by Paul Gilster | Jul 23, 2021 | Culture and Society |
Milan M. ?irkovi?’s work has been frequently discussed on Centauri Dreams, as a glance in the archives will show. My own fascination with SETI and the implications of what has been called ‘the Fermi question’ led me early on to his papers, which explore the theoretical, cultural and philosophical space in which SETI proceeds. And there are few books in which I have put more annotations than his 2018 title The Great Silence: The Science and Philosophy of Fermi’s Paradox (Oxford University Press). Today Dr. ?irkovi? celebrates Stanislaw Lem, an author I first discovered way back in grad school and continue to admire today. A research professor at the Astronomical Observatory of Belgrade, (Serbia), ?irkovi? obtained his PhD at the Dept. of Physics, State University of New York in Stony Brook in 2000 with a thesis in astrophysical cosmology. He tells me his primary research interests are in the fields of astrobiology (habitable zones, habitability of galaxies, SETI studies), philosophy of science (futures studies, philosophy of cosmology), and risk analysis (global catastrophes, observation selection effects and the epistemology of risk). He co-edited the widely-cited anthology Global Catastrophic Risks (Oxford University Press, 2008) with Nick Bostrom, has published three research monographs and four popular science/general nonfiction books, and has authored about 200 research and professional papers.
by Milan ?irkovi?
This year we celebrate a centennial of the birth of a truly great author and thinker who is still, unfortunately, insufficiently well-known and read. Stanislaw Lem was born in 1921 in then Lwów, Poland (now Lviv, Ukraine). That was the year ?apek’s revolutionary drama R.U.R. premiered in Prague’s National Theatre and defined the word “robot”, Albert Einstein was awarded the Nobel Prize in physics for his work on the photoelectric effect in the course of which he effectively discovered photons, and one Adolf Hitler became the leader of a small far-right political party in Weimar Germany.
All three of these central-European developments have exerted a strong influence on Lem’s life and career. His studies of medicine, inspired by both his father’s distinguished medical career and his early-acquired mechanistic view of human beings, have been interrupted three times due to the chaos of WW2 and post-war changes. He narrowly escaped being executed by German authorities during the war for his resistance work. Finally, when he was on the verge of acquiring a diploma at the famous Jagiellonian University of Krakow, in 1949, he abandoned the pursuit in order to avoid the compulsory draft to which physicians were susceptible in the new communist Poland. He did some practical medical work in a maternity ward, but very quickly left medicine for good and became a full-time writer.
The apex of Lem’s creative career spans about three decades, from The Investigation published in 1958, to the publication of Fiasco and Peace on Earth in 1987. During that period, he published his greatest novels, in particular Solaris (1961), The Invincible (1966), His Master’s Voice (1968), and The Chain of Chance (1976), along with numerous short story anthologies, the most important being The Cyberiad (1965), as well as the Ijon Tichy and Pilot Pirx story cycles.
Image: Polish science fiction writer Stanislaw Lem. Credit: Wojciech Zemek.
Finally, several works in the Borgesian meta-genre of imaginary forewords, introductions, and book reviews, notably The Perfect Vacuum of 1971. This has been complemented by very extensive non-fiction writing, mainly in several fields of philosophy of science, futures studies, and literary criticism. The last two decades of Lem’s life were characterized by essayistic and publishing activity, as well as receiving innumerable prizes and awards, but no original fiction writing. Lem passed away peacefully on March 27, 2006, at the age of 84 in his home in Krakow.
Lem was obssessed by the theme of Contact: from his very first science-fiction novel, The Astronauts in 1951 (which he himself denounced as “childish”) to the last, great and deeply disturbing Fiasco, which is a kind of literary and philosophical testament. Nowhere, however, is his thought more in touch with the practical aspects of our SETI/search for technosignatures projects as in His Master’s Voice (originally published in 1968, that is only 8 years after the original Ozma Project! Translated into English by Michael Kandel only in 1983).
It is a brilliant work, perhaps the best novel ever written about SETI, but also a dense tract indeed. So, instead of many examples, I shall concentrate upon this one as a case study for the tremendous usefulness of reading Lem for anyone interested in astrobiology/SETI studies.
The study of the motives and ideas relevant for these fields would require a book-length treatment, as is obvious from the list of auxiliary topics Lem masterfully weaves into the narrative: from the ontological status of mathematical objects to the psyche of the Holocaust survivors, from preconditions for abiogenesis to the origin of the arrow of time. It is a challenging text in more than one sense; there is almost no dialogue and no manifest action beyond the recounting of a SETI project that not only failed but was never truly comprehended in the first place.
Image: A 1983 English edition of His Master’s Voice from Harcourt Brace Jovanovich, one of many editions available worldwide.
And this is a book whose plot should not be spoilt, since it is not as widely read as it should be half a century later. Without revealing too much, His Master’s Voice is set at a time when neutrino astrophysics is advanced enough to be able to detect possible modulations (imagined to have occurred near the end of the 20th century in the continued Cold War world). A neutrino signal repeating every 416 hours is discovered from a point in the sky within 1.5° of Alpha Canis Minoris. An eponymous top-secret project is then formed in order to decrypt the extraterrestrial signal, burdened by all the Cold-War paranoia and heavy-handed bureaucracy of the second half of the twentieth century. The project has its ups and downs, including some quite dramatic and literally threatening the survival of human civilization, but it is—obviously—mostly unsuccessful. The protagonist, a mathematical genius and cynic named Peter Hogarth, is neither a hero nor a villain; the SETI plot ends in anticlimactic uncertainty.
An intriguing consequence of Lem’s scenario is a realization that, while detectability generally increases with the progress of our astronomical detector technology, it does so very unevenly, in jumps or bursts. Although the powerful source of the “message” in the novel (presumably an alien beacon) had been present for a billion years or more, it became detectable only after a sophisticated neutrino-detecting hardware was developed. And even then, the detection of the signal happened serendipitously. Thus, in a rational approach to SETI—not often followed in practice, alas—the issue of detectability should be entirely decoupled from the issue of synchronization (the extent to which other intelligent species are contemporary to us).
Fermi’s paradox does not figure explicitly in His Master’s Voice (in contrast to many other of Lem’s works, especially his late and in my opinion equally magnificent Fiasco), and for an apparently obvious reason: “the starry letter” has always been here, or at least long enough on geological timescales. Detectability is, at least in part, a function of historical human development.
And there is a very real possibility, in the context of the plot, that “the letter” does not originate with intentional beings at all. The fulcrum of the book is reached when three radical hypotheses are presented to weary researchers, including the one attributing the signal to purely natural astrophysical processes! But even in this revisionist case, there are other problems, especially in light of the fact that the signal manifests “biophilic” properties: it helps complex biochemical reactions, and scientists in the novel speculate about whether it helped the abiogenesis on Earth. If it did so, the same necessarily occurred on many other planets in the Galaxy, so even if we abstract the mysterious Senders, it is natural to ask: where are our peers? This leads to more severe versions of Fermi’s paradox. In the same time, it makes us think about the various forms directed panspermia could, in fact, take when we reject our anthropocentric thinking.
There is another key lesson. While the discovery of even a single extraterrestrial artefact (and Lem’s neutrino message can surely be regarded as an artefact in the sense of the contemporary search for technosignatures), would be a great step forward, it would not, at least not immediately, resolve the problem. If one could conclude, as some of the protagonists of His Master’s Voice do, that there exist just two civilizations in the Galaxy, us and the mysterious Senders, that would still require explanation. Two is, in this particular context, sufficiently close if not equal to one.
