The Role of Tholin in the Atmospheric and Surface Chemistry on Titan

Dark reddish organic solids, called tholins, have been produced simulating the chemistry present in Titan's atmosphere. Its optical constants matched the geometric albedo in the optical region of Titan after an adjustable correction factor. The precursor molecules to tholin have been studied by pretrapping the species in the discharge and then analyzing the products by FTIR and GC/MS. This technique only gives information about the stable precursor molecules because the unstable radicals and ions trapped in cold baths react on bringing to room temperature. The build up of the tholins has also been determined by pyrolysis GC/MS. Cassini/Huygens mission revealed the absence of extensive methane/ethane seas on the surface. We expect methane to disappear from Titan only in a million years or so due to photolysis by the VUV. However the mixing ratio of methane stays the same over the astronomical period of time.

All the previous work has not produced tholin in the temperature range 135 to 178 K where tholin are produced by magneotspheric charged particle, then passing through lower temperature at 70 K and finally to the ground at 95 K. While descending to the ground, it gets coated and processed on the way by other sources of energy such as long UV and cosmic ray. It is therefore expected that the stable products of CH₄ (C₂H₆, C₂H₂ and H₂) react with Titan tholin to replenish the CH₄ supply in Titan's atmosphere. Furthermore, the explosive materials on the surface of the tholin could incorporate atmospheric C₂H₆ and therefore might reduce the deposition rate of C₂H₆ onto the ground of Titan.

The result of a laboratory experiment simulating to produce Titan tholin in the temperature range close to about 178 K and then exposing the tholin to stable product of CH₄ under Titan conditions will be reported.