The spatial morphology of Europa's near-surface O ₂ atmosphere

Abstract

Results from a three-dimensional ballistic model of Europa's O ₂ atmosphere are presented. Hubble Space Telescope (HST) ultraviolet observations show spatially non-uniform O ₂ airglow from Europa. One explanation for this is that the O ₂ atmosphere is spatially non-uniform. We show that non-uniform ejection of O ₂ alone cannot reproduce the required morphology, but that a non-uniform distribution of reactive species in Europa's porous regolith can result in a non-uniform O ₂ atmosphere. By allowing O ₂ molecules to react with Europa's visibly dark surface material, we produced a spatially non-uniform atmosphere which, assuming uniform electron excitation of O ₂ over the trailing hemisphere, compares favorably with the morphology suggested by the HST observations. This model, which requires a larger source of O ₂ than has previously been estimated, can in principal be tested by the New Horizons observations of Europa's O ₂ atmosphere.