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W. H. Smyth (AER), M. L. Marconi (FPRI)
The presence of a tenuous atmosphere on Europa consisting of a column of (2-15) x 1014 cm-2 of O2 has been inferred from the measurements of OI 1304~Å~emissions (Hall et al., {\em Nature} 373, 677-679, 1995 and {\em Ap.~J.} 499, 475-481, 1998). Model calculations (Shematovich and Johnson, {\em Adv.~Space Res.} 27, 1881-1888, 2001 and Shematovich et al., preprint, 2004) have since elucidated the structure of the oxygen atmosphere and shown that such columns are obtainable for the expected sputtering rates at Europa. Hydrogen, however, is also produced in substantial amounts by sputtering of Europa's largely H2O surface by energetic heavy ions and to a lesser extent by sublimation of H2O. We have recently calculated (Marconi and Smyth, {\em EOS} 85, JA308, 2004) the spatial distributions of the water group species (H2, H, O2, O, H2O, OH) in Europa's atmosphere using a 2-D hybrid fluid/kinetic model (Marconi, {\em Icarus} 166, 410-424, 2003). We find that while oxygen dominates near the surface, hydrogen is the principal species at higher altitudes. We also find that hydrogen is the most abundant of the escaping species, and hence, an important source for the Europa neutral gas tori about Jupiter. The spatial structure of the most abundant Europa neutral tori and their plasma sources have therefore been calculated with the AER Neutral Cloud Model (Smyth and Combi, {\em Ap.~J.} 328, 888-918, 1988). We find that the abundances of the neutral clouds of Europa are comparable to those for Io. These calculations will be presented, and some implications of Europa's neutral clouds on the thermal plasma, energetic plasma, and the production of energetic neutral atoms in the plasma torus as measured by the Voyager (Bagenal, {\em JGR} 99, 11,043-11,062, 1994), Galileo (Mauk et al., {\em JGR} 109, A09S12 doi:10.1029/2003JA010270, 2004), and Cassini (Mauk et al., {\em Nature} 421, 920-922, 2003) spacecrafts will be discussed.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.