The Far-UV Albedo of Europa From HST Observations

Abstract

We present an analysis of Europa's far-UV spectral albedo using observations during the 1999–2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk-integrated observations show that the far-UV spectrum in the ~130 to 170-nm range is relatively flat or slightly blue (increasing albedo with decreasing wavelength) for the studied hemispheres: the leading, trailing, and anti-Jovian hemispheres. At Lyman-α (121.6 nm), the albedo of the trailing hemisphere continues the blue trend, but it reddens for the leading hemisphere. Also at this wavelength, the albedo of the leading hemisphere, which is higher than the trailing hemisphere at near-UV and visible wavelengths, is lower than the trailing hemisphere, exhibiting spectral inversion. We find no evidence of a sharp water-ice absorption edge at 165 nm on any hemisphere of Europa, which is intriguing since such an absorption feature has been observed on the icy Saturnian satellites.