Exploring Deep-Sea Brines as Potential Terrestrial Analogues of Oceans in the Icy Moons of the Outer Solar System

Abstract

Several icy moons of the outer solar system have been receiving considerable attention and are currently seen as major targets for astrobiological research and the search for life beyond our planet. Despite the limited amount of data on the oceans of these moons, we expect them to be composed of brines with variable chemistry, some degree of hydrothermal input, and be under high pressure conditions. The combination of these different conditions significantly limits the number of extreme locations, which can be used as terrestrial analogues. Here we propose the use of deep-sea brines as potential terrestrial analogues to the oceans in the outer solar system. We provide an overview of what is currently known about the conditions on the icy moons of the outer solar system and their oceans as well as on deep-sea brines of the Red Sea and the Mediterranean and their microbiology. We also identify several threads of future research, which would be particularly useful in the context of future exploration of these extra-terrestrial oceans. The icy moons of the Outer Solar SystemIcy moons are natural satellites that are characterised by a surface that is composed predominantly of ice, which may contain a sub-surface ocean,123 caister.com/cimb Curr. Issues Mol. Biol. Vol. 38 Deep-Sea Brines Antunes et al. ! 124 caister.com/cimb Curr. Issues Mol. Biol. Vol. 38 Deep-Sea Brines Antunes et al.Sub-surface ocean Contrary to previous assumptions, the ocean of Enceladus is believed to have a global distribution rather than being restricted to polar regions (Patthoff and Kattenhorn, 2011). The energy required to maintain liquid water most likely originates from the dissipation of tidal energy from the friction of the sub-surface ocean with the internal silicate interior (Nimmo et al., 2007).Information regarding the composition of the ocean has been obtained from plume analysis, which showed that particle emissions are dominated by water ice and is rich in sodium and potassium salts (0.5 to 2 % by mass), but also contains sodium bicarbonate/ sodium carbonate (Postberg et al., 2011). The ocean is expected to contain low concentrations of ammonia, methane, carbon dioxide and molecular hydrogen (Waite et al., 2017). These elements are potentially produced as a result of geochemical reactions occurring at the interface of the chondrite-like core and the ocean at temperatures below 100°C (Waite et al., 2017). The circulation of the water would drive the chemical evolution of both the rock material and the ocean water, producing a chemical gradient, which could be used by microorganisms to generate energy (Barge and White, 2017). The presence of silica nanoparticles in the plumes is also evidence for hydrothermal reactions occurring in the interior, which may be the source of molecular hydrogen (Sekine et al., 2015).

Subsurface Science and Search for Life in Ocean Worlds

“…Water activity (a W ) ranges from zero to 1 (pure water), and while a low a W is not necessarily immediately deleterious (Barbosa-Cnovas & Juliano 2007), no microbial organisms are known to reproduce below the presently accepted limit of ∼0.6a W (Grant 2004). However, depending on the ionic composition, even saturated brines can be habitable (Hallsworth et al 2007;Cray et al 2013;Klempay et al 2021), as evidenced by halophilic organisms living at low a W in salterns (Lee et al 2018) and deep hypersaline anoxic brine pools (Merlino et al 2018;Antunes et al 2020;Fisher et al 2021). Brines may also act to preserve organics for long periods of time (Perl & Baxter 2020;Klempay et al 2021;Perl et al 2021).…”

Habitability Models for Astrobiology

“…The cross-comparison of variants of similar types of environment (e.g., salterns), as well as slightly different settings (e.g., high salinity biotopes with different pH, temperature, or chemical conditions), could also prove useful. The subsurface oceans of Europa or Enceladus could be compared with deep seawater, hydrothermal systems, or deep-sea brines (Antunes et al, 2020). Planetary atmospheres could be compared with high altitude or near-space regions.…”

Habitability Models for Planetary Sciences

“…The subsurface oceans of Europa or Enceladus could be compared with deep seawater, hydrothermal systems, or deep-sea brines (Antunes et al ., 2020). Planetary atmospheres could be compared with high altitude or near-space regions.…”