Reconstructing Europa's Atmosphere From H2+ Pickup Ions Detected During the Juno PJ45 Flyby

Abstract

We use a suite of single‐species () and multispecies () Direct Simulation Monte Carlo models of Europa's atmosphere to interpret the pickup ions detected by Juno at distances beyond Europa radii during the Perijove 45 flyby. The observations require contributions from both Europa's atmosphere and a background torus; the latter has local densities ranging from on the inbound leg (anti‐Jovian hemisphere) to on the outbound leg (sub‐Jovian hemisphere)—an asymmetry in the torus consistent with recent plasma‐advection modeling. We show that the magnitude and spatial structure of the observed signatures are reproduced to within over along both legs of the flyby by a purely thermal atmosphere embedded within the torus. Best‐fit source rates of yield surface densities of , radial column densities of , and line‐of‐sight column densities at the limb of . Assuming a 2:1 stoichiometric ratio, the implied source rates produce column densities of , slightly less than inferred from remote‐sensing observations. Contrary to initial interpretations, a nonthermal component is not required to reproduce the data. Even when one is included, the atmosphere becomes predominantly thermal through recycling of non‐condensing at the surface—an effect amplified when collisions with the denser population redirect otherwise escaping molecules back to the ice, where they thermalize. Atmospheric escape from Europa supplies Jupiter's magnetosphere with neutral hydrogen and oxygen at roughly equal mass loss rates of .