The search for life on planets beyond our Solar System is too often depicted as a binary process. One day, so the thinking goes, we’ll be able to directly image an Earth-mass exoplanet whose atmosphere we can then analyze for biosignatures. Then we’ll know if there is life there or not. If only the situation were that simple! As Alex Tolley explains in his latest essay, we’re far more likely to run into results that are so ambiguous that the question of life will take decades to resolve. Read on as Alex delves into the intricacies of life detection in the absence of instruments on a planetary surface.
by Alex Tolley
“People tend to believe that their perceptions are veridical representations of the world, but also commonly report perceiving what they want to see or hear.” [17]
Evolution has likely selected us to see dangerous things whether they are there or not. Survival favors avoiding a rustling bush that may hide a saber-toothed cat. We see what we are told to see, from gods in the sky that may become etched as a group of bright stars in the sky. The post-Enlightenment world has not eradicated those motivated perceptions, as the history of astronomy and astrobiology demonstrates.
There have been some famous misperceptions of life and ETI in the past. Starting with Giovanni Schiaparelli’s perceptions of channels (canali) on Mars, followed by Lowell’s creation of a Martian civilization from whole cloth based on his interpretation of canali as canals. As a consensus seemed to be building that plants existed on Mars, in 1957 and later in 1959 Sinton claimed that he had detected absorption bands from Mars indicating organic matter probably pointing to plant life. These “Sinton bands” later proved to be the detection of deuterium in the Earth’s atmosphere.
I note in passing that Dr. Wernher Von Braun assumed that the Martian atmosphere had a surface pressure 1/12th of Earth’s, based on a few telescopic and spectrographic observations and calculations. This assumption was used in his The Mars Project [23] and Project Mars: A Technical Tale [24], to design the winged landers that were depicted in the movie “The Conquest of Space”. How wrong those assumptions proved!
Briefly, in 1967, regular radio pulses discovered by astrophysicist Jocelyn Bell were thought to be a possible ET beacon, perhaps influenced by the interest in Frank Drake’s initial Project Ozma search for radio signals that was started in 1960. They were quickly identified as emitted by a pulsar, a new degenerate stellar type.
Not to be outdone, nn 1978, Fred Hoyle and Chandra Wickramasinghe published their popular science book – Lifecloud: The Origin of Life in the Universe [5]. In the chapter “Planets of Life”, they made the inference that the spectra they observed around stars most closely matched cellulose (a macromolecule of simple hexose sugars, composed of the common elements carbon, hydrogen, and oxygen, and the major material of plants). This became the basis of their claims of the ubiquity of life, panspermia, and cometary delivery of viruses to Earth. Again, this assertion of the ubiquity of life proved to be incorrect based on faulty logical inference, which was in turn based on the incorrect interpretation of cellulose as the molecule identified from the light. It remains a cautionary tale.
Figure 1. A) Illusory lines between objects are interpreted as canals. – Source [16] B) The assumption that lichen-like plants were abundant and produced teh dark areas on Mars. Source [16] C) The spectral fit that convinced Hoyle and Wickramasinghe that they had detected cellulose around other stars. – Source [5] D) View of a purported “fossil” in the famous Mars meteorite, Allan Hills 84001. Doubters argue that the feature is too small to be a sign of Mars life. (Image credit: NASA).
The latest possible misinterpretation may be the results of the Hephaistos Project that claimed 7 stars had anomalous longer wavelength intensities that might indicate a technosignature of a Dyson sphere or swarm [19]. Almost immediately a natural explanation appeared suggesting a data contamination issue with a distant galaxy in the same line of sight.
These historical observational misinterpretations are worth bearing in mind, as the odds are we’ll find life beyond our Solar System, if only because of the vast number of planets that have conditions that could bear life on their surfaces, perhaps even an “Earth 2.0” We will be restricted to data from the electromagnetic (em) spectrum with no hope of acquiring the ground truth from probes sent to those systems.
Returning to the early search for life, the limitations of 1960s technology optical astronomy were highlighted in the US publication of Intelligent Life in the Universe by Iosif Samuilovich Shklovsky, with additional content by Sagan [4], as well as by earlier papers by Sagan [3]. Carl Sagan noted that hypothetical Martian astronomers would not be able to confirm the detection of life and intelligence on Earth using the many terrestrial techniques available at the time, and highlighted some of the issues, including the resolution needed to detect human artifacts, and the ambiguity of spectral data. He asserted that Martian astronomers required ground truth, i.e. a probe in Earth’s orbit or a lander.
From the paper [3]
Moreover, with Earth at 1 km resolution “no seasonal variations in the contrast of vegetation could be detected…. [I]t is estimated that better than 35 m resolution, with global coverage, would be needed to detect life on a hypothetical Earth with no intelligent life.
The best telescopic resolution based on a hypothetical solar gravitational line telescope (SGL) would be far too low to detect life without ambiguity. A simulated image is shown in Figure 2 below.
Figure 2. A 1024×1024 pixel image simulation of an exoplanet at a distance of up to 30 parsecs, imaged with a possible solar gravitational lens telescope, has a surface resolution of a few 10s of km per pixel, Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II. – Source [26]
Given the limitations outlined by Sagan and Shklovskii, how might we detect that exoplanet life soon? In an influential 1967 paper, “Life Detection by Atmospheric Analysis”, Hitchcock and Lovelock argued that a living planet would create disequilibria in the atmospheric gases of the planet [1].
