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Is Most Life in the Universe Lithophilic?

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

WorldSurface CThermal
Depth (km)
at 120C (with
0C at
Depth (km) at
0C with
surface temp
Depth (km) at
120C with
surface temp
Mars-636.4-10.6 **11-196-1018-29
Moon-18 *1.17 ***10315118

* 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.


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


Comments on this entry are closed.

  • Andrew Palfreyman January 11, 2019, 17:11

    The statements made about Martian life seem premature, bearing in mind that not a single microscopic assay has yet been done over the first few centimetres of surface depth.

    • Alex Tolley January 11, 2019, 22:58

      That is why it is a bold [and testable] claim. ;)

  • Christian G January 11, 2019, 18:24

    The key question of course, concerns the conditions that lead to abiogenesis. Certainly existing extremophiles show that life can adapt to and survive in environments previously thought unlikely, but it seems probable that life coming into existence has more stringent requirements. And of course, the possibility of lithopanspermia complicates things further.

    • Alex Tolley January 11, 2019, 23:05

      I don’t disagree. Let me be clear that I am not making a claim that life can evolve on worlds that are dry. I think that conditions have to be more conducive for abiogenesis to occur. What I am saying is that once life has emerged, even if the planet now looks uninhabitable, like Mars, that life will retreat and survive in the crust as long as there is available water for life to use. There is also a case to be made that panspermia may be the start of life on some worlds and that such life can exist in the rocks.

      You may recall that posts on CD have indicated that planets may move into and out of their HZ. Their atmospheres may even be stripped. What I am trying to convey is that once life starts, the lithosphere may prove a longer lasting, stable habitat, even after surface conditions are no longer supportive of life.

  • Robin Datta January 11, 2019, 21:07

    Thank you, Alex, for this synopsis of the topic.

    The zoology that I had to learn involved specialization in just one species, but from readings elsewhere I have come to view biology as an extension of chemistry through organic chemistry, biochemistry and molecular cell biology. The few billion years it took for life to appear on earth can perhaps be attributed to the time it took for panspermia to seed the earth, or the time it took to cook up the primordial “proto-life” from the “non-living” here on earth.

    Mutational variation with natural selection makes organisms almost unrecognizable in reference to their origins; absence of conditions needed for the incubator do not preclude the presence of derivative life forms.

    Recognizable intelligence is in quite another category from physics-chemistry-biology. And here again, absence of the conditions conducive to biology do not exclude intelligence, albeit post-biological.

    • Alex Tolley January 12, 2019, 0:46

      If the Earth formed 4.5 bya, the earliest signs of life appear around 3.5 bya. A mere billion years, and that may be an overestimate because we cannot find older rocks. It has been argued that life appeared as soon as it was possible, so maybe life may either emerge relatively easy or seed easily.

      I do see articles about how we are homing in on what LUCA might have been like, which is fascinating. Hopefully working backward to determine even earlier forms that are just at the point of becoming living. My guess is that the transition might be abrupt from non-living to living, rather than a gradual change. A phase change if you will.

      Intelligence is a whole different ball of wax, as the arguments in the AI community indicate. That is a continuum from the most basic life forms, all the way to human level and beyond. Funnily enough, I was just watching a lecture by David Grinspoon (posted on FB by ljk) which included things that we might watch for as indicative of intelligent (human technological level) life on exoplanets.

  • Michael C. Fidler January 11, 2019, 23:31

    One way of finding subsurface life or organic material is the lava tubes on the Moon and Mars. If subsurface life/H20 or oil/methane exists it may of found it way thru the lava and could be sampled via robotic probes. The Moon and Mars should have huge and long lava tubes because of the lower gravity.

    “Water takes a deep dive into an oceanic tectonic plate.”
    This is the place that we should be drilling for deeper subsurface life on Earth: https://media.nature.com/w800/magazine-assets/d41586-018-07335-8/d41586-018-07335-8_16268680.jpg
    The trenches are dragging the sea water down and microbes with it and may go much deeper for life then anywhere else. What about viruses? The oceans are teaming with them, so many that only a small percentage have been classified, what have the boreholes found?

    Please read the comment in the last post on Barnard’s star b.
    What could be the most common place to find life and the most common type of planet is snowball Super Earths. We have very little info on these type of planets because most of the planets discovered so far have been of short period and inside the snowline, even radial velocity favors larger planets. So could a tecno civilization develop on an ice covered planet? Volcanic and tectonics may be more powerful on these planets, creating areas that minerals can be accessed. This would be a good area for simulation of such planets especially if tidally locked.

    • Michael C. Fidler January 12, 2019, 11:27

      Mud Volcanoes on Mars!
      Fluids mobilization in Arabia Terra, Mars: depth of pressurized reservoir from mounds self-similar clustering.

      “Arabia Terra is a region of Mars where signs of past-water occurrence are recorded in several landforms. Broad and local scale geomorphological, compositional and hydrological analyses point towards pervasive fluid circulation through time. In this work we focus on mound fields located in the interior of three casters larger than 40 km (Firsoff, Kotido and unnamed crater 20 km to the east) and showing strong morphological and textural resemblance to terrestrial mud volcanoes and spring-related features. We infer that these landforms likely testify the presence of a pressurized fluid reservoir at depth and past fluid upwelling. We have performed morphometric analyses to characterize the mound morphologies and consequently retrieve an accurate automated mapping of the mounds within the craters for spatial distribution and fractal clustering analysis. The outcome of the fractal clustering yields information about the possible extent of the percolating fracture network at depth below the craters. We have been able to constrain the depth of the pressurized fluid reservoir between ~2.5 and 3.2 km of depth and hence, we propose that mounds and mounds alignments are most likely associated to the presence of fissure ridges and fluid outflow. Their process of formation is genetically linked to the formation of large intra-crater bulges previously interpreted as large scale spring deposits. The overburden removal caused by the impact crater formation is the inferred triggering mechanism for fluid pressurization and upwelling, that through time led to the formation of the intra-crater bulges and, after compaction and sealing, to the widespread mound fields in their surroundings.”


      Maybe this is where the methane is coming from!

      Alex Tolly thank you for the giving Thomas Gold’s view on the deep hot biosphere, and do you have any references for David Grinspoon thoughts on what we might watch for as indicative of intelligent (human technological level) life on exoplanets?

      • Robin Datta January 12, 2019, 15:08


        Dysonian Approach to SETI: A Fruitful Middle Ground?

