Sputtering of Ices

Abstract

Data obtained from the exploration of the outer solar system has led to a new area of physics: electronically induced sputtering of low-temperature, condensed-gas solids, here referred to as ices. Icy bodies in the outer solar system are bombarded by relatively intense fluxes of ions and electrons, as well as the background solar UV flux, causing changes in their optical reflectance and ejection (sputtering/desorption) of molecules from their surfaces. The low cohesive energies of ices lead to relatively large sputtering rates by both momentum transfer (‘knock-on’ collisions) and the electronic excitations produced by the incident particles. Such sputtering produces an ambient gas about an icy body, often the source of the local plasma. This chapter focuses on the ejection of material by ionizing radiation from a surface that is predominantly a molecular condensed gas solid. The incident radiation types considered are photons, electrons and ions with the emphasis on the ejection processes. This radiation also produces the chemical effects described in the chapters of sections II and III. The induced-chemistry can produce both more refractory and more volatile products and so affect the molecular ejection rate. The emphasis in this chapter is on the production of gas-phase species from icy surfaces in space. We describe the physics and chemistry leading to the ejection of atoms and molecules, give semi-empirical expressions based on these processes, and describe some applications.