Is Most Life in the Universe Lithophilic?

by Paul Gilster | Jan 11, 2019 | Astrobiology and SETI | 90 comments

Seeking life on other worlds necessarily makes us examine our assumptions about the detectability of living things in extreme environments. We’re learning that our own planet supports life in regions we once would have ruled out for survival, and as we examine such extremophiles, it makes sense to wonder how similar organisms might have emerged elsewhere. Pondering these questions in today’s essay, Centauri Dreams regular Alex Tolley asks whether we are failing to consider possibly rich biospheres that could thrive without the need for surface water.

By Alex Tolley

Image: An endolithic lifeform showing as a green layer a few millimeters inside a clear rock. The rock has been split open. Antarctica. Credit: https://en.wikipedia.org/wiki/Endolith#/media/File:Cryptoendolith.jpg, Creative Commons).

A policeman sees a drunk man searching for something under a streetlight and asks what the drunk has lost. He says he lost his keys and they both look under the streetlight together. After a few minutes the policeman asks if he is sure he lost them here, and the drunk replies, no, and that he lost them in the park. The policeman asks why he is searching here, and the drunk replies, “this is where the light is” – The Streetlight Effect

I’m going to make a bold claim that we are searching for life where the starlight can reach, and not where it is most common, in the lithosphere.

One of the outstanding big questions is whether life is common or rare in the universe. With the rapid discovery of thousands of exoplanets, the race is now on to determine if any of those planets have life. This means using spectroscopic techniques to find proxies, such as atmospheric composition, chlorophyll “red edge”, and other signatures that indicate life as we know it. There is the exciting prospect that new telescopes and instruments will give us the answer to whether life exists elsewhere within a decade or two.

The search for life on exoplanets starts with locating rocky planets in the habitable zone (HZ). The HZ is defined as potentially having liquid surface water, which requires an atmosphere dense enough to ensure that water is retained. While complex, multicellular life that visibly populates our planet is the vision most people have of life, as I have argued previously [13], it is most likely that we will detect the signatures of bacterial life, particularly archaean methanogens, as prokaryotes were the only form of life on Earth for over 85% of its existence. Most worlds in the HZ will probably look more like Venus or Mars, either too dry and/or with an insufficient atmosphere to allow surface water. Such worlds will be bypassed for more attractive Earth analogs.

This is particularly important for the most common star type, the M-dwarfs. These stars are often downgraded as having habitable planets due to the flaring of their stars which can strip atmospheres and irradiate the surface. This reduces the likelihood for life at the surface, and for many, is a showstopper.

However, if life established well below the surface, these factors affecting the surface become relatively unimportant. All stars, including M-dwarfs, may well have a retinue of living worlds, but with their life undetectable by current means.

Despite mid-20th-century hopes for multicellular life to be found on Mars or Venus, it is now clear that the surfaces of these planets are devoid of any sort of multicellular based ecosystems. Venus’ surface is too hot for any carbon-based life to survive. The various Martian orbiters and landers have found no multicellular life, and so far no unambiguous evidence of microbial life on or near the surface. The Moon is the only world where surface rock samples have been returned to Earth, and these samples suggest, unsurprisingly, that the lunar surface is sterile [10,12].

NASA’s mantra for the search for life, echoing the HZ requirement, is “Follow the water!” On its face, this makes the lunar surface unlikely as a habitat, similarly Mars, although Mars’ does have an abundance of frozen water below the surface. This leaves the subsurface icy moons as the current favorite for the discovery of life in our solar system, particularly around any hypothetical “hot vents” that mimic Earth’s.

However, when following the trail of liquid water, we now know that the Earth has a huge inventory of water in the mantle, providing a new source of water for the crustal rocks. This water is most likely primordial, sourced from the chondritic material during formation.[6,9] If the Earth has primordial water in the mantle, so might the Moon, as it was formed from the same material as the Earth. A recent analysis of lunar rocks indicates that the bulk of the water in the Moon is also primordial, with concentrations only an order of magnitude less than the water in the Earth’s mantle [1]. While we know Mars has water just below the surface, the same argument about primordial water deep within Mars also follows.

The question then becomes whether this water is in a form suitable for life. Is there a zone in these worlds where water is both liquid and at a temperature below the maximum we know terrestrial thermophiles can survive?

