The Mars 2020 mission will seek the signs of ancient life on Mars and will identify, prepare, document, and cache a set of samples for possible return to Earth by a follow-on mission. Mars 2020 and its Perseverance rover thus link and further two long-held goals inThe Mars 2020 Mission Edited by Kenneth A

Roth et al. (2014a) reported evidence for plumes of water venting from a southern high latitude region on Europa -spectroscopic detection of off-limb line emission from the dissociation products of water. Here, we present Hubble Space Telescope (HST) direct images of Europa in the far ultraviolet (FUV) as it transited the smooth face of Jupiter, in order to measure absorption from gas or aerosols beyond the Europa limb. Out of ten observations we found three in which plume activity could be implicated. Two show statistically significant features at latitudes similar to Roth et al., and the third, at a more equatorial location. We consider potential systematic effects that might influence the statistical analysis and create artifacts, and are unable to find any that can definitively explain the features, although there are reasons to be cautious. If the apparent absorption features are real, the magnitude of implied outgassing is similar to that of the Roth et al. feature, however the apparent activity appears more frequently in our data.

The surface of Europa could contain the compositional imprint of a underlying interior ocean, but competing hypotheses differ over whether spectral observations from the Galileo spacecraft show the signature of ocean evaporates or simply surface radiation products unrelated to the interior. Using adaptive optics at the W.M. Keck Observatory, we have obtained spatially resolved spectra of most of the disk of Europa at a spectral resolution ∼ 40 times higher than seen by the Galileo spacecraft. These spectra show a previously undetected distinct signature of magnesium sulfate salts on Europa, but the magnesium sulfate is confined to the trailing hemisphere and spatially correlated with the presence of radiation products like sulfuric acid and SO 2 . On the leading, less irradiated, hemisphere, our observations rule out the presence of many of the proposed sulfate salts, but do show the presence of distorted water ice bands. Based on the association of the potential MgSO 4 detection on the trailing side with other radiation products, we conclude that MgSO 4 is also a radiation product, rather than a constituent of a Europa ocean brine. Based on ocean chemistry models, we hypothesize that, prior to irradiation, magnesium is primarily in the form of MgCl 2 , and we predict that NaCl and KCl are even more abundant, and, in fact, dominate the non-ice component of the leading hemisphere. We propose observational tests of this new hypothesis.

The ocean in Jupiter's moon Europa may have redox balance similar to Earth's. On Earth, low‐temperature hydration of crustal olivine produces substantial hydrogen, comparable to any potential flux from volcanic activity. Here we compare hydrogen and oxygen production rates of the Earth system with fluxes to Europa's ocean. Even without volcanic hydrothermal activity, water‐rock alteration in Europa causes hydrogen fluxes 10 times smaller than Earth's. Europa's ocean may have become reducing for a brief epoch, for example, after a thermal‐orbital resonance ∼2 Gyr after accretion. Estimated oxidant flux to Europa's ocean is comparable to estimated hydrogen fluxes. Europa's ice delivers oxidants to its ocean at the upper end of these estimates if its ice is geologically active, as evidence of geologic activity and subduction implies.

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