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W. H. Smyth (AER), M. L. Marconi (PBR)
The existence of substantial circumplanetary neutral clouds (or gas tori) of undetermined gas species has been inferred for Europa from energetic ion data from the Galileo Energetic Particle Detector (Lagg et al., GRL 30, 1556, doi:10.1029/2003GL017214, 2003) and from copious energetic neutral atoms (ENAs) imaged by the Cassini Ion Neutral Camera (Mauk et al., Nature 421, 920, 2003). We have recently undertaken two-dimensional kinetic model calculations (Smyth and Marconi, BAAS 36, 1083, 2004) for the water-group species in Europa's atmosphere and determined that the most important gas tori in the vicinity of Europa's orbit are for H2 followed by O. Neutral cloud model calculations have therefore been performed to explore their abundances, spatial distributions, and ability to impact the energetic and thermal plasma. These model calculations show that the neutral population of Europa's atmosphere resides almost entirely in its gas tori, not in its gravitationally-bound satellite atmosphere. The densities for Europa's gas tori are peaked about the satellite's position, asymmetrically distributed about Jupiter, and extend radially inward to Io's orbit. The neutral population of Europa's H2 gas torus is larger than that for Io's combined O and S gas tori while the neutral population for Europa's O gas torus is comparable to that for Io's S gas torus. In the vicinity of Europa's orbit, the H2 gas torus population is adequate to alter significantly the energetic ion populations and pitch-angle distributions and to create a large source of ENAs as measured by the Galileo and Cassini spacecrafts. Europa's gas tori will impact the thermal plasma and outward radial transport by introducing sinks for Iogenic plasma and by creating a substantial secondary peak in the pickup ion production rate at Europa's orbit that dominates that for Io beyond the plasma torus ramp region. Model calculations illustrating these results will be presented.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.