Constraining Europa's subsolar atmosphere with a joint analysis of HST spectral images and Galileo magnetic field data

Abstract

We constrain Europa's tenuous atmosphere on the subsolar hemisphere by combining two sets of observations: oxygen emissions at 1304 Å and 1356 Å from Hubble Space Telescope (HST) spectral images, and Galileo magnetic field measurements from its closest encounter, the E12 flyby. We describe Europa's atmosphere with three neutral gas species: global molecular (${\mathrm{O}}_2$) and atomic oxygen (O), and localized water (${\mathrm{H}}_2\mathrm{O}$) present as a near-equatorial plume and as a stable distribution concentrated around the subsolar point on the moon's trailing hemisphere. Our combined modelling based on the ratio of OI 1356 Å to OI 1304 Å emissions from Roth (2021) and on magnetic field data allows us to derive constraints on the density and location of ${\mathrm{O}}_2$ and ${\mathrm{H}}_2\mathrm{O}$ in Europa's atmosphere. We demonstrate that $50\%$ of the ${\mathrm{O}}_2$ and between $50\%$ and $75\%$ of the ${\mathrm{H}}_2\mathrm{O}$ abundances from Roth (2021) are required to jointly explain the HST and Galileo measurements. These values are conditioned on a column density of $\mathrm{O}$ close to the upper limit of $6\times 10^{16}{\mathrm{m}}^{-2}$ derived by Roth (2021), and on a strongly confined stable ${\mathrm{H}}_2\mathrm{O}$ atmosphere around the subsolar point. Our analysis yields column densities of $1.2\times 10^{18}{\mathrm{m}}^{-2}$ for ${\mathrm{O}}_2$, and $1.5\times 10^{19}{\mathrm{m}}^{-2}$ to $2.2\times 10^{19}{\mathrm{m}}^{-2}$ at the subsolar point for ${\mathrm{H}}_2\mathrm{O}$. Both column densities however still lie within the uncertainties of Roth (2021). Our results provide additional evidence for the existence of a stable ${\mathrm{H}}_2\mathrm{O}$ atmosphere at Europa.

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