The Urey prize lecture: Volatile processes in the outer Solar System

Abstract

The outer Solar System is dominated by the giant planets, but contains also a handful of lunar-sized bodies with surfaces and atmospheres composed of material more volatile than water ice. The best studied of these bodies, Titan, has a surface hidden from view but which may be dominated by a massive ocean of liquid hydrocarbons and nitrogen. The progressive destruction of methane over time by photolysis drives a series of chemical changes in the ocean and atmosphere; some of the evidence for such changes may be detectable in data from Cassini. Much less is known of volatile processes on Triton and Pluto/Charon, but enough can be inferred that a cosmochemistry of the outer Solar System can begin to be assembled. Titan's density and composition are strong indicators of formation in a high pressure disk around Saturn. Pluto/Charon's bulk density and new inferences on the atmospheric composition support an origin in the solar nebula. Triton may be an intriguing hybrid case whose study will really begin in August 1989. Critical to an understanding of outer Solar System chemistry and its relationship to the primordial solar nebula are the composition of comets and molecular abundances on grains in giant molecular clouds. The spacecraft and novel ground-based observations of Halley and other recent comets have provided critical new data, but the level of detail required by most solar nebula models (akin to that derived from meteorites) must await CRAF and sample return missions at the turn of the new millenium.