        • Alex Tolley January 12, 2019, 19:26

          A paper that provokes a lot of discussion. Written in 2011, I think it is now almost positing a strawman argument, as search for artifacts is being done, most notably for Dyson spheres/swarms as posts on CD establish.

          That we haven’t found evidence of Dyson spheres, especially significant numbers in other galaxies still leave the Fermi Question unsolved. But we should definitely pursue these avenues with SETI.

          • Wojciech J January 14, 2019, 19:25

            As I understand there are numerous Dyson Sphere candidates but we are unable to confirm them.

            • ljk January 16, 2019, 11:08

              Including galaxy NGC 5907, which has a LOT of what astronomers are claiming to be red dwarf stars – a lot more than should be typical for a galaxy of its type and age.

              Lots of infrared signatures, which is what a Dyson Shell would look like from a distance. Now imagine a whole lot of them making up a Kardashev Type 3 civilization, one that spans an entire galaxy.

              Anyone looking for Dyson Shells giving that stellar island a proper look?

        • Michael Fidler January 13, 2019, 6:47

          Good paper and makes a good point in the difference between orthodox SETI and Dysonian. Any ETs looking for intelligent life on earth before the 20th century would have to look closely. What is the one structure that would stand out for the last 4500 years? The Giza Pyramids and possibly other ancient stone astronomical structures that may be much older.
          What contact have we had over the ages and to a much more significant degree in the last 70 years that may indicate advanced civilizations with very long term projects in making contact? Radio communication in these cases have been short term and close in and usually of the interference type. But structures have been observed of unusual character and some very large and bizarre.
          So could they be telling us what to look for and that radio is not the formal way of communicating.

      • Alex Tolley January 12, 2019, 19:37

        So there is likely water, and therefore habitat, a mere 2-2.5 km below the surface. That makes drilling much more practical.

        A talk by Grinspoon on thinking about how to detect intelligent/technological ET.

    • Alex Tolley January 12, 2019, 11:29

      While lava tubes are relatively accessible, they are not deep enough to ensure liquid water is present. They may be nearly as dry as the surface. On the Moon, we are talking many kilometers deep below the surface.

      Regarding viruses. I would assume that they are indeed at depths. However, viruses cannot replicate with a host and do not metabolize. This makes it hard[er] to determine their existence, although we determine their presence in the oceans by extracting the DNA and RNA.

      Snowball planets may well have life beneath the glaciers. They do require surface water in abundance to get in that state, as Earth is believed to have done in the past, and liquid water beneath the ice was present to allow oceanic life to continue. However, if we look outwards, Mars is certainly frigid, but the water is largely absent from the surface, although in fairness you could argue that the “snowball” is still in effect, just covered in sand and rock, rather than exposed and maintaining a high albedo. If planets are never out of a snowball state, they are in effect similar to icy moons, albeit with less think surface ice. However, I think that “snowball” conditions imply that they have surface water and that this was maintained by a dense atmosphere. Do we know of any exoplanets that are in a permanent snowball condition, rather than a temporary one?

      As for technological civilization developing on a snowball planet, I have no idea. Can complex life develop on a world that is permanently 100% ice covered? Our world depends on photosynthesis for its main energy flow. A snowball planet would be hampered in that regard. An existing technological civilization could colonize such a world, although as I and others have suggested, why bother? Build space habitats instead.

      • Andrei January 14, 2019, 14:04

        “Can complex life develop on a world that is permanently 100% ice covered?”
        I seriously doubt it can, complex life as far as we know developed in quite more benign environments. (Either one or several, depending on which hypothesis you prefer.)
        Compared to life that get the energy from sunlight the chemosytesis era were slow, here on Earth life were only microscopic one cell organisms until photosynthesis came around – and it still took eons before cambrian to happen. By then there were algae and quite likely smaller simpler plants like moss and lichens.

    • Alex Tolley January 12, 2019, 11:49

      Re: Barnard’s star.
      I note in teh article that the mass of the planet (3.2 Me) makes it far more likely it is a gas planet, rather than a rocky one. However, that doesn’t mean that a hypothetical planet of the right size couldn’t be an ice world, even one with life. The speculation was that life would be at volcanic vents that ensured liquid water around it. It isn’t clear to me how different this is from an icy moon.

      Where I think my viewpoint differs is the nature and location of life. The focus on life at hot vents is less on the bacteria, and more on the complex life that feeds on the bacteria. This complex life must live where there is cooler, liquid water. What I am suggesting is that life is restricted mostly to prokaryotes and that it lives mainly in the tiny pores in the rocks. In the case of worlds that are very cold or very dry on the surface, that life may be living mostly well below the surface, although not exclusively so.

  • AlexT January 12, 2019, 4:08

    “Is Most Life in the Universe Lithophilic?”….

    Misleading header for article, someone, who less informed about present scientific ET life search “achievements” , can wrongly understand that we have already found ET lithophilic life :-) That is wrong conclusion…
    So if we will follow article logic , we can ask more common question:
    “Is Most Life in the Universe – Earth like?” base ananswered question, that is bothering human civilization since stone ages (or even earlier).
    So – why lithophilic only? Why Other types or subtypes of life matter on the Earth less important?

    • Alex Tolley January 12, 2019, 11:03

      Do you have a better title for the piece given your interpretation of the content?

      • jlandau January 12, 2019, 13:42

        Yes, I’d like to see a response to that myself. The title is clear and well-chosen.

      • AlexT January 12, 2019, 18:56

        I suppose that question : “Is Most life on the planet Earth Lithophilic?” at least closer to reality as we know it today :-)
        We know nothing about the life outside the Earth, may be “most life” is “vacuumphilic” , may be it is not exists outside the Earth at all …

      • Robin Datta January 13, 2019, 0:18

        The window of time for abiogenetic emergence or panspermic seeding of life would appear to be much smaller than that for the existence of lithospheric life following the genesis or seeding. It inplies a potintial for persistent biologic life in rocky bodies that have an appropriate history.

        Although it is physically imposible to “leave no stone unturned”, any object of approprate size and shape with an appropriate history could be fair game.

        Viruses, although metabolically inactive, can reveal much about their hosts: host surface defenses from viral membranes and capsules, origins of that life from chirality and the nucleotide set, host metabolic pathways from the viral genome and proteins, etc. And above all, the presence of viruses indicates their origin from life somewhere.

        • Alex Tolley January 13, 2019, 13:40

          host surface defenses from viral membranes and capsules, origins of that life from chirality and the nucleotide set

          Good point. Genetic code too.

