Umov,N.1905.Phys.Z.,6,674-676.vonSeeliger,H.1887.Abh.Bayer.Akad.Wiss.Math.Naturwiss.Kl.,16:405–516. .Hapke,B.W.(2012).ISBN978-0-521-88349-8,pp264-265.2.Minnaert,M.(1941).Asrophys.J.,93,403-410.3.Rosenbush,V.etal.(2015).ISBN978-1-107-04390-9,pp340-359.4.Nelson,R.M.etal.(1998).Icarus,131,223-230;Nelson,R.M.etal.,(2000),Icarus,147,545-558;

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SynopsisWeexplaintheunusualreflectanceandpolarizationbehaviorobservednearsmallphaseanglesonhighlyreflectiveAtmosphereless SolarSystemBodies(ASSBs)suchastheGalileansatellitesIo,EuropaandGanymedeandtheasteroids44Nysa,64Angelina.Thereflectanceandpolarizationpropertiesareconsistentwithahighlyporoussurface.

Ourlaboratorymeasurementsofaluminaparticulatematerials,analogoustotheregoliths oftheseASSBs,suggestthattheseASSBsscatterelectromagneticradiationasiftheywereextremelyfinegrainedwithvoidspace>~95%,havinggrainsizesoftheorderofthewavelengthoftheincidentlight.SuchhighporosityportendssignificantconsequencesforeffortstodeploylandersonhighASSBssuchasEuropa.AspacecraftlandingonitsEuropamightsinkintoit’sporoussurface.

Ourinvestigationsofthereflectanceofaluminaparticulatesarealsogermanetothefieldofterrestrialgeo-engineering,particularlytosuggestionsthatearth’sradiationbalancecanbemodifiedbyinjectinghighlyreflectiveAl2O3 particulatesintothestratospheretoreflectincidentsunlighttherebyoffsettingtheatmosphericwarmingeffectofanthropogenicgreenhousegasemissions.Seeourcompanionpaper(GC43H-1158)

Figure 3. Polarization Phase Curves of the five smallest Al2O3 particle size fractions. The figures are vertically displaced by 1% for each particle size. The solid line is a linear least square fit to the data of PSIMTSAC2 function:

DiscussionUnderstandingthereflectancepropertiesofhighreflectanceparticulateensemblesisofgreatusein

understandingthoseregionsoftheoutersolarsystemwherehighalbedoobjectsexist(suchastheGalileansatellitesIo,EuropaandGanymede)andselectedreflectivelocalesonobjectssuchasCeresandPluto.TheselaboratorymeasurementswillbeofinteresttoallobserversofASSBs,particularlythosewhomakeobservationsoftrans-Jovianobjectsfromearthorearthorbit.Suchobservationsoftrans-Jovianobjectsarelimitedtophaseanglesα<~10o.Thereflectancechangewithrespecttophaseangleprovidesanimportantconstraintonsurfacetextureaftermakingcorrectionsforrotationalchanges.Thepolarizationphasecurvesmayalsoprovideanindependentconstraintonregolithtexture.Thisreflectancebehaviorwereportmightbegeneralized(withcaution)tosuchlocalesintheoutersolarsystemwherehighreflectanceisreported,howeverwecautionthatthispolarizationbehaviorisonlyobservedforAl2O3 particlescomparableinsizeorsmallerthanthewavelengthoftheincidentlightwhichexhibitequantmorphology.Itmightbetemptingtoassumethatthepolarizationeffectswereportherecanbegeneralizedtoallhighalbedoparticulatesofsmallparticlesize.Wehavenotinvestigated,asofyet,otherparticulatematerialsofdifferentcompositionandalbedo.Wesuggestthatcautionbeobservedwhengeneralizingtheseresults,notingthatShkuratov etal.,2002(Fig18)reportsimilarpolarizationbehaviorinNiCr spheresoflargeparticlesize(60µm)andwenotefurthermorethattheseeffectshavealsobeenreportedforlowalbedoBoronCarbide(B4C)oflargeparticlesize(Ovcharenko etal.,2006).Theconjecturethatthereflectancebehaviorreportedheremightapplytoallhighalbedoparticulatesofsub-wavelengthparticlesize,whileconsistentwiththetheorythatunderliesCB,remainstobeconfirmedforabroadrangeofhighreflectanceparticulates.WenotethestrikingsimilaritybetweenthepolarizationphasecurveswereportforhighporosityassemblagesofreflectiveparticulatesofAl2O3 (awatericereflectanceanalog)andthepolarizationphasecurvesoftheGalileansatellitesreportedbyRosenbush etal.,(1997fig.5,2015)wherewatericehasbeenreportedbasedonspectroscopicevidenceforahalfcentury(Moroz,1969).Wehavenotasyetspecificallydeterminedwhetherthesepropertiesarecausedbysmallparticlesize,highvoidspaceorboth.However,wesuggestthatthispolarizationphasecurvedatafromthelaboratory,whencomparedtothetelescopicpolarizationphasecurveobservationsreportedbyRosenbush andcolleagues,isevidencethatEuropamayhaveanextremelyporousregolithoffineparticles.Theempiricalexpressions1and2abovecanbeusedsimultaneouslytorefineandconstrainthesurfacemodelsdevelopedbytheastronomicalobservers.ThismaybethemosteffectiveapproachtounderstandingoutersolarsystemobjectsparticularlytheirpotentialashazardstolifeonEarth.

AcknowledgementsThisworkwaspartiallysupportedbyNASA’sCassiniSaturnOrbiterVisualandInfraredMappingSpectrometerinstrumentteam.

! = cos(' 2) )(+ ⋅ -./ + 1 ⋅ -2/3

Figure 1. The configuration sketch of thegoniometer that we use in our experiment.

Figure 3. Model fit to the measured phase curves. The two panels are the sameexcept that the right panel is plotted in log-log scale. We used above three-component model to fit data at a<8 deg. This model does not have any physicalimplications, but only for a mathematical description of the phase curves. The linearcomponent describe the phase curve between phase angles 4 deg and 10 deg; thetwo exponential components describe the curve at <0.2 deg and between 0.2 degand 2 deg. This model appear to fit the data well.

Model 1, double-exponential-linear model (DEL):

Figure 5. Model fit to the measured phase curve using asecond empirical model above, which was adopted fromNelson et al. (2018). Left panel is in linear scale and rightpanel is in log-log scale. Overall this model fits the phasecurves well, just missing the opposition effect at phase angle<0.1 deg.

Model 2, PSIMTSAC1 (Nelson et al., 2018):

Figure 4. The wavelength dependence of the best-fit parameters forthe DEL model. A decreasing trend is clearly visible for the amplitudeparameter of both exponential components. If the two exponentialcomponents represent two opposition effects (OE), the shadow-hiding(SH) and coherent backscattering (CB), then both OEs have theirstrengths decreasing with increasing grain size, or equivalently withdecreasing wavelength, for grain size much larger than wavelength.Relatively shallower phase slope outside of OE with increasing grainsize is also evident, possibly related to increasing albedo with grainsize.

Abstract No. 1952, Thursday, March 22, Location 47