Europa's tenuous atmosphere

Europa's tenuous atmosphere was discovered by HST/GHRS in atomic oxygen 1304 and 1356 Ang emissions that were inferred to be produced by Jovian plasma electrons dissociatively exciting molecular oxygen. Molecular oxygen is expected to be the dominant end product of a water-derived atmosphere and to be uniformly distributed around the satellite. The properties and spatial distribution of Europa's far-UV emissions are diagnostic for the specific characteristics of Europa's magnetospheric interactions and atmosphere. Although Europa's plasma interaction was expected to resemble Io's, recent published HST/STIS studies of a time-average image of Europa's far-UV emissions have yielded unexpected, puzzling results. The time-averaged emissions appear enhanced around a specific area, and if true, the molecular oxygen atmosphere would be highly asymmetric as well as associated with a particular surface region on Europa. Therefore, many fundamental questions regarding the nature of Europa's thin, water-derived atmosphere, and of its plasma interactions, induced magnetic field, and emission excitation processes still remain, and new questions are furthermore being posed. In the present work we are addressing these questions by analyzing the same set of HST/STIS far-UV images of Europa concentrating on the temporal variations of the emissions that were not reported in the literature. We will show how the derived distribution of Europa's emissions and variations with magnetic System III longitude and plasma conditions do have similar properties to those of Io, but they also have an additional "twist". We will compare our findings to the previously published results, and to emission distributions predicted from published models of the Europa's magnetospheric interactions. We will then explore what effect may make Europa's interactions so unique.