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J. R. Spencer, K. L. Jessup (Lowell Obs.), G. E. Ballester (LPL, U. Arizona), E. Lellouch (Obs. Paris Meudon), M. Lopez-Valverde (Inst. Astrofis. Andalucia), M. Richter (U.C. Davis)
Io's SO2 atmosphere appears to be globally asymmetric, being up to ten times denser on the anti-Jupiter hemisphere. At least on that hemisphere, the atmosphere is probably dominated by sublimation of surface frost rather than direct volcanic supply. Evidence for this picture includes: (i) The concentration of the atmosphere at low latitudes, where frost temperatures are expected to be highest, as previously inferred from Lyman-alpha images of Io (Feldman et al. 2000, GRL 27, 1787); (ii) Our new HST UV long-slit spectra, which show that the anti-Jovian SO2 column density varies smoothly with latitude and/or local time, peaking near the subsolar point, and is not strongly concentrated over the Prometheus plume or other volcanos (Jessup et al. 2003, Icarus, submitted); (iii) The tenfold larger equatorial SO2 column density seen on the anti-Jupiter hemisphere in our new spectra, compared to previous disk-resolved UV spectroscopy of the Jupiter-facing side (McGrath et al. 2000, Icarus 146, 476); (iv) A correlation between longitudinal variations in the strength of 19 \mum SO2 gas absorption lines and the global distribution of optically-thick SO2 frost seen in the near-UV, with both peaking on the anti-Jupiter hemisphere; and (v) a decrease in anti-Jovian 19 \mum line strength between Nov.\ 2001 and Dec.\ 2002, quantitatively consistent with the expected reduction in SO2 vapor pressure due to frost cooling as Io and Jupiter approach aphelion.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.