Observations of water in comets with Herschel

M. de Val-Borro and the HssO team

October 3, 2015

Outline

1 Herschel instruments

2 Herschel cometary observationsH2O excitationD/H measurements in 2 JFC and 1 OCC

3 Model predictions for D/H ratio

4 Prospects for HDO measurements

Herschel Space Observatory

3.5m passively cooled telescope on orbit around L2

active between 14 May 2009 – 29 April 2013mission’s post-operations until 2017 to refinecalibration and data archivalstudy molecular chemistry of the universe3 complementary instruments in IR and sub-mm:

HIFI high-resolution heterodynespectrometer 150–610 µm

PACS camera and medium-resolutionspectrometer 55–210 µm (60–85,85–130 and 130–210 µm)

SPIRE camera and imaging FTS 194–672µm (250, 350 and 500 µm)

Herschel/HIFI (Heterodyne Instrument for the Far Infrared)

far-IR and sub-mm spectrometer5 bands in the 480–1150 GHzdual frequency bands 1410–1910 GHzHIFI observes two polarizationssimultaneously

WBS - 1.1 MHz resolutionHRS - 140 kHz resolution

Wide range of chemical and dynamical studiesHIFI’s high spectral resolution and sensitivity allows for thedetection of multiple rotational water lines

90 h for comets from HssO programaccurate determinations of water production rates in cometsstudy of extended coma emissionresolve H2O line profiles to constrain excitationsearch for water emission in distant comets and MBC

Water in comets

Most primitive material remaining fromsolar nebulaWater main component of cometarynucleusQH2O has been estimated from the groundthrough water high vibrational bands andthe OH radical110–101 ortho-H2O at 557 GHz wasobserved in several comets by SWAS andOdinOther ortho- and para-H2O, HDO andH18

2 O transitions observed by Herschel(e.g., Hartogh et al. 2010)MIRO has detected H2O, H17

2 O and H182 O

in comet 67P/Churyumov-Gerasimenko

D/H in H2O in the Solar System(pre-Herschel view)

Mean D/H measured in several OCC is3 × 10−4

Earth ocean (Vienna Standard MeanOcean Water) is 1.56 × 10−4

Protosolar D/H ratio in H2 is2.5 × 10−5

Cometary D/H ratios represent a factor of ∼ 12 enrichment overthe protosolar value and ∼ 2 enrichment over the Earth ocean valueD/H enrichment in Enceladus is similar to OCCsFrom isotopic measurements and dynamical models most probablesource of Earth H2O was ice-rich reservoir in outer asteroid beltComets could have contributed less than 10% of the Earth water

Model predictions for D/H in Solar SystemKavelaars et al. 2011

Enceladus

5 x 104 yr

f =

(D

/H) H

2O

/ (

D/H

) H2

0 5 10 15 20 250

5

10

15

20

25

30

Heliocentric distance (AU)

35

105 yr

2 x 105 yr

4 x 105 yr

6 x 105 yr

LL3

(low)

1.5 x 105 yr

2.5 x 105 yr

1.7 x 106 yr

Uranus - Neptune

formation region

D/H values measured

in comets

LL3 (high)

f evolution in gas phase prior to condensationLL3 high value taken as initial highly enriched protosolar value dueto low temperature non-equilibrium chemistryIsotopic exchange reactions between H2 and HDO led to a gradualreduction of D/H ratio in water

Comet 103P/Hartley 2

EPOXI’s MRI camera

JFC (6.45 year period)Target of NASA’s EPOXI mission on 4Nov 2010Elongated nucleus with 18 h periodPerihelion on 28 Oct 2010 atrh = 1.05 AUClosest approach to Earth on 20 Oct2010 at 0.12 AUTypical QH2O = 1028 s−1 (active)Herschel observed far-IR and sub-mmspectrum and imaged thermal dust at70-672 µm (Oct 24–Nov 17)

HIFI Observations of 103P/Hartley 2 on Nov 17.28–17.64Hartogh et al. 2011

20 days post-perihelion (rh = 1.095 AU, ∆ = 0.212)Observing sequence

10 32-min scans of HDO 110–101 at 509.292 GHz10 6-min scans of H2O and H18

2O 110–101 at 556.936 and547.676 GHz5 16-min on-the-fly maps of the H2O 110–101 transition

Single-point observations in frequency switched mode(94.5 MHz throw)Similar beam sizes (FWHM 38.1, 38.7 and 41.6′′, ∼6500 km)Spectra acquired with WBS and HRS simultaneouslyAll lines were observed in H+V polarizations

