Spatially resolved spectroscopy of Io's Pele plume and SO2 atmosphere

We report on the first successful, spatially resolved spectroscopic observa tions of Io's SO2 atmosphere. Observations made with the Hubble Space Teles cope Faint Object Spectrograph on 1 August 1996 using the 0.26 " aperture h ave provided detections of SO2 gas in absorption in three locations on the Io disk: the Pele volcano, the Ra volcano, and T3 (a control region at lati tude 45 degrees S, longitude 300 degrees W). The column densities of SO2 at these three locations have been determined by best-fit models of the geome tric albedo and vary by a factor of five, with N-SO2 = 3.25 x 10(16), 1.5 x 10(16), and 7 x 10(15) cm(-2) for Pele, Ra, and T3, respectively. Thus, SO 2 gas is found to be present, and collisionally "thick" (N greater than or similar to 6 x 10(14) cm(-2)), in all three locations. The factor of five d ifference in column densities among the three targets provides the first di rect evidence that the Io atmosphere is spatially inhomogeneous. Models of the SO2 gas band absorption at different temperatures give best-fit models with temperatures of T = 280 (Pele), 150 (Ra), and 200 K (T3). Addition of SO to the models in the amounts N-SO similar to 2.5 x 1015 (Pele), 5 x 10(1 4) (Ra), and 1.5 x 10(15) cm(-2) (T3) provides improved (chi(2)) fits to th e data for all three locations and gives reasonably good agreement with the previous detection of SO in the Io atmosphere at an abundance similar to 0 .1 times that of SO2. We set an upper limit of 2 x 10(14) cm(-2) on the abu ndance of CS2. Observations with the I-IST WFPC2 obtained on 24 July 1996, 7 days earlier than our FOS spectra, showed an active plume over the Pele v olcano. If Pele was still active on 1 August, our results imply that the re gions of highest SO2 gas density on Io may be associated with active volcan ic plumes and not sublimation from the visibly bright SO2 frost patches com mon on the surface of the satellite. As a result of the positive detection of atomic sulfur emission from the SI] 1900 Angstrom multiplet from two of our three targets (similar to 3.6 kR, Pele; similar to 1.6 WR, T3; <1.5 kR, Ra) our spectra also provide the first concurrent measurements of S, SO, a nd SO2 gases in the atmosphere and give a ratio of S/SO2 abundance of simil ar to 0.003-0.007. The SO2 distribution we observe falls off much more slow ly with latitude than the best available sublimation atmosphere models, but matches well the latitudinal distribution of the more realistic sublimatio n-driven atmosphere models that include hydrodynamic flow and photochemistr y. (C) 2000 Academic Press.