“Living systems maintain themselves in a state of relatively low entropy at the expense of their nonliving environments. We may assume that this general property is common to all life in the solar system. On this assumption, evidence of a large chemical free energy gradient between surface matter and the atmosphere in contact with it is evidence of life. Furthermore, any planetary biota which interacts with its atmosphere will drive that atmosphere to a state of disequilibrium which, if recognized, would also constitute direct evidence of life, provided the extent of the disequilibrium is significantly greater than abiological processes would permit. It is shown that the existence of life on Earth can be inferred from knowledge of the major and trace components of the atmosphere, even in the absence of any knowledge of the nature or extent of the dominant life forms. Knowledge of the composition of the Martian atmosphere may similarly reveal the presence of life there.”
This proxy has become the core approach for searching for biosignatures of exoplanets, as we are rapidly evolving the technology to detect gas mixtures via transmission spectroscopy of these worlds.
We have no hope of sending probes to those worlds within the foreseeable future, and the speed of light limits when we could even receive information from a probe that landed on such a planet. Therefore, unlike our system, where samples can be both locally analyzed or returned to Earth, only remote observation is currently possible for exoplanets.
For terrestrial worlds like our contemporary Earth, the signature would be the presence of both oxygen (O2) from oxygenic photosynthesis and methane (CH4) from methanogenic bacteria or archaea. So strong is this idea, that my first post on Centauri Dreams was to review a paper that discussed what the atmospheric biosignature would be for an early Earth before photosynthesis had made O2 the 2nd most common gas in the atmosphere, a period that encompassed most of Earth’s history [2].
However, there are increasing concerns from the astrobiology community about this approach because of possible false positives. For example, O2 could be entirely generated by photolysis of water, and even CH4 might be sufficiently produced by geologic means to maintain that disequilibrium, creating false positives or at best ambiguity of the spectral analysis as a biosignature.
Astrobiologists have continued to explore other possible biosignatures, usually with terrestrial life as the template. For example, Sara Seager published a catalog of small, detectable molecules that included some that are only made by life forms. This included phosphine (AKA phosphane, PH3). In 2021, Greaves reported that PH3 had been discovered in the spectra of Venus’s atmosphere [27]. As PH3 is principally produced by life on Earth, this set off a flurry of observations, experiments, and even a soon-to-be-launched probe to the temperate zone of the Venusian atmosphere. Unfortunately, in this case, confirmation of the signal was not made. It was also suggested that the very similar spectral signature of sulfur dioxide was the culprit. The original observation remains controversial, but at least we will eventually get the ground truth we need. But as we will see later, there is a theoretical abiotic route to PH3 production via Venusian volcanic emissions which adds ambiguity to the finding as a biosignature.
In 2018, Sara Walker published a long paper on the issue of dealing with false positives for any biosignature [7]. Much of the paper relied on the use of Bayesian statistical methods, although a potential flaw was the issue of assigning the prior probabilities. Much of the paper dealt with data that would need some sort of sample, even ground truth, such as a sample taken by an in situ probe, as originally suggested by Sagan and Shklovskii. Purely electromagnetic spectrum (em) data would exclude these sample analyses.
A 2021 paper by Green et al [8] suggested that the detection of life should be viewed on a scale of increasing certainty, rather than a binary true or false determination. In other words, ambiguity was to be encompassed:
“The Community Workshop Report argues (with reasonable grounds) that the first detection of an extraterrestrial biosignature will likely be ambiguous and require significant follow-on work.”
They suggested a new Confidence of Life Detection scale (CoLD), to indicate reliability based on NASA’s Technology Readiness Level (TRL) approach: Confidence is increased with confirmation and as abiotic causes are ruled out. A modified chart is shown in Figure 3.
Figure 3. CoLD scale. Printed with permission in the Vickers et al paper.
At about the same time, NASA sponsored a biosignature workshop [9] to assess and report on life detection to address the recognized problems of reliability of life detection as an immature science. Their effort was especially targeted at communicating a possible life detection. It should be noted that NASA did a very poor job of this in the past, notably the press conferences on the “Martian microbes” from the ALH84001 meteorite in 2006, and the arsenic-based bacteria in 2010, billed as a possible different alien biology that could exist on another world [28]. Nasa clearly wanted to avoid such premature announcements and tread a more cautious approach. [In an ironic twist, researchers have discovered arsenic metabolism in some deep sea marine microbes [29].
This has gained importance because of the increasingly attention-grabbing approach of articles on the discovery of exoplanets that resemble Earth, often using the “Earth 2.0” label. It is highly unlikely any exoplanet with similar dimensions and orbits in the habitable zone (HZ) is a terrestrial-type verdant world suitable for eventual colonization if or when our starships can reach them.
The workshop produced this Standards of Evidence scale published in 2022:
Table 1. Standards of Evidence Life Detection scale, produced by a community wide Effort [9] Credit: Walker et al [14]
In 2023, Smith, Harrison, and Mathis published an extensive critique of the reliability of biosignatures for life detection. In their essay [10] they state in the abstract that:
“Our limited access to otherworlds suggests this observation is more likely to reflect out-of-equilibrium gases than a writhing octopus. Yet, anything short of a writhing octopus will raise skepticism about what has been detected.”
In the introduction, they state that atmospheric gas disequilibria are byproducts of life on Earth, and not unique, for example, abiotic production of O2 in the atmosphere. The following includes a critique of the Krissansen-Totton et al paper that I sourced in my first Centauri Dreams post:
“Often these models don’t rely on any underlying theory of life, and instead consider specific sources and sinks of chemical species, and rules of their interactions. Astrobiologists label these sources, sinks, or transformations as being due to life or nonlife, tautologically defined by the fluxes they influence. For example, defining life via biotic fluxes of methane from methanogenesis [6,7] and defining abiotic fluxes via rates of serpentinization and impacts.”
