Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p52c..05m&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P52C-05
Physics
6008 Composition, 6060 Radiation And Spectra, 6061 Remote Sensing, 6062 Satellites, 6218 Jovian Satellites
Scientific paper
The first ultraviolet images of the surfaces of Europa and Ganymede were made with the Hubble Space Telescope in 1999 and 2000. Both satellites exhibit atmospheric emissions produced by the interaction of plasma electrons trapped in the Jovian magnetosphere with their very tenuous oxygen atmospheres. The Ganymede emissions are clearly auroral in nature, while the oxygen emission from Europa appears to be correlated more closely with visibly bright surface regions. Both satellites exhibit very low (~ few percent) reflectivities at UV wavelengths (120-170 nm). At the wavelength of the strongest solar emission line in this wavelength range, HI Lyman-α\ (121.57 nm), Europa exhibits a surface reflectivity pattern that is anti-correlated in brightness with the visibly dark surface regions, and with the oxygen emission. At face value, the Europa observations imply either that the contaminant that darkens the surface at visible wavelengths has a lower reflectivity than the brighter visible regions (thought to be purer water ice), or that there is more hydrogen gas over the darker surface regions. Both explanations involve interaction of the trapped Jovian magnetospheric plasma with the surface of the satellite. The observations of Europa haev been challenging to explain, especially given the lack of UV reflectivity measurements for possible surface contaminants.
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