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.
We should always remember that water may be necessary for life, but it is insufficient to ensure it.
The geothermal heat flow as a means to extend the HZ of planets, especially tidally locked ones around M_Dwarfs is very interesting. We have talked before about how radiogenic heating could sustain a deep crustal biosphere in worlds with a rocky core well beyond the conventional HZ. [If there is extant life on Mars, that is most likely where we would find it.]
However, while life seems abundant around the hot, smoker vents in Earth’s oceans, the overall geothermal energy flow is 3 orders of magnitude below that of the solar flux. Earth’s biosphere is driven by solar energy through photosynthesis that both fixes carbon and releases oxygen that allows for the more energetic aerobic metabolisms of complex organisms. If geothermal energy was the main energy harvesting route for life in subglacial oceans, life would be very sparse indeed. The geologic emission of useful carbon compounds is the main source of energy for terrestrial anaerobes isolated from other sources of energy.
The paper does raise the very interesting issue of how life survived the “snowball Earth” condition. Wikipedia has a succinct section about how extant life may have survived this [near] global glacial condition. Survival of life through frozen periods. Note that the main energy for life, photosynthesis, would largely have been blocked during this glaciation and reduced life to existing chemotrophically, such as around hot vents.
But note that life, and subsequently photosynthesis had already evolved when the deep glaciation was in place. This is very different from a world with a subglacial ocean evolving in that condition. It relies on the possibility (probability?) that abiogenesis occurred in such a location, and if so, was this easy, or a fluke? The far later evolution of eukaryotes seems perhaps even a lower probability given the time it took for this type of cell to evolve.
Life in cold, dark, subsurface oceans would evolve more slowly than on Earth, further delaying the appearance of even anaerobic complex life, and almost certainly no oxygenic photosynthesis would evolve, at least not as we see on Earth.
Three orders of magnitude is even a little optimistic. The authors talk about 30 to 60 mW of heat flow per square meter on Proxima Centauri – though they cite estimates TRAPPIST-1 e and f have 160-180 mW/m^2. Compare solar energy on Earth at around 1000 W/m^2 (the solar constant is theoretically higher, but conditions vary).
I see maybe a little hope you might be able to dispute their analysis, which comes from an N-body simulation done in 2018 (ref. 43). There is a later paper (Hamish, 2020: https://arxiv.org/pdf/2008.02825.pdf ), which found that resonance considerations apply not only to the orbits, but to the depth of subsurface oceans. If I interpret it correctly, the moons might absorb much more energy than otherwise predicted if their oceans have just the right viscoelastic resonance with the orbital forcing. It would be a most interesting test of the “Gaia hypothesis” to see if an organism could manipulate the thickness of the ice or the viscosity of the water to maintain global heat homeostasis.
There may be two other processes that may effect this. Taking the earth as an example, we travel through our galaxy spiral arms about every 135million years. There is also a oscillation of the Sun and solar system above and below the galactic plain of 66 million years, when the earth passes through the plane of our galaxy every 33 million years.
Mass extinction seem to correlate with these cycles because of cometary impacts, which may bring more water and cause interior heating and plate tectonic movement from large impactors. There may also be a relation to interstellar impacts from hot radioactive material if passage takes place near recent supernova or neutron star mergers.
Europa may have been ice free for millions of years if something like this took place. Our moon may have the best fossil history for asteroid, cometary and interstellar radioactive impacts.
The cold M dwarf planets activity would depend on the orbit of the star around our galaxy with ones orbiting in the galactic plane and closer to the galactic center having the most active impacts.
https://www.universetoday.com/14082/comet-strikes-increase-as-we-pass-through-the-galactic-plane/
https://centauri-dreams.org/2007/07/30/galactic-drift-and-mass-extinction/
Scientists discover 36-million-year geological cycle that drives biodiversity.
https://phys.org/news/2023-07-scientists-million-year-geological-biodiversity.html
The main point is that gigantic forces beyond earth and the planets around cold M dwarfs may have caused life to form and also caused its rapid evolution. Deep impacts creating geothermal plums and water plus organic material replenishment may continue for tens of billions of years to trillions of years due to comets and interstellar radioactive hot elements from supernova and neutron star mergers. This will be a continuing process and increase with such occurrences as starburst activity and when the galactic black hole at the center of our galaxy becomes an active quasar.
