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 (O2) as an atmospheric constituent, but an expected sublimated water (H2O) component remained undetected. Here we present an analysis of high-sensitivity spectra and spectral images acquired by the Hubble Space Telescope revealing H2O in Ganymede’s atmosphere. The relative intensity of the oxygen emissions requires contributions from the dissociative excitation of water vapour, indicating that H2O is more abundant than O2 around the subsolar point. Away from the subsolar region, the emissions are consistent with a pure O2 atmosphere. Eclipse observations constrain atomic oxygen to be at least two orders of magnitude less abundant than these other species. The higher H2O/O2 ratio above the warmer trailing hemisphere compared with the colder leading hemisphere, the spatial concentration in the subsolar region and the estimated abundance of ~1015 molecules of H2O per cm2 are consistent with sublimation of the icy surface as source.
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L.R. appreciates the support from the Swedish National Space Agency (SNSA) through grant number 154/17 and the Swedish Research Council (VR) through grant number 2017-04897. J.S. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 884711).
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Apurva Oza and Audrey Vorburger for their contribution to the peer review of this work.
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Extended Data Fig. 1 Count rates at the two oxygen multiplets as a function of time in the COS exposures.
A sharp increase of the OI1304 Å emission (blue) from light scattered in the geocorona can be seen towards the end. No increase due to geocoronal scattered light is present at OI1356 Å (red). For the analysis the last 520 s in each exposure are removed in the processing and only the counts left of the vertical dotted lines are used.
Extended Data Fig. 2 Model atmosphere parameters and results for the corresponding oxygen intensities.
The temperature of the electrons is assumed to be Te = 100 eV. Maps of the model atmospheres are shown in Extended Data Figure 3. Note that the O2 atmosphere produces the vast majority of the OI1356 Å emissions. For the OI1304 Å emissions, in contrast, O2, O and H2O all have relevant contributions to the signal.
The H2O atmosphere is scaled for the best-fit on the trailing hemisphere. The O2 and O atmosphere are assumed to be identical on the trailing and leading hemispheres. The derived H2O density for the leading hemisphere is lower by a factor of 6.
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Roth, L., Ivchenko, N., Gladstone, G.R. et al. A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations. Nat Astron 5, 1043–1051 (2021). https://doi.org/10.1038/s41550-021-01426-9