Europa's ice-related atmosphere: The sputter contribution

Abstract

Europa, Jupiter's innermost icy satellite, is embedded well within Jupiter's magnetospheric plasma, an intense flux of ions and electrons that approximately co-rotate with Jupiter. The plasma can be thought of as consisting of two populations: The cold, thermal plasma containing charged particles with energies ranging from 1 eV to 1 keV, and the hot, energetic plasma containing charged particles with energies ranging from 10 keV to 100 MeV. When the charged particles interact with Europa's surface, they not only chemically and physically alter the icy surface, but also liberate material from the surface through a process called sputtering, which in turn forms a tenuous atmosphere.

In this paper we calculate the sputter contribution to the atmosphere by modeling the formation of Europa's ice-sputtered atmosphere ab initio. We consider the species H, H₂, O, OH, H₂O, O₂, HO₂, H₂O₂, and O₃, all of which are related to the water-ice surface. Whereas the ice sputter yields of H2O, H2, and O2 have been well established, the ice sputter yields (and the resulting density profiles) of H, O, OH, HO₂ and O₃ are small and largely unknown. As model input we use available plasma ion and electron energy spectra as well as available water-ice sputter yields. Based on first principles, i.e., without applying any scaling to observed data, we calculate atmospheric densities ab initio.

Our results match available observational data and previously published modeling efforts well. Europa's exosphere is dominated by thermally accommodated O₂ close to the surface (below a few 100 km), and the much lighter H₂ molecules at higher altitudes. The water-ice related species that stick to the surface (freeze out) are liberated by cold and hot plasma sputtering in about equal amounts. In addition, in the case of H₂, O₂, and H₂O₂, electrons contribute almost as significantly to the sputter yield as ions do.