Space Exploration and the History of Solar-System Volatiles

The assemblage of volatiles degassed from each planetary object simultaneously reflects the preaccretion history of the solid solar-system material which accreted to produce that object and the internal differentiation history of the object. The key questions are: 1) What are the differences in bulk composition among planetary objects and how did they arise? and 2) How have volatiles within each object been redistributed among its interior, its surface, and its atmosphere?-a) What is the total volatile inventory which was degassed to each surface? b) When did supply occur and what were the energy sources? and c) What was the geochemical fate of each constituent? In comparative studies of planetary-surface and atmospheric volatiles, one of the hardest problems is disentangling differences among planetary-surface volatile inventories which are due to initial differences in bulk composition among objects from those due to differences in subsequent internal differentiation and atmospheric history. In the area of internal differentiation and degassing, the greatest puzzles involve understanding apparent differentiation of surprisingly small objects and surprisingly early differentiation of both large and small objects. The answers to these questions are most efficiently obtained by a balanced program of space exploration which includes strong missionsupporting programs of Earth-based observations, theoretical studies, and laboratory experiments. The study of "planetary atmospheres" cannot be undertaken in isolation from studies of other important sinks for degassed volatiles.