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[39.10] Cassini UVIS Measurements of Hydrogen Exospheres at the Icy Saturnian Satellites

A. R. Hendrix, C. J. Hansen (JPL/CalTech)

Measurements of the icy Saturnian satellites from the Cassini Ultraviolet Imaging Spectrograph (UVIS) reveal atomic hydrogen in emission in regions close to these small bodies. The hydrogen Lyman alpha (1216 Å) radiation is measured in levels greater than the background interplanetary hydrogen (IPH). Lyman beta (1025 Å) hydrogen has also been measured. The hydrogen exospheres are measured at Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, and Iapetus. The measured hydrogen distributions are fit with exospheric models. The surface temperatures of the icy satellites are too cold to expect water vapor atmosphere driven by sublimation. The hydrogen enhancement often appears in greater abundance near the dayside limb than near the night side limb. A possible source of the exospheres is photodesorption of the surface water ice. We investigate whether ejection of hydrogen from the icy surfaces by charged particles is also a likely source. The loss of hydrogen appears to be consistent with previous HST measurements of ozone in the surfaces of Rhea and Dione.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.