Table 1 below shows some estimates for Earth, Mars and the Moon where a suitable liquid water temperature range exists. The estimated thermal gradients are used to suggest the depths where life might start to be found as temperatures and pressures result in liquid water, and the maximum depth life might survive.

On Earth, the reference planet, the high thermal gradient, and warm surface suggest life can be found at any depth, up to about 5 – 6 km. The Moon, due to a low thermal gradient might only have a habitable zone starting at 15 km below the surface but reaching down to nearly 120 km. Mars is intermediate, with a habitable zone 6-29 km in extent.

Table 1. Estimates of thermal gradients and range of depths where water is liquid, but below 120C as a current approximate maximum for thermophiles

* Assumes the Moon surface temperature would be the same as the Earth without an atmosphere
** [7]
*** [8]

So we have 2 possible rocky worlds in our solar system that may have water reservoirs in their mantles due to primordial asteroids and therefore liquid water in their lithospheres deep below the surface, protected from radiation and with fairly constant temperatures within the range of terrestrial organisms. So our necessary condition of liquid water may exist in these worlds, rather than at the surface.

Given that liquid water may be found deep below the surface, is there any evidence that life exists there too?

In 1999, the iconoclast astrophysicist and astronomer Thomas Gold published a popular account of his theory that fossil fuels were not derived from biological sources, but rather from primordial methane that was contaminated by organisms living deep within the Earth’s crust.[4,5]. While his theory remains controversial, his suggestion that organisms live in the lithosphere has been proven correct. [11]. Bores have shown that microorganisms have been found living at least 4 km below the surface. It has been suggested that the biomass of these organisms may exceed that of humanity on Earth, so life in the lithosphere is not trivial compared to that on the surface of our planet.

Figure 1. Illustration of the search for life in the lithosphere. At this time, life has been found at depths of nearly 4 km, but absent at 9 km where the temperatures were too high.
1. Deep-sea, manned submersibles and remotely operated vehicles collect fluid samples that exit natural points of access to the oceanic crust, such as underwater volcanoes or hydrothermal vents. These samples contain microbes living in the crust beneath.
2. Drilling holes into the Earth’s crust allows retrieval of rock and sediment cores reaching kilometers below the surface. The holes can then be filled with monitoring equipment to make long-term measurements of the deep biosphere.
3. Deep mines provide access points for researchers to journey into the Earth’s continental crust, from where they can drill even deeper into the ground or search for microbes living in water seeping directly out of the rock.
Source: [11]

From the article:

To date, studies of crustal sites all over the world—both oceanic and continental—have documented all sorts of organisms getting by in environments that, until recently, were deemed inhospitable, with some theoretical estimates now suggesting life might survive at least 10 kilometers into the crust. And the deep biosphere doesn’t just comprise bacteria and archaea, as once thought; researchers now know that the subsurface contains various fungal species, and even the occasional animal. Following the 2011 discovery of nematode worms in a South African gold mine, an intensive two-year survey turned up members of four invertebrate phyla—flatworms, rotifers, segmented worms, and arthropods—living 1.4 kilometers below the Earth’s surface.

With our existence proof of a deep, hot biosphere in Earth, is it possible that similar life could exist in the lithospheres of other rocky worlds in our solar system, including our Moon?

Mars is particularly attractive, as there is evidence Mars was both warmer and wetter in the past. There was geologic activity as clearly evident by the Tharsis bulge and the shield volcanoes like Olympus Mons. We know there is frozen water below the surface on Mars. What we are not certain of is whether Mars’ core is still molten and hot, and what the areothermal gradient is. One of the scientific goals of the Insight lander, currently on Mars, is to determine heat flow in Mars. This will help provide the data necessary to determine the range of the habitable zone in the lithosphere.

In contrast, we do have samples of Moon rock. An analysis of the Apollo 11 samples showed that organic material was present, but there was no sign of life except for terrestrial contamination [10, 12]. Since then, very little effort has been applied to look for life in the lunar rocks. The theory that the Moon is desiccated, hostile to life, and sterile, seems to have deterred further work. The early analyses indicated that methane (CH4) is present in the Apollo 11 samples. This may be primordial or delivered subsequently by impacts from asteroids or comets. If we ever discovered pockets of natural gas, even petroleum, on the Moon, this would be a staggering confirmation of Gold’s theory.

