Probing Ganymede’s Atmosphere With HST Lyα Images In Transit Of Jupiter

By Keith Cowing
Status Report
January 15, 2023
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Probing Ganymede’s Atmosphere With HST Lyα Images In Transit Of Jupiter
(Top) Full STIS spectral images over the full 25”x25” detector with the dispersed signal from the 6”x6” aperture box centered near y pixel = 350. The horizontal axis contains both spatial and spectral information. Counts above and below the 6” (or ∼240 pixel) high trace are instrumental only. Photons in the bright box at left are interpreted to be all Lyman-α emissions, thus representing a monochromatic image. (Botton) Zoom-in of the Lyman-α image with units converted to kiloRayleigh (kR) and centered on Ganymede’s disk. The direction of Ganymede’s north pole (NP) is shown by the black arrow. —

We report results from far-ultraviolet observations by the Hubble Space Telescope of Jupiter’s largest moon Ganymede transiting across the planet’s dayside hemisphere.

Within a targeted campaign on 9 September 2021 two exposures were taken during one transit passage to probe for attenuation of Jupiter’s hydrogen Lyman-α dayglow above the moon limb.

The background dayglow is slightly attenuated over an extended region around Ganymede, with stronger attenuation in the second exposure when Ganymede was near the planet’s center. In the first exposure when the moon was closer to Jupiter’s limb, the effects from the Ganymede corona are hardly detectable, likely because the Jovian Lyman-α dayglow is spectrally broader and less intense at this viewing geometry.

The obtained vertical H column densities of around (1−2)×1012~cm−2 are consistent with previous results. Constraining angular variability around Ganymede’s disk, we derive an upper limit on a local H2O column density of (2−3)×1016~cm−2, such as could arise from outgassing plumes in regions near the observed moon limb.

Lorenz Roth (1), Gregorio Marchesini (1), Tracy M. Becker (2,3), H. Jens Hoeijmakers (4), Philippa M. Molyneux (2), Kurt D. Retherford (2,3), Joachim Saur (5), Shane R. Carberry Mogan (6), Jamey R. Szalay (7) ( (1) Space and Plasma Physics, KTH Royal Institute of Technology, Stockholm, Sweden, (2) Southwest Research Institute, San Antonio, TX, USA, (3) University of Texas at San Antonio, San Antonio, TX, USA, (4) Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Lund, Sweden, (5) Universität zu Köln, Köln, Germany, (6) Space Sciences Laboratory, University of California, Berkeley, CA, USA, (7) Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA)

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Space Physics (
Cite as: arXiv:2301.05583 [astro-ph.EP] (or arXiv:2301.05583v1 [astro-ph.EP] for this version)
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Submission history
From: Lorenz Roth
[v1] Fri, 13 Jan 2023 14:42:16 UTC (2,917 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