First Measurement of the HDO/H 2 O ratio in a Jupiter Family Comet N. Biver and D. Bockele-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 Q40% 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) (52030 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 Bockele-Morvan et al N chemical fractionation in the presolar cloud or solar nebula ? (Charnley & Rodgers 2008), but no observational evidence yet in the ISM (Grin et al. 2009)