So where should we look?

Although the Moon is in our proverbial backyard, the expected depth of liquid water starts well below the bottom of the deepest craters.. This suggests that either deep boring would be necessary, or we must hope for impact ejecta to be recoverable from the needed depths. The prospects for either seem rather remote, although scientific and commercial activities on the Moon might make this possible in this century.

Despite its remoteness, Mars may be more attractive. Sampling at the bottom of crater walls and the sides of the Valles Marineris may give us relatively easy access to samples at the needed depths. Should the transient dark marks on the sides of crater walls prove to be liquid water, we would have samples within easy reach. The recent discovery of a possible subsurface water deposit just 1.5 km beneath the surface of Mars might be another possible target to reach.

The requirement that water is a necessary, but insufficient, condition for life has focused efforts on looking for life where liquid surface water exists. Because of the available techniques, exoplanet targets will be those that satisfy the HZ requirements. While these may prove the first confirmation of extraterrestrial life, they cannot answer some of the fundamental questions that we would like to know, for example, is abiogenesis common, or rare, and is panspermia the means to spread life. For that, we will need samples of such life. For the foreseeable future, that means sampling the solar system. We have 2 nearby worlds, and Gold suggested that there might be 10 suitable Moon-sized and above worlds that might have deep biospheres [5]. That might be ample.

To date, our search for life beyond Earth has been little more than looking for fish in the waves lapping the shore. We need to search more comprehensively. I am arguing that this search needs to focus on the habitable regions of lithospheres of any suitable rocky world. We might start with signs of bacterial fossils in exposed rock strata and ejecta, and then core samples taken from boreholes to look for living organisms. Finding life, especially that from a different genesis would indicate that life is indeed ubiquitous in the universe.

References

1. Barnes, J. J., Tartèse, R., Anand, M., Mccubbin, F. M., Franchi, I. A., Starkey, N. A., & Russell, S. S. (2014). The origin of water in the primitive Moon as revealed by the lunar highlands samples. Earth and Planetary Science Letters, 390, 244-252. doi:10.1016/j.epsl.2014.01.015

2. Davies, P. C., Benner, S. A., Cleland, C. E., Lineweaver, C. H., Mckay, C. P., & Wolfe-Simon, F. (2009). Signatures of a Shadow Biosphere. Astrobiology, 9(2), 241-249. doi:10.1089/ast.2008.0251

3. Davies, P. C. (2011). ? The eerie silence: Renewing our search for alien intelligence. ? Boston: Mariner Books, Houghton Mifflin Harcourt.

4. Gold, T. (1992). The deep, hot biosphere. Proceedings of the National Academy of Sciences, 89(13), 6045-6049. doi:10.1073/pnas.89.13.6045

5. Gold, T. (2010). ? The deep hot biosphere: The myth of fossil fuels. New York, NY: Copernicus Books.

6. Hallis, L. J., Huss, G. R., Nagashima, K., Taylor, G. J., Halldórsson, S. A., Hilton, D. R., . . . Meech, K. J. (2015). Evidence for primordial water in Earth’s deep mantle. Science, 350(6262), 795-797. doi:10.1126/science.aac4834

7. Hoffman N.(2001) Modern geothermal gradients on Mars and implications for subsurface liquids. Conference on the Geophysical Detection of Subsurface Water on Mars (2001)

8. Kuskov O (2018) Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 1–Bulk Composition. Solar System Research, 2018, Vol. 52, No. 6, pp. 467–479.

9. Mccubbin, F. M., Steele, A., Hauri, E. H., Nekvasil, H., Yamashita, S., & Hemley, R. J. (2010). Nominally hydrous magmatism on the Moon. Proceedings of the National Academy of Sciences, 107(25), 11223-11228. doi:10.1073/pnas.1006677107

10. Nagy, B., Drew, C. M., Hamilton, P. B., Modzeleski, V. E., Murphy, S. M., Scott, W. M., . . . Young, M. (1970). Organic Compounds in Lunar Samples: Pyrolysis Products, Hydrocarbons, Amino Acids. Science, 167(3918), 770-773. doi:10.1126/science.167.3918.770

