Downward oxidant transport through Europa's ice shell by density-driven brine percolation

Jupiter's moon Europa is thought to have an ocean beneath its ice shell and the habitability of the internal ocean depends on the availability of redox gradients. Downward transport of radiolytic materials produced at the surface through the ice shell sets the flux of oxidants into the ocean. Here, we propose that oxidants are transported through the ice shell by the drainage of near-surface brines formed concurrently with chaotic terrains. We estimate that Europa's porous regolith contains $3.7\times10^{14}$ to $5.6\times10^{18}$ moles ($1.2\times10^{13}-1.8\times10^{17}$ kg) of trapped \Oxy. Simulations of coupled melt-migration and eutectic phase behavior show that brines drain before they refreeze, delivering $\sim$85\% of the surface oxidants to the ocean on timescales of $2\times10^4$ years. From the distribution of chaotic terrains and from Europa's surface age we estimate that brine drainage could deliver \Oxy\, to the ocean at rates of $2.0\times10^6$ to $1.3\times10^{10}$ mol/yr. Tidal heating is thought to maintain large subsurface oceans on several Jovian and Saturnian satellites \cite{Nimmo2016,Lunine2017} and these icy ocean worlds are candidates in the search for extraterrestrial life \cite{Gaidos1999,Domagal-Goldman2016}. Jupiter's moon Europa will be visited by both the Europa Clipper and JUICE missions \cite{Grasset2013,Pappalardo2015} because it is of particular interest in this search. The habitability of Europa's interior ocean depends, among other conditions, on the availability of redox gradients \cite{Chyba2001a,Pasek2012,Vance2016,Russell2017}. Sufficient oxidant fluxes into the ocean are feasible if oxidants produced by irradiation at the surface \cite{Carlson1999} can be transported through the ice shell. While Europa's ice shell is generally thought to be convecting \cite{Pappalardo1998,McKinnon1999}, it likely has a conductive lid that prevents surface entrainment (Figure~\ref{fig:fig1}a). The presence of such a lid is consistent with the limited observational evidence for direct subduction of the irradiated surface \cite{Kattenhorn2014} and theoretical arguments against subduction \cite{Johnson2017,Howell2019}.Another mechanism to connect the surface to the ocean is the breaching of Europa's ice shell by large impacts \cite{Bray2014,Cox2015}. Although impact breaching may once have been common, there is scant observational evidence for impact-driven oxidant transfer in the last 30-70~Ma \cite{Zahnle2008,Bierhaus2009,Steinbruegge2020}. This leaves poorly defined processes of resurfacing or crustal thickening as potential transport mechanisms for oxidants \cite{Greenberg2010}. Consequently, the oxidant flux into Europa's ocean is currently not understood and presents a major obstacle to assessing its habitability \cite{Hand2007}.