The spatial morphology of Europaʹs near-surface O2 atmosphere

Results from a three-dimensional ballistic model of Europaʹs O2 atmosphere are presented. Hubble Space Telescope (HST) ultraviolet observations show spatially non-uniform O2 airglow from Europa. One explanation for this is that the O2 atmosphere is spatially non-uniform. We show that non-uniform ejection of O2 alone cannot reproduce the required morphology, but that a non-uniform distribution of reactive species in Europaʹs porous regolith can result in a non-uniform O2 atmosphere. By allowing O2 molecules to react with Europaʹs visibly dark surface material, we produced a spatially non-uniform atmosphere which, assuming uniform electron excitation of O2 over the trailing hemisphere, compares favorably with the morphology suggested by the HST observations. This model, which requires a larger source of O2 than has previously been estimated, can in principal be tested by the New Horizons observations of Europaʹs O2 atmosphere.