The origin and fate of O$_2$ in Europa's ice: an atmospheric perspective

TL;DR

This paper explores the origin and behavior of oxygen in Europa's ice, proposing a new model that emphasizes thermal desorption as a key process influencing the observed oxygen atmosphere, with implications for Europa's potential habitability.

Contribution

It introduces a simple thermal desorption model that challenges previous paradigms, explaining Europa's oxygen atmosphere through trapped O$_2$ release rather than solely radiolysis.

Findings

Thermal desorption likely plays a significant role in Europa's O$_2$ atmosphere.

Orbital variations in O$_2$ are explained by temperature-dependent release from ice.

Radiolytic products may contribute to oxidant supply for Europa's ocean.

Abstract

The early prediction and subsequent detection of an O$_2$ atmosphere on Europa, coupled with the discovery that Europa has an ocean under its ice mantle, has made this moon a prime astrobiologic target, soon to be visited by the JUICE and Europa Clipper spacecraft. In spite of the considerable number of observational, modeling, and laboratory efforts, understanding the physics leading to the observed morphology of Europa's near surface O$_2$ atmosphere has been problematic. This is the case as the observed emissions depend on the local incident plasma ion flux, the local temperature and composition of the regolith, as well as on the near surface electron temperature and density. Here we rely heavily on earlier reviews briefly summarizing the observational, laboratory and simulation efforts. Although it is agreed that radiolysis of the surface ice by the incident Jovian plasma is the ultimate source of observed O$_2$, a recent, simple model of thermal desorption from a regolith permeated with O$_2$ has changed the usual paradigm. This suggests that the observed orbital dependence of the local source of the near-surface O$_2$ atmosphere is due to thermal release of O$_2$ likely trapped on the ice grains at dangling bonds with a smaller contribution due to direct sputtering. This could also impact our understanding of the suggestion that the radiolytic products in Europa's regolith might be a source of oxidants for its underground ocean.