Exospheric O2 densities at Europa during different orbital phases

Europaʹs exosphere is a mixture of different species among which sputtered H2O and H2 dominate in the highest altitudes and O2, formed mainly by radiolysis of ice and subsequent release of the produced molecules, prevails at lower altitudes. Europaʹs O2 exosphere has been demonstrated through both observation and simulation-based techniques to be spatially non-uniform. In the present study we investigate Europaʹs exospheric O2 characteristics under the external conditions that are likely in the Jupiterʹs magnetospheric environment, applying the Europa Global model of Exospheric Outgoing Neutrals (EGEON, Plainaki et al., 2012) for different configurations between the positions of Europa, Jupiter and the Sun. We demonstrate for the first time that the spatial distribution of Europaʹs exosphere is explicitly time-variable due to the time-varying relative orientations of solar illumination and the incident plasma direction. We show that the O2 release efficiency depends both on solar illumination and plasma impact direction. The modeled densities at different orbital phases of Europa are compared, a posteriori, with the analysis results from two observations in order to validate the model. Using the outputs of EGEON we also calculate the supply of neutral oxygen atoms of exospheric origin to Europaʹs neutral cloud.