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[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