Liquid water on cold exo-Earths via basal melting of ice sheets

Abstract

Liquid water is a critical component of habitability. However, the production and stability of surficial liquid water can be challenging on planets outside the Habitable Zone and devoid of adequate greenhouse warming. On such cold, icy exo-Earths, basal melting of regional/global ice sheets by geothermal heat provides an alternative means of forming liquid water. Here, we model the thermophysical evolution of ice sheets to ascertain the geophysical conditions that allow liquid water to be produced and maintained at temperatures above the pressure-controlled freezing point of water ice on exo-Earths. We show that even with a modest, Moon-like geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths. Furthermore, subglacial oceans may persist on exo-Earths for a prolonged period due to the billion-year half-lives of heat-producing elements responsible for geothermal heat. These subglacial oceans, often in contact with the planet’s crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period.

Habitability and sub glacial liquid water on planets of M-dwarf stars

“…To conclude, basal melting and subglacial oceans 1 combined with the extended HZ of M-dwarfs 2 , 16 may provide habitable environments on as much as one planet per star, compared with merely one habitable planet per 100 stars, if only G-type host stars and a conservative HZ model are considered. For closely orbiting locked planets of M-dwarfs, surface liquid water may be found on the night side which is also protected from the energetic radiation of the host star in its early evolutionary stages.…”

“…Producing liquid water by subglacial melting of local or global ice sheets by geothermal heat may be an alternative to radiative heat from the host star. Ojha et al 1 have shown that liquid water may be produced and maintained at temperatures above freezing even on planets with modest (as low as 0.1 Earth’s) geothermal heat produced by radiogenic elements. They find that subglacial oceans or lakes of liquid water can form by basal melting and persist under ice sheets on Earth-sized exoplanets even for surface temperatures as low as 200 K expected e.g.…”

“…Subglacial liquid water may exist out of the traditional HZ also for K- and G-stars, but its contribution to the abundance of habitable planets is most significant for M-dwarfs, as they are the majority of stars, yet their planets might be endangered by the host’s energetic radiation. As a conservative outer limit for the HZ extended by basal melting, one may use the flux on Trappist 1g, the coldest exoplanet calculated by Ojha et al 1 , which is 25% of the flux received by Earth (0.25 S 0 ). This value is roughly coincident with the outer boundary set by the 1D model, as well as with the Early Mars hypothesis 10 (a phenomenological border derived from the flux received some 4 billion years ago by Mars, building on the evidence that early Mars did have liquid water on its surface).…”

“…in Enceladus and Europa, moons of Saturn and Jupiter, respectively, as a subglacial ocean due to tidal heating. Ojha et al 1 modeled the evolution of ice sheets and the conditions that allow liquid water to be produced and maintained at temperatures above freezing on Earth-like exoplanets. Applying their results to enhance the definition of habitability by including subglacial liquid water, we suggest more extended HZ boundaries so there may exist a wider circumstellar region with a potential for subglacial liquid water and life in colder planets and their satellites outside the traditional HZ.…”

A Bayesian Analysis of Technological Intelligence in Land and Oceans

“…In this scenario, vital water-rock reactions might be suppressed, thereby stymieing access to nutrients, substrates, chemical energy, and miscellaneous sources of disequilibria (Noack et al 2016;Journaux et al 2020;Journaux 2022). However, this drawback may be mitigated by recent numerical models, which imply that the slow transport of salts, nutrients, and other substances into the ocean is feasible even when highpressure ices exist (Choblet et al 2017;Journaux et al 2017;Kalousová & Sotin 2018;Hernandez et al 2022;Ojha et al 2022).…”