11. Offord, C. (2018) Life Thrives Within the Earth’s Crust. The Scientist, October 1, 2018.

12. Oyama, V. I., Merek, E. L., & Silverman, M. P. (1970). A Search for Viable Organisms in a Lunar Sample. Science,167(3918), 773-775. doi:10.1126/science.167.3918.773

13. Tolley, A Detecting Early Life on Exoplanets. Centauri Dreams, February 2018

14. Way, M. J., Genio, A. D., Kiang, N. Y., Sohl, L. E., Grinspoon, D. H., Aleinov, I., . . . Clune, T. (2016). Was Venus the first habitable world of our solar system? Geophysical Research Letters, 43(16), 8376-8383. doi:10.1002/2016gl069790

15. Woo, M. The Hunt for Earth’s Deep Hidden Oceans. Quanta Magazine, July 11, 2018

Radiolytic H2: Powering Subsurface Biospheres

by Paul Gilster | Jul 2, 2021 | Astrobiology and SETI | 62 comments

Although we’ve been focusing lately on photosynthesis, radiolysis — the dissociation of molecules by ionizing radiation — can produce food and energy for life below the surface and in deep oceans. Our interest in surface conditions thus needs to be complemented by the investigation of what may lie within, as Alex Tolley explains in today’s essay. Indeed, biospheres in a planet’s crust could withstand even the destruction of all surface life. The possible range of microorganisms well beyond the conventional habitable zone defined by liquid water is wide, and while detecting it will be challenging, we may be able to investigate the possibilities in our own system with landers, looking to a day when interstellar probes are possible to explore exoplanet interiors.

by Alex Tolley

“There may be only one garden of Eden here for large life forms such as ourselves. But living beings small enough to populate tiny pore spaces may well exist within several – and perhaps many-other planetary bodies.”

– Thomas Gold, The Deep Hot Biosphere, 1999 [1]

Thomas Gold was probably wrong about subsurface microbes being the source of fossil fuels using fossil methane (CH4), but he was the first to suggest that the newly discovered microbes in the Earth’s crust might be common in other planetary bodies. This essay will explore whether the molecules and energy available from the radiolysis of water (H2O) might support similar biospheres in other worlds in space.

Follow the water, but don’t forget the energy

NASA’s mantra of “follow the water” is important when searching for life, because liquid water is required for carbon-based terrestrial life. Life can still exist if the water freezes, but it will be in a non-metabolizing state and dormant. [Even in frozen water, such as the snow on mountains, a speck of dark material can melt a tiny volume of water around it, allowing microbes to live in these microscopic habitats.]

But while liquid water is necessary, it is insufficient to support life. Inoculate microbes in a dark, sealed flask of distilled water and they will die or go dormant, unable to acquire the energy needed for metabolism. [This is why you can keep containers of distilled water for a long time, even if bacteria contaminate the contents before sealing.]

The rich surface biosphere on Earth is powered by the sun. Photosynthesis fixes the sun’s energy from carbon dioxide (CO2) and water. Before photosynthesis evolved energy was anaerobically harvested from molecules that could liberate energy when respired. Bacteria living in the ocean’s dark, hot smoker vents metabolise the molecules erupting from the mantle and in turn provide the food and energy for the complex life living near these vents.

By the mid-1990s it was accepted that microorganisms discovered kilometers down in the crust were active in the interstices between the mineral grains. Water percolating in these rocks was responsible for keeping these microorganisms actively metabolizing rather than being in a dormant state. But what were they using for food and energy where it was lightless? Carbon was available as CO2 and CH4. The archaea kingdom of anaerobic organisms include all the methanogens that can convert CO2 to CH4 extracting energy and the using the carbon for metabolism. These were the dominant forms of life on and in the early Earth, and possibly the source of the traces of seasonal CH4 detected on Mars.

However, there is another more energetic molecule, molecular hydrogen (H2) that can be used for metabolism. As an electron donor, it can be coupled with an electron acceptor to be part of an energy harvesting metabolism. If H2 is a metabolic energy source, what is its source in the crustal biosphere?

The standard explanation is that some forms of the serpentinization reactions can produce H2 as well as the better known production of CH4.

However there is another source of H2, created by radiolysis of H2O by decaying radioactive elements such as unstable isotopes of potassium (K), thorium (Th), and uranium (U).

