Geology and activity around volcanoes on Io from the analysis of NIMS spectral images

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The last two successful flybys of Io by Galileo in 2001 (orbits I31, I32) allowed the Near Infrared Mapping Spectrometer to enrich its collection of IR spectral image cubes of the satellite. These data cover hemispheric portions of Io, several volcanic centers as well as their surroundings with a spatial resolution ranging from 2 to 93 km pixel −1. They map thermal emission from the hot-spots and the distribution of solid SO 2 in the 1.0–4.7 μm spectral range. We obtain maps of SO 2 abundance and granularity from the NIMS data using the method of Douté et al. (2002, Icarus 158, 460–482). The maps are correlated to distinguish four different physical units that indicate zones of SO 2 condensation, metamorphism and sublimation. We relate these information with visible images from Galileo's Solid State Imaging System and with detailed mapping of the thermal emission produced by Io's surface. Our principal goal is to understand the mechanisms controlling how lava, pyroclastics and gas are emitted by different types of volcanoes and how these products evolve. The 800 km diameter white ring of fallout created by a violent “Pillanian” eruption during summer of 2001 is at least partly composed of solid SO 2 and has enriched preexisting regional deposits. Orange materials have been recently or are currently emplaced 240 km south from the main eruption site, possibly as sulfur flows. A similar event may have taken place in the past at Ababinili Patera (12.5° N, 142° W). Carefull study of SO 2 maps covering the Emakong region also suggests that sulfur forms the bright channel-fed flow emerging from the south eastern side of the caldera. Within the main caldera of Tvashtar Catena completely cooled patches of crust exist. Elsewhere, the caldera is still cooling from previous episodes of flooding. We confirm that Amirani emits constantly large amount of SO 2 gas by interaction of fresh lava with the volatiles of the underlying plains. Nevertheless SO 2 frost is not the major component of the bright white ring seen in the SSI images. Over the whole Gish Bar region, SO 2 frost seems barely stable and is constantly regenerated. The stability increases along gray filamentary structures which could be faults filled with materials having peculiar thermal properties. Northwest of Gish Bar Patera, a localized bright deposit shows an unusual spectral signature potentially indicative of H 2O molecules forming ice crystals or being trapped in a nonidentified matrix. The Chaac region may present a thickened old crust reducing the geothermal flux to levels lower than 0.5 W m −2 and thus creating a cold trap for SO 2. Looking at the abundance and degree of metamorphose of SO 2, we establish the relative age of different flows and ejecta for the Sobo Fluctus. Finally the assumption that the white patches in visible images indicate SO 2 rich deposits is once again challenged. In the Camaxtli region we identify a topographically controlled compact white deposit showing only moderate SO 2 abundance. In contrast, we detect two spots of quite pure SO 2 ice on the gray flanks of Emakong. Furthermore, the close association of fumarolic SO 2 and red S 2 already noted for several volcanic centers is observed at Tupan.