Titan and the cosmochemistry of the outer solar system.

Abstract

Modeling of physical and chemical processes in solar nebula environments is applied to the data base of gases in Halley's comet to infer the conditions under which that comet formed. Assuming that the chemistry in the inner nebula is catalyzed by reactions on grains, the abundances of the volatile carbon species CH₄, CO, and CO₂ in Halley could have been supplied by the solar nebula. The NH₃ abundance in Halley is, however, too high to have been derived from the nebula. An alternative is, that the surrounding giant molecular cloud is a possible source for ammonia. The frictional heating, sublimation and recondensation of grains free-falling into the solar nebula from a surrounding interstellar cloud is examined. The expansion of the sublimating gas from the grain surface and abundance of cold grains implies that most of the gas returns to the solid phase near nebular ambient temperatures ($\sim$50 K). Such a process could lead to at least two populations of grains: (1) essentially unaltered interstellar grains and (2) a component with more volatile gases at nebula ambient temperatures, yielding volatile-rich amorphous phases. Plausible models of the early history of Titan suggest that ammonia and water were present in liquid form at the surface. Thermodynamic modeling showed that such an ocean could have reacted with silicates. Ammonia-water fluids enriched in potassium would have been brought to the surface through the cryogenic equivalent of volcanism. Later impacts would have released the ⁴⁰Ar produced by decay of the ⁴⁰K into the atmosphere. The abundance of atmospheric ⁴⁰Ar may be dominated by this source. A means is proposed for putting limits on the history of Titan's atmosphere by considering the breakup of bolides during atmospheric entry and the resulting modification of the crater size-frequency distribution at the surface. The size of the bolide which can reach the surface unfragmented depends on the atmospheric pressure. Given observation of the minimum crater size and relative surface age a rough determination of the evolutionary trend of atmospheric pressure in Titan's history is achievable. This is a key scientific issue since contrasting models of both increasing and decreasing atmospheric density have been proposed.