Session 54P. Io, Callisto, and Ganymede II
Contributed Poster Session, Thursday, October 15, 1998, 5:00-6:30pm, Hall of Ideas

## [54P.11] Io's Disk-Integrated Spectral Characteristics: A Re-examination of Longitudinal Compositional Variations

Ross A. Beyer (LPL), Deborah L. Domingue (JHU/APL), Arthur L. Lane (JPL)

Ultraviolet spectra taken by IUE over 18 years (1979--1996) demonstrate no discernable temporal variations on disk-integrated spatial scales. Intimate and linear mixing modeling of these spectra coupled with visible spectra [1] show SO2 is more abundant on the leading side, commensurate with earlier findings [2,3]. Three component modeling was performed on these combined spectra using seven candidate end-members: fine- and medium-grained SO2 frost [4], sulfur, forsterite, fayalite, Allende meteorite, and Apollo lunar soil~#70017 [5]. SO2 was present in all viable results. The second most abundant component was alternatively sulfur or fayalite. The third component was present at 0%--20% levels with the most common being fayalite, forsterite, and the Apollo sample.

Quantitative estimates of SO2 abundance have varied by 30%--60% depending on whether the estimates were made from UV or IR observations. UV observations have estimated 20% global coverage [4] while IR based estimates range from 85% on the leading and 50% on the trailing side [3]. McEwen {\it et al.} (1988) calculated disk-integrated SO2 abundance on Io using both intimate and linear mixing models. The components they chose were SO2 frost and hypothetical constituents based on component analysis of the Voyager color data set. Both our intimate and linear mixing model results estimate 25% ± 8% SO2 on the trailing side, on the low end of the range estimated from the Voyager data analysis [6]. While the SO2 abundance on the leading side varies more with longitude, ranging from 37% ± 16% at 15\circ--45\circ longitude and 60% ± 16% at 135\circ--165\circ longitude, well above previous mixing model estimates based on IUE, 4.08-mm, and Voyager data sets [6].

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\noindent [1] Nelson & Hapke, 1978, {\it Icarus} {\bf 36}, 304; [2] Nelson {\it et al.}, 1980, {\it Science} {\bf 210}, 7784; [3] Howell {\it et al.}, 1984, {\it Icarus} {\bf 57}, 83; [4] Nash {\it et al.}, 1980, {\it GRL} {\bf 7}, 665; [5] Wagner {\it et al.}, 1987, {\it Icarus} {\bf 69}, 14; [6]~McEwen {\it et al.}, 1988, {\it Icarus} {\bf 75}, 450

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