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[47.09] Cassini UVIS Observations of Saturn's Icy Satellites

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

Over the past year Cassini's UltraViolet Imaging Spectrograph (UVIS) has obtained numerous observations of Saturn's icy satellites at far (110 to 190 nm) and extreme (55 to 110 nm) ultraviolet wavelengths. This region of the spectrum is sensitive to the presence of water, CO2 and NH3 ice and a variety of gases. Saturn's icy moons experience a different radiation and dust environments ranging from distant Phoebe in the solar wind to Enceladus at the peak of the E ring to Mimas deep in Saturn's magnetosphere.

The composition of Enceladus' atmosphere (detected by the Cassini Magnetometer team) may be determined by an occultation of gamma Orionis on July 14. The UVIS spectrum of the star will be compared before and after the occultation to look for the absorption signature of atomic and molecular oxygen, carbon monoxide and atomic nitrogen. If none of these species are detected we can set upper limits on the amount of these gases in Enceladus' atmosphere. A similar technique applied to the occultation of lambda Scorpius in March allowed us to compute an upper limit for the presence of molecular oxygen of 5 x 10⁷ molecules O2/cm3. Gamma Orionis is substantially brighter at FUV wavelengths than lambda Scorpius, and the UVIS will be configured for full spectral resolution, so we anticipate more sensitive detection thresholds.

We will also present comparative results for the composition of the surfaces (distribution of water ice on Phoebe is heterogeneous, the bright side of Iapetus is more similar to Phoebe than the dark side), predominant grain size of ice on surfaces dominated by water ice, and general uv albedo. These data sets will eventually allow comparison of the efficacy of processes that affect the uppermost surface.

This work was partially supported by JPL, California Institute of Technology, under a contract with NASA.

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