An Endogenic Origin of Titan,s Methane

Details An Endogenic Origin of Titan,s Methane An Endogenic Origin of Titan,s Methane

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p43b..04z&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P43B-04

Other

1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 6281 Titan

Scientific paper

Methane is abundant in the atmosphere and at the surface of Titan. Rapid photochemical dissociation of methane in the atmosphere implies its supply from the interior of the satellite, an implication that is confirmed by the difference in the fractionation of carbon and nitrogen isotopes [1]. Recent models for giant planet formation do not support the concept of high-pressure sub-nebulae [2] in which methane forms through grain-catalyzed reactions [3]. Therefore, we start from the assumption that most of the carbon in the Saturnian system was accreted from the surrounding solar nebula in the form of macromolecular organic carbon particles, SiC, graphite, nano-diamonds and amorphous carbon grains, as it is found in the unequilibrated chondrites and in the interstellar medium. We further assume that the relatively low abundances of noble gases in Titan,s present atmosphere indicate that the planetesimals that formed the satellite came from a warm (T > 75K) environment where methane, even if present, was not trapped directly [1,4]. Following these assumptions, we develop the idea that Titan,s methane could be endogenic [4]. Here we examine the possible chemical pathways for producing methane in Titan,s interior. The icy planetesimals that formed Titan ensured the incorporation of water ice in the growing satellite, together with reduced rocky components represented by solar nebula condensates, which included Fe-rich metal, presolar grains and the carbon species mentioned above. Decay of radionuclides and heat released during accretion supplied energy to melt ice. Interaction of water with rocks caused mineral oxidation and formation of ferrous silicates, magnetite, phosphates and other salts. Hydrogen was produced in all oxidation reactions and the water-iron reaction provided a major source of H2. Deep below the surface, hydrothermal reactions caused disproportionation of organic compounds through the overall pathway: organic matter + H2O > CO2 + CH4 [c.f., 5]. Although both O-bearing carbon species (e.g., carboxylic acids, carbonates) and H-rich compounds (aliphatic hydrocarbons) form through C disproportionation, the H2-rich environment favored generation of H-rich organic compounds and methane. In addition, high-temperature hydrogenation of inorganic carbon also led to formation of methane. The resulting gas could be stored as clathrates in Titan's watery mantle for subsequent release to the atmosphere. We cannot exclude the possibility that methane generation continues to the present time. References: [1] Niemann H.B. et al. (2005) Nature, in press. [2] Canup R. and Ward W.R. (2002) Astron. J. 124, 3404-3423. [3] Prinn R.G. and Fegley B. (1981) Ap. J. 249, 308-317. [4] Owen T., Atreya S. and Niemann H.B. (2005) Uspekhi Physicheskikh Nauk 175, 664-668. [5] Price L.C. and DeWitt E. (2001) GCA 65, 3791-3826.

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