Figure 7.9. Low-resolution NIMS hot spot image {inset), with white arrows showing the correlation of the I27D hot spot of Lopes et at. (2001) with the bright flow field of Tsui Goab Fluctus in the Culann-Tohil region as imaged by the SSI during October 2001. This is the only location of potentially active, primary sulfur effusive volcanism detected during the Gatiteo mission. (See also color section.)

suggested it was a likely site for sulfur volcanism (Pieri et at., 1984), and if the correlation between bright yellow materials and sulfur holds true, then the 19941995 event at Ra may be an example of an explosion-dominated style eruption including sulfur flows. However, no repetition of such an event has since been detected, either by Gatiteo, HST, or Earth-based telescopes.

An ^290 km long, yellow and white-gray flow extends north-east from the dark caldera-like Emakong Patera, which Williams et at. (2001b) suggested might be part of a large primary or secondary sulfur flow field making up the Bosphorus Regio area of Io. Although the colors of the Emakong flows match those of sulfur that has undergone radiation exposure (e.g., Nash, 1987), and the flow is fed by a dark curvilinear channel (consistent with hot sulfur), no surface changes were detected at Emakong during the Gatiteo mission. The best evidence for active sulfur volcanism occurred during the February 2000 fly-by, when NIMS detected a weak hot spot at Tsui Goab Fluctus (Figure 7.9), a bright flow field adjacent to an apparently inactive small shield volcano in the Culann-Tohil region (Williams et at., 2004). The temperature measured by NIMS (~260±95°C) falls with the range of molten sulfur, and there is no indication of any adjacent silicate volcanic activity. However, there was no evidence of surface changes in Tsui Goab Fluctus after the February 2000 event (SSI

Possible Sites of Effusive Sulfur Dioxide Volcanism Balder Patera

Tohil Patera

Figure 7.10. Galileo SSI images showing possible sites of effusive S02 volcanism on Io. (left) Balder Patera in the Chaac-Camaxtli region (Williams et al., 2002), site of a proposed glaciallike flow (Smythe et al., 2000). (right) Tohil Patera in the Culann-Tohil region (Williams et al., 2004), the south-west section of which has an enhanced S02 signature and flow-like margins in its interior. (See also color section.)

coverage was of low-resolution), so if fresh sulfur flows were emplaced, they did not cover any new terrain.

Evidence for effusive S02 volcanism is scant; most surface changes that show variations in S02 content resolvable by NIMS are in the form of regional variations in the plains (Doute et al., 2001, 2002, 2004), which are likely due to redistribution and/ or recrystallization of explosively emplaced S02 snow produced by freezing of volcanic gases (Carlson et al., 1997). However, NIMS detected a strong signature of S02 confined to the floor of Balder Patera in the Chaac-Camaxtli region (Williams et al., 2002), which SSI shows to have a homogeneous white-colored patera floor (Figure 7.10). It is unclear why the floor should be so enriched in S02 relative to the surrounding plains. Smythe et al. (2000) proposed that an S02 glacial-like flow may have erupted and flooded the patera floor. Although the dynamics of such a flow have not yet been explored, mapping in the Culann-Tohil region has detected another region of possible effusive S02 material. The south-east section of Tohil Patera contains a white material in which high-resolution SSI images show has apparent flow margins (Williams et al., 2004); NIMS indicates that this region also has a signature of enhanced S02, although not as abundant as that at Balder Patera. Although these images are intriguing, additional assessment of the potential for S02 flows must await further study.

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