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J. R. Spencer (Lowell Obs.), K. S. Noll (S.T.Sc.I.), C. Henry (U. Oklahoma), R. E. Johnson (U. Virginia), A. Lane (J.P.L.), D. Domingue (A.P.L.), W. Calvin (U.S.G.S. Flagstaff)
We present the first global disk-resolved maps of the distribution of Ganymede's 2600 Åabsorption band, due probably to O3 (Noll et al. 1997), obtained with STIS on HST. These data complement earlier disk-integrated UV spectra by IUE (Nelson et al. 1987) and HST (Noll et al. 1996), and Galileo UVS spectra of selected regions of the surface (Hendrix et al. 1999).
STIS global image cubes of the leading and trailing hemispheres of Ganymede, obtained in November 1998, cover 1900 - 4000 Åwith a resolution of 640 km x 150 km x 15 Å. We map the 2600 ÅO3 band strength by dividing a 2500 ± 80 Åalbedo image by the average of a 2070 ± 80 Åand 2930 ± 80 Å``continuum'' albedo image. Fitting a polynomial to each albedo spectrum gives similar results.
The band depth maps show the expected strong band on the trailing hemisphere and very weak band on the leading hemisphere, but without an obvious dependence on terrain type, or distance from the antapex within each hemisphere. On the trailing side, O3 abundance is stronger at mid-latitudes (±40o)than at either the equator or higher latitudes, and is thus different from the equatorially-concentrated trailing-side O2 absorption (Calvin and Spencer 1997). The ±60o - 70o latitudes, where Hendrix et al. (1999) see a strong O3 band, may be too foreshortened to be well seen in our data. Implications of these results for O3 formation on Ganymede will be discussed.