          Tangentially, suppose Earth was subject to panspermia from alien viruses with different genetic codes? Can we be sure that the terrestrial code that generates the capsid coat proteins from the viral DNA is producing the same proteins the virus started with? Maybe quantum computing could test the possibility, otherwise, it is nearly intractable AFAICS. A better bet would be to try to snare viruses in deep space heading sunward to determine if their coat proteins are using the same code on their DNA. If it was different…

          The same technique could be used for any other organism that we detect coming in from interstellar space. But that is another topic.

          • Andrei January 15, 2019, 6:37

            Virus cannot procreate without a host cell, they are devolved life. There has been some suggestion that something like what you describe could be used to send messages within the genetic code.

            How life started is still unsolved. But the RNA hypothesis have got a new iteration, and now look more promising.
            We always known that the mitochondria resemble small organism cells in themselves, they cannot live on their own today. But then they have devolved too, as they are in a safe environment.
            Perhaps we’re symbiotic life too, no better than lichen! :)

  • Bruce D. Mayfield January 12, 2019, 12:51

    I’d have to respectfully answer the article’s speculative title question with a no. Can rock-bound earth biomass outweigh 8 billion humans? Maybe so, but we amount to only a tiny fraction of earth’s total biomass. Yeah, this point is earth centric thinking, but lithospheric life doesn’t dominate here, so why expect it to dominate elsewhere? It’s an interesting question, but it amounts to speculations plied on top of many other speculations. A logical tower needs a sturdy foundation.

    For lifefroms to grow abundantly isn’t freedom of motion important? Life locked in solid rock may have roles to play, but how can it become very widespread?

    • Alex Tolley January 12, 2019, 18:57

      Re: Biomass. From The Scientist article:

      …Earth’s current biomass,16 though most estimates are now below 15 percent. Before the rise of land plants around 400 million years ago, though, deep biomass could have outweighed life on the surface by an order of magnitude…

      Some points.
      1. Up to 15% of the Earth’s biomass is pretty significant.
      2. Before land plants arrived (10% of Earth’s history), the subsurface biomass was far larger.
      3. Any random planet in the HZ will likely still be in the pre-land plant evolutionary phase, where the prokaryotes in the crust will represent the dominant biomass.
      4. Planets that were living and now dead on the surface will again be dominated by subsurface prokaryotes. Earth will be in that state again within 2 bn years.

      Therefore I think that there is a reasonable claim that subsurface prokaryotes are the most likely form of life in the galaxy.

      As microbiologists tend to say: Life on Earth is mostly bacteria (by numbers and species) with a small number of complex life forms. ;)

      Re: motility. I think the best time frame to look at this is over many thousands of years, rather than human lifetimes. It is estimated that bacteria can reach the deepest levels within thousands of years, a mere eyeblink in Earth’s history. I see this as analogous to technological expansion across the galaxy. From a human perspective, we worry about travel times to the stars, limited to c, and probably much less. Yet at c, civilization can fill the galaxy well within a million years, just 0.4% of 1 galactic rotation, and perhaps 0.007% of the age of the galaxy. For a long-lived observer, that would appear to be very fast, while it is a very long time for human observers.

      But to address the motility argument directly. Plants are not mobile, other than being able to spread seeds by wind and animals. Their roots can penetrate rocks in some cases. Lichens use acids to penetrate rocks. Terrestrial plants clearly dominate the planet given enough time. Bacteria living in colonies and biofilms are hardly very motile, yet they can spread over time. The limitation for lithophilic prokaryotes is food to metabolize. For prokaryotes living in the cold, deep ocean muds, the limitation is temperature, not food. Yet given enough time, bacteria in rocks will migrate and fill their ecological space. Once occupied, that environment is going to be much more stable than conditions on the surface. Even a major asteroid impact will not eliminate these organisms.

      • Bruce D. Mayfield January 13, 2019, 0:46

        Alex Tolley and Gary Wilson below, some of my objection was well answered, I must admit. (The motility issue.)

        The problem I still have is with the fundamental abiogenesis origin of life explanation. This is assumed to be true by so many that it is taken as an unquestionable, well established fact. But abiogenesis is not the only viable explanation for how life occurs …

        • Ron S. January 13, 2019, 11:53

          Such as…?

          • Gary Wilson January 13, 2019, 13:51

            Somewhere in the universe abiogenesis must have occurred surely. For me, panspermia is just passing the buck. Abiogenesis may be a rare event or it may occur whenever the correct precursor molecules form in the presence of available energy and water. I suggest the latter is far more likely to be true. I don’t believe the Earth is a “special location” and I don’t believe panspermia really explains anything regarding the actual origins of life.

          • Bruce D. Mayfield January 13, 2019, 16:35

            Intelligent design, since you asked. The forbidden solution.
            The possibility that DNA itself is an artifact of an ETI.

            • Ron S. January 13, 2019, 18:13

              No. You said “viable explanation”, not explanation. And, “forbidden”? Really?

              On your second sentence you need to explain how ET arose without abiogenesis. Shifting the location of abiogenesis explains nothing, as Gary also mentions.

            • Alex Tolley January 13, 2019, 18:20

              Like the “engineers” in Prometheus or the Martians in Mission to Mars? The problem is that this also passes the buck, requiring the original abiogenesis to be passed to ET. We don’t want to get into an “engineers [turtles] all the way down” situation.

              A naturalistic explanation requires abiogenesis at some point. Ideally on Earth. Panspermia from a nearby genesis in the solar system is a 2nd best. If panspermia was to prove the dominant mechanism for spreading life, we would want to know how abiogenesis occurred to start the process off.

        • AlexT January 14, 2019, 3:48

          Abiogenesis – is only explanation for how life occurs, panspermia and ETI gene engineering – is only possible scenario for how life can develope and spread around universe, but in the beginnig there was important misteriuos transformation from “dead” matter to the first live organism(s).
          There is serious chance that this first life was started on our planet…
          Even if you believe DNA was created by advanced ETI – you should somehow explain how those super-ETI evolved from “dead” matter.

      • Michael C. Fidler January 14, 2019, 11:41

        Here is where it may of all started:
        Biomarker for life found in space for the first time.(Methyl chloride)

        The good news is that a molecule thought to be a biomarker for life has been found for the first time in abundance in a comet and around a young star.


