Origin and Evolution of Planetary Atmospheres

The atmospheres of the terrestrial planets and satellites are generally quite thin, much thinner than might be expected given their relatively high abundances of geochemical volatiles and popular estimates of the available mass of volatile-rich impactors. In some cases a specific deficiency (e.g., water on Venus) implies a specific loss process (a runaway greenhouse effect that promoted irreversible hydrogen escape; same planet). But more generally the absence of atmophiles is broad-based and so argues for broad-based volatile loss. This talk will address the hypothesis that extant planetary atmospheres represent the small residue left between relatively large rates of supply and escape.