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Dramatic changes in Jupiter's ultraviolet spectrum were recorded with the Hubble Space Telescope Faint Object Spectrograph (FOS) following the impact of comet Shoemaker-Levy 9 fragment G on 18 July 1994. A ratio of the 18 July FOS spectrum, taken 3 hours after the G impact, to one obtained on 14 July, before the impacts began, effectively removes the solar spectrum and normal jovian absorptions, as well as systematic instrumental effects. This ratio shows strong absorption bands of S$_2$, CS$_2$, and NH$_3$, as well as evidence for H$_2$S in the form of continuous absorption peaked near 210nm. These observations represent the first detections of S$_2$ and CS$_2$ in the Jupiter atmosphere, and only the second detection of S$_2$ in any astronomical object. The estimated column abundance of S$_2$, {\cal N}$\sim 10^{19}$ cm$^{-2}$, corresponds to a mass of sulfur of $m \sim 10^{14}$g in our 0.9 arcsec field of view. This lower limit is significantly greater than the mass of sulfur expected to be present in a 1 km diameter impactor. There is no evidence for oxygen-bearing molecules, particulary SO$_2$ and SO, in the 18 July spectrum, which is surprising given the high abundance of sulfur. A second post-impact FOS spectrum was obtained near the limb of Jupiter on 21 July, approximately 30 minutes after the impact of fragment S. In addition to the presence of all molecules identified in the 18 July spectrum, we identify strong, narrow emission lines from MgII, MgI, SiI, FeI, and CS. Lower limits for the abundances of the metal atoms are about 1 part in 10$^6$ of a 10$^{14}$ g impactor, consistent with expected abundances of these elements in the impactors. FOS spectra obtained on 9 August and 23 August show strong temporal evolution of the G impact site, with the disappearance of S$_2$ and an increase in NH$_3$ relative to the CS$_2$.
\noindent This work is supported by NASA through grant GO-5376.01-93A from STScI, which is operated by AURA under NASA contract NAS5-26555.
