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

TL;DR

This study combines HST spectral images and Galileo magnetic data to constrain Europa's subsolar atmosphere, revealing the presence and distribution of oxygen and water vapor with specific column densities.

Contribution

It introduces a joint analysis method that integrates spectral and magnetic data to better constrain Europa's atmospheric composition and distribution, especially for water vapor.

Findings

Approximately 50% of the oxygen and water vapor abundances are needed to explain observations.

Column densities of O2 and H2O are quantified, supporting the existence of a stable water atmosphere.

Results are consistent with previous estimates within uncertainties.

Abstract

We constrain Europa's tenuous atmosphere on the subsolar hemisphere by combining two sets of observations: oxygen emissions at 1304 {\AA} and 1356 {\AA} 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 ($\mathrm{H_2O}$) 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 {\AA} to OI 1304 {\AA} emissions from Roth (2021) and on magnetic field data allows us to derive constraints on the density and location of $\mathrm{O_2}$ and $\mathrm{H_2O}$ in Europa's atmosphere. We demonstrate that $50\%$ of the $\mathrm{O_2}$ and between $50\%$ and $75\%$ of the $\mathrm{H_2O}$ 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 $6 \times10^{16}~\mathrm{m}^{-2}$ derived by Roth (2021), and on a strongly confined stable $\mathrm{H_2O}$ atmosphere around the subsolar point. Our analysis yields column densities of $1.2 \times10^{18}~\mathrm{m}^{-2}$ for $\mathrm{O_2}$, and $1.5 \times10^{19}~\mathrm{m}^{-2}$ to $2.2 \times10^{19}~\mathrm{m}^{-2}$ at the subsolar point for $\mathrm{H_2O}$. Both column densities however still lie within the uncertainties of Roth (2021). Our results provide additional evidence for the existence of a stable $\mathrm{H_2O}$ atmosphere at Europa.