And this shows, finally, the true gift of Lem’s thought to astrobiology and SETI studies: a capacity to go one step beyond in strangeness, to kick us sufficiently strongly out of the grooves of conventional thinking, to disturb us—and offend us, if necessary—and make us reject the comfortable and usual and mundane. In a general sense, all philosophy should do the same for us; that it usually does not is indeed discouraging and depressing. From time to time, however, a thinker passes with a bright torch illuminating the path and indicating how clueless we in fact are.
Lem was just such a figure. Reading him is indeed the highest form of celebration of reason and wisdom.
by Paul Gilster | Jul 2, 2021 | Astrobiology and SETI |
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
Holm, N., Oze, C., Mousis, O., Waite, J., & Guilbert-Lepoutre, A. (2015). Serpentinization and the Formation of H2 and CH4 on Celestial Bodies (Planets, Moons, Comets). Astrobiology, 15(7), 587-600. https://doi.org/10.1089/ast.2014.1188
Lollar, B. S., Onstott, T. C., Lacrampe-Couloume, G., & Ballentine, C. J. (2014). The contribution of the Precambrian continental lithosphere to global H2 production. Nature, 516(7531), 379-382. https://doi.org/10.1038/nature14017
Sauvage, J. F., Flinders, A., Spivack, A. J., Pockalny, R., Dunlea, A. G., Anderson, C. H., Smith, D. C., Murray, R. W., & D’Hondt, S. (2021). The contribution of water radiolysis to marine sedimentary life. Nature Communications, 12(1). https://doi.org/10.1038/s41467-021-21218-z
Sauvage, J. F., (2018) Sedimentary Catalysis of Radiolytic Hydrogen Production, Open Access Dissertations. Paper 704. https://digitalcommons.uri.edu/oa_diss/704
Cepelewicz, J, (2021, May 24). Radioactivity May Fuel Life Deep Underground and Inside Other Worlds, Quanta Magazine, Accessed online June 2021 https://www.quantamagazine.org/radioactivity-may-fuel-life-deep-underground-and-inside-other-worlds-20210524/
Tolley, A., Detecting Early Life on Exoplanets. February 23, 2018
https://www.centauri-dreams.org/2018/02/23/detecting-early-life-on-exoplanets/
Tolley, A., Is Most Life in the Universe Lithophilic? – AMT prior article JANUARY 11, 2019 https://www.centauri-dreams.org/2019/01/11/is-most-life-in-the-universe-lithophilic/
Tarnas, J., Mustard, J., Sherwood Lollar, B., Bramble, M., Cannon, K., Palumbo, A., & Plesa, A. C. (2018). Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming. Earth and Planetary Science Letters, 502, 133-145. https://doi.org/10.1016/j.epsl.2018.09.001
by Paul Gilster | May 28, 2021 | Astrobiology and SETI |
Put a rocky, Earth-sized planet in the habitable zone of a Sun-like star, and good things should happen. At least, that seems to be the consensus, and since there are evidently billions of such planets in the galaxy, the chances for complex life seem overwhelmingly favorable. But in today’s essay, Centauri Dreams associate editor Alex Tolley looks at a new paper that questions the notion, examining the numerous issues that can affect planetary outcomes. Just how long does a planetary surface remain habitable? Alex not only weighs the paper’s arguments but runs the code that author Toby Tyrrell used as he examined temperature feedbacks in his work. Read on for what may be a gut-check for astrobiological optimists.
by Alex Tolley
The usual course of the discussion about planet habitability assumes that the planet is in the habitable zone (HZ), probably in the continuously habitable zone (CHZ). The determination if the planet is inhabitable concerns the necessary composition and pressure of the atmosphere to maintain a surface temperature able to support liquid water. As stars usually increase their luminosity over time, we see charts like the one below for Earth showing the calculated range of average surface temperature. Atmospheric pressure and composition can be modified to determine the inner and outer edges of the HZ or CHZ.
Image: Life on an exoplanet in a globular cluster. Credit: David Hardy (Astroart.org).
However, these charts say nothing about the various factors that may upset the stability of the climate, especially the geological carbon cycle, where volcanic outgassing of carbon dioxide (CO2) is approximately balanced by weathering of rocks to ultimately sequester carbon as carbonate rocks such as limestone. Imbalances can create significant changes in greenhouse gas (GHG) composition of the atmosphere with temperature impacts [4].
Figure 1. Possible bounds of Earth surface temperature over 4.5 By of changing solar output and the impact of atmosphere. The early Earth would have required a different energy trapping atmosphere to maintain an inhabitable temperature during its early history to prevent freezing. Continued increase in solar luminosity will render the surface too hot for life even with no atmospheric trapping of the sun’s heat. Source: Kasting 1988 [3]
We have much evidence of widely fluctuating average surface temperatures for the Earth, from a possibly hot Archaean eon, several local temperature maximums including the end of the “Snowball Earth”, the Permian/Triassic extinction, and the Eocene thermal maximum. Earth has also cooled, most notably during the “Snowball Earth” period that lasted millions of years and several extreme glaciations over its history.
Before even considering the many other possible factors that may preclude an inhabitable planet, there is a question of just how stable are planetary surface temperatures, especially when subjected to shocks due to excess CO2 emissions from very active volcanism, or conversely from excess weathering depleting the atmospheric CO2 pressure.
The Journal Paper
A new paper [1] by Prof. Toby Tyrrell looks at this question in a very different way. He posits that for the many planetary types and conditions in the galaxy, we should assume a wide variety of possible temperature feedbacks and simulate the average surface temperature of a planet over a long period of geologic time (3.0 By) to simulate how frequently planets could continuously maintain an inhabitable surface temperature throughout this period.
Tyrrell on randomly configured feedbacks:
“It is assumed that there is no inherent bias in the climate systems of planets as a whole towards either negative (stabilising) or positive (destabilising) feedbacks. In other words, it is assumed here that the feedback systems of planets are the end result of a set of processes which do not in aggregate contain any overall inherent predisposition either towards or against habitability.”
His model is very simple. He assumes a randomly chosen number of feedback values (change in temperature over time for a specified temperature) within an inhabitable temperature range. Figure 2 below shows one example of the model showing random feedbacks, calculated temperature attractors, runaway temperature zones, and a time course of temperature impacted by random temperature perturbations. The values for temperatures between each feedback node are interpolated from the 2 surrounding nodes. If the feedback slope is negative and if the 2 points straddle a 0 feedback, the model calculates a temperature attractor at that point, so that temperatures between the 2 feedback nodes tend to stabilize the temperature at the attractor position.
If, however, the slope is positive, the temperature will be destabilized and driven towards the upper node if above the 0 feedback level, and conversely to the lower node if below the 0 level. He sets a minimum (-10C) and maximum(60C) surface temperature range that if the calculated temperature extends beyond those boundary temperatures or is in feedback that will lead to runaway feedback to either a very low or very high temperature, the model assumes the planet is no longer habitable on the surface. The model adds 2 other important elements. Firstly there is a long-term forcing (e.g. increased solar output), which for the Earth is a positive one as the sun continues to increase its output over time. The second is to introduce small, medium, and large temperature perturbations (i.e. shocks) that introduce noise into the model and can flip the climate between attractor temperatures and also into runaway temperature conditions where the feedbacks positively reinforce the temperature change. Figure 2 below is extracted from the paper to indicate an example. [Annotations added for clarity.]
