Mars Clay Findings Are GOOD: The Great Filter Could Be Behind Us

NASA has announced that Curiosity and Perseverance have found the strongest-yet evidence of life on Mars: vivianite and greigite. On Earth these minerals are found in sediments associated with microbial life.

Great Filter-Doomers will find this alarming. If life evolves easily (and quickly - in thermodynamics that's the same thing) then it should be everywhere. And yet, where is everyone? The galaxy appears to be dead. There must be a Great Filter between the appearance of life, and technological civilization leaving its solar system of origin.


It's worth pointing out that until about 700 MA ago, Earth was a boring microbial planet. There was basic multicellular life but until the Ediacaran, nothing that you could actually see with the naked eye, or that had complex organization. In fact for over a billion years it was stuck in a simple oscillation of build up oxygen, die back, build up oxygen, die back, encoded in the banded iron formations (in different redox mixes) visible in extremely old rocks like the ones above in Australia. An alien visitor would have sampled the boring microbial soup and moved on. Visiting a hundred million years later, it would have observed exactly the same situation. Life on Earth was very much like a "blinker" in Conway's Game of Life, with little sign it would ever break out of it. Based on our N of 1, thermodynamically, the appearance of life is likely (it was almost immediately after the Earth cooled!) - and similarly, the appearance of multicellular life is UNlikely.


For that reason, if there is a filter, there's a big question is whether the Filter is behind humanity, or in front of it. The discovery of microbial life on Mars, especially extinct microbial life, would be good news, in the same way that discovering an extinct civilization would be bad news. A thought experiment may help illustrate.

Imagine you send out a fleet of near lightspeed von Neumann probes. As they cover the galaxy, the reports come back to Earth: thousand then millions of planets with oceans of bacteria and/or blue-green algae, some living, some extinct, embedded in clay like the ones on Mars. But absolutely nothing multicellular, anywhere, besides our freakish Earth. Everywhere, simple one-dimensional ecosystems, some "blinking" forever like Earth almost did, but no dusty ruined cities, or eerily silent half-built Dyson spheres, or even alien cockroaches. Nothing beyond a Kardashev 0.001![1] Time to uncorck the champagne! The Great Filter is behind us! We're the first!

Now imagine the opposite case: ghost planet after ghost planet, civilizations that blossomed and then burnt out. Some of them had even sent out their own probes and learned their fate. We would be looking at our own future.

If we assume this Mars finding really is extinct microbes, we now have N=2 for the denominator of how frequently life evolves, and N=1 - where it never got past the microbe stage - a 50% rate of the Great Filter being behind us.[2] Assuming the principal of mediocrity, 50% of the aliens we're not seeing are microbes embedded in clay. 50% is hardly a guarantee of our eternal future among the stars but it moves the needle in the optimistic direction. You might think it would be boring to explore the Solar System and find only microbes on Mars, Venus, under the ice of Europa and Enceladus - but such discoveries should make you happy for humanity's future, especially if they're extinct.


FOOTNOTES

[1] Recently there have been efforts to establish a continuous Kardashev scale (rather than having only discrete classifiers for civilizations which surpassed certain benchmarks, "1" being the power output of a star); one paper assigns 2023 Earth a 0.7276 (Zhang et al 2023.) For very low Kardashev numbers, we could relate the SQ (sentience quotient) for intelligence to the Kardashev scale. Calculating a simple upper bound for the Kardashev number of "algal Earth" - assume a number of cells per meter of seawater equal to that during an algal bloom, times the surface area of Earth's ocean, times the energy budget of an algal cell, divided by the power of the Sun:

5.1x10^14 m^3 x 0.7 x 10^11 algal cells/m^3 x 10^-11 Watts/algal cell
divided by 3.84x10^26 Watts = 10^-12 Kardashev

-70 is the lower bound for SQ, to single-celled organisms, so we can say that -70 SQ converts to 10^-12 Kardashev. Humans have an SQ about +13. However human civilization cooperates to control more energy than a single human, so +13 does not correspond to 0.7276, but whatever the SQ of the human race as a whole, does. You can't get the Kardashev of a single human just by dividing 0.7276 by 8 billion because of the non-zero-sum effects of civilized cooperation.


[2] As written before, Venus had oceans until about a billion years ago. I would have liked to include it here as another microbial blinker planet that ran out of time before its own Ediacaran, with the evidence of both phosphine and microbe-sized UV absorbers in its upper atmospheres, as the remnant of its ecosystem. This would give us a denominator of 3, and even more confidence that the Great Filter is behind us. However, the famous phosphine paper failed multiple attempts at replication and two papers (Jiang et al 2024 and Egan et al 2025) have advanced good candidate abiotic explanations for what the absorbers could be.

