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

A planet’s Ly α emission is sensitive to its thermospheric structure. Here we report joint Hubble Space Telescope and Cassini cross-calibration observations of the Saturn Ly α emission made 2 weeks before the Cassini grand finale. To investigate the long-term Saturn Ly α airglow observed by different ultraviolet instruments, we cross-correlate their calibration, finding that while the official Cassini/UVIS sensitivity should be lowered by ∼75%, the Voyager 1/UVS sensitivities should be enhanced by ∼20% at the Ly α channels. This comparison also allowed us to discover a permanent feature of the Saturn disk Ly α brightness that appears at all longitudes as a brightness excess (Ly α bulge) of ∼30% (∼12 σ ) extending over the latitude range ∼5°–35° N compared to the regions at equator and ∼60° N. This feature is confirmed by three distinct instruments between 1980 and 2017 in the Saturn north hemisphere. To analyze the Ly α observations, we use a radiation transfer model of resonant scattering of solar and interplanetary Ly α 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 α 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 α bulge region, at latitudes between ∼5° and ∼35°, the observed brightening and line broadening support seasonal effects, variation of the temperature vertical profile, and potential superthermal atoms that require confirmation.