Liquid Water Stability Zones on Super-Mars Exoplanets: Implications for Subsurface Astrobiology

The search for life in the universe has primarily been limited to the surface of terrestrial bodies and the oceans of icy worlds. However, in the age of the James Webb Space Telescope and the Astro2020 Decadal Survey recommendations, where exoplanets are increasingly becoming an important area of research, it is necessary to further characterize the types of environments where extraterrestrial life may be found. On Earth, we see life in many forms and locations, including microbial life deep in the crust. The deep biosphere is estimated to be one-tenth to one-third of Earth's total current biomass and is dominated by autotrophic and heterotrophic metabolisms including methanogenesis and sulfur cycling. In terrestrial bodies far from their host star, stellar flux is lower, and after the first few billion years, the heat of accretion begins to dwindle. This leaves radioactive decay as a major source of long-lived thermal energy that exists independently from a planet's orbit. The decay process can also act as a catalyst for radiolysis, which can provide electron donors essential for metabolic processes to occur. We therefore propose subsurface pore space on rocky planets as another potentially habitable zone to consider in the search for life. We utilize a pre-existing thermal evolution model to constrain the history of a hypothetical Earth-sized exoplanet in a Mars-like orbital radius over the course of 10 Gyr given different endmember budget scenarios of radioactive heat-producing elements and construct a box model to consider potentially habitable zones in the subsurface. We show that the major limiting factor to habitability in the subsurface rests with porosity and explore whether viscous relaxation processes and water-rock interactions close all possible deep pore spaces at a rate faster than fissures and pore spaces can be created or maintained. Additionally, we show that if porosity is not a limiting factor, energy from rock and water interactions could produce oxidants and reductants that could support and sustain metabolisms in the deep subsurface of terrestrial planets over long periods of geologic time.