And:
“This leads to the conclusion that most exoplanet biosignatures are futile if our goal is to detect life outside the solar system with confidence.”
Figure 4 below shows the 4 approaches they suggest to firm up the strength of biosignatures.
1. Biological research on terrestrial life,
2. Looking for life with probes sent to planets in our system
3. Experimental work on abiogenesis and molecular outcomes
4. SETI via technosignatures
Figure 4. The 4 approaches to explore biosignatures. Credit: Smith, Harrison & Mathis.
They conclude:
“As astrobiologists we believe the search for life beyond Earth is one of the most pressing scientific questions of our time. But if we as a community can’t decide how to formalize our ideas into testable hypotheses to motivate specific measurements or observational goals, we are taking valuable observational time and resources away from other disciplines and communities that have clearly articulated goals and theories. It’s one thing to grope around in the dark, or explore uncharted territory, but do so at the cost of other scientific endeavors become increasingly difficult to justify. One of the most significant unification of biological phenomena–Darwin’s theory of natural selection–emerged only after Darwin went on exploratory missions around the world and documented observations. It’s possible the data required to develop a theory of life that can make predictions about living worlds simply has not been documented sufficiently. But if that’s the case we should stop aiming to detect something we cannot understand, and instead ask what kinds of exploration are needed to help us formalize such a theory.”
One month later, Vickers, Peter, et al. published “Confidence of Life Detection: The Problem of Unconceived Alternatives.” [11]. This paper aimed to demolish the idea of using Bayesian probabilities as there was little hope of even conceiving of novel ways life may arise, nor the abiotic mimics of possible signatures.
From the abstract:
“It is argued that, for most conceivable potential biosignatures, we currently have not explored the relevant possibility space very thoroughly at all. Not only does this severely limit the circumstances in which we could reasonably be confident in our detection of extraterrestrial life, it also poses a significant challenge to any attempt to quantify our degree of (un)certainty.”
From the introduction:
“(…) the problem of unconceived abiotic explanations for phenomena of interest……., we stress that articulating our uncertainty requires an assessment of the extent to which we have explored the relevant possibility space. It is argued that, for most conceivable potential biosignatures, we currently have not explored the relevant possibility space very thoroughly at all.”
From section 2 – The challenges of known and unknown false positives:
“As Meadows et al. (2022, p. 26) note, “[I]f the scope of possible abiotic explanations is known to be poorly explored, it suggests we cannot adequately reject abiotic mechanisms.” Conversely, if it is known to be thoroughly explored, we probably can reject abiotic mechanisms.”
In effect, they are reiterating the problems of inference. For example, it was thought since Roman times that all swans were white as no examples of differently colored swans had been seen in the European, Asian, and African continents. This remained the case until black swans were discovered in Australia.
Today the more popular phrase that covers the issue of inference is “Absence of Evidence is not Evidence of Absence”.
They critique Green’s CoLD scale and suggest that the Intergovernmental Panel on Climate Change (IPCC) approach using a 2D scale of scientific consensus and strength of evidence may be more suitable.
Figure 5. IPCC framework for climate as a biosignature confidence scale. Credit: Vickers et al.
As Vickers was sowing doubts about biosignature detection reliability and how best to handle the uncertainties, Sara Walker and collaborators published “False Positives and the Challenge of Testing the Alien Hypothesis” [12] repeating their earlier argument for Bayesian methods, and the need for definitive biosignatures to avoid false positives. Those definitive biosignatures may be based on the Assembly Theory of the composition of organic molecules [13]. I would include the complementary approach, even though it does allow for false positives [14]. However, these approaches require samples that will not be available for exoplanets. Walker ends with the suggestion that the approach of levels (or ladders) of certainty such as the Confidence of Life Detection (CoLD) scale, and the community Standards of Evidence Life Detection scale should be used because we need more data to understand planetary types and life, and that data will improve the probabilities of evaluating the specific probability of life on a planet, especially with the rapidly increasing number of exoplanets.
Whatever the pros and cons of different approaches, it appears that continuing research and cataloging of exoplanets will help narrow down the uncertainties of life detection. Ideally, several orthogonal approaches can be used to triangulate the probability that the biosignature is a true positive.
Sagan was right in that we need the ground truth of close observation and samples to validate electromagnetic data. While ground truth can eventually be acquired for planets in our solar system, we don’t have that for exoplanets, nor will we have that for the foreseeable future, unless that low probability SETI radio or optical signal is detected. In time, with a catalog of exoplanet data, it might be possible to collect enough examples to determine if there are abiotic mimics of different gas disequilibria, or other phenomena like the chlorophyll “red edge”. But we cannot know with certainty, and any abiotic mimic reduces the confidence of biotic interpretations.
Therefore, biosignatures from exoplanets will remain uncertain indications of life. We cannot escape from this. It will be up to the community and the responsible media to make this clear. [And good luck with the media].
As if this issue were not relevant, Payne and Kalteneggar just published a paper indicating that the O2 + CH4 signature is stronger in the last 100-300m than today, principally due to the greater partial pressure of O2 in the atmosphere during that period [15]. It was covered by the press as “Earth Was More Attractive to Aliens Back When Dinosaurs Roamed” [21]. C’est la vie.
References
1. Hitchcock, D. R., and J. E. Lovelock. “Life Detection by Atmospheric Analysis.” Icarus, vol. 7, no. 1–3, Jan. 1967, pp. 149–59. https://doi.org/10.1016/0019-1035(67)90059-0.