Figure 2. Extracted from the Tyrrell paper and annotated. The left chart shows 9 randomly created feedbacks. Where 2 adjacent feedbacks are connected by a negative slope and cross the zero feedback line, a temperature attractor is created, in this example there are 2 attractors. At either end are zones where the temperature would cause a runaway increase or decrease in temperature and these are indicated by grayed areas. The chart also indicates that over the long term, there is a negative forcing that reduces the average temperature over time. The right-hand chart shows the temperature over time. The 2 attractors are shown, as is the starting temperature [blue square]. The various perturbations are indicated both in time and size by the red triangles below the chart axis. The gray bands show the runaway temperature conditions and the black bands the start of uninhabitable conditions. The 500 My display shows the temperature flipping between the 2 attractors, with each flip due to larger temperature perturbations. A few temperature perturbations approach but do not cross into the runaway temperature zones.
With the parameters he uses, the model demonstrates that with repeated runs, only a few percent of planet runs enjoy a 3 billion year period where surface temperatures stay within the inhabitable temperature range. Once the range is exited, surface life ends and the planet becomes lifeless on the surface.
Figure 3 shows how rarely planets can maintain inhabitable conditions over the entire 3 billion year time period.
Figure 3. The probability of a planet always surviving as inhabitable over 3 billion years over several runs with the same feedback conditions but with no temperature perturbations and with random temperature perturbations. The gray (H1 – chance alone) hypothesis is pure random perturbations withwout feedbacks and the red (H2 – mechanism alone) – feedbacks but without large perturbations – are compared with the simulation results. The most important result is that a planet that can maintain surface inhabitable conditions is quite rare.
Tyrrell:
“The initial prospects for Earth staying habitable could have been poor. If so, this suggests that elsewhere in the Universe there are Earth-like planets which had similar initial prospects but which, due to chance events, at one point became too hot or too cold and consequently lost the life upon them. As techniques to investigate exoplanets improve and what seem at first to be ‘twin Earths’ are discovered and analysed, it seems likely that most will be found to be uninhabitable.”
His conclusion is ominous for astrobiologists. Even if we discover many planets that are in the HZ, and confirm that their atmospheres could support an inhabitable surface, those planets are either frozen or too hot to allow life to exist on the surface. The vast majority of apparently suitable worlds will prove lifeless and appear as if abiogenesis (or even panspermia) has failed to ignite an evolutionary progression to complex life and even possibly technological civilization.
This conclusion is enough to dampen any astrobiologist’s day and suggests that the search for biosignatures may be as disappointing as the results of SETI.
While the paper shows the results using the values of the published model and code, the supplementary information includes a considerable analysis of the model, for example, extending the inhabitable range, and several other parameters. However, the broad conclusion remains robust. Maintaining an inhabitable temperature over 3 billion years is unlikely.
Tyrrell acknowledges the simplicity of his approach and suggested in a recent SETI Institute webinar that he hopes to apply his approach with a more sophisticated planetary climate model to determine if his findings hold up.
Given the proxy indications of Earth’s paleotemperatures (see figure 4 below) showing wide ranges and some close misses to survival shown by the mass extinctions, why did Earth life survive? Tyrrell argues that the anthropic principle has to be invoked. Just as the universe we live in needs the exact constants for life and we couldn’t be in any universe without those conditions, so we technological humans cannot investigate unless the Earth had maintained a continuous inhabitable surface temperature.
A Critique
Figure 4 below shows the estimated temperature fluctuations in the paleotemperature proxy data. Have we just been lucky that there do not appear to be any clear multiple attractor temperatures?
Figure 4. A chart of paleotemperature of Earth. For 3 billion years Earth’s average surface temperature has fluctuated in a range of less than 30C.
An obvious question is whether his model reflects reality. The random nature of the feedbacks coupled with the temperature perturbations might lead to many situations where even small temperature perturbations will tip the surface temperature beyond the acceptable range. As we can see from Figure 2, the upper-temperature attractor is within 10C of a runaway temperature increase, making habitability susceptible to even relatively small temperature perturbations.
Fortunately, the model code has been placed online and the source code available to experiment with.
Observing several runs it became clear that the model would quickly fail if the current temperature at an attractor temperature was near the boundary range so that even a modest temperature perturbation could push the temperature outside the range. How serious was this effect?
Figure 5. The most benign model. The 2 feedbacks are at the temperature range extremes and result in an attractor at 25C that is maintained across the temperature range. The long-term temperature forcing is set to 0. Only the infrequent large temperature perturbations, average size 32C are operative.
I created an experiment (also suggested at bottom of page 4 of the paper) where the planet would always have the most favorable conditions for a stable surface temperature. Just 2 feedbacks were created at each end of the range, with the calculated attractor in the middle of the range at 25 C, so that any perturbation would have to exceed 35C in either direction to exit the inhabitable conditions. I removed the long-term feedback too. I also removed both the small and medium-sized perturbations, leaving just the rare, large perturbations. The probability of timing and size of the perturbations was left as per the model. By starting the planet’s temperature at the attractor, the inhabitable conditions would be maintained at the attractor temperature unless a random large perturbation exceeded the 35C size.
The results for different perturbation probabilities are shown in figure 6. The average survival time of planetary runs and the %age survival plotted against perturbation probability demonstrate what might be intuitively guessed. Tyrrell’s purely mechanistic run (H2) with optimal feedback and no perturbations had all planet runs complete the 3 By survival. This is consistent with figure 6 where expected large perturbations = 0.
Figure 6. Survival times and %age survival of planets without an attractor and with a single attractor at 25C. With an attractor, survival times are greatly enhanced, especially as the expected number of perturbations increases.
Inspection of the model’s large temperature perturbation distribution indicated that the average size was 32C with a standard deviation of 16C. For the stable model planet I was testing, the temperature would be perturbed beyond either range boundary by a value of just 0.25 standard deviations, i.e. that about 40% of all randomly selected sample perturbations would trigger a surface temperature outside the inhabitable range. When that happened depended on the random timing and would dictate the survival time of inhabitable temperatures. As a control to determine the frequency of perturbations, a model world was created with no attractor temperature so that it sat on a temperature knife edge. Any perturbation would cause runaway heating or cooling. The impact of the lack of the stabilizing temperature attractor on survival time and average % of planets surviving for 3 By is evident.
Given the importance of the large perturbations, just how reasonable are the size of the perturbations and the maximum inhabitable temperature range.?
It is hypothesized that the “Snowball Earth” temperature ranged from deepest glaciation to a temperature maximum could have been as high as 100C (-50C to 50C). The chart of paleotemperature suggests for over the last few billion years an average surface temperature range of 26C (-10C to16C). That the Cryogenian glaciation period encompassing the “Snowball Earth” could have had a surface temperature of -50C, yet life quickly reemerged more vigorous than ever (the Cambrian “explosion”) after the glaciers melted, suggests that the lower temperature bound of -10C may be too conservative. As for the upper bound, it has been suggested that the Archaean eon may have had surface water temperatures of 70-80C. While most complex life has an effective upper limit of 60C, extremophiles have been found at 122C. For complex life, while the resilient tardigrades can withstand extreme temperatures for short periods, the inhabitable surface range is reasonable for complex life. However, we should bear in mind that ecological refugia can provide safety for complex life, for example around undersea vents to resist freezing, and migration to the poles to escape the equatorial temperatures and hence live in regimes that remain below the average surface temperature.
These points were acknowledged in the Tyrrell paper that discussed the limitations and caveats to the model.
Tyrrell:
Geographical variability implies that more extreme average global surface temperatures might be required to force extinction everywhere. Microbial life can potentially survive periods of inhospitable surface conditions within refuges, such as in subsurface rocks or deep in an ice-covered ocean at hydrothermal vents, emerging later to recolonise the surface; evidence from Neoproterozoic Snowball Earth events suggests however that eukaryotic photosynthetic algae persisted through the events and therefore that surface habitability was maintained at some locations. Other environmental conditions can affect habitability, but only temperature (and therefore water availability) are considered here.
