DETECTION OF A HYDROGEN CORONA IN HST Lyα IMAGES OF EUROPA IN TRANSIT OF JUPITER

Abstract

ABSTRACT We report far-ultraviolet observations of Europa in transit of Jupiter obtained with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope on six occasions between 2014 December and 2015 March. Absorption of Jupiter’s bright hydrogen Ly α dayglow is detected in a region several moon radii above the limb in all observations. The observed extended absorption provides the first detection of an atomic hydrogen corona around Europa. Molecular constituents in Europa’s global sputtered atmosphere are shown to be optically thin to Ly α . The observations are consistent with a radially escaping H corona with maximum densities at the surface in the range of (1.5–2.2) × 10 3 cm −3 , confirming the abundances predicted by Monte Carlo simulations. In addition, we search for anomalies around the limb of Europa from absorption by localized high H 2 O abundances from active plumes. No significant local absorption features are detected. We find that an H 2 O plume with line-of-sight column density in the order of 10 16 cm −2 , as inferred by Roth et al. would not be detectable based on the statistical fluctuations of the transit measurements, and hence is not excluded or further constrained. The presence of plumes with line-of-sight column densities of >2 × 10 17 cm −2 can be excluded at a 3- σ level during five of our six observations.