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R. E. Johnson (Engineering Physics, University of Virginia, USA)
Energetic ion bombardment decomposes ice on the surface of Europa producing an O2 atmosphere. Similarly the incident particle radiation decomposes Na-containing minerals putting Na into the gas phase. Here I use the data of Brown and Hill (Nature 1996) to determine the escape rate of Na from Europa's. This rate is compared to the ion implantation rate and shows that Europa is a net source of Na, like the moon, Mercury and Io. Using the energetic particle flux and sputter-induced decomposition data, an estimate of the surface concentration of Na is given. The results for oxygen and sodium are compared and implications for other surface species are discussed.