Other
Scientific paper
Sep 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998baas...30.1086l&link_type=abstract
American Astronomical Society, DPS meeting #30, #38.P10; Bulletin of the American Astronomical Society, Vol. 30, p.1086
Other
Scientific paper
Bombardment by energetic (500 keV) S(+) and O(+) sputters significant amounts of water and water products ( 10(2) -10(3) H_2O molecules/ion) [Shi et al., J. Geophys. Res., 100, 26387, 1995] from Europa's icy regolith and creates a tenuous O_2 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 O_2 column density (based on the HST observations) and nonvolatiles with long residence times (e.g., salts and organics) as 1% of the H_2O 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, SO_2, and CO_2, may indicate the presence of such materials.
Johnson Robert E.
Killen Rosemary Margaret
Lewis William S.
Waite H. Jr. J..
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