        Microorganism floating around in space looking for a restaurant, find new one just opened around a young star. They fill up on all the nice organic molecules floating around the new star and decide to stay the night. End up in a comet with lots more organic compounds. Finally reach a home impacting on superearths! (there may be variations on this depending on how much krypton resulting in super Microorganism evolving into Superman on the Superearths)

        Ok, just thinking out loud but could be where all that oil was deeep impacted into earth so instead of just diamonds, it let the oil and the microorganisms squeeze thru those cracks. Maybe even large oceans of oil deep in the earth from Titan size comets! Moon, Mars, Mercury and Venus boiled of all the surface goo long ago. Hey, they still find meteorites that are just slim and goo, so who knows!

        See also: The Sun and Solar System Debris, by William R. Corliss.
        Page 203 Paragraph X3. Petroleum and coal-like nature (Kerogen) of organic matter in carbonaceous chondrites.

        • Alex Tolley January 14, 2019, 13:04

          Sara Seager needs to amend her list of molecules that can be used as biosignatures (if her team has not already done so.)

  • Gary Wilson January 12, 2019, 16:49

    I think the lithosphere might harbor life in less hospitable places than Earth provides Bruce. I think that might be the key point. Clearly the Martian surface is less hospitable than much of Earth’s surface but an enormous amount of microbial life might exist below ground near water sources as Alex has stated. Overall if we tend to think there will be many more inhospitable but not impossible places for life outside of Earth than there are earthlike worlds in habitable zones then I would be willing to make a small wager that lithospheric life might be the more common form.

    • John Marcello January 13, 2019, 9:45

      Maybe in this paradigm it makes more sense to consider HZ to mean “hospitable zone”, instead of the more compatible “habitable zone.”

  • galacsi January 12, 2019, 17:27

    This article meets some of my thinking about the Moon. But First the figure of -18°C on the surface is average and the depth at 0C will be very different at poles and at equator. I am sure 0°C will be much closer to the surface at the equator.
    The second point is that if there is water in the moon, near the surface it will exist in the form of ice and will form like a plug for the gases emitted by the interior of the moon. Like argon or helium. Or may be methane and other gases. The water vapor will give ice. And under the surface, the Moon must behave like a football ball or a better comparison a waterbed. First with a surface layer of desiccated rock and then a layer of rocks full of ice, then gases and finally rocks impregnated with liquid water. this may explain why the moon sounds like a bell when it is hit on the surface. This means that it is not necessary to go to the poles to get water, but that it is better to drill near the equator. If I am right it would be great news for the future of humanity on the Moon.

    • Alex Tolley January 13, 2019, 13:48

      I am sure 0°C will be much closer to the surface at the equator.

      I did not use the official. average temperature (although it is very similar) but rather the average surface temperature of the Earth if it did not have a GH atmosphere. As for local variation, it is important to know what the average stable temeperature is. This will lie some distance from teh surface. When energy was expensive in teh US in teh 1970’s, and some architects like Malcolm Wells were advocating, and building, Earth sheltered and underground homes, the rule of thumb was that the average stable temperature a few meters below the surface was about 50-55F year round. With the extremes of temperature on the lunar surface, especially at the equator, the stable temperature will be below the surface. I don’t know what that depth or temperature would be. It would be nice to have a map with isotherms for various depths.

  • Brian Schmidt January 12, 2019, 19:49

    Great article! My question is, what about Mercury? Mean surface temp at the equator is 67C, near the poles it’s -73C, so a lithospheric temp above 0C should be at or near the surface everywhere except high latitudes.

    There’s presumably a dessicated zone a few 10s/100s of meters thick at the surface due to daytime heat, with that zone being shallower or virtually nonexistent at high latitudes. Below that zone (and below the cryosphere at very high latitudes), liquid water could be present in the lithosphere.

    I suppose Mercury may not have been considered because it wasn’t a likely place for life to originate on its own. However, Mercury might be the best current candidate for transpermia from an Earth meteorite bearing microbes. A lot easier for an impact of a microbe-bearing meteorite to get a few 10s of meters down than it is to get a few kilometers down.

    • Alex Tolley January 13, 2019, 0:14

      I think Tom Gold would put Mercury in the deep hot biosphere category as he believed that this was where life started. I think that abiogenesis or panspermia needs either more clement conditions. Mercury was probably never a planet with sufficient conditions for abiogenesis or panspermia to take told. Having said that, we might look for subsurface life on Mercury, especially if we ever start to mine it for resources.

      • Brian Schmidt January 13, 2019, 18:57

        “Mercury was probably never a planet with sufficient conditions for abiogenesis or panspermia to take told.”

        I’ll buy that regarding abiogenesis, but for the reasons I described above, it might be the best case for transpermia, at least since Mars entered modern climatic conditions several billion years ago.

        I’ll also add that Tom Gold turned out to be right in some sense about a hot biosphere here on Earth, but the rest of his theories are unproven and I think not generally accepted. My point being that this doesn’t have to be argued from his perspective while still being relevant to the general concept of a lithosphere being a good place to look for life.

        • Alex Tolley January 14, 2019, 12:38

          Re: panspermia on Mercury. Suppose a rock with bacteria plowed into Mercury today. Even if it impacted in a good site and burrowed itself just beneath teh surface, would that be sufficient for panspermia to work? OTOH, if a world like Mercury had a hydrated surface, I think that such a rock would be more likely to seed the lithosphere. Maybe many such rocks would eventually get lucky in seeding Mercury despite its surface conditions. The probabilities just seem to favor it working when the surface is more favorable to life, allowing life to establish and penetrate down into the lithosphere. But as I said, I would look in case serendipity strikes.

          • Alex Tolley January 14, 2019, 12:48

            Igneous rocks are perhaps the most hostile environment in which deep subsurface microbes exist. Due to the processes needed to create igneous rocks, i.e. extreme temperature and pressure, these habitats are effectively sterilized at their creation. Microbes can only colonize the rocks when they have been removed from the hostile surroundings, usually by tectonic processes. Once the rocks cool to below the temperature threshold for microbial life (as far as we know it) the microbes must be transported there by the infiltration of groundwater from above. The groundwater penetrates microscopic fissures and spaces between the crystals and the microbes begin to take hold.

            source: Deep subsurface microbes.

            The suggestion that water must infiltrate igneous rocks for microbes to colonize suggests to me that if the surface is baked, it will be hard for panspermia to operate as a seeding mechanism. However, once established, if there is sufficient subsurface water, then microbes can colonize worlds like Mercury where sedimentary rock is [presumably] absent.

