First Measurement of the HDO/H 2 O 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 H 2 O by Herschel PACS 70  m SPIRE 500  m km 1x1, km 5400 km PACS 1661 GHz HIFI 557 GHz Nucleus seen by EPOXI Comet 103P/Hartley 2 in november 2010 A 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 spectrumPACS, spectrum, SED mode HIFI spectra

Problems to avoid to measure an accurate HDO/H 2 O ratio: Time Variability: Comet outgassing can vary quickly:  Outbursts: e.g. 17P/Holmes Q H2O x10000 ~12h  Periodic variation: e.g. C/2007 N3 Q±40% in 42h,  Irregular activity: Odin before Herschel obs. => Q H2O (t) in 103P

Opacity effect Coma sampling The main H 2 O line corresponds to a column density ~2000x higher than HDO  high opacity of submm lines of H 2 O (  =5 to 40)  Properly model the water emission ?  Use optically thinner proxy, e.g. H 2 18 O Problems to avoid to measure an accurate HDO/H 2 O 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 H 2 O in comet 103P/Hartley 2:  17 november 2010: Alternate 11 observations of H 2 O (557 GHz) + H 2 18 O (548 GHz) with 10 of HDO (509 GHz) and five maps of H 2 O: total min (~8h) Same receiver  similar field of view (~39’’ ) Observed variation: smooth decrease by ~20% of Q H2O during the observation

GHz GHz S/N = 10S/N = 60 Hartogh et al. (2011) Result:

Analysis of the observations Excitation model : collisions with H 2 O + e - + infrared pumping, gas temperature determined by other observation (e.g. methanol lines at IRAM/CSO)  the HDO/H 2 18 O production rate ratio is not very sensitive to the model parameters (similar transition: J KaKc = )  H 2 O maps still need to be fully interpreted (opacity not well modeled at the center – variable T?) Hypothesis : 16 O/ 18 O = 500 (+/- 10%) (VSMOW) (520±30 in 4 comets with Odin) =>D/H = (1.61 ± 0.24) x 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 (Earth value x2) New D/H measurements in water from Oort cloud comets From 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/H 2 O high-res IR observations comet 8P/Tuttle (Keck, VLT - Villanueva et al. 2008) D/H ~3-4 x Other upper limits in Oort cloud comets: HDO at CSO + H 2 O/H 2 18 O 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 JS OC OC = Oort Cloud JF = Jupiter family 103P

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…

14 N/ 15 N ratio in 17P/Holmes: 14 N/ 15 N = 139 ± 26 consistent value in CN HCN and other major parent of CN are equally enriched in 15 N Isotopic ratios in volatiles : nitrogen Measurements in HCN Bockelée-Morvan et al 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)