Dynamic models are often unstable without tuning. The simplest example is Wolfram’s linear cellular automata with 3 cell states determining the next cell state. With just 8 possible rules for the 3 state combinations, there are 256 combinations of rules, yet just 6 (2.3%) do not converge on static states. The random feedback combinations may reflect a similar outcome, but where the majority of conditions will easily slip out of the inhabitable temperature range, rather than the benign experimental planet conditions I tested.
Conclusion
Dr. Malcolm (Jurassic Park):
“Life Finds a Way”
Given the results from my experiment with the optimal feedbacks for a stable climate, if feedbacks are more stabilizing on average than the hypothesized randomly assigned feedbacks, planets with inhabited surfaces possibly may not be quite as rare as the author’s model indicates. Tyrrell notes that average surface temperatures hide the variability of temperatures and exclude possible refugia, such as undersea hot vents, and lithosphere life warmed by the planet’s core. If we can accept that the Earth was populated by unicellular bacteria and eukaryotes for most of its history, and that the Earth’s complex biota may have even taken a major loss during the Cryogenian period, it seems likely that inhabitable worlds will have some sort of life assuming abiogenesis is easily achievable. While our climate history may be a lucky chance, history does not seem to indicate some attractor temperatures, but rather a single attractor that is subject to GHG source and sink imbalances that last for some time. The hypothesized extreme volcanism that ended the Permian resulted in the greatest extinction event in the fossil record and lasted for 2 million years. Our current fossil fuel burning that is increasing the atmospheric CO2 levels while very much like a temperature shock is not believed to be able to cause a runaway heating as happened on Venus. However, it is suggested that sometime in the next billion years, the Earth’s atmosphere will need to have no CO2 to stay habitable. Well before then, autotrophs will not be able to fix carbon and the complex life biosphere will collapse.
Once life starts, it is tenacious. A reset back to extremophiles may well be recoverable given time allowing new complex life forms to emerge under the right conditions and genetic “accidents”. However, there may be many more possible wrinkles to the sustainability of habitability, and eventually, surface life may be unable to survive. For subsurface life, the story may be very different. The lithospheric life might survive all other life until our sun destroys Earth billions of years in the future.
It would be interesting to modify the model so that rather than stopping when the temperature is outside the range, that the instances of these periods are recorded as possible reset conditions for refuge (e.g. lithosphillic) life to restart the evolutionary process rather than assuming the planet is “sterile”.
Time will tell when astrophysicists have cataloged and characterized a statistically useful sample of Earth-like worlds in the HZ that can test the model hypothesis of rare survival of surface inhabitablity over billions of years.
References
1. Tyrrell, Toby. “Chance Played a Role in Determining Whether Earth Stayed Habitable.” Communications Earth & Environment, vol. 1, no. 1, (2020), doi:10.1038/s43247-020-00057-8.
2. Ibid Supplementary Information
3. Kasting, James, et al “How Climate Evolved on the Terrestrial Planets”, Scientific American, (1988)
4. Berner, R. A. & Caldeira, K. “The need for mass balance and feedback in the geochemical carbon cycle”. Geology 25, 955-956 (1997).
by Paul Gilster | Sep 18, 2020 | Astrobiology and SETI |
Keith Cooper’s The Contact Paradox is as thoroughgoing a look at the issues involved in SETI as I have seen in any one volume. After I finished it, I wrote to Keith, a Centauri Dreams contributor from way back, and we began a series of dialogues on SETI and other matters, the first of which ran here last February as Exploring the Contact Paradox. Below is a second installment of our exchanges, which were slowed by external factors at my end, but the correspondence continues. What can we infer from human traits about possible contact with an extraterrestrial culture? And how would we evaluate its level of intelligence? Keith is working on a new book involving both the Cosmic Microwave Background and quantum gravity, the research into which will likewise figure into our future musings that will include SETI but go even further afield.
Keith, in our last dialogue I mentioned a factor you singled out in your book The Contact Paradox as hugely significant in our consideration of SETI and possible contact scenarios. Let me quote you again: “Understanding altruism may ultimately be the single most significant factor in our quest to make contact with other intelligent life in the Universe.”
I think this is exactly right, but the reasons may not be apparent unless we take the statement apart. So let’s start today by talking about altruism before we explore the question of ‘deep time’ and how our species sees itself in the cosmos. I think we have ramifications here for how we deal not only with extraterrestrial contact but issues within our own civilization.
I’m puzzled by the seemingly ready acceptance of the notion that any extraterrestrial civilization will be altruistic or it could not have survived. Perhaps it’s true, but it seems anthropocentric given our lack of knowledge of any life beyond Earth. What, then, did you mean with your statement, and why is understanding altruism a key to our perception of contact?
I think so much that is integral to SETI comes down to our assumptions about altruism. How often do we hear that an older extraterrestrial society will be altruistic, as though it’s the end result of some kind of evolutionary trajectory. But there’s several problems with this. One is that the person making such claims – usually an astrophysicist straying into areas outside their field of expertise – is often conflating ‘altruism’ with ‘being nice’.
And sure, maybe aliens are nice. I kind of get the logic, even though it’s faulty. The argument is that if they are still around then they must have abandoned war long ago, otherwise they would have destroyed themselves by now, ergo they must be peaceful.
And it’s entirely possible, I suppose, that a civilisation may have developed in that direction. In The Better Angels of Our Nature, Steven Pinker attempted to argue that our civilization is becoming more peaceable over time, although Pinker’s analysis and conclusions have been called into question by numerous academics.
I hope so. I think the notion is facile at best.
It’s what human societies should always aim for, I truly believe that, but whether we can achieve it or not is another question. When it comes to SETI, we seem to home in on the most simplistic definitions of what an extraterrestrial society might be like – ‘they’ve survived this long, they must be peaceful’. A xenophobic civilization might be at peace with its own species, but malevolent towards life on other planets. A planet could be at peace, but that peace could be implemented by some 1984-style dystopian dictatorship where nobody is free. Neither of which is particularly ‘nice’, and we could think of many other scenarios, too.
Nevertheless, this myth of wise, kindly aliens has grown up around SETI – that was the expectation, 60 years ago, that ET would be pouring resources into powerful beacons to make it easy for us to detect them. To transmit far and wide across the Galaxy, and to maintain those transmissions for centuries, millennia, maybe even millions of years, would require huge amounts of resources. When we consider that the aliens may not even know for sure whether they share the Universe with other life, it’s a huge gamble on their part to sacrifice so much time and energy in trying to communicate with others in the Universe.
If we look at what altruism really is, and how that may play into the likelihood that ET will want to beam messages across the Galaxy given the cost in time and energy, then it poses a big problem for SETI. ET really needs to help us out – to display a remarkable degree of selfless altruism towards us – by plowing all those resources into transmitting signals that we’ll be able to detect.
One of the forms that altruism can take in nature is kin selection. We can see how this has evolved: lifeforms want to ensure that their genes are passed on to later generations, so a parent will act to protect and give the greatest possible advantage to their child, or nieces and nephews. That’s a form of altruism predicated by genes, not ethics. Unless some form of extreme panspermia has been at play, alien life would not be our kin, so they would be unlikely to show us altruistic behaviour of this type.
But we haven’t exhausted all the forms altruism might take. Is there an expectation of mutual benefit that points in that direction?