First measurement of D/H in H2O in a JFC

110–101 HDO (509 GHz) S/N ∼ 10110–101 H18

2O (548 GHz) optically thin isotopologue reference

HIFI HRS H2O 556.936 GHz Nov 17.27 UT

Check pointing and derive excitation parametersLine peaks approximately 10′′ westward of the nucleusQH2O = 1028 s−1

PACS spectral maps on Nov 4.55 UT (2.5 h before CA)(89.9, 179.5 and 180.5 µm water lines)

179.5 µm (1669 GHz) 180.5 µm (1661 GHz)

10” offset corresponds to 1100 km at the cometIcy grains accelerated in anti-solar directionQH2O = 1.2 × 1028 s−1

PACS Photometry on Nov 4.47 UTMeech et al. 2011

Blue Red

2′ × 2′ image of dust coma 70, 100 and 160 µmAssymetric coma, negligible contribution nucleus thermal emissionQdust = 200 kg s−1, Qdust/Qgas ∼ 2 (Bockelee-Morvan et al 2010)

HDO and H182 O excitation models

collision excitation with H2O and electrons dominate theexcitation in the inner coma (Crovisier 1984)solar infrared pumping of vibrational bands lead tofluorescence equilibrium in the outer comaself-absorption effects are negligiblestandard Haser distribution with isotropic outgassinglevel populations depend on collisional rates and Tkin

H2O–H2O cross section are derived from Itikawa 1972Tkin = 50 K from CH3OH mmTkin ∼ 70–85 K from ro-vibrational lines in near-IR at scalesof 0.5-2′′ (Mumma et al 2011)electron density xne = 0.2 wrt measurements in 1P/Halleyvexp = 0.6 km s−1 from half-width of optically thin linesortho-to-para ratio 2.8 (consistent with IR Mumma et al 2011)

HDO level population from Monte Carlo code

101 102 103 104 105 106

r [km]

10-4

10-3

10-2

10-1

100

rela

tive p

opula

tion

000101

111

110

202

212

211

HIFI observations sample molecules with an excitation stateintermediate between LTE and fluorescence equilibrium

D/H ratio in 103P/Hartley 2 from HIFINov 17.27 UT

Assuming VSMOW 16O/18O = 500 ± 50 (520 ± 30 in 4comets with Odin):(D/H)H2O = (1.61 ± 0.24) × 10−4

HDO/H182O production rate ratio is not very sensitive to the

model parametersunexpected result: factor of two smaller than in OCCs(2.96 ± 0.25) × 10−4

close to terrestrial VSMOW D/H value (1.558 ± 0.001) × 10−4

larger than the protosolar value (2.1 × 10−5) and the ISMvalue (1.6 × 10−5) in H2JFC expected to have higher D/H as they formed in Kuiperbelt in colder environment

Model predictions for D/H in cometsKavelaars et al. 2011Model  PredicKons  for  D/H  in  comets  

Kaveelars et al. (2011)

JF

Takes into account planet migration

J S OC OC

OC = Oort Cloud JF = Jupiter family 103P  

D/H ratios in the solar system

comets present diversity in D/H ratiosOort cloud comets have twice the value of the Earth’s oceanJFC 103P/Hartley 2 and the CI values in carbonaceouschondrites are consistent with VSMOW

Explaining the low D/H ratio in 103P/Hartley 2

D enhancement in H2O predicted to increase with distance fromthe Sun (not yet confirmed by observations)

1 103P/Hartley 2 may not come from the Kuiper beltFraction of JFC originate in the OCIs it a Trojan (Horner et al. 2007) originating near Jupiter?Perhaps OCCs did not form in the vicinity of the giant planetsor do not represent the solar system (Levison et al. 2010)

2 D/H ratio with heliocentric distance not as expectedIn the early phase of the solar system formation material wasmixed over large distances (Walsh 2011).Region of solar nebula with terrestrial D/H ratio includes theKuiper belt

Radial mixing of planetesimalsGrand Tack model (Walsh et al. 2011)

Evolution of small body populationInward then outward type II migrationof Jupiter and SaturnExplain Mars’ mass and distribution ofS/C/D asteroids in main belt

Radial mixing of planetesimalsGrand Tack model (Walsh et al. 2011)

Distribution of 100-km planetesimalsat the end of the giant planetmigration phaseSubstantial radial mixingTNOs on eccentric Earth-crossingorbits

C/2009 P1 (Garradd)

long-period comet (P = 127 000 yr, i = 106◦)perihelion on Dec 23, 2011HIFI HDO observations conducted on Oct 6, 2011 rh = 1.88AU ∆ = 1.76 AUSame observing strategy as for comet 103P/Hartley 2

