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Fig. 6.4. Eruption at Tvashtar Catena on Io imaged by the Galileo spacecraft. This pair of images captures a dynamic eruption at Tvashtar Catena, a chain of volcanic bowls on Io. A change has occurred in the location of hot lava over a period of a few months in 1999 and early 2000. This style of volcanism on Io is unpredictable and short-lived. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.4. Eruption at Tvashtar Catena on Io imaged by the Galileo spacecraft. This pair of images captures a dynamic eruption at Tvashtar Catena, a chain of volcanic bowls on Io. A change has occurred in the location of hot lava over a period of a few months in 1999 and early 2000. This style of volcanism on Io is unpredictable and short-lived. (Credit: NASA/JPL/University of Arizona/PIRL)

a large surface area can be affected. Late in its mission, Galileo detected a large plume deposit ring south of the Karei area. The interior radius of this ring was 415 km, and its exterior radius averaged 690 km. The area covered by the deposit was almost 933,000 km2 [316]!

As noted, plumes can occur on a continuous basis. Both Voyagers 1 and 2 observed prominent plumes coming from a feature known as Masubi. During the Galileo mission, distinct surface changes were noted since the Voyager observations, and Galileo observed two distinct eruptions that produced prominent rings around Masubi. The first eruption occurred between orbits 9 and 10, leaving a prominent dark ring. However, by the next observation on orbit 11, the rings had begun to fade. Eight months later, on orbit 15, the rings had completely disappeared. So, the rings from the fallout can be quite short-lived [317].

The most prolific plume on Io appears to be from the feature known as Prometheus. This most persistently active plume was observed by both Voyagers 1 and 2, and by the Galileo spacecraft on every favorable encounter. According to Geissler et al., Prometheus produces a dusty plume 50-150 km high that deposits a bright ring of SO2 ~250 km in diameter, as well as fainter rings both interior and exterior to the bright ring. The center of the bright ring had migrated 85 km to the west since the Voyager encounters. The plume apparently derives from the distal end of lava flows issuing from a fissure, and these flows had grown in length since Voyager. The plume either moved along with the flow, or another plume developed in a different place along the flow (Fig. 6.5). We see that not only can activity occur continuously, but it can also move around contributing further to the surface changes on Io [318].

Another continuous plume is one associated with Pele, having been first observed by Voyager 1. Due to its consistent high-temperature thermal emission, giant plume, and enormous red ring, Pele is one of Io's most distinctive plumes (Fig. 6.6). The Hubble Space Telescope detected plumes here in 1998, 1996, and

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