Kuiper prize lecture: Escape of atmospheres, ancient and modern

Abstract

The escape of gases from atmospheres is discussed from a historical viewpoint, with emphasis on concepts rather than details, and on the interaction of ideas and observations. Although rooted in theoretical ideas from the kinetic theory of gases, the subject remained almost sterile until space-age data could be brought to bear. Major advances followed from the first sounding-rocket data on the Earth's upper atmosphere, from the Mariner 5 results on hydrogen near Venus, and the nitrogen isotopic composition found by Viking at Mars. These results stimulated concepts of diffusion-limited flow, nonthermal escape mechanisms, formation of bound nonthermal coronae, energy-limited escape, and mass fractionation during early blowoff. As a consequence, we not only have acceptable explanations of a wide range of phenomena, but also prospects of explaining the xenon isotopic patterns in meteorites and various atmospheres, and the stil -mysterious behavior of Io's atmosphere and plasma torus.