Probing Ganymede's atmosphere with HST Ly$\alpha$ images in transit of Jupiter

TL;DR

This study uses Hubble Space Telescope Lyα imaging to investigate Ganymede's atmosphere during transit, detecting hydrogen attenuation and setting limits on water vapor outgassing near the moon's limb.

Contribution

First direct ultraviolet observations of Ganymede's atmosphere during transit, providing new measurements of hydrogen column density and constraints on water vapor outgassing.

Findings

Detected hydrogen attenuation around Ganymede consistent with previous measurements.

Derived an upper limit on local water vapor column density near the moon's limb.

Observed variability in hydrogen attenuation depending on Ganymede's position relative to Jupiter.

Abstract

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-$\alpha$ 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-$\alpha$ dayglow is spectrally broader and less intense at this viewing geometry. The obtained vertical H column densities of around $(1-2)\times 10^{12}$~cm$^{-2}$ are consistent with previous results. Constraining angular variability around Ganymede's disk, we derive an upper limit on a local H$_2$O column density of $(2-3)\times 10^{16}$~cm$^{-2}$, such as could arise from outgassing plumes in regions near the observed moon limb.