Mapping SO2 frost on Io by the modeling of NIMS hyperspectral images

We analyze a collection of hyperspectral images of Io acquired by the near infrared mapping spectrometer (NIMS) of Galileo during the G2 to E16 orbits of Jupiter. This analysis leads to the geographical distribution and physical characterization of SO2 frost deposits over about three-fourths of Io's surface. These deposits are excellent tracers of various phenomena, including volcanic production and emission, atmospheric transportation, condensation, metamorphism, irradiation, and sublimation, that occur throughout the SO2 cycle, We assume that the deposits of solid SO2 are optically thick and are geographically mixed with other sulfur-bearing compounds. We first assess the mean moderate backscattering behavior of the SO2 frost (Henyey Greenstein phase function parameter g = -0.27 +/- 0.05) using a sequence of spectra at two different locations over a large range of phase angles. This behavior may indicate a granular texture with many defects or a fluffy texture. Second, a more systematic inversion of the hyperspectral images is achieved based on a linear spectral model of pure SO2 with variable grain size mixed with a spectrally neutral unit. As a result, we produce two global mosaics that map SO2 frost coverage and mean grain size. SO2 deposits are omnipresent on Io's surface at the spatial scale of this study (approximate to 200 km), but the SO2 frost is concentrated within several large areas centered at medium latitudes. These SO2-rich regions (surface coverage higher than 60%) show a longitudinal correlation with plumes located lower in latitude, suggesting that these plumes are the principal sources of SO2 gas. After a possible dynamic condensation around the plumes or at the equator, the gas is remobilized by the solar or thermal fluxes and flows mostly latitudinally toward the coldest and nearest regions devoid of hot-spots. Third, the correlation of the distribution and grain size mosaics distinguishes four different SO2 physical units that indicate relative regional variations of condensation, metamorphism, and sublimation. Finally, comparisons with Voyager ultraviolet (A. S, McEwen, T. V. Johnson, D, L. Matson, and L. A. Soderblom, 1988, Icarus, Vol. 75, pp. 450-478) and Galileo visible (P. Geissler, A. S, McEwen, L. Keszthelyi, R. M. C. Lopes-Gautier, J. Granahan, and D. P. Simonelli, 1999, Icarus, Vol. 140, pp. 256-282) observations demonstrate molecular contamination of SO2 at medium and high latitudes and that these contaminated SO2 deposits may be optically thin. (C) 2000 Academic Press.