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S. Douté (IGPP UCLA), R. Lopes-Gautier (JPL), B. Schmitt (CNRS LPG), R. Carlson (JPL), P. E. Geissler (LPL University of Arizona), Galileo NIMS Team
The surface of Io is shaped by various interrelated volcanic, atmospheric and magnetospheric phenomena. Albedo and color changes, that occur in the visible at short distances from many volcanic centers displaying high thermal emission, constitute the most prominent signs of this intense activity. Liquid materials flow from the vents, gases and solid pyroclastics condense or settle on the surface. Curiously, little is known about the precise chemical nature and physical state of these materials. The high temperature hot-spots detected by the IR spectro-imager NIMS and the camera SSI of Galileo suggest the existence of large silicate lava fields but their spectral detection in the visible and near infrared is still ambiguous. The white, yellow, red, orange or brown hues that give Io its colorful appearance suggest abundance of sulfur-bearing compounds such as SO2, native and short chain sulfur, sulfur oxides. Nevertheless none of these compounds have been clearly identified, except SO2 thanks to its numerous and distinctive absorption bands in the NIR. Other components, like H2S, Na2S, Na2SO4, and NaCl, have been proposed from thermochemical models of volcanic gases for example, but still wait to be detected. The analysis of NIMS IR observations at different spatial scale can give us some clues to understand the real nature of Io's surface. In our presentation we will give an updated review of what we have found so far in terms of identification, mapping and physical state of the materials, with an emphasis on the SO2. This work was begun at the Laboratoire de Planétologie de Grenoble with the support of the French Programme National de Planétologie, and continued under contract with NASA through the Jupiter System Data Analysis Program.
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