HST/FOS Observations of Io in the Near Ultraviolet

K.L. Jessup, G.E. Ballester, J.T. Clarke (University of Michigan), D.F. Strobel (John Hopkins University), N. Schneider (University Colorado), M. McGrath (Space Telescope Institute), M. Combi (University of Michigan), J. Ajello (JPL), J.L. Luhmann (IGPP, UCLA), X. Zhu (John Hopkins University)

We present initial results arising from the comparison of four

sets of Hubble Space Telescope/Faint Object Spectrograph near

ultraviolet observations obtained in the four year period from

August 1993 until October 1996. As an initial diagnositc, we have

obtained the ratios of: small apeture (0.3") pre-COSTAR FOS/BL

spectra of Pele and a known bright region, obtained on August 1993;

disk integrated spectra of Io immediately after eclipse egress and 1

hour post egress obtained on October 1996 with FOS/RD; and disk

integrated spectra of Io at eastern and western elongation obtained on

1994 with FOS/BL and again in October 1996 with FOS/RD. With these

ratios, we have indentified the relative differences in the SO2

absorption signature within each individual data set, and temporally

between each data set. Future work on these spectra will include

careful evalutation and modeling of the geometric albedoes associated

with each spectra. Comprehensive analysis and comparision of these

results should provide constraints useful in separating the sublimation

and volcanic components of Io's SO2 atmosphere, identifying the

dominant source of SO2 gas and determining limits of the abundance

and spatial distribution of the SO2 atmospheric component.

This research has been supported by the STScI grants GO-5438 and

GO-6786, and by the Univeristy of Michigan.