Abstract
Europa is the innermost of Jupiter's three large icy moons. The existence of a torus of neutral gas in Europa's orbit has been inferred from in-situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa’s global atmosphere and consists primarily of molecular hydrogen. Recently, the Juno spacecraft confirmed the torus through measurements of H2+ ions.  However, the neutrals in this torus have never been observed more directly. Here we present observations by the highly sensitive Cosmic Origins Spectrograph of the Hubble Space Telescope (HST/COS) from 2020 and 2021. COS scanned the equatorial plane of the Jupiter system across the orbital distance of Europa between 8 and 10 planetary radii west of the planet . We report constraints from the COS high-resolution spectra on the primary neutral gasses (H2, H, O, and O2) near Europa's orbit and compare them to simulation results from the neutral torus model developed by Smith et al. (2019).
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