Evidence For A Sublimated Water Atmosphere On Ganymede From Hubble Space Telescope Observations

Observation of Ganymede’s leading hemisphere. For details on the panels see caption of Figure 3. The estimated surface temperature (142 K) is lower as the albedo is higher. The measured profile of the line ratio is qualitatively similar to the trailing hemisphere but decreases only to ∼1.8 on the disk. The derived column density ratio NH2O/NO2 (bottom) is accordingly lower as expected for the colder surface, if sublimation is the source

Ganymede's atmosphere is produced by charged particle sputtering and sublimation of its icy surface.

Previous far-ultraviolet observations of the OI1356-A and OI1304-A oxygen emissions were used to derive 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 dissociative excitation of water vapor, indicating that H2O is more abundant than O2 around the sub-solar point.

Away from the sub-solar 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 to the colder leading hemisphere, the spatial concentration to the sub-solar region, and the estimated abundance of ∼1015 H2O/cm2 are consistent with sublimation of the icy surface as source.

Lorenz Roth, Nickolay Ivchenko, G. Randall Gladstone, Joachim Saur, Denis Grodent, Bertrand Bonfond, Philippa M. Molyneux, Kurt D. Retherford

Comments: submitted to Nature Astronomy (reformatted for arXiv)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Space Physics (physics.space-ph)
Cite as: arXiv:2106.03570 [astro-ph.EP] (or arXiv:2106.03570v1 [astro-ph.EP] for this version)
Submission history
From: Lorenz Roth
[v1] Mon, 7 Jun 2021 12:51:25 UTC (905 KB)

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