Constraining Europa's Subsolar Atmosphere With a Joint Analysis of HST Spectral Images and Galileo Magnetic Field Data

Abstract

AbstractWe constrain Europa's tenuous atmosphere on the subsolar hemisphere by combining two sets of observations: oxygen emissions at 1,304 and 1,356 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 (O2) and atomic oxygen (O), and localized water (H2O) present as a near‐equatorial plume and as a stable distribution concentrated around the subsolar point on the moon's trailing hemisphere. Our combined modeling based on the ratio of OI 1,356 to OI 1,304 Å emissions from Roth (2021; https://doi.org/10.1029/2021gl094289) and on magnetic field data allows us to derive constraints on the density and location of O2 and H2O in Europa's atmosphere. We demonstrate that 50% of the O2 and between 50% and 75% of the H2O abundances from Roth (2021; https://doi.org/10.1029/2021gl094289) are required to jointly explain the HST and Galileo measurements. These values are conditioned on a column density of O close to the upper limit of 6 × 1016 m−2 derived by Roth (2021; https://doi.org/10.1029/2021gl094289), and on a strongly confined stable H2O atmosphere around the subsolar point. Our analysis yields column densities of 1.2 × 1018 m−2 for O2, and 1.5 × 1019 to 2.2 × 1019 m−2 at the subsolar point for H2O. Both column densities, however, still lie within the uncertainties of Roth (2021; https://doi.org/10.1029/2021gl094289). Our results provide additional evidence for the existence of a stable H2O atmosphere at Europa.