2. Tolley, Alex Detecting Early Life on Exoplanets. Centauri Dreams February 23, 2018.
https://www.centauri-dreams.org/2018/02/23/detecting-early-life-on-exoplanets/
3. Kilston, S. D., Drummond, R. R., & Sagan, C. (1966). A search for life on Earth at kilometer resolution. Icarus, 5(1–6), 79–98. https://doi.org/10.1016/0019-1035(66)90010-8
4. Shklovskii, I.S., and Carl Sagan. Intelligent Life in the Universe. San Francisco, CA, United States of America, Holden-Day, Inc., 1966.
5. Hoyle, Fred, and N. Chandra Wickramasinghe. Lifecloud: The Origin of Life in the Universe. HarperCollins Publishers, 1978.
6. Stirone, S., Chang, K., & Overbye, D. (2020). Life on Venus? Astronomers see a signal in its clouds. The New York Times. https://www.nytimes.com/2020/09/14/science/venus-life-clouds.html
7. Walker SI, et al Exoplanet Biosignatures: Future Directions. Astrobiology. 2018 Jun;18(6):779-824. doi: 10.1089/ast.2017.1738. PMID: 29938538; PMCID: PMC6016573.
8. J. Green, T. Hoehler, M. Neveu, S. Domagal-Goldman, D. Scalice, and M. Voytek, 2021, “Call for a Framework for Reporting Evidence for Life Beyond Earth,” Nature 598:575-579, https://doi.org/10.1038/s41586-021-03804-9.
9. “Independent Review of the Community Report From the Biosignature Standards of Evidence Workshop.” National Academies Press eBooks, 2022, https://doi.org/10.17226/26621.
10. Smith, Harrison B., and Cole Mathis. “Life Detection in a Universe of False Positives.” BioEssays, vol. 45, no. 12, Oct. 2023, https://doi.org/10.1002/bies.202300050.
11, Vickers, Peter, et al. “Confidence of Life Detection: The Problem of Unconceived Alternatives.” Astrobiology, vol. 23, no. 11, Nov. 2023, pp. 1202–12. https://doi.org/10.1089/ast.2022.0084.
12, Foote, Searra, Walker, Sara, et al. “False Positives and the Challenge of Testing the Alien Hypothesis.” Astrobiology, vol. 23, no. 11, Nov. 2023, pp. 1189–201. https://doi.org/10.1089/ast.2023.0005.
13. Tolley, A (2024) “Alien Life or Chemistry? A New Approach Alien Life or Chemistry? A New Approach, Centauri Dreams https://www.centauri-dreams.org/2024/01/24/alien-life-or-chemistry-a-new-approach/
14, Tolley, A (2018) “Detecting Life On Other Worlds”, Centauri Dreams https://www.centauri-dreams.org/2018/08/10/detecting-life-on-other-worlds/
15. R C Payne, L Kaltenegger, Oxygen bounty for Earth-like exoplanets: spectra of Earth through the Phanerozoic, Monthly Notices of the Royal Astronomical Society: Letters, Volume 527, Issue 1, January 2024, Pages L151–L155, https://doi.org/10.1093/mnrasl/slad147
16. Ley, Willy, and Wernher Von Braun. The Exploration of Mars. 1956
17. Leong, Y.C., Hughes, B.L., Wang, Y. et al. Neurocomputational mechanisms underlying motivated seeing. Nat Hum Behav 3, 962–973 (2019). https://doi.org/10.1038/s41562-019-0637-z
18. BBC News. “Arsenic-loving Bacteria May Help in Hunt for Alien Life.” BBC News, 2 Dec 2010, https://www.bbc.co.uk/news/science-environment-11886943.
19. Sankaran, Vishwam. “Dyson Spheres: Alien Power Plants May Be Drawing Energy From 7 Stars in the Milky Way.” The Independent, 17 May 2024, https://www.independent.co.uk/space/aliens-dyson-spheres-milky-way-power-plants-b2546601.html.
20. “Astrobiology at Ten.” Nature, vol. 440, no. 7084, Mar. 2006, p. 582. https://doi.org/10.1038/440582a.
21. Nield, David. “Earth Was More Attractive to Aliens Back When Dinosaurs Roamed.” ScienceAlert, 10 Nov. 2023 https://www.sciencealert.com/earth-was-more-attractive-to-aliens-back-when-dinosaurs-roamed.
22. Choi, Charles Q. “Mars Life? 20 Years Later, Debate Over Meteorite Continues.” Space.com, 10 Aug. 2016, https://www.space.com/33690-allen-hills-mars-meteorite-alien-life-20-years.html.
23, Von Braun, Wernher. The Mars Project. University of Illinois Press, 1953.
24, Von Braun, Wernher. Project Mars: A Technical Tale. Apogee Books, 2006.
25. Sinton, W M, “Radiometric Observations of Mars,” Astrophysical Journal, vol. 131, p. 459-469 (1960).
26. Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II. https://arxiv.org/abs/2002.11871
27. Greaves, J.S., Richards, A.M.S., Bains, W. et al. “Phosphine gas in the cloud decks of Venus.” Nat Astron 5, 655–664 (2021). https://doi.org/10.1038/s41550-020-1174-4
28. NASA (2010) NASA-Funded Research Discovers Life Built With Toxic Chemical URL: https://www.prnewswire.com/news-releases/nasa-funded-research-discovers-life-built-with-toxic-chemical-111207604.html
29. Saunders, J. K., Fuchsman, C. A., McKay, C., & Rocap, G. (2019). “Complete arsenic-based respiratory cycle in the marine microbial communities of pelagic oxygen-deficient zones.” Proceedings of the National Academy of Sciences, 116(20), 9925-9930. https://doi.org/10.1073/pnas.1818349116
It is hard for me to swallow the idea that the fact that Mars is red because it covered with hematite which iron rusted from oxygen is nothing but a false positive. We do know that Mars had a thicker atmosphere in the past. We know the photo dissociation O2 molecular oxygen into two O1 single oxygen molecules is combined with O2 to make O3 or ozone which shielded the cyanobacteria from UV radiation. If this doesn’t happen faster than the solar wind stripping, then it would be harder for life to survive unless is goes into an ancient sea. Mars also was thought to have a magnetic field in the past which might not have strong enough to deflect the solar wind. The Sun was thirty percent less bright four billion years ago since it increased by 7 percent every billion years. The temperature we lower, and therefore, the Jeans escape was less, so the planet had a much thicker atmosphere. It may have had ancient seas and rivers.
