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[2.05] The Atmospheric Signature of Io's Prometheus Plume and Anti-Jovian Hemisphere: Evidence for a Sublimation Atmosphere

K.L. Jessup, J. Spencer (Lowell Observatory), G. E. Ballester (LPL, U. Arizona), M. Vigel (MIT), R. Howell (Geology and Geophysics, U. Wyoming), F. Roesler (Dept. of Physics, U. Wisconsin), R. Yelle (LPL, U. Arizona)

We obtained limb-to-limb spectra of Io's anti-jovian hemisphere in the 1900-3200 Å/ wavelength region using the Hubble Space Telescope Space Telescope Imaging Spectrograph (HST/STIS) instrument with the 0.1" slit centered over the Prometheus plume and tilted roughly 45^o to the spin axis (Jessup et al. 2003). We report the SO₂, SO, and S₂ column densities, and SO₂ gas temperature, as a function of latitude and terrain type. Peaking at column densities ~1-2x10¹⁷cm^-2 at the equator, the SO₂ atmosphere decreased smoothly in density with increasing distance from the subsolar point; while strong peaks in density were not observed over Prometheus or other volcanic centers. These behaviors strongly imply that Io's anti-jovian SO₂ atmosphere is sublimation driven. The limb-to-limb SO absorption signature was weak and fairly invariable, upper limits of 4 x10¹⁵ and 2 x10¹⁵ cm^-2 were set for the equatorial and mid- to high-latitude regions, respectively. Because of this and the smooth decrease in the SO₂ density, the upper limit of the SO/SO₂ ratio ranged from ~3% at the equator to ~10% at the highest latitudes observed. S₂ gas absorption was not detected, an upper limit of 10¹⁵ cm^-2 was found over the Prometheus region indicating an S₂/SO₂ ratio ~0.1%, which is significantly lower than the 12 - 20 % S₂/SO₂ ratio observed at Pele (Spencer et al. 2000) confirming the plume classifications first suggested by McEwen and Soderblom (1983). In addition to the absorption signatures, continuum emission was detected near 2800 Å/ for the first time. The emission peaked in brightness over the Prometheus plume, similar to the morphology of the visible emissions recently observed on Io (Geissler et al. 2001). Because the brightness of the emission was directly correlated with the SO₂ abundance it is likely that the observed emission is continuum emission resulting from the electron impact on SO₂ gas

The author(s) of this abstract have provided an email address for comments about the abstract: k.l.jessup@lowell.edu

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.