Chemical Disequilibrium as a Biosignature in an Antarctic Subglacial Lake Analog to Ocean Worlds

Measurements of plume material from Enceladus yield strong evidence for hydrothermal activity and complex organics [1] in a subsurface ocean. There are also now several lines of evidence for plumes on Europa [2], suggesting that samples of subsurface waters may be accessible to future missions. These developments motivate further attempts to sample plumes and explore the chemistries of subsurface waters on ocean worlds.

To look for life in these subsurface ocean environments, generalized biosignatures that don't assume terrestrial biochemistry may prove valuable. Here, we explore chemical disequilibrium as a potential general biosignature [3] in a terrestrial analog for ocean worlds, Subglacial Lake Whillans (SLW), West Antarctica. We use the measured chemistry of SLW to calculate its total chemical disequilibrium. This calculation indicates that much of the disequilibrium results from the coexistence of oxygen and a number of organic molecules including methane, acetate and formate. Coexisting oxygen and pyrite embedded in the bedrock and sediments are another source of disequilibrium. Chemolithotrophs in the lake likely oxidize pyrite to fix carbon dioxide, while heterotrophs sustain themselves with the disequilibria between organics and oxygen [4].

This disequilibrium is maintained by a combination of abiotic and biotic sources and sinks. Abiotic sources and sinks include oxygen inputs from melting ice at the lake lid, and spontaneous redox reactions, while life depletes chemical disequilibrium by catalyzing redox reactions. Constraints on the abiotic sources and sinks of chemical disequilibrium and the total measured chemical disequilibrium allows for a calculation of the biological impact on the chemical disequilibrium of the lake. This biotic impact is free energy based, and is thus a generalizable biosignature metric because free energy is required by all life.

[1] Postberg et al. 2018, Nature.

[2] Jia et al. 2018, Nature Astronomy.

[3] Krissanson-Totten et al. 2016, Astrobiology.

[4] Vick-Majors et al. 2016, Frontiers in Microbiology.