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Naturally occurring ices lie on both interstellar dust grains and oncelestial objects, such as those in the outer solar system. These ices arecontinu- ously subjected to irradiation by ions from the solar wind and/orcosmic rays, which modify their surfaces. As a result, new molecular speciesmay form which can be sputtered off into space or planetary atmospheres. Wedetermined the experimental values of sputtering yields for irradiation ofoxygen ice at 10 K by singly (He+, C+, N+, O+ and Ar+) and doubly (C2+, N2+ andO2+) charged ions with 4 keV kinetic energy. In these laboratory experiments,oxygen ice was deposited and irradiated by ions in an ultra high vacuum chamberat low temperature to simulate the environment of space. The number ofmolecules removed by sputtering was observed by measurement of the icethickness using laser interferometry. Preliminary mass spectra were taken ofsputtered species and of molecules formed in the ice by temperature programmeddesorption (TPD). We find that the experimental sputtering yields increaseapproximately linearly with the projectile ion mass (or momentum squared) forall ions studied. No difference was found between the sputtering yield forsingly and doubly charged ions of the same atom within the experimentaluncertainty, as expected for a process dominated by momentum transfer. Theexperimental sputter yields are in good agreement with values calculated usinga theoretical model except in the case of oxygen ions. Preliminary studies haveshown molecular oxygen as the dominant species sputtered and TPD measurementsindicate ozone formation.

Sputtering of Oxygen Ice by Low Energy Ions