Far-UV Eclipse Observations of Ganymede's Atmosphere with New Horizons Alice: New Constraints to the Atomic Oxygen Component

Ganymede's atmosphere is a surface-bounded-exosphere composed mainly of molecular oxygen. The bulk density of the O2 atmosphere is inferred from the diagnostic ratio between far-UV auroral emission line brightnesses observed with the Hubble Space Telescope (HST), but this measurement is relatively uncertain owing to a lack of information available for the energies of the electrons in Ganymede's magnetosphere that dissociatively excite the OI 130.4 nm and 135.6 nm emissions. Only a few other species such as H have been detected, and the abundance of atomic oxygen has been constrained only in relation to lower limits for the O2 density based on the line ratios. The New Horizons (NH) spacecraft observed Ganymede with the Pluto-Alice (P-Alice) instrument during the Jupiter flyby in spring of 2007. HST Advanced Camera for Surveys (ACS) far-UV images of Ganymede complement the P-Alice far-UV spectroscopy at this time. OI 130.4 nm and 135.6 nm emissions were detected in both data sets. The ACS Ganymede images are consistent with previous Space Telescope Imaging Spectrograph (STIS) imaging. P-Alice observed two Ganymede eclipse events, viewing the sunlit sub-Jupiter and nightside anti-Jupiter hemispheres, separately, from before ingress through after egress. Through comparisons of the P-Alice spectra in sunlight to those in eclipse we are able to disentangle the component of the OI 130.4 nm emission line brightness attributed to processes other than electron impact dissociation of O2, namely a combination of solar reflectance and solar resonant scattering by atomic oxygen atoms. The CII 133.5 nm solar emission feature in the sunlit hemisphere dataset also disappears in eclipse and was not detected in the nightside hemisphere spectra, as expected, providing distinct fits to the solar reflectance component. We thereby provide the first meaningful constraint on the atomic oxygen atmosphere revealed through the solar resonant scattering emission source. We briefly discuss the potential of similar far-UV measurements with the upcoming Jupiter Icy Moon Explorer (JUICE) Ultraviolet Spectrograph (UVS) for better understanding Ganymede's atmosphere and auroral interaction processes.