Okay, so what about quid pro quo? That’s a form of reciprocal altruism. Consider, though, the time and distance separating the stars. It could take centuries or millennia for a message to reach a destination, and there’s no guarantee that anyone is going to hear that message, nor that they will send a reply. That’s a long time to wait for a return on an investment, if there even is a return. Why plow so many resources into transmitting if that’s the case? What’s in it for them?
So if kin selection and reciprocal altruism are not really tailored for interstellar communication, then it seems more unlikely that we will hear from aliens. Of course, there is always the possibility of exceptions to the rule, one-off reasons why a society might wish to broadcast its existence. Maybe ET wants to transmit a religious gospel to the stars to convert us all. Maybe they are about to go extinct and want to send one last hurrah into the Universe. But these would not be global reasons, and we shouldn’t expect alien societies to make it easy for us to discover them.
Good point. Why indeed should they want us to discover them? I can think of reasons a society might decide to broadcast its existence to the stars, though I admit that it’s a bit of a strain. But aliens are alien, right? So let’s assume some may want to do this. I like your mention of reciprocal altruism, as it’s conceivable that an urge to spread knowledge, for example, might result in a SETI beacon of some kind that points to an information resource, the fabled Encyclopedia Galactica. What a gorgeous dream that something like that might be out there.
Curiosity leads where curiosity leads. I wonder if it’s a universal trait of intelligence?
It’s interesting that you describe the Encyclopedia Galactica as a ‘dream’, because I think that’s exactly what it is, a fantasy that we’ve imagined without any strong rationale other than falling back on this outdated idea that aliens are going to act with selfless altruism. As David Brin argues, if you pump all your knowledge into space freely, what do you have left to barter with? And yet it is expectations such as receiving an Encyclopedia Galactica that still drive SETI and influence the kinds of signals that we search for. I really do think SETI needs to move on from this quaint idea. But I digress.
It’s certainly worth keeping up the SETI effort just to see what happens, especially when it’s privately funded. But I want to circle back around. I’ve always had an interest in what the general public’s reaction to the idea of extraterrestrial civilization really is. In the 16 years that I’ve been writing about this and talking to people, I’ve found a truly lopsided percentage that believe as a matter of course that an advanced civilization will be infinitely better than our own. This plays to a perceived disdain for human culture and a faith in a more beneficent alternative, even if it has to come from elsewhere to set right our fallen nature.
Put that way, it does sound a bit religious, but so what — I’m talking about how human beings react to an idea. Humans construct narratives, some of them scientific, some of them not.
I’m also talking about the general public, not people in the interstellar community, or scientists actively working on these matters. As you would imagine with COVID about, I’m not making many talks these days, but when I was fairly active, I’d always ask audiences of lay people what they thought of intelligent aliens. The reaction was almost always along two lines: 1) The idea used to seem crazy, but now we know it’s not. And 2) it would be something like an European Renaissance all over again if we made contact, because they would have so much to teach us.
A golden age, with its Dantes and Shakespeares and Leonardos. Or think of the explosion of Chinese culture and innovation in the Tang Dynasty, or Meiji Japan, all this propelled by the infusion not of recovered ancient literature and teaching, as in the European example, but materials discovered in the evidently limitless databanks of the Encyclopedia Galactica.
I ran into these audience reactions so frequently in both talks to interested audiences and just conversations among neighbors and friends that I had to ask what was propelling the Hollywood tradition of scary movies about alien invasion? What about Independence Day, with its monstrous ships crushing the life out of our planet? So I would ask, if you believe all this altruistic stuff, why do you keep going to these sensational movies of death and destruction?
The answer: Because people think they’re fun. They’re a good diversion, a comic book tale, a late night horror movie where getting scared is the point. Whole film franchises are built around the idea that fear is addictive when experienced within the cocoon of a home or theater. Thus the wave of horror fiction that has been so prominent in recent years. It’s because people like being scared, and the reason for that goes a lot deeper into psychiatry than I would know how to go. I admit I may not believe in Cthulhu, but I love going to Dunwich with H. P. Lovecraft.
Keith, as we both know — and you, as the author of The Contact Paradox would know a lot more about this than I do — there is an active lobby against messaging to the stars: METI. I’ve expressed my own opposition to METI on many an occasion in these pages, and the discussion has always been robust and contentious, with the evidently minority position being that we should hold back on such broadcasts unless we reach international consensus, and the majority position being that it doesn’t matter because sufficiently intelligent aliens already know about us anyway.
I don’t want to re-litigate any of that here. Rather, I just want to note that if the anti-METI position gets loud pushback in the interstellar community, it gets even louder pushback among the general public. In my talks, bringing up the dangers of METI invariably causes people to accuse me of taking films like Independence Day too seriously. From what I can see from my own experience, most people think ETI may be out there but assume that if it ever shows up on our doorstep, it will represent a refined, sophisticated, and peaceful culture.
I don’t buy that idea, but I’m so used to seeing it in print that I was startled to read this in James Trefil and Michael Summers’ recent book Imagined Life. The two first tell a tale:
Two hikers in the mountains encounter an obviously hungry grizzly bear. One of the hikers starts to shed his backpack. The other says, “What are you doing? You can’t run faster than that bear.”
“I don’t have to run faster than the bear — I just have to run faster than you.”
Natural selection doesn’t select for bonhomie or moral hair-splitting. The one whose genes will survive in the above encounter is the faster runner. Trefil and Summers go on:
So what does this tell us about the types of life forms that will develop on Goldilocks worlds? We’re afraid that the answer isn’t very encouraging, for the most likely outcome is that they will probably be no more gentle and kind than Homo Sapiens. Looking at the history of our species and the disappearance of over 20 species of hominids that have been discovered in the fossil record, we cannot assume we will encounter an advanced technological species that is more peaceful than we are. Anyone we find out there will most likely be no more moral or less warlike that we are…
That doesn’t mean any ETI we find will try to destroy us, but it does give me pause when contemplating the platitudes of the original The Day the Earth Stood Still movie, for example. It’s so easy to point to our obvious flaws as humans, but the more likely encounter with ETI, if we ever meet them face to face, will probably be deeply enigmatic and perhaps never truly understood. I also argue that there is no reason to assume that individual members of a given species will not have as much variation between them as do individual humans.
It’s a long way from Francis of Assisi to Joseph Goebbels, but both were human. So what happens, Keith, if we do get a SETI signal one day. And then, a few days later, another one that says, “Disregard that first message. The one you want to talk to is me?”
I’m hesitant to rely too closely on comparisons with ourselves and our own evolution, since ultimately we are just a sample of one, and we could be atypical for all we know. I see what Trefil and Summers are saying, but equally I could imagine a world, perhaps with a hostile environment, where species have to work together to survive. Instead of survival of the fittest, it becomes survival of those who cooperate. And suppose intelligent life evolves to be post-biological. What role do evolutionary hangovers play then?
I think the most we can say is that we don’t know, but that for me is enough of a reason to be cautious both about the assumptions we make in SETI, and about the possible consequences of METI.
But you’re right about our flawed assumption that aliens will exist in a monolithic culture. Unless there’s some kind of hive mind or network, there will likely be variation and dissonance, and different members of their species may have different reactions to us.
If we detected two beacons in the same system, I think that would be great! Why? Because it would give us more information about them than a single signal would. Since we will have no knowledge of their language, their culture, their history or their biology, being able to understand their message in even the most general sense is going to be exceptionally difficult.
So, if we detect a signal, we might not be able to decipher it or learn a great deal. But if we detect two different, competing beacons from the same planet, or planetary system, then we will know something about them that we couldn’t know from just one unintelligible signal, which is that they are not necessarily a monolithic culture, and that their society may contain some dissonance, and this may influence how, and if, we respond to their messages.