Figure 1. Radiolysis of water in the interstices of rock when water is present.

Besides creating H2, radiolysis also produces oxidants which in turn react with the rocks, most notably with sulfides, producing sulfates.

Li et al

“We have demonstrated that the S-MIF-bearing dissolved sulfate in the saline fracture waters at Kidd Creek originates from sulfides in the Archaean host rocks. The most likely mechanism for sulfate production in these anoxic fracture water systems is the indirect oxidation of sulfide minerals by oxidants from radiolytic decomposition of water” [3]

Radiolysis vs Serpentinization

Experiments on the radiolysis of water suggested that radiolysis was not an important source of H2 compared to serpentinization. Serpentinization occurs wherever high iron (Fe) igneous rocks from the mantle, water and heat interact. The ocean ridges between the plates are important zones where this takes place.

However, later experiments with oceanic sediments showed that radiolysis production of H2 was catalyzed by the minerals increasing production of H2 many fold. In the sediments on the ocean floor it was found that radiolysis was the main source of H2 as an energy source for microbes.

Sauvage et al: [9]

“Radiolytic H2 has been identified as the primary electron donor (food) for microorganisms in continental aquifers kilometers below Earth’s surface. […] all common marine sediment types catalyse radiolytic H2 production, amplifying yields by up to 27X relative to pure water. […] Comparison of radiolytic H2 consumption rates to organic oxidation rates suggests that water radiolysis is the principal source of biologically accessible energy for microbial communities in marine sediment older than a few million years.”

Moreover, radiolytic H2 is as dominant a source of food and energy as marine photosynthesis powered by the sun.

Sauvage et al: [10]

“[…] radiolytic H2 production in marine sediment locally produces as much electron donor (food) as photosynthetic carbon fixation in the ocean.”

In summary globally, radiolysis can provide both food and energy comparable to that of the marine photosynthetic organisms.

Methanogens & Sulfur-reducing bacteria

In anoxic environments where both archaeal methanogens and sulfur-reducing bacteria coexist, the biomass and types of the latter are greater than the former. One reason may be that the available energy from the reduction of sulphur from sulfates is greater than the reduction of carbon to CH4 from CO2.

CH4 can be created by the reduction of carbon dioxide.

Serpentinization is a geologic source. However, archaean methanogens are believed the dominant source of methane in the atmosphere on the early Earth reducing carbon dioxide anaerobically to methane [12]. It is the uncertainty of the source of the methane detected on Mars that intrigues astrobiologists.

Sulfate-reducing bacteria such as Desulfovibrio and Desulfobacter use the H2 to reduce sulfates created by the radiolysis oxidants on mineral sulfides to again reduce the sulfur to silfides.

As table 1 indicates, this is a more energetic reaction than methanogenesis and may account for the very many different bacteria utilizing H2 and sulphate as an energy source.The radiolytic oxidants also react with CH4 to form simple organic molecules such as formate and acetate which can be used as food sources by bacteria, further indicating the value of radiolysis in maintaining a subsurface habitat.

Habitability

As I have noted in previous posts, the search for life has typically been focused on surface-living, complex, aerobic life, as the low hanging fruit of detectability. This restricts the search to planets in the habitable zone (HZ). However, as unicellular life dominated Earth’s history and anaerobic respiration was dominant until the evolution of photosynthesis, and the Great Oxidation Event increased the partial pressure of oxygen in the atmosphere, such worlds may give rise to false negatives when analyzing the atmosphere by spectroscopy. Furthermore, as professor Tyrrell has suggested, surface life on Earth-like worlds may have a low probability of being sustained over 3 billion years due to events perturbing the surface temperature into runaway conditions.

Unlike the variable conditions on the surface, subject to wide ranges of conditions and vulnerable to cosmic and geologic disruption that saw 5 major extinctions on Earth, as well as an ongoing 6th extinction in the Anthropocene, conditions in the crust are far more stable, and less vulnerable to the disruptions on the surface. Such crustal biospheres once established may survive even after surface life has been extinguished, especially once the star’s luminosity renders the surface uninhabitable.

Such a biosphere may even allow for an evolutionary reset starting with microorganisms should surface conditions become uninhabitable for a temporary period and subsequently returning to habitability.