            • ljk January 16, 2019, 11:19

              What about Venus? It did have a global ocean at one time long ago apparently. Did any life forms that could retreat under the surface where it would be safe from the very hostile conditions above?

              Just thinking about this makes me realize how little we have actually explored Venus and the other worlds of our Sol system. We have literally barely scratched their surfaces, and most of those “scratches” have not been in the search for native life, alive or fossilized.

              • Alex Tolley January 16, 2019, 15:35

                Venus is possibly the most difficult planet to retrieve or analyze rock samples, and drilling is going to be even harder.

                I don’t know how its very hot surface impacts the availability of liquid water within the bounds of extremophile acceptable temperatures, but clearly, they cannot be close to the surface. The subsurface temperature profile may always be above that for life. However, it is possible that signs of fossil life might still be present in the rocks. But finding and retrieving the samples is going to be out of reach for some time. If Venus has life, it is probably in the high atmosphere. A probe looking for life in the upper atmosphere seems far more doable to me. [Of course, that may just be looking where the light is…]

  • Laintal January 13, 2019, 16:28

    Hi All

    This post from you Paul seems to fit in well with this new planet around Barnard B, there is a good graphic in there too.


    Another interesting post

    • Alex Tolley January 13, 2019, 18:35

      Michael Fidler posted a link to that previously. The planet has an estimated surface temperature of 103K. Although just outside the HZ, it may be a case that the star (type M4) may have had a long, pre-main sequence period where its luminosity was higher, and any surface water perhaps not entirely frozen. It would be interesting if a planetologist like Dr. Ramirez could provide such a climate analysis. Perhaps such early conditions were more conducive to abiogenesis and the planet’s crust now hosts a subsurface biosphere beneath any surface ice.

  • George King January 13, 2019, 19:06

    Alex’ piece reminded me of an article a while back here on Centauri Dreams about a possible relationship between the genesis of life and entropy.

    I couldn’t find the earlier article here, but this article appears to discuss the same basic concept:


    It just seems unlikely to me that life is unique to this particular fleck of rock within the cosmos, which was formed from processes of star and system formation that themselves are ubiquitous throughout the observed universe.

    The universe perhaps may be inherently infused as much with spirit — or prana — as it is with matter. Science is better at answering “how” questions than necessarily other type questions, which are addressed elsewhere more intuitively.

    Alex’ focus on how extensively life is present here in what some might view as “lifeless rock” augurs well for the presence of life elsewhere, as a ubiquitous process as discussed in the earlier article.

    Albeit perhaps harder to find from afar life.

    • Alex Tolley January 14, 2019, 12:30

      I can also recommend Stuart Kauffman’s The Origins of Order: Self-Organization and Selection in Evolution. He looks at self-organizing principles that emerge from computer simulations of toy models against a range of biological functions, from metabolism to cell differentiation.

      • Michael C. Fidler January 20, 2019, 10:27

        Complex molecules emerge without evolution or design.

        In biology, folded proteins are responsible for most advanced functions. These complex proteins are the result of evolution or design by scientists. Now, a team of scientists led by University of Groningen Professor of Systems Chemistry, Sijbren Otto, have discovered a new class of complex folding molecules that emerge spontaneously from simple building blocks.

        A striking conclusion drawn from the discovery of this new folding molecule is that complexity can emerge spontaneously. ‘This is interesting for origin-of-life research: apparently, you can get these complex molecules before biological evolution has started.’ The formation of the new molecule is actually driven by folding, explains Otto. ‘That is quite special. The energy level of this molecule is very low. This drives the equilibrium from a “random” mixture of small rings towards this specific very stable 15-mer.’


        • Alex Tolley January 20, 2019, 20:29

          Self-organization is almost an “order for free” phenomenon. While this specific example is a little distant from mainstream approaches to the origin of life, it is another example in the growing library of self-organizing systems (chemical entry).

  • Patient Observer January 13, 2019, 23:23

    Deep stuff. For whatever reason, my father was convinced that natural gas was of nonbiological origin. IIRC, Gold predicted that natural gas would be present in large meteor craters; released by substantial fracturing of deep rock formations. Again, IIRC, that prediction was confirmed.

    It would be hard to conceive that life could evolve to multicellular level within rock or rock fractures (those nematodes in SA gold minds must be colonizers from the surface). But a Star Trek episode would disagree:


    I though the episode was silly but the message was nevertheless positive.

    • Alex Tolley January 14, 2019, 12:19

      It would be hard to conceive that life could evolve to multicellular level within rock or rock fractures

      I agree. The size constraints would prevent that. It may even prevent the evolution of eukaryotes. The only possibility that comes to mind to overcome that constraint is isolated pockets in the crust, rather like enclosed cave systems.

      [ As an aside, John Varley[?] wrote of a nightclub in such a deep crustal pocket…then a small burp in the crust removed it. A very vivid picture that has stayed with me for decades. ]

    • NS January 15, 2019, 2:23

      Compared to what? As I recall in 1967 Star Trek’s competition was stuff like “Lost in Space” and “Voyage to the Bottom of the Sea”. “Devil in the Dark” raised the possibility of silicon-based life, and not only did we humans not kill the “monster” in the end, it was strongly implied that WE were the monsters. Very broadening to my then 11-year-old mind.

  • Alex Tolley January 14, 2019, 14:35

    It is an interesting issue whether complex anaerobes can evolve. Clearly, complex life can be anaerobic – even humans can do a little anaerobic respiration. But the energy differential between aerobes and anaerobes is so high that any such organisms would be either sessile or very slow moving.

    Note that my CD post on the Permian extinction Climate Change and Mass Extinctions: Implications for Exoplanet Life included data on the relatively anoxic conditions in the oceans that wiped out most marine species as temperatures rose. The anoxic conditions at the bottom of the Gulf of Mexico have reduced the fauna there, wiping out the shrimps and oysters that lived on the sea floor. Typically these conditions just result in bacterial oozes, and the generation of gases like CH4 and H2S from the anaerobic respiration.

  • hiro January 14, 2019, 19:32

    Has anyone ever tested synthetic cells/simple organisms “surviving” in extreme environments? Maybe not 100% artificial but it’s more like natural simple organisms having some fraction of synthetic cells inside the bodies perhaps more like mutant zombies (is this the right word to use)?

    • Alex Tolley January 15, 2019, 14:04

      I’m not sure what you are asking here. We can measure in situ environmental factors with live extremophiles, and we have tested bacteria in the lab to determine limits for temperature, acidity, and radiation. Bacteria are easy to experiment on, and in the last decade or so, it is “cheap” to sequence their DNA and run genomic studies to elucidate mechanisms for extremophile traits.

