37th DPS Meeting, 4-9 September 2005
Session 29 Planet and Satellite Formation
Poster, Tuesday, September 6, 2005, 6:00-7:15pm, Music Foyer

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[29.09] Volatiles in Saturn and Titan: The new lights from Cassini-Huygens

F. Hersant, D. Gautier (LESIA, Obs. Paris), J.I. Lunine (LPL, Univ. Arizona), G. Tobie (LPG, Univ. Nantes)

In 1995, the Galileo probe raised the problem of the origin of volatile elements in giant planets by measuring that noble gases, C, N and S were all enriched in Jupiter by a factor of about 3 with respect to their solar abundances. This suggested that the planetesimals who formed Jupiter had a composition different from that of comets of the Oort cloud in which the N2/CO was found drastically lower than the value corresponding to a solar C/O abundance (Cochran and Cochran, 2000, Cochran 2002). An interpretation of the enrichment in heavy elements found in Jupiter was proposed by Gautier et al. (2001), based on the assumption that all volatiles were trapped by clathration in the feeding zone of the forming planet.

On the basis of the clathration theory, Hersant et al. (2004) have reproduced, the abundances of C, N and S in Saturn available in 2004, and predicted that only Xe is substantially enriched in Titan's atmosphere. However, the C/H ratio measured by the CIRS instrument aboard Cassini (Flasar et al. 2005) has found a C/H ratio about twice higher than predicted by Hersant et al. (2004), and no noble gas at all was detected in Titan's atmosphere by the GCMS aboard Huygens (Niemann et al. 2005). These unexpected results are discussed and are found in fact still compatible with the clathration theory. It is however concluded that the C/H ratio in Saturn depends upon the relative proportions of CO, CO2, and CH4 in the feeding zone of the forming Saturn, values which are unfortunately quite uncertain. Moreover, even if Xenon is trapped as clathrate hydrate in the deep interior of Titan, internal thermodynamical processes may preclude it to escape to the atmosphere of the satellite. Other tests for identifying the origin of volatiles in Saturn and Titan are proposed. Sending a probe into Saturn in order to measure abundances of Ar, Kr and Xe and various isotopic ratios appears to be a key objective of future space missions.

References: Cochran, A.L., and Cochran, W.D. 2002, Icarus, 154, 381-390; Cochran, A.L. 2002, Astrophys. J., Lett., 576, L165-L168.; Hersant, F., Gautier, D., and Lunine, J. I., 2004, PSS, 52, 623-641; Flasar, F.M., 2005, Science, 307, 1247- 1251; Niemann, H., et al., 2005, Nature, submitted.

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