Hubble Space Telescope Observations of Variations in Ganymede's Oxygen Atmosphere and Aurora

P M Molyneux,S E Milan,N P Bannister,S W H Cowley,J D Nichols,C Paty,D Grodent,E J Bunce,J T Clarke,J.‐C Gérard

doi:10.1029/2018ja025243

Abstract

AbstractWe present high‐sensitivity Hubble Space Telescope (HST) Cosmic Origins Spectrograph and HST Space Telescope Imaging Spectrograph measurements of atmospheric OI 130.4‐nm and OI] 135.6‐nm emissions at Ganymede, which exhibit significant spatial and temporal variability. These observations represent the first observations of Ganymede using HST Cosmic Origins Spectrograph and of both the leading and trailing hemispheres within a single HST campaign, minimizing the potential influence of long‐term changes in the Jovian plasma sheet or in Ganymede's atmosphere on the comparison of the two hemispheres. The mean disk‐averaged OI] 135.6‐nm/OI 130.4‐nm observed intensity ratio was 2.72 ± 0.57 on the leading hemisphere and 1.42 ± 0.16 on the trailing hemisphere. The observed leading hemisphere ratios are consistent with an O2 atmosphere, but we show that an atomic oxygen component of ~10% is required to produce the observed trailing hemisphere ratios. The excess 130.4‐nm emission on the trailing hemisphere relative to that expected for an O2 atmosphere was ~11 R. The O column density required to produce this excess is determined based on previous estimates of the electron density and temperature at Ganymede and exceeds the limit for an optically thin atmosphere. The implication that the O atmosphere is optically thick may be investigated in future by observing Ganymede as it moves into eclipse or by determining the ratio of the individual components within the 130.4‐nm triplet.

Highlights

Ganymede’s tenuous oxygen atmosphere was discovered via Hubble Space Telescope (HST) Goddard HighResolution Spectrograph detection of the atomic oxygen emission multiplets OI 130.4 nm and OI] 135.6 nm (Hall et al, 1998)
We present high-sensitivity Hubble Space Telescope (HST) Cosmic Origins Spectrograph and HST Space Telescope Imaging Spectrograph measurements of atmospheric OI 130.4-nm and OI] 135.6-nm emissions at Ganymede, which exhibit significant spatial and temporal variability
We have analyzed the OI] 135.6-nm/OI 130.4-nm intensity ratio at Ganymede extracted from HST/STIS and HST/COS data and find a range of values that are best explained by a significant difference in the atmospheric compositions of the satellite’s leading and trailing hemispheres

Summary

Introduction

Ganymede’s tenuous oxygen atmosphere was discovered via Hubble Space Telescope (HST) Goddard HighResolution Spectrograph detection of the atomic oxygen emission multiplets OI 130.4 nm and OI] 135.6 nm (Hall et al, 1998). The observed double-peaked profile of the 135.6-nm feature indicated that the emissions originated from two distinct regions near Ganymede’s poles. This detection, combined with the discovery of an intrinsic magnetic field at Ganymede strong enough to carve out a magnetosphere around the moon (Kivelson et al, 1996, 1997), hinted at the existence of auroral processes. As a result of the tidal locking, the same (trailing) hemisphere always faces upstream relative to the bulk flow of the Jovian plasma in which the satellite is embedded, which has an orbital velocity significantly larger than that of Ganymede. The difference in auroral latitude on the two hemispheres is consistent with the impinging plasma compressing the upstream (trailing hemisphere) magnetosphere towards Ganymede and stretching out the downstream (leading hemisphere) magnetosphere into a magnetotail, as predicted by magnetohydrodynamic (MHD) models (Duling et al, 2014; Jia et al, 2008, 2009; Paty & Winglee, 2004, 2006; Paty et al, 2008; Payan et al, 2015)

Results

Discussion

Conclusion