S

JÈu i- m smaller plumes produce circular rings averaging 150-200km in radius that are yellow or white in color. Evidence indicates that these plumes are the most numerous ones and normally coat their surroundings with frosts of fine-grained SO2. The repeated eruptions of these smaller plumes are most likely significant contributors to Io's resurfacing rate. Larger plumes are much less numerous, but produce orange or red sulfur rich rings, oval in shape, that typically range in diameter from 500 to 550 km (Fig. 6.3). The larger plumes probably account for most of the dust ejection. Both types of plumes can occur as a single episode, or can be somewhat continuous over a period of time [310]. It is theorized that the two classes of plumes are driven by different volatiles. The smaller plumes are powered by the explosive volatilization of SO2. These plumes contain silicate ash and or sulfur compounds that cause the discoloration of the resulting deposits. The larger plumes probably contain significant quantities of sulfur, evidenced by extensive red or orange £ deposits [311]. According to Harland, Susan Kieffer's comprehensive analysis and

£ model showed that sulfur dioxide at a depth of several kilometers would be heated c to 1,400 K upon direct contact with silicate lava. Once boiled under pressure, the

O gaseous sulfur dioxide would force open pre-existing cracks in the crust and vent

•> © explosively upon reaching the surface [312].

,0 Galileo observed four volcanic eruptions that were described as explosive [313].

q © Some of the larger plumes can be truly spectacular. A plume seen by the Galileo

O spacecraft over the volcano Tvashtar in late 2000, rose ~400 km high [314] (Fig. 6.4)!

Galileo later detected an even larger plume that rose ~500 km over a feature provisionally named Thor. Apparently these plumes rise so high because of the intensity of their eruption and Io's low surface gravity. The Galileo spacecraft may have actually flown through the plume of Thor, since the Galileo Plasma Subsystem detected SO2 near closest approach at this time [315]. Due to the great height of these plumes,

Fig. 6.3. Arizona-sized Io eruption. The right frame was captured by the Galileo spacecraft five months after it captured the left one. During this time a new dark spot 400 km in diameter, roughly the size of Arizona, developed in the right half of the frame around a valcanic center named Pillan Patera. (Credit: NASA/JPL/University of Arizon/PIRL)

Fig. 6.3. Arizona-sized Io eruption. The right frame was captured by the Galileo spacecraft five months after it captured the left one. During this time a new dark spot 400 km in diameter, roughly the size of Arizona, developed in the right half of the frame around a valcanic center named Pillan Patera. (Credit: NASA/JPL/University of Arizon/PIRL)

~~ Tvashtar Catena 125 {26 Nov 1999) 127 (22 Feb 2000)

+ C21 low-resolution color visible wavelength data + fire fountain sketch + IR data of active lava flow

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