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O. Mousis (DESPA, Obs. Meudon, France; LPG, U. Nantes, France), D. Gautier, A. Coustenis (DESPA, Obs. Meudon, France)
We propose a new interpretation of the D/H ratio in the atmospheric CH4 in Titan inferred from ground-based observations by Orton (1992) and from ISO/SWS 1997 observations by Coustenis et al. (2001). Our analysis assumes that planetesimals which formed Titan were produced in the feeding zone of Saturn, prior to the completion of the planet and the formation of its subnebula (Mousis et al. 2001a). We follow the scenario described by Mousis et al. (2001b) where planetesimals located in the feeding zone of Saturn contained methane originating from deuterium enriched ices infalling from the presolar cloud onto the nebula disk. These ices vaporized, subsequently exchanged their deuterium with hydrogen in the nebula, and finally formed clathrates hydrates of CH4. Relics of these planetesimals were embedded in the subnebula of Saturn and subsequently formed Titan. We argue that the present atmospheric methane, which is continuously photodissociated, is replenished by a kind of cryovolcanism from a reservoir located in the interior of Titan. The plausibility of this statement is reinforced by the recent laboratory work of Loveday et al. (2001) who have shown that methane clathrates hydrates are stable up to a pressure of 10 GPa. These authors suggest that a thick layer of clathrate hydrate of methane is located just below the water ice crust described by models of interiors (Grasset et al. 2000). Retrieving, through a turbulent model of the solar nebula, the whole story of D/H in CH4 from its initial interstellar value down to that measured in Titan's atmosphere today, permits us to estimate that the value of D/H in the presolar cloud was not higher than about 300 ppm. According to our analytical model of nebula derived from the one of Dubrulle (1993), the value of D/H in CH4 in comets of Oort should be only slightly higher than that in Titan. Measuring D/H in CH4 in these objects is thus a key test of the validity of our scenario.
References : Coustenis, A., et al., 2001. To be submitted for publication. See also paper in this DPS meeting. Dubrulle, B. 1993. Icarus 106, 59-76. Grasset, O., et al. Planet. Space Science, 48, 617-636. Loveday, et al. Nature, 410, 661-663. Mousis, O, et al. 2001a. Accepted for publication in Icarus. Mousis, O, et al. 2001b. Submitted for publication in Icarus. Orton, G. 1992. Proc. of the Titan Symp., Toulouse, France, 9-12 Sept. 1991, ESA-SP 338, 81.
The author(s) of this abstract have provided an email address for comments about the abstract: Olivier.Mousis@chimie.univ-nantes.fr