For me, the name of the game is information. Learn as much about them as we can before we embark on making contact, because the more we know, then the less likely we are to be surprised, or to make a misunderstanding that could be catastrophic.
Just so. But there, you see, is the reason why I think we have to be a lot more judicious about METI. It’s just conceivable that, to them as well as us, content matters.
But look, I see you’re headed in a direction I wanted to go. If information is the name of the game, then information theory is going to play a mighty role in our investigations. So it’s no surprise that you dwell on the matter in The Contact Paradox. Here we’re in the domain of Claude Shannon at Bell Laboratories in the 1940s, but of course signal content analysis applies across the whole spectrum of information transmittal. Shannon entropy measures disorder in information, which is a way of saying that it lets us analyze communications quantitatively.
Do you know Stephen Baxter’s story “Turing’s Apple?” Here a brief signal is detected by a station on the far side of the Moon, no more than a second-long pulse that repeats roughly once a year. It comes from a source 6500 light years from Earth, and Baxter delightfully presents it as a ‘Benford beacon,’ after the work Jim and Greg Benford have done on the economics of extraterrestrial signaling and the understanding that instead of a strong, continuous signal, we’re more likely to find something more like a lighthouse that sweeps its beam around the galaxy, in this case on the galactic plane where the bulk of the stars are to be found.
Baxter’s story sees the SETI detection as a confirmation rather than a shock, a point I’m glad to see emerging, since I think the idea of extraterrestrial intelligence is widely understood. No great revolution in thought follows, but rather a deepening acceptance of the fact that we’re not alone.
Anyway, in the story, the signal is investigated, six pulses being gathered over six years, with the discovery that this ETI uses something like wavelength division multiplexing, dividing the signal into sections packed with data. Scientists turn to Zipf graphing to tackle the problem of interpretation – as you present this in your book, Keith, this means breaking the message into components and going to work on the relative frequency of appearance of these components. From this they deduce that the signal is packed with information, but what are its elements?
Shannon entropy analysis looks for the relationships between signal elements, so how likely is it that a particular element will follow another particular element? Entropy levels can be deduced – how likely are not just pairs of elements to appear, but triples of elements? In English, for example, how likely is it that we might find a G following an I and an N? Dolphin languages get as high as fourth-order entropy by this analysis, as you know. Humans get up to eighth or ninth. Baxter’s signal analysts come up with a Shannon entropy in the range of 30 for ETI.
Let me quote this bit, because I love the idea:
“The entropy level breaks our assessment routines… It is information, but much more complex than any human language. It might be like English sentences with a fantastically convoluted structure – triple or quadruple negatives, overlapping clauses, tense changes… Or triple entendres, or quadruples.”
We’re in challenging territory here. In the story, ETI is a lot smarter than us, based on Shannon entropy. The presence of this kind of complexity in a signal, in Baxter’s scenario, is evidence that the detected message could not have been meant for us, because if it were, the broadcasting civilization would have ‘dumbed it down’ to make it accessible. Instead, humanity has found a signal that demonstrates the yawning gap between humanity and a culture that may be millions of years old. If we find something like this, it’s likely we would never be able to figure it out.
Would something like this be a message, or perhaps a program? If we did decode it, what would it mean? An ever better question: What might it do? Baxter’s story is so ingenious that I don’t want to give away its ending, but suffice it to say that impersonal forces may fall well outside our conventional ideas of ‘friendly’ vs. ‘hostile’ when it comes to bringing meaning to the cosmos.
But let’s wrap back around to Shannon and Zipf, and the SETI Institute’s Laurance Doyle, to whom you talked as you worked on The Contact Paradox. Doyle told you that communication complexity invariably tells us something about the cultural complexity of the beings that sent the message. And I think the great point that he makes is that the best way to approach a possible signal is by studying how communications systems work right here on Earth. Thus Claude Shannon, who started working out his theories during World War II, gets applied to the question of species intelligence (dolphins vs. humans) and now to hypothetical alien signals.
In a broader sense, we’re exploring what intelligence is. Does intelligence mean technology, or are technological societies a subset of all the intelligent but non-tool making cultures out there? SETI specifically targets technology, which may itself be a rarity even in a universe awash with forms of life with high Shannon entropy in communications they make only among themselves.
A great benefit of SETI is that it is teaching us just how much we don’t know. Thus the recent Breakthrough Listen breakdown of their findings, which extends the data analysis to a much wider catalog of stars by a factor of 220, all at various distances and all within the ‘field of view,’ so to speak, of the antennae at Green Bank and Parkes. Still more recent work at the Murchison Widefield Array tackles an even vaster starfield. Still no detections, but we’re getting a sense of what is not there in terms of Arecibo-like signals aimed intentionally at us.
So how do you react to the idea that, in the absence of information to analyze from an actual technological signal, we will always be doing no more than collecting data about a continually frustrating ‘great silence?’ Because SETI can’t ever claim to have proven there is no one there.
That’s one of my unspoken worries about SETI; how long do we give it before we start to suspect that we’re alone? People might say, well, we’ve been searching for 60 years now – surely that’s long enough? Of course, modern SETI may be 60 years old, but we’ve certainly not accrued 60 years’ worth of detailed SETI searches. We’ve barely scratched the tip of the iceberg bobbing up above the cosmic waters.
So how long until we can safely say we’ve not only seen the tip of the iceberg, but that we’ve also taken a deep dive to the bottom of it as well? Maybe our limited human attention spans will come into play long before then, and we’ll get bored and give up. I think we can also be too quick to assume that there’s no one out there. Take the recent re-analysis of Breakthrough Listen data, which prompted one of the researchers, Bart Wlodarczyk-Sroka of the University of Manchester, to declare:
“We now know that fewer than one in 1600 stars closer than about 330 light years host transmitters just a few times more powerful than the strongest radar we have here on Earth. Inhabited worlds with much more powerful transmitters than we can currently produce must be rarer still.”
Except that we don’t know that at all. All we can say was that there was no one transmitting a radio signal during the brief time that Breakthrough was listening. We could have easily missed a Benford Beacon, for instance. It’s a problem of expectation versus reality – we expect these powerful, omnipresent beacons, and when we don’t find them we jump to the conclusion that ET must not exist, rather than the possibility that our expectation is flawed.
The Encyclopedia Galactic is a similar kind of expectation that isn’t just a fanciful notion, but is a concept that actively influences SETI – we expect ET to be blasting out this guide to the cosmos, so we tailor SETI to look for that kind of signal, rather than something like a Benford Beacon. It also biases our thinking as to what we might gain from first contact – all this knowledge given to us by peaceful, selflessly altruistic beings. It would be lovely if true, but I think it’s dangerous to expect it.
Case in point: Brian McConnell recently wrote on Centauri Dreams about his concept for an Interstellar Communication Relay – basically a way of disseminating the data detected within a received signal, giving everybody the chance to try and decipher it [see What If SETI Finds Something, Then What?]. He rightly points out that we need to start thinking about what happens after we detect a signal, and the relay is a nifty way of organising that, so that should we detect a signal tomorrow, we will already have procedures in hand.
I won’t comment too much on the technical aspects, other than to say that if a message contains a Shannon entropy of 30, then it probably won’t matter how many people try and make sense of the message, we won’t get close (A.I., on the other hand, may have a bit more luck).
The Interstellar Communication Relay is an effort to democratize SETI. My cynical side worries, however, about safeguards. The relay relies on people acting in good faith, and not concealing or misusing any information gleaned from a signal. McConnell proposes a ‘copyleft license’, a bit like a creative commons license, that will put the data in the public domain while preventing people commercialising it for their own gain. I can see how this makes sense in the Encyclopedia Galactica paradigm – McConnell refers to entrepreneurs being allowed to make “games and educational software” from what we may learn from the alien signal.
