McEwen, A., Geissler, P., Belton, M., Porco, C., Pappalardo, R., Johnson, T., Squyres, S., Williams, D., and Head, J. (2001). "Cassini and Galileo Satellite Imaging Results: Giant North Polar Plume on Io." AGU Spring Meeting Abstracts P51A-04.


The imaging experiments of both Galileo (SSI) and Cassini (ISS) acquired images of the Galilean satellites during the joint encounter. These include both sunlit images (reflected light) and eclipse images of Io, Europa, and Ganymede. Much of the Galileo data is still being returned and most of the Cassini images have not yet been analyzed. Auroral glows in eclipse are described by Geissler et al. (this conference). We focus here on the most interesting result to date: the new giant plume over Tvashtar Catena (63 N, 127 W). Cassini eclipse sequence images acquired from Dec 29 2000 through Jan 5 2001 all show a North Polar plume-like feature extending about 3 pixels (180 km) above the limb. This feature and the giant Pele plume are both best seen in UV images acquired in the penumbra. The plumes are not detected in visible to near-IR reflected light. The North Polar feature is also apparent in clear-filter eclipse images. This plume-like feature was at first puzzling: is it a volcanic eruption or an auroral glow related to magnetospheric interactions? On December 16 2000 an intense brightening at 3.8 microns was detected at the location of Tvashtar by F. Marchis and colleagues (see Unfortunately the Cassini eclipse images are all centered at about longitude 300, almost 180 degrees from the longitude of Tvashtar. A plume emanating from Tvashtar (27 degrees from the limb) must be 380 +/- 60 km high to explain the north polar feature. Only the plume of Pele has ever been observed to reach this height. An intense eruption at Tvashtar with probable lava fountains was observed in November of 1999, and new dark deposits suggested activity of a plume no more than 100 km high. Hence, we did not at first consider Tvashtar to be a likely source for the plume. But, the first full Galileo image of Io made available (3 days before abstract deadline) showed a dramatic surface change: There is a new giant ring (~1200 km diameter) surrounding Tvashtar. This is about the same size as the ring surrounding Pele, so a plume of height comparable to Pele is feasible. The Tvashtar plume deposits are not bright red like those of Pele, implying some difference in plume composition or polar condensation. This plume may have had significant effects on the population, density, and distribution of neutral and charged particles in the Jupiter system during the joint observations.