Sputtering of ices in the outer solar system

Abstract

Exploration of the outer solar system has led to studies in a new area of physics: electronically induced sputtering of low-temperature, condensed-gas solids (ices). Many of the icy bodies in the outer solar system were found to be bombarded by relatively intense fluxes of ions and electrons, causing both changes in their optical reflectance and ejection (sputtering) of molecules from their surfaces. The small cohesive energies of the condensed-gas solids afford relatively large sputtering rates from the electronic excitations produced in the solid by fast ions and electrons. Such sputtering produces an ambient gas about an icy body, often the source of the local plasma. This colloquium outlines the physics of the sputtering of ices and its relevance to several outer-solar-system phenomena: the sputter-produced plasma trapped in Saturn's magnetosphere; the ${\mathrm{O}}_2$ atmosphere on Europa; and optical absorption features such as S${\mathrm{O}}_2$ in the surface of Europa and ${\mathrm{O}}_2$ and, possibly, ${\mathrm{O}}_3$ in the surface of Ganymede.