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

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 Lyα 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 Lyα. 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.

Detection of a hydrogen corona at Callisto

“…The vertical column densities derived for Callisto's H corona are similar (visit 1) or ∼ 3× higher (visit 2) compared to the vertical column density found for Europa of (2.3-3.5) ×10 11 cm −2 [Roth et al, 2017] [Smyth and Marconi, 2006;Marconi, 2007], and directly from ion sputtering and radiolysis in the icy surface . Loss of H from the atmosphere is dominated by thermal escape and by reactions with the surface, leading to a dip in density at low altitudes (<50 km) in the modeled abundance profiles [Liang et al, 2005;Marconi, 2007].…”

“…The vertical column densities derived for Callisto's H corona are similar (visit 1) or ∼3× higher (visit 2) compared to the vertical column density found for Europa of (2.3–3.5) ×10 11 cm −2 [ Roth et al , ]. The vertical column density derived for Ganymede's H corona of ∼2.4 × 10 12 cm −3 [ McGrath et al , ] is somewhat higher.…”

“…Our H abundances are in reasonable agreement with the results of Marconi [] for Ganymede, which should provide a good first‐order estimate also for Callisto. No hemispheric differences have yet been detected at Europa or Ganymede, due to a lack of orbital coverage in the published data [ Feldman et al , ; Roth et al , ].…”

An Impacting Descent Probe for Europa and the Other Galilean Moons of Jupiter

“…Na and K were measured (Brown, 2001) in the extended exosphere of Europa, at distances of 6 -13 R E , with transverse column densities of N C (Na) = (1.5 -3.7) · 10 9 cm -2 and N C (K) = (4.5 -13) · 10 7 cm -2 , and a Na/K ratio in Europa's extended atmosphere of 25±2. Recently, Roth et al (2017) reported HST observations of the H corona at Europa from which they derived maximum exospheric densities at the surface in the range of (1.5-2.2)·10 3 cm −3 , confirming the abundances predicted by Monte Carlo simulations (Smith and Marconi 2006;Wurz et al, 2014). A recent review of the chemical 6 composition and densities profiles for the relevant release processes was given by Vorburger and Wurz (2017).…”

Europa’s Optical Aurora

“…In their December 2012 dataset, Roth et al (2014) detected localized off-limb hydrogen and oxygen emissions near Europa's south pole, which they used to infer an active water plume. A lack of similar detections in the other 19 visits indicates that such events are rarely detectable in this type of observation, though subsequent work by other groups using a variety of methods support the presence of sporadic plume activity originating from a few distinct surface locations (Sparks et al 2016;McGrath and Sparks 2017;Jia et al 2018 (Roth et al 2017). In addition, both Na and K have been observed in Europa's extended atmosphere (> 5R Europa ); the relative abundance of these species is different from that of Io and of micrometeorites, suggesting an endogenic origin of this alkali material (Brown and Hill 1996;Brown 2001).…”