A. S. McEwen, ¹ M. J. S. Belton, ² H. H. Breneman, ³ S. A. Fagents, ⁴ P. Geissler, ¹ R. Greeley, ⁴ J. W. Head, ⁵ G. Hoppa, ¹ W. L. Jaeger, ¹ T. V. Johnson, ³ L. Keszthelyi, ¹ K. P. Klaasen, ³ R. Lopes-Gautier, ³ K. P. Magee, ³ M. P. Milazzo, ¹ J. M. Moore, ⁶ R. T. Pappalardo, ⁵ C. B. Phillips, ¹ J. Radebaugh, ¹ G. Schubert, ⁷ P. Schuster, ⁸ D. P. Simonelli, ⁹ R. Sullivan, ⁹ P. C. Thomas, ⁹ E. P. Turtle, ¹ D. A. Williams ⁴

During late 1999/early 2000, the solid state imaging experiment on the Galileo spacecraft returned more than 100 high-resolution (5 to 500 meters per pixel) images of volcanically active Io. We observed an active lava lake, an active curtain of lava, active lava flows, calderas, mountains, plateaus, and plains. Several of the sulfur dioxide-rich plumes are erupting from distal flows, rather than from the source of silicate lava (caldera or fissure, often with red pyroclastic deposits). Most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust, but rapidly emplaced channelized flows are also found at all latitudes. There is no evidence for high-viscosity lava, but some bright flows may consist of sulfur rather than mafic silicates. The mountains, plateaus, and calderas are strongly influenced by tectonics and gravitational collapse. Sapping channels and scarps suggest that many portions of the upper ~1 kilometer are rich in volatiles.

¹ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
² National Optical Astronomy Observatories, Tucson, AZ 85719, USA.
³ Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
⁴ Department of Geology, Arizona State University, Tempe, AZ 85287, USA.
⁵ Department of Geological Science, Brown University, Providence, RI 02913, USA.
⁶ NASA Ames Research Center, Moffett Field, CA 94035, USA.
⁷ Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA.
⁸ DLR, Institut fur Weltraumsensorik und Planetenerkundung, 12489 Berlin, Germany.
⁹ Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, USA.