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Abstract Juno flew over the northern mid‐latitudes of Ganymede during orbit 34 of the Juno mission, reaching an altitude of 1,053 km (16:56:07.972 UTC) at a sub spacecraft latitude/longitude of 33.66N, 57.5W degrees on 7 June 2021. Between 16:43 and 17:02 UT, Juno pierced Ganymede's magnetosphere at a velocity relative to Ganymede of 18.57 km s −1 . Juno's instrumentation provided a unique opportunity to sample the local environment of Ganymede and its magnetosphere. We present measurements of the composition of the polar ionospheric outflow and the energetic electrons that penetrate Ganymede's atmosphere and produce its aurora. When these new observations are combined with modeling, conclusions can be drawn that affect our understanding of the atmosphere of Ganymede. The measured JADE precipitating plasma electrons provide an energy flux beyond that needed to create the observed oxygen emissions measured by UVS, but the electron energy spectrum is optically thin to the sparse atmosphere and does not provide the observed oxygen ultraviolet emission unless the O 2 column density is increased by over an order of magnitude compared to previous atmospheric models. More than 99% of the electron energy flux passes through the atmosphere into the ice, thereby increasing the H 2 and O 2 content of the atmosphere. The increased H 2 and O 2 production is largely responsible for increasing the oxygen column density to a level that produces within known uncertainties the OI135.6 and OI130.4 nm emissions when bombarded by the electron energy flux observed by JADE. This suggests that past modeling efforts have underestimated the density of the atmosphere by over an order of magnitude.
Waite et al. (Mon,) studied this question.