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M.L. Marconi (Fresh Pond Research Institute)
Recent HST UV observations indicate that Europa has a substantial O2 atmosphere with column 2.4 to 14 x 1014 cm-2 (Hall et al., ApJ 499, 1998). Galileo PLS plasma measurements have detected pickup ions in the water group region near M/Q = 16 (Paterson et al., JGR 104, 1999), suggesting the presence of water group species in Europa's atmosphere. In addition, the Cassini ENA imager has detected a large neutral cloud, comparable in amount of material to Io's neutral clouds, near Europa's orbit (Mauk et al., Nature 421, 2003), which suggests that Europa's atmosphere has a substantial escaping flux of neutrals (Smyth et al.,this meeting). Due to the high water-ice coverage of Europa's surface, low surface temperature (temperature below about 120 K), but large high energy particle flux(Paranicas et al., JGR 105, 2000), it appears reasonable that the sputtering of water-ice by energetic ions is the main source of the atmosphere (Ip et al., Adv. Sp. Res. 26, 2000).
A spherical hybrid fluid/kinetic model for Europa's atmosphere , including the likely major consitituents, H2O, OH, O, H, O2, and H2, has been developed. The species are assumed to be sputtered from the surface by energetic ions (spectra and flux from B. Mauk, private communication , 1999; and yields from R.E. Johnson, private communication, 2001). Neutral-neutral collisions, chemistry, and photochemistry as well as ion-neutral collisions and electron impact dissociation, using a simplified plasma model, are taken into account. The resulting tenuous quasi-collisional atmosphere is composed mainly of O2 and H2 at levels comparable to the lower end of the range estimated by Hall et al. (1998). Preliminary results for the structure and composition of the atmosphere as well as escape rates for atmospheric species will be presented. This work was in part funded by the NASA Planetary Atmospheres Program.
The author(s) of this abstract have provided an email address for comments about the abstract: marconi@freshpond.org
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.