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A.S. McEwen (LPL, University of Arizona), Galileo SSI Team
Three close flybys of Io by the tenacious Galileo spacecraft have revealed a world unlike any other in the Solar System. Repeat imaging at moderate resolution (100-500 m/pixel) shows that the long-lived Prometheus and Amirani flow fields are being emplaced from many individual lava breakouts, similar to compound flows growing by inflation of pahoehoe lava. The I27 (Feb 2000) images of Prometheus reveal bright streaks associated with the new lava breakouts, but no central vent for the plume, which suggests that the plume may be produced by the combined effect of many (>10) new breakouts interacting with volatiles such as SO2 in the surrounding plains. Many paterae on Io are very different from calderas seen on the terrestrial planets as they do not usually reside on shield volcanoes and are characterized by irregular and angular boundaries. The formation of paterae may be closely related to the unique vertical tectonics of Io, and the dense mafic to ultramafic lavas may then follow the resulting fractures and erupt in the topographic lows. The high mountains appear to be another consequence of the tectonics, and are often closely associated with paterae. The highest-resolution (5-20 m/pixel) images of Io's bright plains reveal unusual textures that are difficult to interpret. Nevertheless, these textures plus apparent sapping alcoves, the long-lived Prometheus-type plumes, and the presence of bright fresh flows all suggest that the extensive plains at all latitudes are rich in volatiles such as SO2 or other sulfurous materials. However, the current resurfacing of Io is dominated by silicate lava flows, so it is unclear why volatile-rich plains are so ubiquitous, especially near the equator. Perhaps, as on the terrestrial planets, much of the resurfacing is accomplished by rare events, or maybe the present-day rate of silicate volcanism on Io is unusual.