It is a possibility that there was life and the red color of Mars is an example. We might have to actually go there and get soil samples from the polar ice caps and lake beds or soil that was once ancient seas. If the oxygen did not come from life, then we will have false positive. We don’t have to worry about those, because we have not ever found any yet in exoplanets. Once we do, there are a many more criteria we can use to rule them out like are there the other biosignature gases there like Methane, nitrogen, What is the mass of the planet and it is in the life belt, The size of the star, etc.
If we do find life in the form of fossils on Mars, then it is pretty hardy.
@Geoffrey
I am not sure, but are you questioning whether the iron oxides on the rock surfaces are possibly a positive for extinct/[extant?] life on Mars?
So let’s back up first to iron oxides on earth. The famous banded iron formations start appearing in the late Archaean and seem to increase around the time of the great oxygenation event between 2 and 3 bya. The iron oxides appear as grey bands and are believed to be due to the pulses of higher oxygenation in the oceans due to the oxygenic photosynthesis of the cyanobacteria. Couple this with the evidence of liquid water on Mars in the Noachian (4.5 – 3.9 bya) and perhaps this implies life once existed.
The problems I see with this is hypothesis are:
1. Life had to at least get started by the end of this period, or at least remain possible in refugia.
2. The timetable is earlier than the evolution of oxygenic photosynthesis on Earth. This doesn’t mean that it cannot have evolved earlier on Mars.
3. We have no evidence of iron oxides through the rocks, just surface Fe2O3. This is very obvious where the Perseverance rover took drill samples. The holes show no evidence of iron oxides below a millimeter or so below the surface. This suggests that the oxides were created transiently, or possibly over time without sedimentation in an ocean.
4. There is evidence of sedimentation and the Opportunity rover found those hematite spherules (“blueberries”) that have been determined to be similar to terrestrial deposits that are abiogenic.
Iron oxides do form in water, but also by other means, such as contact with water in the air, and even abrasion with sand. There are alkali brines on Mars and the radiation produces peroxides that are in the regolith (which we need to remove to use the regolith for agriculture if desired). So we really cannot assume that the source of the oxygen is from free O2 in the atmosphere that is biogenic. The amount of iron oxides is believed to be millimeters to possibly meters in depth across Mars. Idk what that represents, but I would assume that the needed oxygen is readily sourced from the water that once flowed on Mars and the remaining water that still may exist in its atmosphere that may be replenished by seasonal melting of glaciers.
You may recall the transient excitement at the discovery of local methane in the atmosphere, but so low in concentration that is could be geologic or biotic in origin. As I have posted before, in the Martian crust liquid water probably exists which could mean a deep biosphere just as we have on Earth. We really need to recover those Perseverance samples if we can, and the Exomars mission should be able to drill more deeply for samples. However, if life can only be found deeper in the crust, then a different approach may be needed, such as sample collection at the crater wall avalanches, or even deep drilling when looking for subsurface glaciers, or possibly a polar drilling mission.
What I don’t think is likely is that the iron oxides on the surface are related to biology rather than chemistry. Maybe we have discarded the evidence of the ALH84001 “Mars meteorite” too quickly and newer, or future analytic techniques, may shift the thinking toward life.
The photo dissociation of H2O water vapor and carbon dioxide CO2 produces oxygen, but does that process make enough to explain Mars surface being red. Samples of Martian rock and ice will tell us.
Perhaps in summary but different words, our indicators for exoplanet life if positive do not necessarily clinch the argument, and it is unclear how much consensus is needed to close the argument with exoplanet data. Especially since we have no off-world positive conclusions thus far.
With some worlds here in the solar system we have some essential chemistry deposited or produced on the surface thanks to elemental abundances. But there are others, like our moon, that are bereft of even these ( O, C, N,…). Yet this chemical agents exist in nebular form, often in circumstellar disks. And in solids of various pre-planetary origin such as carbonaceous rich objects. once located in the inner solar system, farther out – or perhaps even mixed with an interstellar medium ( our Oort cloud and analogous others).
As I recall from decades back, space science committees prioritized comet sample returns higher than human landings on the moon. Considering how each impacted the search for extra-terrestrial life, it now seems to make a certain sense.
In the last few years there was interest in the discovery of phosphine in the mid layers of the Venusian atmosphere with implications for bacteria life. Fair enough. But I remember from decades back a similar stir about its presence in the atmosphere of Jupiter; it was along with non-equilibrium carbon monoxide abundances. Some efforts were made to determine a correspondence between Great Red Spot brightness and increased phosphine concentration within vs. the surrounding atmosphere ( a detergent effect if not evidence of life), but no difference across the boundary obtained thus far. Or else the matter has been shelved.
Still, it does raise a question or two. If phosphine is considered a life indicator in the case of Venus, then why not for Jupiter which could be a very rich biochemical stewpot as well? Whatever their meaning, the sensor receptivity is likely stronger for jovian investigations than turned toward exoplanets tens of parsecs away.