    • Patient Observer January 15, 2019, 18:49

      Is the question in reference to work in which genes were removed one by one from a bacterium to obtain the smallest viable genome? I recall that Craig Venter’s team was involved in that project as part of an effort to produce novel organisms. They even came up with some new amino acids but I have no idea where the work lead.

      • hiro January 15, 2019, 23:45

        Removing natural cells and then injecting the synthetic cells (to replace the natural ones) inside the body, let’s see how far one can do this before a simple organism fails all its functions and dies.

        • Alex Tolley January 16, 2019, 12:36

          As Patient Observer notes, Craig Venter created a minimal synthetic bacterium with 151 genes. This is close to a discovered organism with just 182 genes.

          Smallest genome clocks in at 182 genes

          Venter’s creation might survive in a lab, but I doubt it would last long in the wild, competing with other organisms. Experiments on other organisms like the gut bacteria E. coli and yeast shows that knocking out even a few genes can be lethal, or that knocked out genes prevent some facet of viability.

          I am not aware of synthesizing organisms to increase the extremeness of the environment they live in, but certainly, there has been a lot of work improving enzyme performance for industrial use. One can evolve enzymes to maximize some desired reaction.

  • Wojciech J January 14, 2019, 20:01

    I do remember reading two seperate studies on

    A:the percentage and approximate mass of Earth ejecta transported to Mars,Moon, Venus and Europa
    B:the possibility of life in rocks surviving ejection from Earth’s atmosphere

    Will have to dust up my memory on these two and post them.

    Also without spoilers:Stephen Baxter in his books Proxima and Ultima deals with this subject a bit, leading to some far reaching fun to read speculations….

  • Wojciech J January 15, 2019, 7:10

    Seeding Life on the Moons of the Outer Planets via
    R. J. Worth123, Steinn Sigurdsson123, Christopher H. House24
    Accepted for publication in Astrobiology on Nov. 10, 2013

    “We performed n-body
    simulations of such ejecta to determine where in the Solar System rock from
    Earth and Mars may end up. We find that, in addition to frequent transfer of
    material among the terrestrial planets, transfer of material from Earth and
    Mars to the moons of Jupiter and Saturn is also possible, but rare. We expect
    that such transfer is most likely during the Late Heavy Bombardment or
    during the next one or two billion years. At this time, the icy moons were
    warmer and likely had little or no icy shell to prevent meteorites from reaching
    their liquid interiors. We also note significant rates of re-impact in the first
    million years after ejection. This could re-seed life on a planet after partial or
    complete sterilization by a large impact, which would aid the survival of early
    life during the Late Heavy Bombardment.”


    If you read till the end, there are interesting tables with numbers on mass of material transferred from Earth and Mars to other bodies in Solar System-it’s quite a lot.

    • Alex Tolley January 15, 2019, 14:00

      A useful reference on transfer rates and times. As the vast majority of the impact ejecta on Earth fall back to Earth within 10 my, with exponential decay, it would be interesting to have some better comparison of data regarding the best way to overcome a post abiogenesis bombardment. Does Earth originated ejecta offer a better haven for life to reseed Earth, or would ejecta from another world be better, however that world acquired life?

    • AlexT January 16, 2019, 5:34

      I sure, as first step we have to find this so much desired in this community ET life at least somewhere outside the Earth, after we find it and carefully study it we can beging to build valuable theory how this ET life evolved.
      Meanwhile all discussion in this topic in best case – is science fiction only, especially accounting the fact that bold titles of this article is falce if we apply it to single life type we know today (the Earth life). Why should it be correct for whole Universe?

      • Alex Tolley January 16, 2019, 16:44

        Without putting forward ideas on what ET life may be like and where it lives, how can we effectively look for it?

        Atmospheres with out of equilibrium gases is a possible biosignature based on Earth life. Worlds with surface water are based on Earth life requirements. Carbon as the main polymer backbone is an Earth life feature. How will we search if we don’t have an idea what to look for and the traces it leaves?

        Bacteria are the most abundant form of life on Earth. They are relatively simple. They evolve quickly to changing conditions. If there were other life forms billions of years ago, they were apparently eliminated by bacteria.

        Paul Davies’ idea of a “shadow biosphere” is worth examining to broaden our horizons of what life is. Yet so far, we have not found any evidence of such a biosphere. maybe we don’t have a good idea of what to look for?

        Looking for life via [litho]panspermia is certainly one idea to find ET life. If ‘Oumuamua is from another star system, then it may have organisms within its mass. If we could reach it, or other extra-solar objects like it, it would be worth testing it for signs of life using the techniques we have, however Earth-centric.

        Call it SciFi if you like, but propose search methods that are Earth-life agnostic.

        • AlexT January 18, 2019, 6:44

          …lives, how can we effectively look for it?

          The good question, but basically I am sure we should look for everything unusual in the new ET worlds, and if you prefer not to loose something important during long search it will be better not to put limits on the research methods using problematic theory and speculative. For example only, if the header of your this article was printed like: “Can Life in the Universe be Lithophilic?” – so I could fully agree with this header, and it could fully explain my position (please understand, I use this example to try to explain my position on the issue only) .

          … possible biosignature based on Earth life.

          Sorry, but I fully support approach for biosignature searching based on Earth’s life pattern, mostly because it is only life type we know, we can study it, we can build scientific (not sci-fi) grounded methodology to search it.
          Most important for our egoistic , Humancentric point of view – it is most valuable life type we should search, we have to find the possible future colonization destinations for human race.
          Due to fact we do not know any different (from Earth life) type of live organisms – we can imagine endlessly every other possible variants, most of them will be false.
          May be it will be more clever to try to create something different in our laboratory, so it will allow to build the new additional search pattern. For example we should not exclude some type of “cybernetic / robotic” live forms (semiconductor based or quantum computer or … based intelligent systems) … Our own technology development is directly pointing to realism of this option.

          …If ‘Oumuamua is from another star system, then it may have organisms within its mass

          Sorry, once again, may be – may not be :-)
          The endless versions about question – “what is ‘Oumuamua?”.
          We an suppose everything, but seas that will never get the right answer, but I am wandering why almost everyone among SETi community try to use it to prove … hmmm… what?
          Meanwhile, we are found lot of stones in the space, but no sign of ET life among those stones, not non-lithophilic nor lithophilic, nothing. Extrapolating our experience I can conclude that most probable – there is no any life on (inside) ‘Oumuamua dead body… mostly because space conditions are not “Earth’s life” friendly .
          It is very easy to make sensation from the thing that none can check, all human religion uses the exactly same approach.

