The atmospheric signature of Io's Prometheus plume and anti-jovian hemisphere: evidence for a sublimation atmosphere

Using the Hubble Space Telescope's Space Telescope Imaging Spectrograph we have obtained for the first time spatially resolved 2000-3000 Angstrom spectra of Io's Prometheus plume and adjoining regions on Io's anti-jovian hemisphere in the latitude range 60degrees N-60degrees S, using a 0.1 slit centered on Prometheus and tilted roughly 45degrees to the spin axis. The SO2 column density peaked at 1.25 x 10(17) cm(-2) near the equator, with an additional 5 x 10(16) cm(-2) enhancement over Prometheus corresponding to a model volcanic SO2 output of 10(5) kg s(-1). Apart from the Prometheus peak, the SO2 column density dropped fairly smoothly away from the subsolar point, even over regions that included potential volcanic sources. At latitudes less than +/-30degrees, the dropoff rate was consistent with control by vapor pressure equilibrium with surface frost with subsolar temperature 117.3 +/- 0.6 K, though SO2 abundance was higher than predicted by vapor pressure control at mid-latitudes, especially in the northern hemisphere. We conclude that, at least at low latitudes on the anti-jovian hemisphere where there are extensive deposits of optically-thick SO2 frost, the atmosphere is probably primarily supported by sublimation of surface frost. Although the 45degrees tilt of our slit prevents us from separating the dependence of atmospheric density on solar zenith angle from its dependence on latitude, the pattern is consistent with a sublimation atmosphere regardless of which parameter is the dominant control. The observed drop in gas abundance towards higher latitudes is consistent with the interpretation of previous Lyman alpha images of Io as indicating an atmosphere concentrated at low latitudes. Comparison with previous disk-resolved UV spectroscopy, Lyman-alpha images, and mid-infrared spectroscopy suggests that Io's atmosphere is denser and more widespread on the anti-jovian hemisphere than at other longitudes. SO2 gas temperatures were in the range of 150-250 K over the majority of the anti-jovian hemisphere, consistent with previous observations. SO was not definitively detected in our spectra, with upper limits to the SO/SO2 ratio in the range 1-10%, roughly consistent with previous observations. S-2 gas was not seen anywhere, with an upper limit of 7.5 x 10(14) cm(-2) for the Prometheus plume, confirming that this plume is significantly poorer in S-2 than the Pele plume (S-2/SO2 < 0.005, compared to 0.08-0.3 at Pele). In addition to the gas absorption signatures, we have observed continuum emission in the near ultraviolet (near 2800 Angstrom) for the first time. The brightness of the observed emission was directly correlated with the SO2 abundance, strongly peaking in the equatorial region over Prometheus. Emission brightness was modestly anti-correlated with the jovian magnetic latitude, decreasing when Io intersected the torus centrifugal equator. (C) 2003 Elsevier Inc. All rights reserved.