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Figure 8.8. (a) Voyager image of the brightness of the Prometheus plume (from Strom and Schneider, 1982). Note the general sickle shape of the contours and the presence of a small signal near the surface on the left. (b) DSMC simulated number density contours (normalized by 5 x 1016m~3) of gas with a surface temperature of 108 K on the left and 106 K on the right. Select gas flow streamlines are also shown. (c) Normalized column density contours of 1-nm particles entrained in the gas flow. Notice the low altitude "dust cloud'' circled on the left reflecting a settling time through the local atmosphere under the canopy of ~ 1,200 s. No cloud is seen on the right reflecting a settling time of only a couple of hundred seconds there. (See also color section.)

Figure 8.8. (a) Voyager image of the brightness of the Prometheus plume (from Strom and Schneider, 1982). Note the general sickle shape of the contours and the presence of a small signal near the surface on the left. (b) DSMC simulated number density contours (normalized by 5 x 1016m~3) of gas with a surface temperature of 108 K on the left and 106 K on the right. Select gas flow streamlines are also shown. (c) Normalized column density contours of 1-nm particles entrained in the gas flow. Notice the low altitude "dust cloud'' circled on the left reflecting a settling time through the local atmosphere under the canopy of ~ 1,200 s. No cloud is seen on the right reflecting a settling time of only a couple of hundred seconds there. (See also color section.)

on a timescale of weeks, obscuring a large portion of Pele's red ring, Three years later, most of Pillan's pyroclastic deposits had vanished beneath a renewed red ring from Pele and bright frost from nearby Kaminari, Do the plume deposits simply coat the surrounding surfaces, or do they bury them? Estimates of the deposition rates of plume sediments vary greatly, ranging from 103 kg s-1 for Pele (Spencer et al., 1997) to 107kgs-1 for both Pele and Pillan (Cataldo et al., 2002), A globally averaged resurfacing rate of ~1mmyr_1 is required in order to account for the erasure of impact craters on Io (Spencer and Schneider, 1996), equivalent to the emplacement of 1,000m3 s-1, Plume deposit thicknesses of order 5mmyr-1 are needed to account for this resurfacing, given that roughly one-fifth of Io's surface was coated by plume fallout during the Galileo mission, 0nly massive plumes, with dust production rates of order 105kgs-1, could produce such deposition, Much of the resurfacing could instead be accomplished by the eruption of silicate lava flows: Amirani alone is estimated to have erupted 50-500 m3 s-1 of new lava flows between subsequent Galileo observations, assuming that the flows were 1-10m thick (Keszthelyi et al., 2001), However, silicate lava flows cover only a small fraction of the surface, and the most powerful volcanic upheavals are confined to lava lakes like Loki, In contrast, plume deposits mantle areas that are hundreds to thousands of times greater than the lava flows and patera that produced them, This redistribution of material may account for the apparent absence of flooded and partially destroyed impact craters on Io,

There is no doubt about the significance of the contribution of volcanic plumes to Io's atmosphere, Voyager 1 first identified S02 in Loki's plume that was attributed to volcanic outgassing (Pearl et al,, 1979), The earliest Earth-based detections of atmospheric S02 and S0 showed spectral broadening and Doppler shifting that was ascribed to volcanic plumes (Lellouch et al., 1990, 1992, 1996), As pointed out in Chapter 10, gases such as NaCl and S2 that have negligible vapor pressure at Io's surface temperature require volcanic sources, The patchy nature of the tenuous atmosphere can best be explained by a combination of sublimation and volcanic venting (Ingersoll, 1989; Lellouch 1996; see also Chapter 10),

0nce lofted into the atmosphere, the gas and dust ejected from plumes can be ionized by sunlight and impacting charged particles, swept away by the Jovian magnetic field, and spread throughout the Jovian system, Variations in the composition and mass of the neutral clouds and plasma torus are believed to be caused by volcanic eruptions on Io (Brown and Bouchez, 1997), Moses et al, (2002a) suggest that measurements of the S: 0 ratio in the plasma torus may be an effective means of remotely monitoring giant plume eruptions on Io, Correlations between the flux of ^10-nm dust particles recorded by the Galileo Dust Detector and the record of giant plume eruptions on Io during the 5-year tenure of the spacecraft indicate that ejection of dust from the most energetic plumes is chiefly responsible for the dust streams emanating from Io into interplanetary space (Kruger et al,, 2003),

The electrical currents that connect Io to Jupiter directly impinge on the plumes near the sub-Jupiter and anti-Jupiter points, Io is an effective electrical generator, powered by the magnetic field of Jupiter as it sweeps past the conducting satellite (Chapter 11), Part of Io's conductivity is through its ionosphere, but the conduction of current into the interior of Io through plumes is an intriguing possibility (Gold, 1979). Spokes and filaments have been seen in Prometheus that appear similar to plasma-arc discharges observed in the laboratory (Peratt and Dessler, 1988), although they could instead result from multiple sources along the lava flow. Powerful currents could produce interesting disequilibrium chemical reactions within the plumes and possibly heat the surface of the satellite near the plume sources. These effects would be local, however, as the power generated globally by electrical induction is two orders of magnitude less than that derived from tidal heating.

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