Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p32a..07h&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P32A-07
Other
6218 Jovian Satellites, 6223 Callisto
Scientific paper
Callisto is an interesting end member among the Galilean satellites, in that the leading hemisphere (centered on 90 W) is darker than the trailing hemisphere (centered on 270 W) in the visible and near-UV. The surface alteration processes considered to contribute to its spectral reflectance are micrometeoroid bombardment, radiation-induced chemistry and sputtering, and implantation of neutrals from Io. Callisto's heavily cratered surface exhibits relatively dark and reddish spectra throughout the UV and infrared; micrometeoroid bombardment may play a dominant important alteration role at this icy moon (Buratti, Icarus 92, 312, 1991). Callisto's distance from Jupiter calls into question the importance of radiation in affecting its albedo and surface chemistry. Its relatively thick atmosphere (Liang et al., JGR 110, 2005) could shield the surface from both neutrals and slow ions. We present results from an investigation of the full set of Galileo Ultraviolet Spectrometer (UVS) data of Callisto, in the 210-320 nm wavelength range. We are interested in understanding how UV spectra can contribute to answering questions about Callisto's composition and effects of radiation and micrometeoroid bombardment. The nature of Callisto's dark, red UV reflectance spectra is investigated. Analyses of UVS Callisto data during the Galileo mission suggested the presence of H2O2 (Hendrix et al., LPSC XXX, 1999), similar to that detected on Europa. We describe the spatial distribution of the absorbing species in the UV and investigate whether the red shape of the Callisto UV spectrum is due to H2O2 or whether other species, such as carbonaceous or meteoritic materials, are involved. Pre-Galileo disk-integrated HST (Noll et al., GRL 24, 1139, 1997) and IUE (Lane and Domingue, GRL 24, 1143, 1997) observations showed a broad absorption feature superimposed on the general red spectrum, attributed to SO2, on the leading hemisphere relative to the trailing hemisphere. The Galileo UVS data also suggest a leading hemisphere absorption feature in ratio spectra (Hendrix et al., LPSC XXIX, 1998) but its presence and distribution was not investigated using the full Galileo data set until now. Here we will address whether the band correlates with large-scale surface features such as the Valhalla impact basin and whether the band has been correctly identified. Since this feature is very broad and not particularly distinctive, we examine whether the absorbing species could be some carbon-containing molecule. The presence of CO2 on Callisto and the dark nature of the surface suggest that carbon-based products will be present across the surface and could be chemically modified by radiation or meteoritic bombardment to produce an absorption feature.
Hendrix Amanda R.
Johnson Robert E.
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