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R.E. Johnson (U. Virginia), R.W. Carlson (JPL), J.F. Cooper (GSFC), C. Paranicas (APL), M.H. Moore (GSFC), M. Wong (JPL)
Early observations of the Galilean satellites indicated that their reflectance spectra were modified by energetic particles. However, it has only recently been shown quantitatively that radiolysis, the chemical alteration of a surface by charge-particles, is occurring on these moons. Charged particle irradiation is one of a number of space weathering processes. It can produce new species in the surface, form an atmosphere, and is a source of the local plasma. Hence, understanding the radiation effects is a prerequisite to understanding the origins and evolution of these objects. A key issue is whether the surface reflectance is representative of the intrinsic materials or is simply a patina of absorbing material 'painted on' by the energetic Jovian particles. At Europa, it is also important to know whether there is a relationship between the putative subsurface ocean and the composition of the surface and the atmosphere. Radiolysis due to Jovian particles can alter the optical surface in times scales short compared to cratering time scales and produces chemical changes at micron to meter depths. It competes with downward mixing by meteorite gardening, sublimation and burial, and subduction, so that materials altered by the energetic ions and electrons can exist at depths much greater than the radiation penetration depth [1]. Hence, radiolytic products have been suggested as a possible energy source for subsurface biology. In this paper we combine laboratory data on irradiation processing of materials with data on the Jovian plasma to summarize our understanding of the radiation processing of the Galilean satellites with emphasis on Europa.
[1] R. E. Johnson, R.W. Carlson, J. F. Cooper, C. Paranicas, M. H. Moore, M. Wong, Radiation Effects on the Surfaces of the Galilean Satellites, in Jupiter: Atmosphere, Satellites and Magnetosphere ed. F. Bagenal (Univ of Arizona Press) to be published
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.