The thin crust of our oceans spreading ridges could also record such impacts in active plumes and seamounts in the deep abyss that cover large sections of the Pacific ocean…
Back when Mueller was publishing his analysis, I attended his talk to SETI. I was a bit perturbed about their recalibration of the mass extinctions, as well as their assumption that all the big extinctions were caused by impacts despite the difference of opinions by the experts. Nevertheless, his Fourier analysis did suggest about a 62 my period, with a lesser period of about 1/2 that.
The more recent hypothesis that extinctions are related to the path of the sun as it oscillates above and below the galactic plane is interesting, although the physical causes of the extinctions vary from cosmic ray counts to disturbing the Oort cloud.
However, the mass extinctions do not explain the key points of abiogenesis and the 2 subsequent events of the ingestion and incorporation of bacteria to create mitochondria, and in addition, chloroplasts in plants. There have become accompanied by other changes in the cell such as the nucleus and organelles, all features that bacteria and archaea do not have. These events took 3-4 galactic revolutions and therefore seem unrelated to the path of the sun in the galactic plane. This seems more relevant to the question of the evolution of prokaryotes, eukaryotes, and complex life, although the last may have been easier once colonial organisms appeared.
As I suggested in one of my early CD posts, the timing alone of the evolution of these stages on Earth suggests that prokaryotes will be the dominant life form on exoplanets. Once complex life was rapidly evolving. other factors would influence the path of evolution. The mass extinction events indicate that this could be quite drastic, e.g. the Permian and Cretaceous extinctions.
But as has been pointed out by many, evolution is contingent. If we look backward, the necessary events that resulted in our human technological civilization are important, yet random and not directed. Now it is possible that different conditions and paths may converge on the same result, but we do not know that. The recent rise of human civilization over the last 15-20 millennia is also a result of cultural factors and chance events. As Jared Diamond has suggested in “Guns, germs, and Steel”, accidents of geography, animal distribution, and culture resulted in the European dominance of the globe whilst other cultures remained in the stone age.
I suspect there is no single theory that can explain everything, but rather chance played a major role. Einstein may have been right when he said: “God does not play dice with the universe”, but I think nature does play dice when it comes to evolution, both natural and cultural.
“Replay the tape a million times … and I doubt that anything like Homo sapiens would ever evolve again,” – Stephen Jay Goul. in his book Wonderful Life.
Contingency and determinism in evolution: Replaying life’s tape
Replaying Evolution
While from a qualitative standpoint, I do believe that this line of reasoning works, I do have a reservation about how it is “calibrate”. An underlying assumption is that exoplanets examined are calibrated based on Earth’s effective temperature with a particular bond albedo. In other words a particular reflectance of radiation back into space. And that would give an effective temperature of 254 K at 1 AU from our sun. Parenthetically, we know that its Greenhouse effects gives a temperature on the surface more like 288 K. At least overall it is over 273. Otherwise we would revert to an iceball.
My own approach in comparing exoplanets would be based on a simpler criterion. If a star has a particular luminosity and effective temperature, calculate how far away you have to be to get an effective temperature for a point source equivalent to 400 K.
Thereafter, you do any kind of dance that you want. Compensate your temperate zone for K or F stars with a higher or lower desired local temperature, adjust the albedo, or greenhouse as you like. For example, if an exoplanet has an ice sheet and a clear atmosphere, the equilibrium temperature at the 400 K region might be different than on Earth. If the atmosphere is more of a greenhouse – ditto. So, it is my contention that it is likely that placing an exoplanet at the same 254 K region obtained with
Earth’s albedo, around a different type of star – or a different evolutionary history, … Well closer examination should produce some big surprises – including prevailing temperature equilibrium.
In the solar system, here are some Bond Albedo values:
Mercury: 0.088
Venus: 0.76
Earth: 0.306
Moon: 0.11
Mars: 0.25
Jupiter: 0.503
Saturn: 0.342
Uranus: 0.3
Neptune: 0.29
Enceladus: 0.88
One would expect that Europa would be similar to that last entry. But clearly, treating invisible exoplanets as though they have the same Bond albedo as Earth, when atmosphere presence, absence or depth are unknown, or whether there are clouds or oceans or land forms similar to Earth’s, … the assumption is going to provide misleading results. Effective stellar temperature at given distance allows for a domain of possible planetary configurations with their effective temperature solutions.