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[74.07] A review of HST/FOS NUV spectra of Io obtained on 1994 and 1996

K-L Jessup, G.E. Ballester, J.T. Clarke (Univ. of Michigan), X. Zhu, D. Strobel (JHU), N. Schneider (Univ. of Colorado), M. McGrath (STScI), M. Combi (Univ. of Michigan), J. Ajello (JPL)

We present an overview of the analysis and detailed modeling of Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) near ultraviolet reflectance spectra of Io obtained on June 1994 and October 1996. The observations record temporal changes in SO₂ abundance on Io’s leading and trailing hemispheres, as well as over a 1.5 hour period initiated at eclipse egress. These variations include a dramatic change in the level of gas absorption and frost reflectance ~1.3 hrs after eclipse egress, as well as significant differences between the leading and trailing hemispheres. As reported previously (Jessup et al. 1997), Io’s albedo at east elongation was ~20% darker than that observed at west elongation in 1994, but in 1996 the east elongation albedo was only ~5% darker than at west elongation. Gas absorption at east elongation was greater than on the west during both the 1994 and 1996 observing campaigns. The relative differences in the gas absorption observed in these spectra are opposite to those observed by Trafton et al.(1996).

We present SO and SO₂ column abundances fitted to each dataset based on an extensive exploratory study of the effects of volcanic SO and SO₂ output, atmospheric and volcanic temperature, and the degree to which any atmospheric SO or SO₂ component is confined or extended. The temperature dependence of the SO₂ absorption column is determined from high resolution laboratory data taken by Freeman et al. (1984) at 213 K and Smith et al. (1998) at 300 K. As reported previously (Jessup et al. 1998), the fits to several of the absorption features are improved when a column of SO is added to the model atmosphere. However, we cannot derive the temperature dependence of the SO absorption cross section from the available laboratory measurements. This prevents us from making absolute statements regarding the contribution of gaseous SO to Io’s reflectance spectrum. The sensitivity of Io’s reflectance spectrum to temperature requires that more high resolution data be taken of the SO and SO₂ absorption cross sections at low and high temperatures.

This research has been funded by NASA Grant NGT5-50154

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