H2O and H182O lines observed simultaneously in band 1a mixer

H182O line more reliable reference for D/H determination

maps serve to constrain the H2O excitationH2O line at 988 GHz observed in the band 4a mixertelescope beam sizes 38.′′1, 38.′′7, and 41.′′6 for H2O, H18

2O andHDO lines (∼ 50 000 km)

C/2009 P1 (Garradd) HRS spectra from Oct 6 2011Bockelee-Morvan et al 2012

110–101 HDO (509 GHz), H182O (548 GHz), H2O (557 and 988 GHz)

phase angle is 30◦

profiles with 30% extended production (Paganini et al 2012)

C/2009 P1 (Garradd) WBS spectral mapsOct 6.56 and 6.60 UT 2011

H2O on-the-fly maps (557 and 988 GHz)Beam diameters are 38′′ and 21′′

C/2009 P1 (Garradd) production rates

H2O maps (557 and 988 GHz)Flat curve expected if nucleus is dominant source

D/H in comet C/2009 P1 (Garradd)Bockelee-Morvan et al 2012

HDO/H182O = 0.215 ± 0.023

Using H2O production rates with 30% extended sourceD/H = (2.06 ± 0.22) × 10−416O/18O = 523 ± 32Variable T profile and isotropic outgassing v 0.5–0.6 km s−1

Significantly higher than 103P D/H = (1.61 ± 0.24) × 10−4

Only 16O/18O ∼ 300 can reconcile D/H with OCCUncertainties take into account 5% relative calibration error

D/H ratios in the solar systemNo single archetypal D/H value for all OCC

Oort-cloud comet 2009 P1 has a D/H ratio that issignificantly higher than the value of the Earth’s ocean16O/18O consistent with terrestrial value

45P D/H measurementLis et al. 2014

JFC observed in August 2011No HDO emission detected with D/H < 2 × 10−4

Consistent with Hartley 2 measurementCanonical OCC D/H 3 × 10−4 excluded at 4.5σFurther confirmation of diversity of D/H

D/H ratios in the solar system

JFC 103P/Hartley 2 and 45P ratios are consistent withVSMOW value

D/H ratios in the solar system

ROSINA measurement in 67P ∼ 3 times VSMOW value(Altwegg et al. 2015)Kuiper belt is not solely composed of Earth-like water

Implications of D/H diversity

Low D/H in JFCs in contradiction with models involvingisotopic exchanges: current understanding of deuteration inthe solar system, or the formation zone of JFCs, have to berevisited

Ices condensed close to the Sun would be more deuterated?e.g., out of equilibirium chemistry at high TH2/H2O/OH/H/O (Thi et al. 2010)D/H is not monotonically increasing away from the Sun (Yanget al. 2012)Revisit origin of JFCs and OCCs

JFC = Trojans formed in the vicinity of Jupiter?90% of OCC from other stars in the Sun’s birth cluster(Levison et al. 2010)?

D/H diversity in the solar system

Diversity in the D/H ratio in the OC populationDichotomy between JFC and OCC?Diversity within the two populations expected from the GrandTack dynamical model involving Saturn and Jupiter innermigration (Walsh et al. 2011)

The reservoir of icy bodies with Ocean-like water is largerthan previously thought and may include the Kuiper beltbased on very small sample

much higher fraction of ocean water could have been deliveredby comets?Alexander et al. (2012) suggests the bulk D/H in cometsmight be compatible with the Ocean’s value, consideringD-enriched organics

Prospects for HDO measurements

HIFI provided unique capability for measurement of D/H ratio(3 comets)Rosetta 67P (JFC)ALMA band 8 gives access to the 110–101 line at 464 GHz inbright cometscomets require scheduling on a short notice and their emissionis extended best suited for compact ALMA configurationsautocorrelation mode is most sensitive for HDO detectioncomplementary observations of water in the IR, or UV/radioobservations of OH, will be used to obtain accurate waterproduction ratesHDO can be observed in the near-IR, or UV for bright comets

Summary

D/H Herschel observations of 103P, C/2009 P1 and 45P110–101 lines HDO (509 GHz) and H18

2O (548 GHz) detectedin 2 objectsOcean like water found for the first time in a JFCHigh D/H values previously measured in OCCs are notrepresentative of all comets.Findings does not fit present models on origin of cometarymaterial and isotopic fractionation with heliocentric distanceDynamical modelling suggests that population ofplanetesimals underwent large-scale mixingIsotopic differences in comets may be linked to the chemicaldiversity observed in JFC and OCCParadigm of maximum 10% cometary water in hydrospherebased on composition arguments needs to be revisitedFurther D/H measurements required to increase sample size