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Arxiv: A patchy CO_2 exosphere on Ganymede revealed by the James Webb Space Telescope Published: 9/20/2024 1:00:39 PM Updated: 9/20/2024 1:00:39 PM

Paper abstract: Jupiter's icy moon Ganymede has a tenuous exosphere produced by sputteringand possibly sublimation of water ice. To date, only atomic hydrogen and oxygenhave been directly detected in this exosphere. Here, we present observations ofGanymede's CO_2 exosphere obtained with the James Webb Space Telescope.CO_2 gas is observed over different terrain types, mainly over those exposedto intense Jovian plasma irradiation, as well as over some bright or darkterrains. Despite warm surface temperatures, the CO_2 abundance overequatorial subsolar regions is low. CO_2 vapor has the highest abundance overthe north polar cap of the leading hemisphere, reaching a surface pressure of 1pbar. From modeling we show that the local enhancement observed near 12 h localtime in this region can be explained by the presence of cold traps enablingCO_2 adsorption. However, whether the release mechanism in this high-latituderegion is sputtering or sublimation remains unclear. The north polar cap of theleading hemisphere also has unique surface-ice properties, probably linked tothe presence of the large atmospheric CO2 excess over this region. These CO2molecules might have been initially released in the atmosphere after theradiolysis of CO_2 precursors, or from the sputtering of CO_2 embedded inthe H_2O ice bedrock. Dark terrains (regiones), more widespread on the northversus south polar regions, possibly harbor CO_2 precursors. CO_2 moleculeswould then be redistributed via cold trapping on ice-rich terrains of the polarcap and be diurnally released and redeposited on these terrains. Ganymede'sCO_2 exosphere highlights the complexity of surface-atmosphere interactionson Jupiter's icy Galilean moons.