          • Alex Tolley January 20, 2019, 20:26

            You seem to want it both ways. You argue against being too Earth-centric:

            …single life type we know today (the Earth life). Why should it be correct for whole Universe?

            But then you say:

            Sorry, but I fully support approach for biosignature searching based on Earth’s life pattern, mostly because it is only life type we know, we can study it, we can build scientific (not sci-fi) grounded methodology to search it.

            When asking questions about ET life, we need to be as agnostic as possible, although grounded with what we know.

            It cannot work in all cases, as a hypothesized machine “life” might not create any of the “biosignatures” we might expect, nor structures or artifacts of technological civilization. Yet we do look for artifacts of technological life, and the Von Neumann replicators are a plausible hypothesis for such machine artifacts.

            The best we can do is keep our options open so that when we find interesting anomalies we do not ignore them.

            For the case for lithophilic life, the premise is that the simplest lifeforms we know are bacteria and that bacteria can live in the crust. Bacteria are also the most likely organisms to facilitate panspermia. Just as bacteria survive even some of our best attempts at sterilization, it is not unreasonable to hypothesize that they might survive in rocks when surface conditions do not remain suitable for most life.

            While the hypothesis may be incorrect, it is a testable one, both in our solar system and potentially, albeit with difficulty, in extra-solar systems. The title is a question, and the content makes the case that the answer is “yes”. If you think the answer should be “no”, make your case.

            • Alex T. January 22, 2019, 4:29

              There is no any contradiction in my position, because two statements related to absolutely different things.
              1. We have to search for the Earth like worlds and life – because it can be possible place for the fututre homo sapience colonization.
              2. The fact that we know somthing about Earth’s life does not mean automatically that we can freely apply single example to whole Universe. We do not have any scientific ground about what alternative life forms can be and what conditions they need, i.e. we do not know what to search.
              By the way , in connection to your article, I am believe that lithophilic life it is the evolution branch of more conventional life that previously appeared on the Earth as result of some misterious (for modern science) process – abiogenesis . I.e. I am sure that lithophilic life cannot evolve without previous long period evolution of more traditional life forms – it is only one of multiple deviations (mutations) of Earth life evolution.

              • Alex Tolley January 22, 2019, 15:15

                I agree with your last point about the likely origin of lithospheric life which I think I have made quite clear in my piece and my comments.

                I disagree with your point 1. Human colonization is so far off in teh future that this shouldn’t even be a consideration. Furthermore, worlds with biocompatible life to terrestrial life might well make those planets off-limits for colonization. Planets are a very sparse resource for colonization, especially given the alternatives.

                While your point 2 is one way of looking at the situation, it is not the only way. If we applied your approach to theoretical physics, all those theories are pointless because you don’t believe they hold any value for shaping experiments.

                • AlexT January 25, 2019, 12:55

                  …Human colonization is so far off in teh future that this shouldn’t even be a consideration…

                  Cannot agree. Same argument “why we should not do that” can be applied to space travel and everything else connected to space exploration – it is too far so should not be taken in account.
                  When “habitable zone expansion” it is one of main properties of the Earth’s life… By the way same your argument we can apply to non-Earth like life too – we should not consider it – because it is too far (in space and time)…

                  …If we applied your approach to theoretical physics…

                  Theoretical physics try to build math. models those models are based on some huge amount of experimental data and nature observation, and scientific experiment only can prove or disapprove theory.
                  When we talking about non-Earth like life models – science do not have any experimental data and no any nature observation, so models are based on our imagination only, in same time none can propose any experiment that can prove or disapprove “non-Earth like life models”. So you cannot limit scientific research by models that cannot be proved by scientific methods…
                  I.e. in present state of biology – it will be better not to put any limits on life searches. be ready to get unusual result and only after you get this result try to build correct model of observed data…
                  I.e. maximum efforts to space exploration by every possible mean and less limits on what to explore – aggressive data collection.

  • Sean Robert Meaney January 15, 2019, 8:51

    In that case Mars is more likely to be accessible simply because 11km deep is within range of cavern formation due to asteroid impact. Asteroid hits dont just extract rock from the initial crater, they create a concave shaped cavern with a focal point related to the initial surface point of impact and the bottom of the initial crater.

    • Alex Tolley January 15, 2019, 13:47

      cavern formation due to asteroid impact

      Interesting. Do you have a reference for that? All a quick search shows up is the cenotes around the Chicxulub impact.

  • Another David January 15, 2019, 12:24

    Nice article Alex, I also have been thinking about this issue and I agree with you, but for a different reason, namely on how the habitable zone itself will change with time. This is particularly dramatic for M dwarfs. M dwarfs will be much more luminous before they reach the main sequence and this period will last from a few hundred million years to around a billion for the least massive ones such as Proxima Centauri.

    Proxima Centauri was once for about a billion years up to 32 times more luminous than it is now so its habitable zone stretched out much further than present, something like one astronomical unit if I recall correctly. If as on Earth, life on another planet was able to get established on the order of a few hundred million years, then it may be common for life just as it was developing face a situation where to survive it either as to go underground, or under the ice if there was substantial water present. Since M dwarfs outnumber all other spectral
    types, these underground or under ice eco-systems could be the most dominant kind.

    • Alex Tolley January 15, 2019, 15:26

      I agree with you. Exoplanets will enter and exit their sun’s HZ, and if life emerges, it will likely retreat into the lithosphere to escape deteriorating surface conditions. As you point out, M-dwarfs can have long pre-sequence periods where their luminosity is much higher. Planets beyond teh outer edge of the HZ might well have lithospheric life. An earlier comment by Michael Fidler regarding Barnard’s star regarding life beneath deep glaciers of frozen worlds (extreme snowball conditions) would play into that scenario. [ Also see my point 4 in reply to one of Bruce Mayfield’s comments.]

  • Patient Observer January 15, 2019, 19:21

    Dear Alex, I and, I am sure, the others posters appreciate your efforts to address every comment. That adds immensely to our knowledge and engagement in this fascinating topic.

  • DJ Kaplan January 16, 2019, 22:29

    “Most life in the universe”, :) deep in the realm of the non-falsifiable. Fertile ground for endless speculation.

