Io’s Atmosphere Silhouetted by Jupiter Lyα

Abstract

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 Lyα wavelength of 121.6 nm. Jupiter’s bright Lyα 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 ∼1 occurs. This method of measuring column densities along lines of sight above the limb uses detailed image simulations and complements Lyα 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 Lyα 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.