A search for liquid water on a planetary surface may be too confining when it comes to the wide range of possibilities for supporting life. We see that in our own Solar System. Consider the growing interest in icy moons like Europa and Enceladus, where there is no possibility of surface water but a potentially rich environment under a thick layer of ice. Extending these thoughts into the realm of exoplanets reminds us that our calculations about how many life-bearing worlds are out there may be in need of revision.
This is the thrust of work by Lujendra Ojha (Rutgers University) and colleagues, as developed in a paper in Nature Communications and presented at the recent Goldschmidt geochemistry conference in Lyon. What Ojha and team point out is that radiogenic heating can maintain liquid water below the surface of planets in M-dwarf systems, and that added into our astrobiological catalog, such worlds, orbiting a population of stars that takes in 75 percent or more of all stars in the galaxy, dramatically increase the chances of life elsewhere. The effect is striking. Says Ojha:
“We modeled the feasibility of generating and sustaining liquid water on exoplanets orbiting M-dwarfs by only considering the heat generated by the planet. We found that when one considers the possibility of liquid water generated by radioactivity, it is likely that a high percentage of these exoplanets can have sufficient heat to sustain liquid water – many more than we had thought. Before we started to consider this sub-surface water, it was estimated that around 1 rocky planet every 100 stars would have liquid water. The new model shows that if the conditions are right, this could approach 1 planet per star. So we are a hundred times more likely to find liquid water than we thought. There are around 100 billion stars in the Milky Way Galaxy. That represents really good odds for the origin of life elsewhere in the universe.”
Image: This is Figure 2 from the paper. Caption: Schematic of a basal melting model for icy exo-Earths. a Due to the high surface gravity of super-Earths, ice sheets may undergo numerous phase transformations. Liquid water may form within the ice layers and at the base via basal melting with sufficient geothermal heat. If high-pressure ices are present, meltwater will be buoyant and migrate upward, feeding the main ocean. The red arrows show geothermal heat input from the planet’s rocky interior. b Pure water phase diagram from the SeaFreeze representation illustrating the variety of phases possible in a thick exo-Earth ice sheet. Density differences between the ice phases lead to a divergence from a linear relationship between pressure and ice-thickness. Credit: Ohja et al.
The effect is robust. Indeed, water can be maintained above freezing even when planets are subject to as little as 0.1 Earth’s geothermal heat produced by radiogenic elements. The paper models the formation of ice sheets on such worlds and implies that the circumstellar region that can support life should be widened, which would take in colder planets outside what we have normally considered the habitable zone.
But the work goes further still, for it implies that planets closer to their host star than the inner boundaries of the traditional habitable zone may also support subglacial liquid water. We also recall that the sheer ubiquity of M-dwarfs in the galaxy helps us, for if water from an internal ocean does reach the surface, perhaps through cracks venting plumes and geysers, we may find numerous venues relatively close to the Sun on which to search for biosignatures.
The key factor here is subglacial melting through geothermal heat, for oceans and lakes of liquid water should be able to form under the ice on Earth-sized planets even when temperatures are as low as 200 K, as we find, for example, on TRAPPIST-1g, which is the coldest of the exoplanets for which Ojha’s team runs calculations.
Such water is found to be buoyant and can migrate through this ‘basal melting,’ a term used, explain the authors, for “any situation where the local geothermal heat flux, as well as any frictional heat produced by glacial sliding, is sufficient to raise the temperature at the base of an ice sheet to its melting point.” Subglacial ice sheets are found on Earth in the West Antarctic Ice Sheet, Greenland and possibly the Canadian Arctic, and the paper points out the possibility of the mechanism at work at the south pole of Mars.
The authors’ modeling uses a software tool called SeaFreeze along with a heat transport model to investigate the thermodynamic and elastic properties of water and ice at a wide range of temperatures and pressures. Given the high surface gravity of worlds like Proxima Centauri b, LHS 1140 b and some of the planets in the TRAPPIST-1 system, water ice should be subjected to extreme pressures and temperatures, and as the paper points out, may evolve into high-pressure ice phases. In such conditions, the meltwater migrates upward to form lakes or oceans. Indeed, this kind of melting and migration of water is more likely to occur on planets where the ice sheets are thicker and there is both higher surface gravity as well as higher surface temperatures.
Image: A frozen world heated from within, as envisioned by the paper’s lead author, Lujendra Ojha.
Beyond radiogenic heating, tidal effects are an interesting question, given the potential tidal lock of planets in close orbits around M-dwarfs. Yet planets further out in the system could still benefit from tidal activity, as the paper notes about TRAPPIST-1:
…the age of the TRAPPIST-1 system is estimated to be 7.6 ± 2.2 Gyr; thus, if geothermal heating has waned more than predicted by the age-dependent heat production rate assumed here, tidal heating could be an additional source of heat for basal melting on the TRAPPIST-1 system. On planets e and f of the TRAPPIST-1 system, tidal heating is estimated to contribute heat flow between 160 and 180 mW m−2. Thus, even if geothermal heating were to be negligible on these bodies, basal melting could still occur via tidal heating alone. However, for TRAPPIST-1 g, the mean tidal heat flow estimate from N-body simulation is less than 90 mW m−2. Thus, ice sheets thinner than a few kilometers are unlikely to undergo basal melting on TRAPPIST-1 g.
So we have two mechanisms in play to maintain lakes or oceans beneath surface ice on M-dwarf planets. The finding is encouraging given that one of the key objections to life in these environments is the time needed for life to evolve given that the young planet should be bombarded by ultraviolet and X-ray radiation, a common issue for these stars. We put in place what Amri Wandel (Hebrew University of Jerusalem), who writes a commentary on this work for Nature Communications, calls ‘a safe neighborhood,’ and one for which forms of biosignature detection relying on plume activity will doubtless emerge building on our experience at Enceladus and Europa.
The paper is Ojha et al., “Liquid water on cold exo-Earths via basal melting of ice sheets,” Nature Communications 13, Article number: 7521 (6 December, 2022). Full text. Wandel’s excellent commentary is “Habitability and sub glacial liquid water on planets of M-dwarf stars,” Nature Communications 14, Article number: 2125 (14 April 2023). Full text.