    • Alex Tolley January 17, 2019, 11:33

      Not falsifiable today, but falsifiable nonetheless.

      Finding no subsurface life elsewhere in the solar system would be a hint that it is wrong. Finding high ratios of CH4 and CO2 in most planets in the HZ with very low H2O in the atmosphere might hint that the hypothesis is correct. This latter might have enough data in less time than it took to confirm the existence of the Higgs particle.

  • Robin Datta January 17, 2019, 15:20

    Subterranean Civilizations

    • Alex Tolley January 17, 2019, 19:48

      I just watched this episode earlier today. I find this a quite an engaging series that covers a lot of ideas well and the graphics are superb.

  • Harry R Ray January 22, 2019, 11:49

    Alex: Check this out! Nature EXCLUSIVE: Tiny animal carcasses found buried in Antarctic lake. “The suprise discovery of ancient crustacians and a tardigrade emerged from a rare mission to drill into a lake sealed off by a kilometre of ice.” Mercer Lake is the lake described above. NOW: “…and a tardigrade…”. So – they CAN die after all! I wonder if they tried to bring it back to life? If not, is the specimen still available for a possible attempt?

    • Alex Tolley January 22, 2019, 15:05

      I wasn’t aware that tardigrades were immortal. My take on the finding was whether the organisms found were freshwater or marine species and whether the lake was dead or whether it might still support a very sparse, ecosystem. The dead tardigrade sample allows for radiocarbon dating to determine its age and therefore infers something about the lake. If they can sequence the DNA, it can be compared to current species to determine its nearest relatives and hence how the Mercer Lake formed.

      What may be reanimated is any bacterial spores, even if none of the bacteria in the sample are alive.

  • ljk January 24, 2019, 17:43

    January 24, 2019

    Earth’s oldest rock found on the Moon

    Houston, TX and Columbia, MD—January 24, 2019. Scientists discover what may be Earth’s oldest rock in a lunar sample returned by the Apollo 14 astronauts. The research about this possible relic from the Hadean Earth is published today in the journal Earth and Planetary Science Letters.

    An international team of scientists associated with the Center for Lunar Science and Exploration (CLSE), part of NASA’s Solar System Exploration Research Virtual Institute, found evidence that the rock was launched from Earth by a large impacting asteroid or comet. This impact jettisoned material through Earth’s primitive atmosphere, into space, where it collided with the surface of the Moon (which was three times closer to Earth than it is now) about 4 billion years ago. The rock was subsequently mixed with other lunar surface materials into one sample.

    The team developed techniques for locating impactor fragments in the lunar regolith, which prompted CLSE Principal Investigator Dr. David A. Kring, a Universities Space Research Association (USRA) scientist at the Lunar and Planetary Institute (LPI), to challenge them to locate a piece of Earth on the Moon.

    Full article here:


    To quote:

    Led by Research Scientist Jeremy Bellucci and Professor Alexander Nemchin, team members working at the Swedish Museum of Natural History and Curtin University in Australia rose to the challenge. The result of their investigation is a 2 gram fragment of rock composed of quartz, feldspar, and zircon, all commonly found on Earth and highly unusual on the Moon. Chemical analysis of the rock fragment shows it crystallized in a terrestrial-like oxidized system, at terrestrial temperatures, rather than in the reducing and higher temperature conditions characteristic of the Moon.

    “It is an extraordinary find that helps paint a better picture of early Earth and the bombardment that modified our planet during the dawn of life,” says Dr. Kring.

    It is possible that the sample is not of terrestrial origin, but instead crystallized on the Moon, however, that would require conditions never before inferred from lunar samples. It would require the sample to have formed at tremendous depths, in the lunar mantle, where very different rock compositions are anticipated. Therefore, the simplest interpretation is that the sample came from Earth.

  • ljk January 29, 2019, 14:09

    Earth’s core may have hardened just in time to save its magnetic field

    This shift both prevented the protective magnetic field from collapsing and recharged it.

    By Carolyn Gramling

    3:02 pm, January 28, 2019

    Earth’s inner core solidified around 565 million years ago — just in time to not only save the planet’s protective magnetic field from imminent collapse, but also to kick-start it into its current, powerful phase, a new study suggests.

    The finding, reported online January 28 in Nature Geoscience, supports an idea previously proposed by simulations that Earth’s inner core is relatively young. It also provides insight into how, and how quickly, Earth has been losing heat since its formation 4.54 billion years ago —key to understanding not only the generation of the planet’s magnetic shield but also convection within the mantle and plate tectonics.

    “We don’t have many real benchmarks for the thermal history of our planet,” says Peter Olson, a geophysicist at Johns Hopkins University who was not involved in the new study. “We know the interior was hotter than today, because all planets lose heat. But we don’t know what the average temperature was a billion years ago, compared with today.” Pinning down when iron in the inner core began to crystallize could offer a window into how hot the interior of the planet was at the time, Olson says.

    Full article here:


    So does this mean that the cores of Venus and Mars did not solidify in time and therefore that is why they have such weak ones?

  • ljk February 12, 2019, 14:30

    11 February 2019

    Controversial fossils suggest life began to move 2.1 billion years ago

    By Michael Le Page

    Burrow-like structures several millimetres in diameter have been found in 2.1-billion-year-old rocks in Gabon, Africa. The structures were made by a moving lifeform of some kind, claim geologist Abderrazak El Albani at the University of Poitiers in France and his team.

    The team do not know what made the trace fossils, but they speculate that it could be something similar to colonial amoeba or slime moulds – organisms made of cells that normally live separately. The trace fossils were found near bacterial mats that the mysterious lifeforms may have been feeding on. “It’s truly amazing,” says El Albani.

    Previously, the earliest evidence of moving lifeforms was just a half a billion years old. There are burrows and tiny footprints in rocks of this age, probably left by small animals.

    The 2.1-billion year-old burrows are very unlikely to have been produced by organisms as complex as animals, which probably appeared only between about 850 and 650 million years ago. In fact, it’s not even clear that organisms as complex as amoeba were around 2.1 billion years ago: they are eukaryotes, and the oldest eukaryotic fossils found so far are about 1.7 billion years old. So if El Albani’s interpretation is correct, these finds challenge the conventional story of life’s evolution.

    Full article here:


  • ljk March 11, 2019, 15:11

    A new turn in the search for the origin of life

    March 8, 2019

    Professor Friedemann Freund, SETI Institute, explores a fascinating new discovery in the search for the origin of life, here.