First Measurement of the HDO/H2O ratio in a Jupiter Family Comet

N. Biver and D. Bockelée-Morvan,… LESIA, Observatoire de Paris

Based on Hartogh et al. (2011) to be published in Nature

(online on 5 Oct. 2011)

Why studying the D/H ratio in cometary water

- Comets are relics of the chemistry in the outer regions of the primitive Solar Nebula- They are relics of the planetesimals that formed outer planets and icy satellites, and may have contributed to Earth oceans The composition of comets can constrain the chemistry and processes at work in the early Solar System

What is the origin of comet material:- interstellar chemistry ?

- product of nebular chemistry ? - role of coma processing + comet aging ?

The study of the chemical diversity of comets is important to investigate chemical differences related to different formation regions or aging

How do the different dynamical classes compare ? long-period comets (Oort cloud) origin: Saturn-Neptune region Jupiter-family comets (Kuiper Belt/Scattered disk) beyond Neptune

Dust seen by Herschel

H2O by Herschel

PACS 70 m

SPIRE 500 m

360 000 km

1x1,5 106 km

5400 km

PACS 1661 GHz HIFI 557 GHz

Nucleus seen by EPOXI

Comet 103P/Hartley 2 in november 2010A JFC at 0.13 AU from Herschel!

Water lines observed with Herschel

About 30 water lines have been detected in comet 103P/Hartley 2

SPIRE spectrum PACS, spectrum, SED mode

HIFI spectra

Problems to avoid to measure an accurate HDO/H2O ratio:

Time Variability: Comet outgassing can vary quickly: Outbursts: e.g. 17P/Holmes QH2O x10000 ~12h Periodic variation:e.g. C/2007 N3 Q±40% in 42h, Irregular activity:Odin before Herschel obs. => QH2O(t) in 103P

Opacity effect

Coma sampling

The main H2O line corresponds to a column density ~2000x higher than HDO high opacity of submm lines of H2O (=5 to 40)Properly model the water emission ?Use optically thinner proxy, e.g. H2

18O

Problems to avoid to measure an accurate HDO/H2O ratio:

Asymetry, jets, in cometary coma no easy comparison if different field of view: Observe at a similar frequency, with the same telescope

HIFI observations to measure D/H in H2O in comet 103P/Hartley 2:

17 november 2010:Alternate 11 observations of H2O (557 GHz) + H2

18O (548 GHz) with 10 of HDO (509 GHz) and five maps of H2O: total 66 + 320 + 80min (~8h)

Same receiver similar field of view (~39’’ )

Observed variation: smooth decrease by ~20% of QH2O during the observation

110-101

509 GHz110-101

548 GHz

S/N = 10 S/N = 60

Hartogh et al. (2011)Result:

Analysis of the observations• Excitation model : collisions with H2O + e- +

infrared pumping, gas temperature determined by other observation (e.g. methanol lines at IRAM/CSO)

• the HDO/H218O production rate ratio is not very

sensitive to the model parameters (similar transition: JKaKc = 110-101)

• H2O maps still need to be fully interpreted (opacity not well modeled at the center – variable T?)

• Hypothesis : 16O/18O = 500 (+/- 10%) (VSMOW)(520±30 in 4 comets with Odin)

=> D/H = (1.61 ± 0.24) x 10-4

D/H(VSMOW) = 1.56 x 10-4

Measurements of D/H in cometary water:

Measurements in Oort cloud comets 1P/Halley, Hyakutake, Hale-

Bopp : D/H ~3 x 10-4 (Earth value x2)

New D/H measurements in water from Oort cloud cometsFrom OD/OH comet C/2002 T7 (LINEAR)(UVES at ESO-VLT - Hutsemekers et al. 2008) From D/H Ly- comet C/2001 Q4 (NEAT)(STIS on the Hubble Space Telescope - Weaver et al.)HDO/H2O high-res IR observations comet 8P/Tuttle(Keck, VLT - Villanueva et al. 2008)

D/H ~3-4 x 10-4

Other upper limits in Oort cloud comets:HDO at CSO + H2O/H2

18O Odin:comet 153P/Ikeya-Zhang

comet C/2004 Q2 (Machholz)

D/H < 2.5 x 10-4

Deuterium in water in the Solar System

153P

Model Predictions for D/H in comets

Kaveelars et al. (2011)

JF

Takes into account planet migration

J SOCOC

OC = Oort CloudJF = Jupiter family103P

Hypotheses to explore:

Ices condensed close to the Sun would be more deuterated? e.g., Out of equilibirium chemistry at high T H2/H2O/OH/H/O (Thi et al. 2010) – but seems incompatible with D/H in carbonaceous chondrites

The origin of JFCs and Oort Cloud comets must be revisited? e.g. JFC = Troyans formed in the vicinity of Jupiter (Horner et al. 2007) e.g. 90% of Oort cloud comets from in other stars vicinity (Levisson et al. (2010), to reproduce the actual population and location of the Oort cloud)

Other explanations?

Why would the D/H ratio in water be terrestrial in JFCs and twice larger in

Oort Cloud comets?

Conclusions…

The D/H ratio in the water of the Jupiter Family comet 103P/Hartley 2 is equal to that of the Earth – in line with the similarities seen between carbocaneous chondrites and cometary material (e.g. 81P/Wild 2)

The difference found with Oort cloud comets (D/H twice higher), will require more theoretical modeling to explain it!

New D/H measurements are planned with Herschel…

• 14N/15N ratio in 17P/Holmes: 14N/15N = 139 ± 26

• consistent value in CN• HCN and other major parent of CN are equally enriched in 15N

Isotopic ratios in volatiles : nitrogenMeasurements in HCN

Bockelée-Morvan et al. 2008

N chemical fractionation in the presolar cloud or solar nebula ? (Charnley & Rodgers 2008), but no observational evidence yet in the ISM (Gérin et al. 2009)