Probing Ganymede’s Atmosphere with HST Lyα Images in Transit of Jupiter

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 2021 September 9 two exposures were taken during one transit passage to probe for attenuation of Jupiter's hydrogen Lyα 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 Lyα 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.