If there is life within the atmosphere of Jupiter, than unless it has remarkable transport, it will be difficult to conceive how it could get out, what with escape velocity or even high speeds in its atmosphere – unless that’s where all those little green men live. But on the other hand, some of the pre-life biosphere could fall into early Jupiter in the planetary circumstellar disk or early jovian satellites. The remaining satellites though well irradiated in the magnetosphere could provide evidence of what biological building blocks were around or even how far biology progressed before the terrestrial planets cooled off, for example. A tall order for generating life? Yes, but more about tall orders in a moment.
Whether it is a question of the solar system or where exoplanets orbit around the other stars, life might not be spelled out unambiguously, but life’s great advance on Earth, based on the “abiogenesis” localized origin assumption, seems to violate some intuitions too. This supposition is founded on the idea that starting with a lunar like surface on Earth, circumstances conspired to allow life’s advance on the planet to jump all, or more Drake equation probability factor-hurdles than we can think of to toss in, … without any fundamental biological precursors available from the solar system or the galactic medium.
A tall order.
So, we look at nearby exoplanets for life indicators and anticipate finding something that indicates microbial presence of life. But not anything as complex as our own because of this a priori notion of isolation. Possibly, if the local returns are consistent with the anticipated results, it might open the door to another hypothesis: that the Earth in its aeons of galactic wandering originated in an interstellar cloud richer in the biologic pre-cursors that make you and me possible.
Otherwise, this search for life seems to resemble the billion dollar lottery invitations offered at the local grocery store. If you win, you’ve got bragging rights, but likely no one available to listen. It works for raising revenue, but as a fundamental biological mechanism….?
@wdk
You pack in a lot of ideas in that comment. Firstly, I think that past assumptions of life on Mars, pulsar emissions as ETI, and the more recent flubs have made scientists more careful. They want to “dot their i’s and cross their t’s” before announcing the discovery of ET life. That caution is warranted. [ One might think that the ufologists would want to be more careful too, but they seem to run with government conspiracies to cover up, as the latest DoJ review has shown.]
It is not just the discovery of ET life either. Think back to “cold fusion”, the “perpetual motion machine”, or the more recent “room temperature superconductor” announcements. All proved embarrassingly wrong, with the last being deliberate fakery.
Until the 1980s, it was assumed that other solar systems would have a retinue of planets like ours. So models changed the arrangement. But reality demonstrated that these systems proved very unlike ours, with some planets wildly more bizarre than the most imaginative Sci-Fi. With that knowledge, we have become more cautious over what conditions rule out dead worlds and imply living ones instead. You mention phosphine which is indeed produced by life on Earth, but we can make phosphine, and a model for its abiotic production on Venus has been published.
I tend to be rather “old school” and want to see something “wriggling in a video” if it is a microbe, or with obviously living structures if sessile. but as we know from fossils, we can be fooled by what appears to be life but is not, and vice versa.
The planets in our system are all reachable and we could explore them thoroughly for life. But exoplanets are another story. Without some means to either go to them or be able to remotely view them with what seems like a very close inspection, then we have a problem. We can only use proxies, and those models need to be validated in some way. If we had some theoretical telescope with millimeter/centimeter resolution, we could certainly hope to identify multicellular life like that we have on Earth, but microbes might be more difficult unless we detect biofilms or colonies that can be resolved, and even then…
[I recall a public meeting at NASA Ames over a Mars landing. They managed to get A C Clarke on the line and he was saying there were trees on Mars based on photos of the surface. He was mistaken, but I can see why, as the forms looked like tree structures.]
But we are human, and while most scientists are cautious, following Feynman’s admonition that the observer is the easiest to fool, some seek publicity and throw caution to the winds for their own motives.
I just watched Lisa Kaltenberger give a very spirited interview in which she advocates caution about life, but nevertheless ends with a mention of her paper on the easier detection of life in prehistoric times as the O2 level was higher than it is now.
Lisa Kaltenberger interviewed on the SETI.org YT channel.
My sense is that unless we get lucky with an unambiguous detection of life in our system (and that is still a decade or more away) we won’t have much to help us with the big question of abiogenesis. If we got lucky with a technosignature – either an unambiguous signal or a artifact – that would settle the question of life, whether biological or post-biological, elsewhere, and imply abiogenesis has happened. What we cannot know without more information are the details and whether each world has its own biogenesis or as a result of [directed] panspermia. These questions and others about abiogenesis and the path of evolution will likely remain unanswered.
By statistics alone, the probability that an exoplanet hosts only primitive life is higher than whether it hosts technological ETI. However, detecting the former (as Alex describes well) is difficult and ambiguous, while detecting the latter (if they are communicative) should be far less ambiguous. Unfortunately, it isn’t possible for us to calculate which of the two are more likely to be high-confidence detection events (or the first).
@Ron
Yes. back in the 1960s and 1970s, the technology was only good enough for ETI messages which some were hopeful that there was a “galactic club” out there. It made sense for SETI to focus on messages. But now that we can detect exoplanets and can make the first planetary atmosphere analyses, SETI has also reached out to astrobiologists and funds some astrobiology research.
It would make a good long-term bet whether we get an unambiguous message or life detection first. IIRC, Shostak to a congressional special hearing that he was sure we would discover life of whatever kind within 50 years (i.e. after he was long gone.)
Idk about about ETI, but I would bet on a high confidence of life detection in that time frame, probably on an exoplanet. (But I too will be gone in 50 years.)
A.T., R.S.,
Understand that my comment is not a protest, but simply comment or exploration on an interesting subject, searching for a gateway if not an exit. With what advances in this area that have been made, it still tries patience. It could very well be that we could say all the same things a decade hence since the detection problem is admittedly tough.
