Thermal Outgassing and Sublimation from the Porous Regoliths of Europa, Ganymede, and Callisto at Depth

Recently, the Juno spacecraft’s JADE and MAG instruments have revealed the thermal nature of Europa’s atmospheric source, long predicted for O₂ also at Ganymede. This implies the icy Galilean moon atmospheres are directly regulated by solar heating and coupled to their surface ice temperatures. However, the precise thermal desorption mechanism of O₂ and H₂ is not yet simulated numerically in exosphere general models to date, as a more detailed surface-atmosphere coupling is required to treat the thermally evolving and porous Galilean satellite surfaces. In this light, we couple a multi-layered implicit thermal solver MultIHeaTS to a volatile evolution model to predict the diurnal evolution of H₂O sublimation and O₂ thermal- desorption fluxes through the upper ∼meter of icy Galilean satellite regoliths. Radiolytic O₂ release is modelled with an Arrhenius law following recent thermal desorption analyses, constraining the activation energy for O₂-H from observations of trapped O₂ bubbles and variability inferred in the surface and atmosphere. Several depths are considered from ~ 1 mm to 1 m, with results estimating outgassing fluxes at depth and at the surface, as a lower limit. Porosity strongly affects the net outgassing flux and should therefore be included in new numerical investigations on the near-surface atmospheres of the Galilean satellites. The outgassing model serves as a benchmark for Saturnian satellites as well as more generally, icy bodies.