The Enigmatic Abundance of Atomic Hydrogen in Saturn's Upper Atmosphere

A planet's Ly alpha emission is sensitive to its thermospheric structure. Here we report joint Hubble Space Telescope and Cassini cross-calibration observations of the Saturn Ly alpha emission made 2 weeks before the Cassini grand finale. To investigate the long-term Saturn Ly alpha airglow observed by different ultraviolet instruments, we cross-correlate their calibration, finding that while the official Cassini/UVIS sensitivity should be lowered by similar to 75%, the Voyager 1/UVS sensitivities should be enhanced by similar to 20% at the Ly alpha channels. This comparison also allowed us to discover a permanent feature of the Saturn disk Ly alpha brightness that appears at all longitudes as a brightness excess (Ly alpha bulge) of similar to 30% (similar to 12 sigma) extending over the latitude range similar to 5 degrees-35 degrees N compared to the regions at equator and similar to 60 degrees N. This feature is confirmed by three distinct instruments between 1980 and 2017 in the Saturn north hemisphere. To analyze the Ly alpha observations, we use a radiation transfer model of resonant scattering of solar and interplanetary Ly alpha photons and a latitude-dependent photochemistry model of the upper atmosphere constrained by occultation and remote-sensing observations. For each latitude, we show that the Ly alpha observations are sensitive to the temperature profile in the upper stratosphere and lower thermosphere, thus providing useful information in a region of the atmosphere that is difficult to probe by other means. In the Saturn Ly alpha bulge region, at latitudes between similar to 5 degrees and similar to 35 degrees, the observed brightening and line broadening support seasonal effects, variation of the temperature vertical profile, and potential superthermal atoms that require confirmation.