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W. S. Lewis, J. H. Waite (Southwest Research Institute), R. E. Johnson (University of Virginia), R. M. Killen (Southwest Research Institute)
Bombardment by energetic (500 keV) S+ and O+ sputters significant amounts of water and water products (~102-103 H2O molecules/ion) [Shi et al., J. Geophys. Res., 100, 26387, 1995] from Europa's icy regolith and creates a tenuous O2 atmosphere [Hall et al., Science, 373, 677, 1995]. Other constituents present in the ice (e.g., salts and organics) are also ejected, as demonstrated by the detection of a tenuous sputter-produced sodium exosphere [Brown and Hill, Nature, 380, 229, 1996]. Ionization of the sputtered neutrals by EUV radiation, electron impact, and charge transfer produces an ionosphere [Kliore et al., Science, 292, 38, 1997]. To determine whether ionized sputter products are likely to be present in Europa's atmosphere at levels detectable with an ion mass spectrometer on a Europa orbiter, we first estimated neutral abundances by scaling volatiles with short residence times as 1% of the O2 column density (based on the HST observations) and nonvolatiles with long residence times (e.g., salts and organics) as 1% of the H2O column. Compared with fluxes obtained using sputtering yields determined in the laboratory for an incident ion flux of 500 keV S+ and O+, our estimated neutral densities represent conservative, lower-limit values. Using these densities, we then estimated ionization and ion loss rates to determine ion densities vs. altitude (20-300 km). Our results indicate that all the molecular ions assumed to be present at the 1% level can be detected by an ion mass spectrometer with a sensitivity of 10-3 cm-3. This provides the exciting possibility of remotely sampling components representative of salts from a sub-surface ocean and organic molecules indicative of prebiotic activity on Europa. Our study also suggests that the already observed species, Na, SO2, and CO2, may indicate the presence of such materials.