Though with regard to planetary concepts “until the 1980s”, my own experience with that question was not necessarily that clean cut. Working in a graduate program earlier devoted primarily to stars, in contrast exoplanets and even brown dwarfs were addressed with extreme skepicism by an accomplished faculty concerned with STARS mainly and their end products. After a symposium lecture on white dwarfs one day in the late 70s, the topic of brown dwarfs was a sidebar over coffee with the same instructor who gave the presentation on his specialty. Jeans Limit argued against brown dwarfs and planets back then, coincidentally as assessed back then, right around 0.08 mass or hydrogen fusion ignition. Circumstellar clouds around stars was acknowledged, but not convincing as a planet formation preliminary. Unstable and no demonstrated results. And if exoplanets did exist – well, they would not be luminous to be detected with late 1970s technology and there was plenty to work on with what was visible. QED.
Of course, there was no guarantee that brown dwarfs would exist or be distributed enough to find one in our galactic vicinity. Stellar spectroscopic binaries toward the end of the decade were converging toward that limit though.
But the fact that we lived in a planetary system argued perhaps for the existence of others. And if there was any argument for exoplanet systems resembling this one was the theoretical dead end of Bode’s Law for anticipating where the next undiscovered planet in the Solar System would show up.
More critical, however, was the argument about which model for stability of circumstellar disks were, lasting long enough to produce planets. In the late 1970s there were questions whether they would disperse before producing any. And further back, the notion that circumstellar disks from which planets could emerge was one of several theories. Stellar close passages might have been more prevalent since the 18th century pioneers in orbital mechanics such as Laplace.
Earlier I had run into a 1940s encyclopedia that provided this rather official explanation for any interested layman.
Remarkably, Olaf Stapledon who had written at least two panoramic space novels, “First and Last Men” and “Star Maker” ( circa 1930-35) seemed to be aware of much of what Hubble had determined about galactic structure. But in each of the books, he explains the close stellar passage explanation for the formation of planets. Collapsing nebulas of gas and dust centered around protostars, in his mind, and his sources, evidently, had nothing to do with solar systems. Just drawn out of stars like the sun like beads.
With some knowledge of the plot line of the two novels, and having seen an old encyclopedia or two that explain the close stellar passage explanation, I found it fortuitous that Stapledon to call this out. But the result with Stapledon’s reasoning, is that planets would be separated by much greater distances than the current separation of detectable exoplanets period. Whether simply for novelistic purposes, philosophy or plot, Stapledon just assumed that life would unfold on these extremely separate planets in other solar systems.
By contrast, we have confirmed orders of magnitude more exoplanets in our vicinity than the analysis Stapledon relied on at the time. I hadn’t noticed any reference to “pan-spermia” in what I have read in the two stories, but maybe that would be his rationale for what is described. Pre-cursors to life I would argue for, at any rate. In addition, I feel more optimistic for life’s prospects with the physical evidence we have today than the one that science of his day suggested.
So, In Situ it is. Not to far from where I live is where The Creator (2023 film) is located. The Taiwan foundries along with NVIDIA are creating AI that will be sent to distant planets in are solar system and eventually very distant exoplanets. Maybe subatomic quantum AI will be developed and this is where our money should be going. The rate of change that is taking place is tremendous and AI will make it 1000s of times faster. So stop worrying about “The Ambiguity of Exoplanet Biosignatures” and start developing the chips that can do it right now in the many planets, moons and dwarf planets in our solar system…
There is a very long blogpost (197 pages) about why we will see superintelligence within a decade. If he is correct (Big If) the world changes.
Situational Awareness – The Decade Ahead.
Today, an AGI level system will be a huge server farm and a superintelligence not much larger as many AGIs will rapidly build the superintelligence. That superintelligence could start sending messages into the galaxy. As the technology continues size reduction, within a few decades it will be small enough to be the payload of an interstellar ship.
Idk about quantum AI, This seems right out of the 3 Body Problem. But if we can pack the technology into the rolled up dimensions posited by string theory, couldn’t we use the technology to travel in space at what appears to be FTL speeds? Then the AI could just “transport” itself to distant destinations and make direct contact with other intelligences. Assuming we can align the AI with human goals, what would such an AI do in the galaxy?
AI is a tool just like any other tool it can be used for good or bad. The human ego prefers to make anything new a dangerous unknown. What you really have is at the level of a wolf that can destroy us or just become a domesticated dog. The problem is we make into a werewolf because in our brain it’s the real ugliness that happened in our primeval being. Anyway this is why I mentioned going through the biolevel first because our brain works on a quantum atomic and subatomic level. Ai may help us figure that one out and could lead to a no chip but a high level intelligence that could fit inside a marble. We do not need a megalithic server farm to find out if microbiological life exist in Europa but a neural level Ai that knows what it is looking at. The tools it need such as microscope and labs could also be shrunk down and use quantum level physics to do the job.
What Ai could do for for us, its master is explore the planets in other solar systems at minimal cost and may be able to communicate in real-time through quantum warp wormholes.
3 or 4 marbles giving a holographic views of dinosaurs on Teegarden’s Star b…
On the one hand, you are arguing for getting the ground truth by having probes reach the worlds. No argument from me that this seems the best validation approach today. But on the other hand, you are arguing for fanciful technologies that include FTL flight and communication that we have no serious idea how to achieve or implement. Any probe constrained by the speed of light to reach exoplanets will require a long time to both reach and communicate its findings back to Earth. A century at least for Trappist 1 exoplanets. In the meantime, our technologies will improve and will have to do as best it can with remote sensing. We might get very lucky if it turns out that microorganisms are drifting through space and can be detected when they enter our solar system. However, while that could confirm life exists elsewhere, it does not tell us the source, nor the variety of life at the source.