So we have evidence that life is in the subsurface crustal rocks and that radiolysis may be an important, if not the most important, source of food and energy for this life. But what about bodies elsewhere?

Other Celestial Bodies

1. Mars

Mars has all the same ingredients as Earth for subsurface microorganisms to live. At some depth below the surface the temperature should be sufficient to create liquid water [13]. While serpenitization can occur, especially to generate CH4, CO2 should be available for methanogens to respire and release CH4. Residual radioactive elements should be able to produce the needed H2 and SO4.for sulfur reducing bacteria. The race is on to determine whether there is an extant microbial biosphere on Mars. Looking for frozen microbes in ejecta from large meteor impacts that have penetrated to the needed depths might be the easiest approach for robotic vehicle discovery. If there are any near surface hot spots from residual volcanism or local concentrations of radioactive elements, these might also be good places to look. Whether earth, Venus, or Mars is the original world where abiogenesis occurred, panspermia between these worlds due to ejecta and microbes propelled by solar radiation, early Mars may have been a home for life.

Tarnas et al: [14]

“We have demonstrated that radiolysis alone produced sufficient quantities of reductants to have sustained a subsurface biosphere during the Noachian for hundreds of millions of years. Given sufficient oxidant availability, this habitat could have fostered chemolithotrophic microbial communities that would have imprinted organic, morphological, and isotopic biosignatures on their habitat’s host rock.”

2. Icy moons

Jupiter’s Europa and Saturn’s Enceladus are 2 icy moons that have subsurface oceans and tidally induced warming. Radiolysis of subsurface water in the core below the ocean sediments should provide the conditions necessary for a microbial biosphere. Analysis of the plumes by a flyby or orbiting probe is one method to search for life, although this is more likely to detect life in the oceans, rather than below the ocean-crust interface. This later will prove a much more difficult target.

3. Titan

Saturn’s moon Titan is believed to have a rocky core, overlaid with a liquid ocean, and topped with hydrocarbons with a dense nitrogen atmosphere. As with the icy moons, below the crust-ocean interface is a possible biosphere.

4. Ceres

Like the icy moons, Ceres has a rocky core overlaid with brines. Evidence of cryovolcanism suggests that these brines must be partially liquid. Unlike the icy moons, there is no tidal heating. This suggests that any biosphere in the core must be powered by radiolysis from any residual radioactive element decay. Catillo-Rogers recently suggested Ceres has the potential to host life as it has the radioactive elements for both heating and radiolysis. [5]

5. Eris

The Trans-Neptunian dwarf planet Eris has a density of 2.52 g/cm^3, indicating that it must be composed of rocky material and ices. If, like Pluto, there is evidence of cryovolcanism then it is possible that a core with radioactive elements and liquid water provides a habitat for a microbial biosphere. If so, then other dwarf planets extending out into the Kuiper belt could also have similar subsurface habitats.

6. Comets and Kuiper belt Objects

Holm focused on serpentinization for the production of CH4 and H2 on celestial bodies [7] He notes that radioactivity could also warm these bodies, making serpentinization possible. However, he did not consider radiolysis that might have been an important contribution to the production of energy rich molecules that could be used for metabolism.

Comets and their parent bodies, such as Transneptunian Objects (Kuiper Belt Objects—KBOs), accreted from a mixture of volatile ices, carbonaceous matter, and rocks in the coldest regions of the protosolar nebula. […] However, the rocky material contained in comets includes radioactive isotopes, whose decay can provide an important source of heat, possibly significantly altering the internal structure of these icy objects after their formation. There is a general agreement that short-lived radioactive isotopes like 26Al and 60Fe could have played a major role during the early evolution of both comets and their parent bodies, possibly leading to the melting of water ice and to the triggering of serpentinization and FTT reactions.

A more recent paper by Bouquet emphasized the importance of radiolysis in icy bodies which not only produced H2, but sulfates to support metabolism [2].

We found that radiolysis can produce H2 quantities equivalent to a few percent of what is estimated from serpentinization. Higher porosity, which is unlikely at the scale of a body’s entire core but possible just under the seafloor, can increase radiolytic production by almost an order of magnitude. The products of water radiolysis also include several oxidants, allowing for production of life-sustaining sulfates. Though previously unrecognized in this capacity, radiolysis in an ocean world’s outer core could be a fundamental agent in generating the chemical energy that could support life.

