A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations

Abstract

Ganymede's atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the O i 1,356 Å and O i 1,304 Å oxygen emissions were used to infer sputtered molecular oxygen (O 2 ) as an atmospheric constituent, but an expected sublimated water (H 2 O) component remained undetected. Here we present an analysis of high-sensitivity spectra and spectral images acquired by the Hubble Space Telescope revealing H 2 O in Ganymede's atmosphere. The relative intensity of the oxygen emissions requires contributions from the dissociative excitation of water vapour, indicating that H 2 O is more abundant than O 2 around the subsolar point. Away from the subsolar region, the emissions are consistent with a pure O 2 atmosphere. Eclipse observations constrain atomic oxygen to be at least two orders of magnitude less abundant than these other species. The higher H 2 O/O 2 ratio above the warmer trailing hemisphere compared with the colder leading hemisphere, the spatial concentration in the subsolar region and the estimated abundance of ~10 15 molecules of H 2 O per cm 2 are consistent with sublimation of the icy surface as source.

A Stable H2O Atmosphere on Europa’s Trailing Hemisphere From HST Images

“…The relative FUV oxygen emissions at neighboring moon Ganymede show similar behavior with decreasing  (OI) E r above the limb and the same hemispheric dichotomy, that is, a lower ratio (   (OI) 2 E r ) above the trailing hemisphere (Molyneux et al, 2018). Using an HST observation of Ganymede directly before and in eclipse of Jupiter, Roth et al (2021) recently showed that resonant scattering by atomic oxygen is negligible, setting an upper limit on the global abundance of O in the atmosphere. This upper limit effectively rules out atomic O as explanation for a lower oxygen emission ratio in bright emission regions on Ganymede's disk.…”

“…Investigating the radial profiles of the ratio in STIS images Roth et al. (2021), also found that systematically decreases from the region around the limb () towards the center of the observed disk on both hemispheres. The found profile is shown to be in agreement with an H 2 O‐dominated atmosphere around the sub‐solar point and a O 2 ‐dominated atmosphere away from the sub‐solar point.…”

“…This upper limit effectively rules out atomic O as explanation for a lower oxygen emission ratio in bright emission regions on Ganymede's disk. Investigating the radial profiles of the ratio  (OI) E r in STIS images Roth et al (2021), also found that  (OI) E r systematically decreases from the region around the limb (   (OI) 2 E r ) towards the center of the observed disk on both hemispheres. The found profile is shown to be in agreement with an H 2 O-dominated atmosphere around the sub-solar point and a O 2 -dominated atmosphere away from the sub-solar point.…”

“…Using an HST observation of Ganymede directly before and in eclipse of Jupiter, Roth et al. (2021) recently showed that resonant scattering by atomic oxygen is negligible, setting an upper limit on the global abundance of O in the atmosphere. This upper limit effectively rules out atomic O as explanation for a lower oxygen emission ratio in bright emission regions on Ganymede's disk.…”

“…Here, we use the same approach as recently used by Roth et al. (2021) for Ganymede and study in detail the relative brightness of the two oxygen emissions in order to constrain the H 2 O and atomic oxygen (O) abundances relative to O 2 in Europa's global atmosphere. First, we compare consecutive exposures taken in and out of eclipse to constrain the O abundance through its possible resonant scattering contribution.…”