Within the solar system, I am all for miniaturizing probes that can be sent as smart sensors all through our system with a relatively low cost. But they will be limited to our system and nearby interstellar space in terms of mission time. I acknowledge there is no way to predict future technologies a century hence, so we may be astounded at what can be achieved in a century, or we may still be constrained by the physical limits we currently know.
IOW, I can imagine a trans-Atlantic tunnel, bridge, or hypersonic aircraft, but not a teleport system to make the trip. In practice, an electronic telepresence is far simpler, and is already achievable today, albeit in a limited fashion.
How about Proxima Centauri swarms to Jupiter’s atmosphere, and easy target…
What about using this concept to penetrate Europa’s ice, like a shotgun’s large buckshot or a cannon’s grapeshot. Something like the Russian Venus entry vehicles but miniaturized a 100 to a 1000 times. Coming in at a 100,000 miles an hour, could these AI probes reach to the ocean below Europa’s ice? Just how dense is the ice of Europa? Europa gravity is 1/7.5 of earth’s and is exposed to the vacuum of space so how would that effect the mostly water ice crust?
Seems unlikely that any probes could survive the impact to return data from beneath the ice! (Such a mission would also seem unlikely to pass planetary contamination review.)
We have military equipment that can handle millions of Gs and could have many probes work as a team on and below ice. Anything built like this would automatically be the cleanest thing on the face of the earth…
We do not know how dense the ice is but Europa Clipper mission will launch in October 2024 and will arrive in 2030 and should answer that…
According AI ChatGPT the range limit in light years of the JWST for indentifying the chemical composition of Earth sized exoplanets through transmission spectroscopy is ten to forty light years. The range limit is 50 to 70 miles under challenging conditions, the planets with thin atmospheres and fainter stars and 100 light years the maximum range limit. Ibid.
Excuse me. I meant 50 to 70 light years.
Backing off a little from positive identification of life within about ten parsecs, within that 10 parsecs our sampling even within assumed habitable zones is just that: a sampling. Principal sources of data are transiting planets with inclinations odds of about 1 out of a 100. Then as stars range from the smallest red dwarfs to the stars like our own (G2), whether doppler shift or astrometric, detecting those measures take longer and longer. For example, the Earth has had less than 30 revolutions since the exoplanet era of astronomy got underway.
Where there seems to be a gap is visual or other spectral band detections.
Notoriously close stars have not revealed many planets. Yet transit data suggest this would be out of character.
Additionally, despite the number of transiting planets discovered to date, we have no guarantee that even systems like Trappist – (almost wrote Transit -) 1have identified all the planets within the system. What distance to write off other planets within the system, I am not sure, but say at 1 AU and the inclinations of planets in our own solar system, beside waiting forever, the planets might miss the line of sight cylindrical FOV most or all the time.
Transiting planets afford great opportunities for getting mass, dimensions and gross chemical nature from density or atmospheric traces, but they might put a brake on further exoplanet research in a given solar system. For example, with even two or three identified hot Neptune like worlds, their significance for a time might be their identification and search for more “habitable” planets farther out might be indefinitely shelved.
Something analogous to a Gaia type observatory might the answer to the problem (And maybe it already exists or operates effectively as such). This would be a 10-30 parsec near space search for additional planets in systems where one or more have already been identified. Likely it would need good IR and visual bands as well as good resolution in terms of fractions of arcseconds squared. And, of course, some spectral resolution as well for chemical identification. Whether this is unique or not these days, I am not sure. Maybe the idea of looking at near stars with some offset or pointing a little in the wrong direction. Unless it has a secondary ( primary ?) mission, the review committee members might all off on it like tea pots on the stove.
Hundreds if not thousands of textbooks train children in the Scientific Method, and we try hard not to notice that sometimes the Scientific Method has three steps, sometimes five, sometimes it’s a line, sometimes a circle, sometimes it’s a personal odyssey of discovery about why the toaster doesn’t work, and sometimes it’s a social process of collective advancement. Sometimes we argue by authority and cite it to Bacon, seldom noting that we need to read a certain interpretation into a few old sentences to get them to say what we mean. Yet one thing every such unique formulation has in common is the lack of a randomized controlled trial documenting that the scientific method actually works for any particular purpose. It ranks above and beyond such validation.
In the current discussions, the urge to control “valuable observational time and resources” seems paramount – but I suspect this is directed more toward the career of the administrator than real concern for the possibility of finding alien life, which surely we can’t predict.
Just TODAY I’ve seen word of a new, beautiful possibility for life, at least in my particular imagination. According to https://www.globaltimes.cn/page/202406/1314711.shtml , parts of the Moon are covered with a few layers of graphene of a high crystalline quality. This seems to be associated with iron, which may have been involved in its catalytic deposition. To review for a moment:
a) the graphene and iron have an organized structure
b) the graphene and carbon may have been ingested from the solar wind
c) graphene is conductive and has been used in solar cell development, so it ought to be able to ‘harvest energy’ from the environment in some sense
Now that’s three out of perhaps 6 to 8 different properties of life. Does the graphene respond to a stimulus, such as by altering its charge in response to a change in solar wind intensity? Is there a homeostatic mechanism that stabilizes the number of layers it forms on a surface? And of special interest, in the presence of solar wind, can a chunk of this moon rock catalyze the spread of graphene layers onto fresh surfaces nearby? Is that something that some samples are better evolved to do than others?
Any given day, someone might be discovering life on the Moon. Or anywhere else, I think. It does help though when a nation actually goes out there with a probe! I don’t think we can tell them in advance what they’ll find – we’ll be lucky enough to notice it when they do.