I worry about this. In The Contact Paradox, I wrote about how even something as innocent as the tulip, when introduced into seventeenth-century Dutch society, proved disruptive (https://en.wikipedia.org/wiki/Tulip_mania). The Internet, motor cars, nuclear power – they’ve all been disruptive, sometimes positively, other times negatively.
How do we manage the disruptive consequences of information from an extraterrestrial signal? Even if ET has the best of intentions for us, they can’t foresee what the effects will be when facets of their culture or technology are introduced into human society, in which case the expectation that ET will be wise and ‘altruistic’ is almost irrelevant. Heaven forbid they send us technology that could be turned into a weapon, and we can’t guarantee that bad actors – after being freely given that information – won’t run off with it and use it for their own nefarious ends. A copyleft license surely isn’t going to put them off.
My feeling is that fully deciphering a signal will take a long, long time, if ever, in which case we shouldn’t worry quite so much. But suppose we are able to decipher it quickly, and it’s more than just a simple ‘greetings’. Yes, we have to think about what happens after we detect a signal, but it’s not just the mechanics of processing that data that we have to think about; we also have to plan how we manage the dissemination of potentially disruptive information into society in a safe way. It’s a dilemma that the whole of SETI should be grappling with I think, and nobody – certainly not me – has yet come up with a solution. But, I think that revising our assumptions, recasting our expectations, and casting aside the idea that ET will be selflessly altruistic and wise, would be a good start.
Well said. As I look back through our exchanges, I see I didn’t get around to the Deep Time concept I wanted to explore, but maybe we can talk about that in our next dialogue, given your interest in the Cosmic Microwave Background, which is the very boundary of Deep Time. Let’s plan on discussing how ideas of time and space have, in relatively short order, gone from a small, Earth-centered universe defined in mere thousands of years to today’s awareness of a cosmos beyond measure that undergoes continuous accelerated expansion. All Fermi solutions emerge within this sense of the infinite and challenge previous human perspectives.
by Paul Gilster | Feb 14, 2020 | Culture and Society |
We usually think about habitability in terms of liquid water on the surface, which is the common definition of the term ‘habitable zone.’ But even in our own system, we have great interest in places where this is not the case (e.g. Europa). In today’s essay, Nick Nielsen begins with the development of complex life in terms not just of a habitable zone, but what some scientists are calling an ‘abiogenesis zone.’ The implications trigger SETI speculation, particularly in systems whose host star is nearing the end of its life on the main sequence. Are there analogies between habitable zones and the conditions that can lead not just to life but civilization? These boundary conditions offers a new direction for SETI theorists to explore.
by J. N. Nielsen
Recently a paper of some interest was posted to arXiv, “There’s No Place Like Home (in Our Own Solar System): Searching for ET Near White Dwarfs,” by John Gertz. (Gertz has several other interesting papers on arXiv that are working looking at.) Here is the abstract of the paper in its entirety:
The preponderance of white dwarfs in the Milky Way were formed from the remnants of stars of the same or somewhat higher mass as the Sun, i.e., from G-stars. We know that life can exist around G-stars. Any technologically advanced civilization residing within the habitable zone of a G-star will face grave peril when its star transitions from the main sequence and successively enters sub-giant, red giant, planetary nebula, and white dwarf stages. In fact, if the civilization takes no action it will face certain extinction. The two alternatives to passive extinction are (a) migrate away from the parent star in order to colonize another star system, or (b) find a viable solution within one’s own solar system. It is argued in this paper that migration of an entire biological population or even a small part of a population is virtually impossible, but in any event, far more difficult than remaining in one’s home solar system where the problem of continued survival can best be solved. This leads to the conclusion that sub-giants, red giants, planetary nebula, and white dwarfs are the best possible candidate targets for SETI observations. Search strategies are suggested.
There are a number of interesting ideas in the above. The first thing that strikes me about this is that it exemplifies what I call the SETI paradigm: interstellar travel is either impossible or so difficult that SETI is the only possibility for contact with other civilizations. [1]
The SETI paradigm is worth noting in this context because Gertz is considering these matters on a multi-billion year time scale, i.e., a cosmological scale of time, and not the scale of time at which we usually measure civilization. Taking our own case of civilization as normative, if terrestrial civilization endures through the red giant and white dwarf stages of our star, that means our civilization will endure for billions of years, and in those billions of years (in the Gertz scenario) we will not develop any of the technology that would allow us to make the journey to other stars, including those other stars that will come within less than a light year of our own star with some frequency over cosmological scales of time. [2] We will, however, according to this scenario, develop technologies that would allow us to migrate to other parts of our own planetary system. I find that this contrast in technological achievement makes unrealistic demands upon credulity, but this is merely tangential to what I want to talk about in relation to this paper.
What most interests me about the scenario contemplated in this paper is its applicability to forms of emergent complexity other than human civilization. What I mean by “other forms of emergent complexity” is what I now call emergent complexity pluralism, which I present in my upcoming paper “Peer Complexity during the Stelliferous Era.” The paper isn’t out yet, but you can see a video of my presentation in Milan in July 2019: Peer Complexity during the Stelliferous Era, Life in the Universe: Big History, SETI and the Future of Humankind, IBHA & INAF-IASF MI Symposium. (Write to me if you’d like a copy of the paper.) In brief, we aren’t the only kind of complexity that may arise in the universe.
The simplest case of an alternative emergent complexity, and the case most familiar to us, is to think of Gertz’s scenario in terms of life without the further emergent complexities that have come to supervene upon human activity, chiefly civilization. In the case of a planet like Earth, possessed of a biosphere that has endured for billions of years and which has produced complex forms of life, one could expect to see exactly what Gertz attributes to technological civilizations, though biology alone could be sufficient to account for these developments. However — and this is a big however — the conditions must be “just right” for this to happen. In other words, something like the Goldilocks conditions of the “Goldilocks Zone” (the circumstellar habitable zone, or CHZ) must obtain, though in a more generalized form, so that each form of emergent complexity may have its own distinctive boundary conditions.
A further distinction should be introduced at this point. The boundary conditions of the emergence of complexity (whether of life, or civilization, or something else yet) may be distinct from the boundary conditions for the further development of complexity, and especially for developments that involve further complexity emerging from a given complexity, in the way that consciousness and intelligence emerged from life on Earth, and civilization emerged in turn from consciousness and intelligence. This distinction has been captured in origins of life research by the distinction between the habitability zone (the CHZ, in its conventional use) and the abiogenesis zone. The former is the region around a star where biology is possible, whereas the latter is the region in which biology can arise.
In a 2018 paper, The origin of RNA precursors on exoplanets, by Paul B. Rimmer, Jianfeng Xu, Samantha J. Thompson, Ed Gillen, John D. Sutherland, and Didier Queloz, this distinction between conditions for the genesis of life and conditions for the development and furtherance of life is made, and the two sets of boundary conditions are shown to overlap, but not to precisely coincide:
“The abiogenesis zone we define need not overlap the liquid water habitable zone. The liquid water habitable zone identifies those planets that are a sufficient distance from their host star for liquid water to exist stably over a large fraction of their surfaces. In the scenario we consider, the building blocks of life could have been accumulated very rapidly compared to geological time scales, in a local transient environment, for which liquid water could be present outside the liquid water habitable zone. The local and transient occurrences of these building blocks would almost certainly be undetectable. The liquid water habitable zone helpfully identifies where life could be sufficiently abundant to be detectable.” [3]
The idea implicit in defining an abiogenesis zone distinct from a habitable zone can be extrapolated to other forms of complexity: boundary conditions of emergence may be distinct from boundary conditions for development and longevity; the conditions for the emergence of civilization may be distinct from the conditions for the longevity of civilization. But let us return to the scenario of life maintaining itself within its planetary system without the assistance of intelligence or technology.
