SO2 spatial distribution and processes

The Galileo NIMS experiment obtained low spatial resolution (120-350 km per pixel) spectral maps on global scales. SO2 is found everywhere, except in hot volcanic areas (Carlson et al., 1997; Doute et al., 2001). The fractional coverage and mean grain size of SO2 frost, assumed to be linearly mixed with a spectrally neutral component (e.g., sulfur), has been mapped by Doute et al. (2001) using numerous NIMS data sets. These two properties provide clues about the physical state of solid SO2 which depends on the relative rates of SO2 condensation, metamorphism, and sublimation (Doute et al., 2001, 2002, 2004). The fractional coverage and mean grain size are combined into a spectral unit map (Doute etal., 2001; Figure 9.8) and can be discussed in terms of four units: I, rich in SO2 with fine-grained frost; II, rich in SO2 with coarsegrained frost; III, depleted in SO2, fine-grained frost; and IV, depleted in SO2, coarsegrained frost.

A layer of relatively fresh SO2 frost (Unit I) covers 50-70% of large areas mostly located at medium and high latitudes (30°-60°) and generally devoid of permanent hot spots. At the margins of these areas, the SO2 coverage remains high but the mean grain size increases substantially, suggesting metamorphism. These SO2-rich regions show a correlation with active plumes, the plumes generally being on the same meridian but at a lower latitude. One exception is Prometheus, the closest field of abundant SO2 being to the east, in Bosphorus Regio (100°-150° of longitude).

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Figure 9.7. Spectrum of Io and equivalent-width maps. Maps of the absorption strength (equivalent width) of the 1.98-^m S02 band {top) and the 3.15- ^m band (middle) are shown at the right, with black signifying more absorption. Note the strong equatorial enhancement of the unknown 3.15- ^m absorber (possibly H20) and its correlation with both the weak, long-path-length S02 feature and the bright deposits in the Io reference map (bottom). (See also color section.)

Nonetheless all these regions likely represent condensation areas of S02 gas migrating from the plumes. The plumes consist of gas and particles that rise to altitudes of tens to hundreds of kilometers before collapsing back and striking the surface at supersonic speeds, generating shock waves and high pressures (Zhang et al., 2003). The pressure increase causes partial dynamic condensation of S02, often forming rings of fallout material. The remaining gas contributes to a net longitudinal flux of volcanic S02 that flows toward medium and high latitudes (Doute et al., 2001,2002; Moreno et al., 1991; Wong and Johnson, 1996a, 1996b). The gas condenses where the average temperatures are sufficiently low (—110 K).

For a given condensation field associated with a plume, the degree of metamorph-ism will depend on the condensation rate compared with the metamorphism timescale. The green (Unit I) regions associated with Pele-Pillan, Marduk, Amirani-Maui, and Prometheus may exhibit the most recent activity and the highest S02 emission rate. Culann, Volund, and Zamama appear to be less active.

The regions depleted in S02 (S02 surface coverage <35%, Units III and IV) represent approximately 60% of the total surface area observed by NIMS. The Jupiter-facing quadrant of the trailing hemisphere contains many of these

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Figure 9.8. Sulfur dioxide spectral unit map. The plumes (P) are sources of fine-grained S02 frost (Unit I, green; see color section) deposits, generally poleward of the low-latitude plumes. Hot spot locations are denoted with stars and crosses, with stars being long-lived hot spots and crosses denoting sporadic thermal features. Metamorphosed S02 snowfields (Unit II) are shown as light green and yellow. S02-poor areas (Units III, IV) occur in the 270 to >360°W longitude region.

S02-poor regions, a finding consistent with Nelson et al.'s (1980, 1987) measurements of the longitudinal distribution of S02, noted previously. The S02-frost depleted regions contain many hot spots and plumes. Recent deposition of hot pyroclastic flow materials from nearby volcanic centers and/or mean temperatures above the stability point of S02 in the range «110-200 K (Doute et al., 2002) may prevent the formation of significant frost deposits. Isum and Mulungu are hot spots centered within an extended area displaying remarkably low S02 frost coverage. These hot spots lack plumes to provide S02 and the regions may also possess higher mean temperatures than elsewhere, resulting in low S02 coverage. Marduk and Zamama, in contrast, are located in regions showing high S02 abundance. This suggests that the condensation/ sublimation ratio is above the average of these thermally active regions. A higher S02 production rate and/or lower regional temperatures could provide an explanation.

0ptically thick but patchy frost deposits lie near the equator of the anti-Jovian hemisphere and are characterized by medium frost coverage (35-50%) and by coarse grains (300-500 ^m), as indicated in the 1.98-^m map discussed above (Figure 9.7). The condensation of volcanic S02 can occur during night-time, but under sunlight equatorial frost sublimates. The sublimational atmosphere creates a high-pressure zone that prevents gaseous S02 from spreading from the plumes to the equator. Io's equatorial temperatures are often close to the S02 instability point, favoring meta-morphism of the frost and perhaps distillation of S02 by repeated sublimation and condensation. Some equatorial regions (e.g., Bosphorus Regio) display an S02 areal abundance that exceeds the usual background level of 45% (Doute et al., 2002). This enhanced concentration may be due to either an intense incoming flux of gas coming from a neighboring plume (dynamic condensation) or a negative thermal anomaly (~110K) causing cold trapping.

Mechanisms controlling the emission of S02 and other compounds from different types of volcanoes, and how these products evolve, can be derived from regional-scale observations at high spatial resolution (Doute et al., 2002; Doute and Schmitt, 2003; Lopes-Gautier et al., 2000; Lopes et al., 2004; Williams et al., 2002). Persistent hot spots such as Prometheus, Culann, Surya, and Tupan are thought to emit a great variety of gases, some of which will condense at Io's surface near their source regions. Associated fields of freshly condensed S02 are easily observed, and deposits of more refractory compounds with higher (e.g., S8) or lower (e.g., NaCl) molecular weight may also be present (although their exact nature is unknown). Three different mechanisms of emission are proposed for the volatile compounds, and supported by the distribution maps. These are (a) the interaction between flowing lava and pre-existing volatile deposits on the surface, (b) the direct degassing from the lava, and (c) the eruption of a liquid aquifer from underground.

The geometric elongation of Prometheus's S02 deposition ring coupled with higher S02 concentration values within its eastern part is the best illustration of mechanism (a). Temporal development of a 95 km long lava field displaced the sublimation front, and thus also the plume and its associated circular ring of deposition.

Amirani also emits a large amount of S02 gas, perhaps by a similar interaction of fresh lava with the volatiles of the underlying plains. Nevertheless, S02 frost is not the major component of the bright white ring surrounding Amirani and seen in visible images. The eruption style is presumably different with the white compounds being degassed from the lava at a single vent (mechanism (b)) and S02 being principally sublimated along the numerous active boundaries of the Amirani flow (mechanism (a)). Mechanism (b) may operate for some Pillanian eruptions like the Thor eruption that occurred during the summer of 2001 (Lopes et al., 2004) and that created a 800 km diameter white ring of fallout partly composed of solid S02.

Mechanism (c) may have been operative inside a small caldera to the east of Chaac and on the north-western flank of the volcanic edifice Emakong. These areas exhibit an extremely deep S02 absorption that is indicative of abundant, pure, and perhaps icy S02 deposits. The S02 is topographically confined by the caldera walls, suggesting sapping or an eruption of an S02 liquid aquifer (Smythe et al., 2000, Lopes et al., 2001).

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