Much of the geologic activity preserved on Europas icy surface has been attributed to tidal deformation, mainly due to Europas eccentric orbit. Although the surface is geologically young (30 - 80 Myr), there is little information as to whether tidall
y-driven surface processes are ongoing. However, a recent detection of water vapor near Europas south pole suggests that it may be geologically active. Initial observations indicated that Europas plume eruptions are time-variable and may be linked to its tidal cycle. Saturns moon, Enceladus, which shares many similar traits with Europa, displays tidally-modulated plume eruptions, which bolstered this interpretation. However, additional observations of Europa at the same time in its orbit failed to yield a plume detection, casting doubt on the tidal control hypothesis. The purpose of this study is to analyze the timing of plume eruptions within the context of Europas tidal cycle to determine whether such a link exists and examine the inferred similarities and differences between plume activity on Europa and Enceladus.
We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e. Europas leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europas atmospheric
ion{O}{1} 1304 A and ion{O}{1} 1356 A emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 $pm$ 14 and 226 $pm$ 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modelling the reflected light from Europas surface. The total emission also shows a clear dependence of Europas position with respect to Jupiters magnetospheric plasma sheet. We derive a lower limit for the O$_2$ column density of 6 $times$ 10$^{18}$ m$^{-2}$. Previous observations of Europas atmosphere with STIS in 1999 of Europas trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian vs sub-Jovian asymmetry with respect to the central meridian on the leading side, and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90 degree west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally-driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europas UV emission can also be caused by inhomogeneous surface properties, inhomogeneous solar illuminations, and/or by Europas complex plasma interaction with Jupiters magnetosphere.
