Io's Atmosphere Silhouetted by Jupiter Lyα

We report results from a new technique for mapping Io's SO2 vapor distribution. The Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope observed Io during four Jupiter transit events to obtain medium resolution far-UV spectral images near the Ly alpha wavelength of 121.6 nm. Jupiter's bright Ly alpha dayglow provides a bright, mostly uniform background light source for opacity measurements, much like during a stellar occultation or transiting exoplanet event. Peaks in the photoabsorption cross-sections for sulfur dioxide occur near 122 nm, with resulting absorptions raising the altitude where a tangential line-of-sight opacity of similar to 1 occurs. This method of measuring column densities along lines of sight above the limb uses detailed image simulations and complements Ly alpha reflectance imaging and other methods for measuring Io's SO2 gas on the disk. Our reported near-terminator limb observations with STIS confirm the findings from previous Ly alpha disk reflectance imaging that Io's polar SO2 density is an order of magnitude lower than found at the equator. We provide constraints for additional attenuation by atmospheric hydrogen atoms produced by charge exchange reactions between magnetospheric protons and Io's atmosphere. Searches for plume-related features provided no definitive enhancements within the signal quality, ruling out unusually high levels of activity for Pele and Tvashtar.