Image: This is Figure 4 from the Rimmer et al. paper. Caption: A period-effective temperature diagram of confirmed exoplanets within the liquid water habitable zone (and Earth), taken from a catalog (1, 42, 43), along with the TRAPPIST-1 planets (3) and LHS 1140b (4). The “abiogenesis zone” indicates where the stellar UV flux is large enough to result in a 50% yield of the photochemical product. The red region shows the propagated experimental error. The liquid water habitable zone [from (44, 45)] is also shown. Credit: Rimmer et al.
Whereas the CHZ is usually defined in terms of a region of space around a star clement for life as we know it, the boundary conditions for alternative emergent complexities will be optimal relative to the emergent complexity in question. That is to say, the wider we construe “habitability” (i.e., the more diverse kinds of emergent complexity that might inhabit a planet or planetary system) the more CHZs there will be, as each form of emergent complexity will have boundary conditions distinctive to itself.
In a planetary system with a large number of rocky worlds spaced relatively close together, these worlds could serve as “stepping stones” for enhanced lithopanspermia. [4] At each stage in the life of the parent star of such a planetary system with life, the life would be distributed among the available planets, and it would flourish into a planetary-scale biosphere on the world with the most clement conditions. When the star began to swell into a red giant, the inner planets would become inhospitable to life, but life could then migrate outward to the cooler planets. And then, when the star cooled down again, life could once again planet-hop nearer to the now-cooler star.
We do not yet know if the boundary conditions for emergent complexity longevity obtain within our own solar system. Is Mars close enough that life, going extinct on Earth, could make the transition to this cooler world, and possibly also further out to the moons of the gas giants? In The Jovian Oceans [5] I suggested that, as the sun grows into a red giant, the outer regions of the solar system will become warmer and the subsurface oceans of some of the moons of Jupiter and Saturn may thaw out and become watermoons (in contradistinction to waterworlds). These regions of our solar system may be clement to life when Earth is no longer habitable, but if life cannot make the journey to these worlds, they may as well not exist at all. We still have a billion years for sufficiently hardy microorganisms to evolve, and for collisions with large bodies to blast microorganisms off the surface of Earth and into trajectories that would eventually result in their impacting on Mars. The chances for this strike me as marginal, but over a billion years we cannot exclude marginal scenarios.
As I have noted in Life: from Sea to Land to Space, the expansion of life from Earth into space (like the expansion of life from the oceans onto land) will open up a vastly greater number of niches to life than could exist on any one planet, so that the opportunities for adaptive radiation are increased by orders of magnitude. But this expansive scenario for life in space is contingent upon the proper boundary conditions obtaining; life must expand into an optimal environment in order for it to experience optimal expansion and adaptive radiation. [6] And as the boundary conditions for the emergence of emergent complexity may be distinct from the boundary conditions for the longevity of emergent complexity, emergent complexity (like a biosphere) may flourish and die on one planet without the opportunity to exploit the potential of other niches. [7]
There are also distinctive boundary conditions for the longevity of civilization. If a civilization is to employ technological means to extend its longevity, whether through journeying to other stars, or, according to Gertz’s scenario, shifting itself within its home planetary system (“sheltering in place”), then the conditions must first be right for a life to arise, and then for civilization to supervene upon life, and finally for civilization to pass beyond its planetary origins by technological means. These boundary conditions might include, for example, an adequate supply of fossil fuels for the civilization to make its original transition to industrialization, and, later, sufficient titanium resources to build spacecraft, and sufficient fissionables to supply nuclear power or to operate nuclear rockets.
It takes a “just right” planetary system for a technological civilization to successfully make a spacefaring breakout from its homeworld — just as being a space-capable civilization is a necessary condition for spacefaring breakout, coming to an initial threshold of technological maturity in the context of favorable boundary conditions is also a necessary condition for being a spacefaring civilization. It also takes a “just right” stellar neighborhood for a spacefaring civilization to make an interstellar breakout from its home system. The boundary conditions for interstellar civilization are subject to change over cosmological scales of time, because stars change their relationships to each other within the galaxy, but there will still be regions in the galaxy with more favorable conditions and regions in the galaxy with less favorable conditions.
As I have noted in other contexts, technology is a means to an end, and usually not an end in itself, so that there is a certain fungibility in the use of technologies: if the resources are unavailable for a particular technology, they may be available for some other technology that can serve in a similar capacity. A marginal technology in favorable boundary conditions, or a superior technology in unfavorable boundary conditions, might do the trick either way. However, there are limits to technological fungibility. The boundary conditions for the longevity of technological civilizations set these limits.
Notes
[1] I have written about the SETI paradigm in my Centauri Dreams post Stagnant Supercivilizations and Interstellar Travel, inter alia.
[2] I discussed interstellar travel by waiting for other planetary systems to pass near our own in the aforementioned Stagnant Supercivilizations and Interstellar Travel.
[3] “The origin of RNA precursors on exoplanets,” by Paul B. Rimmer, Jianfeng Xu, Samantha J. Thompson, Ed Gillen, John D. Sutherland, and Didier Queloz, Science Advances, 01 Aug 2018: Vol. 4, no. 8, DOI: 10.1126/sciadv.aar3302
[4] Cf. two papers on this, “Enhanced interplanetary panspermia in the TRAPPIST-1 system” by Manasvi Lingam and Abraham Loeb, and “Fast litho-panspermia in the habitable zone of the TRAPPIST-1 system”, by Sebastiaan Krijt, Timothy J. Bowling, Richard J. Lyons, and Fred J. Ciesla, and my post Emergent Complexity in Multi-Planetary Ecosystems.
[5] This post also noted two papers, then recent, on habitability zones around post-main sequence stars, “Habitable Zones Of Post-Main Sequence Stars” by Ramses M. Ramirez, et al., and “Habitability of Super-Earth Planets around Other Suns: Models including Red Giant Branch Evolution” by W. von Bloh, M. Cuntz, K.-P. Schroeder, C. Bounama, and S. Franck, both of which are relevant to Gertz’s argument.
[6] René Heller has introduced the concept of superhabitable worlds, i.e., worlds more clement for life than Earth, thus optimal for life (cf., e.g., “Superhabitable Worlds”, by René Heller and John Armstrong), which suggests a similar implicit distinction between merely habitable planetary systems and superhabitable planetary systems, merely habitable galaxies and superhabitable galaxies, and so on.
[7] Freeman Dyson argued for the value of life that can adapt to conditions distinct from the planetary endemism that characterizes life as we know it: “…planets compare unfavourably with other places as habitats. Planets have many disadvantages. For any form of life adapted to living in an atmosphere, they are very difficult to escape from. For any form of life adapted to living in vacuum they are death-traps, like open wells full of water for a human child. And they have a more fundamental defect: their mass is almost entirely inaccessible to creatures living on their surface.” (Dyson, F. J. 2003. “Looking for life in unlikely places: reasons why planets may not be the best places to look for life.” International Journal of Astrobiology, 2(2), 103-110) Dyson’s reasons for favoring life independent of planets does not alter the fact that a lot of interesting chemistry occurs on planets that does not occur elsewhere because other environments do have not large scale geomorphological processes; however, Dyson’s observations do point to the selective value of life that can adapt to habitats without planets.
