Sputtering of Ices

5 Abstract Data obtained from the exploration of the outer solar system has led to a 6 new area of physics: electronically induced sputtering of low-temperature, 7 condensed-gas solids, here referred to as ices. Icy bodies in the outer solar system 8 are bombarded by relatively intense fluxes of ions and electrons, as well as the 9 background solar UV flux, causing changes in their optical reflectance and ejection 10 (sputtering/desorption) of molecules from their surfaces. The low cohesive energies 11 of ices lead to relatively large sputtering rates by both momentum transfer (‘knock12 on’ collisions) and the electronic excitations produced by the incident particles. 13 Such sputtering produces an ambient gas about an icy body, often the source of the 14 local plasma. This chapter focuses on the ejection of material by ionizing radiation 15 from a surface that is predominantly a molecular condensed gas solid. The incident 16 radiation types considered are photons, electrons and ions with the emphasis on the 17 ejection processes. This radiation also produces the chemical effects described in 18 the chapters of sections II and III. The induced-chemistry can produce both more 19 refractory and more volatile products and so affect the molecular ejection rate. 20 The emphasis in this chapter is on the production of gas-phase species from icy 21 surfaces in space. We describe the physics and chemistry leading to the ejection of 22 atoms and molecules, give semi-empirical expressions based on these processes, 23 and describe some applications.