7. Rogue/Free Floating Planets

Rogue planets ejected from their systems would include bodies similar to those in the solar system. Given the prevalence of conditions needed for a subsurface biosphere, especially in bodies at the edge of our system, there seems every reason to believe that these rogue planets should also host subsurface conditions suitable for a microbial biosphere.

Habitable yes, but inhabited?

The above suggests that if radioactive elements can also heat the surrounding material so that water is kept liquid, then almost any celestial body with a rocky material and water could potentially be a microbial habitat in space, irrespective of whether it is in the HZ or not. As suggested earlier, planets in the HZ that have lost surface habitability could retain refugia for life in the crust.

For other non-Earth-like bodies which may have the conditions for a subsurface biosphere the question becomes whether they are living or sterile. Is abiogenesis possible on these worlds, or must they be inoculated by life from living worlds? We don’t yet know the answers to such questions, but it does suggest that astrobiologists take seriously the possibility that any body with a suitable subsurface environment could be inhabited and therefore instruments to detect such life should be included with exploratory probes. As we increase the exploration of our system, landers and rovers should include technologies to detect life, especially within the habitable zone below the surface.

Could we detect subsurface biospheres on exoplanets?

Detection of subsurface biospheres on exoplanets is going to be very difficult. Seager [6] produced a catalog of possible biosignature molecules, of which hydrogen sulphide (H2S) is primarily of biologic origin and therefore its presence is likely an unambiguous biosignature.

Although H2S is likely to have a very low concentration in the atmosphere it has a distinctive IR signal which could be detectable in principle,

As we can currently only analyze exoplanets spectroscopically, if it becomes possible to detect the very small amounts H2S in an otherwise unpromising atmosphere with possibly unsuitable surface conditions for life, then we should attempt to devise the technology to detect the presence of this gas as an unambiguous biosignature.

Based on the terrestrial history of life, it seems likely that on living exoplanets, extant life will be mostly unicellular, possibly even just prokaryotes. On Earth-like worlds geologic processes will ensure that such life will also inhabit a deep, crustal biosphere. Detecting such life will be very difficult, but perhaps not impossible with suitable technology. In the distant future, interstellar probes with landers should be able to detect such life as we explore our stellar neighborhood and catalog and map the forms of life we find.

References

Gold, T. (2021). The Deep Hot Biosphere: The Myth of Fossil Fuels (November 6, 1998) Hardcover. Springer.

Bouquet, A., Glein, C. R., Wyrick, D., & Waite, J. H. (2017). Alternative Energy: Production of H 2 by Radiolysis of Water in the Rocky Cores of Icy Bodies. The Astrophysical Journal, 840(1), L8. https://doi.org/10.3847/2041-8213/aa6d56

Li, L., Wing, B. A., Bui, T. H., McDermott, J. M., Slater, G. F., Wei, S., Lacrampe-Couloume, G., & Lollar, B. S. (2016). Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks. Nature Communications, 7(1). https://doi.org/10.1038/ncomms13252

Lin, L. H., Wang, P. L., Rumble, D., Lippmann-Pipke, J., Boice, E., Pratt, L. M., Lollar, B. S., Brodie, E. L., Hazen, T. C., Andersen, G. L., DeSantis, T. Z., Moser, D. P., Kershaw, D., & Onstott, T. C. (2006). Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome. Science, 314(5798), 479-482. https://doi.org/10.1126/science.1127376

Castillo-Rogez, J. C., Neveu, M., Scully, J. E., House, C. H., Quick, L. C., Bouquet, A., Miller, K., Bland, M., De Sanctis, M. C., Ermakov, A., Hendrix, A. R., Prettyman, T. H., Raymond, C. A., Russell, C. T., Sherwood, B. E., & Young, E. (2020). Ceres: Astrobiological Target and Possible Ocean World. Astrobiology, 20(2), 269-291. https://doi.org/10.1089/ast.2018.1999

Seager, S., Bains, W., & Petkowski, J. (2016). Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry. Astrobiology, 16(6), 465-485. https://doi.org/10.1089/ast.2015.1404

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