Downward Oxidant Transport Through Europa's Ice Shell by Density‐Driven Brine Percolation

Abstract

Tidal heating is thought to maintain large subsurface oceans on several Jovian and Saturnian satellites (Lunine, 2017;Nimmo & Pappalardo, 2016) and these icy ocean worlds are candidates in the search for extraterrestrial life (Domagal-Goldman et al., 2016;Gaidos et al., 1999). Jupiter's moon Europa will be visited by both the Europa Clipper and JUICE missions (Grasset et al., 2013;Pappalardo et al., 2015) 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 (Chyba & Phillips, 2001;Pasek & Greenberg, 2012;Russell et al., 2017). Sufficient oxidant fluxes into the ocean are feasible if oxidants produced by irradiation at the surface (Carlson et al., 1999;Vance et al., 2016) can be transported through the ice shell. While Europa's ice shell is generally thought to be convecting (McKinnon et al., 2016;Pappalardo et al., 1998), it likely has a conductive lid that prevents surface entrainment (Figure 1a). The presence of such a lid is consistent with the limited observational evidence for direct subduction of the irradiated surface (Kattenhorn & Prockter, 2014) and theoretical arguments against subduction (B. Johnson et al., 2017;Howell & Pappalardo, 2019).Another mechanism to connect the surface to the ocean is the breaching of Europa's ice shell by large impacts (Bray et al., 2014;Cox & Bauer, 2015). Although impact breaching may once have been common, there is scant observational evidence for impact-driven oxidant transfer in the last 30-70 Ma (Bierhaus et al., 2009;Steinbrügge et al., 2020;Zahnle et al., 2008). This leaves poorly defined processes of resurfacing or crustal thickening as potential transport mechanisms for oxidants (Greenberg, 2010). Consequently, the oxidant flux into Europa's ocean is currently not understood and presents a major obstacle to assessing its habitability (Hand et al., 2007).

Partially‐Saturated Brines Within Basal Ice or Sediments Can Explain the Bright Basal Reflections in the South Polar Layered Deposits

“…Drainage of an interconnected system of LVNs that would concentrate brines downward through a deposit, such as the SPLD. This process has been observed, for example, in temperate sea-ice (e.g., Nye & Frank, 1973) and has been hypothesized to occur within the Europa ice shell (e.g., Hesse et al, 2022). 2.…”

Brine Volume Fraction as a Habitability Metric for Europa's Ice Shell

“…These measurements suggest that ice in textural equilibrium should be permeable even at very low brine volume fractions and nutrient transport could be permissible at temperatures down to the eutectic. This property has been used previously to justify the transport of oxidants through Europa's ice shell via porosity waves (Hesse et al, 2022); however, we note that sea ice does not possess an equilibrium texture (e.g., Junge et al, 2001Junge et al, , 2004Moore et al, 1994). We thus designate the region of the ice shell where the brine volume fraction is less than 0.06 but water is still stable as nutrient-closed potential habitats.…”

“…Although sunlight is not expected to serve as an energy source to support life at Europa (i.e., organisms inhabiting the ice shell and/or ocean are likely not phototrophic), radiolytically generated oxidants at the surface may represent an analogous energy source for chemotrophic organisms (Chyba, 2000). The oxidant flux from Europa's surface to the ocean is poorly constrained and could be punctuated or continuous depending on the transport mechanism (e.g., brine drainage from chaos terrain or complete overturning of the ice shell) (Hesse et al., 2022; Vance et al., 2016). Estimates assuming the ice shell fully overturns on timescales equal to the age of the surface suggest Europa's ocean could be more oxygenated than Earth's ocean (Greenberg, 2010; Hand et al., 2007).…”

Modeling Virus and Bacteria Populations in Europa’s Subsurface Ocean

“…This DOM is then transported upward through the ocean column via circulation and/or diffusion to the sub-ice biosphere, where it is able to be consumed by DOM-limited heterotrophs. This scenario rests on the assumption that Europa’s biosphere is net reductant limited, based on recent estimates showing that O 2 fluxes from radiolysis could be comparable to or even overwhelm H 2 fluxes from hydrothermal activity [ 22 , 27 ]. Hence, the viral elevator represents the primary mechanism for reduced organic matter to make it to the sub-ice biosphere, promoting the viability of this habitat.…”