A Patchy CO_2 Exosphere on Ganymede Revealed by the James Webb Space Telescope

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