If Threonine and Aspartate are Detected on Europa, They Must Have Been Recently Generated

Important paper for detecting biosignatures elsewhere in the solar system. Truong et al measure the decomposition rate of amino acids in conditions mimicking the under-ice oceans of Europa or Enceladus. Using that data, they infer whether any amino acids detected there are leftovers from early abiotic chemistry, or must have resulted from a more recent process. In particular threonine and aspartate are unstable over time and if detected at concentrations greater than 1 nM, they must have been generated recently. Click through to the paper below.

Truong N, Monroe AA, Glein CR, Anbar AD, Lunine JI. Decomposition of Amino Acids in Water with Application to In-Situ Measurements of Enceladus, Europa and Other Hydrothermally Active Icy Ocean Worlds. arXiv:1904.04407 [astro-ph.EP]

Molecules with MW > 200 Found in Enceladus Vapor

That's bigger than all amino acids and nucleobases. (Note, 9 months old, I had missed this before.) Press release here, paper here. (Postberg et al, Nature. 2018 Jun; 558(7711): 564–568.)

Putting Numbers on Panspermia: Material From Earth Impacting Outer System Moons, and Escaping Solar System

In a simulation, Worth Sigurdsson and House (2013) (WSH) retrodict that Europa, Callisto, Titan, and Enceladus have received 1,900, 370, 510 and 340 metric tons of material from Earth, with 3.4 billion metric tons from Earth ejected from the solar system entirely. Enceladus may be less interesting if it really did only form in the cretaceous, but the others have all been there since the start of the solar system. WSH state explicitly that they didn't try to estimate the viability of life surviving the journey, but it cannot be repeated enough that we now have evidence that living things - metazoans, in fact - can survive uncontrolled re-entry with minimal protection, as some worms that were on board the Columbia were found alive on the ground weeks later. We're now able to start putting bounds at least on local panspermia (within our own solar system), though it would be interesting to estimate the chances for gravitational capture by surrounding stars. This is exciting not only to flesh out the realism of panspermia as traditionally considered, but also the idea of very small, molecule- or cell-sized organic von Neumann probes passively spreading between lower-gravity bodies.

A very basic calculation using water surface area and the time it took for life to appear on Earth, shows that all other things being equal, there is a 1-in-3 chance of indigenous life on Europa. An experiment in reproducing impact conditions and local conditions on these moons, along with adding most-likely-transferred Earth fauna, seems that it would be fairly easy to do - a sort of Miller-Urey experiment for local panspermia.

R.J. Worth, Steinn Sigurdsson, and Christopher H. House. Seeding Life on the Moons of the Outer Planets via Lithopanspermia. Astrobiology, Dec 2013.

Look for RNA-World Rock Strata on the Moon

Schulze-Makuch and Crawford show in Astrobiology that that the Moon may have briefly been habitable - either (two options) about 4.5 billion years ago, or 3.5 billion years ago, for a few tens of millions of years, with an atmosphere and some liquid water (Gizmodo digest here.) Since the moon was formed after an impact with the early Earth, we should assume they had many of the same starting materials. The moon had less surface area and less time, and split from the Earth prior to even the earliest suggested prebiotic activity around 4 billion years ago, so it would have had to develop its own life - it could not have been "seeded."

The Moon with life (although terraformed.) From Techeblog.
Recent work by Tashiro et al suggest that a 4 billion year old rock stratum on Earth shows evidence of biological activity and may even be the fossil result of an RNA-World stage in the evolution of life on Earth. If it existed on Earth, it also could have existed on the Moon. It's not as though that rock stratum is exposed everywhere on Earth (the Tashiro people used samples from northern Labrador, Canada.) But it's interesting to think that the same stratum could have existed on the Moon if prebiotic chemistry took a similar course - and that those strata may be much easier to find and more widespread given the inactivity of the Moon relative to Earth.

Complex Organics from Enceladus

The work is driven by a chemical species that is hard to explain as other than the result of the reaction of these other complex molecules, and they build a model for how it's getting from inside Enceladus out into the plumes. Cassini detected it both in the E ring and the plume itself. Figure 10 from the paper (!!!):


Bonus points for one of the cooler names for a peer-reviewed paper ever. Points off for everyone who's touched this data and not noticed this before! What are you doing over there! You're giving fits to us bio/chemical types who are following this work. SciAm writeup here.


Frank Postberg, Nozair Khawaja, Bernd Abel, Gael Choblet, Christopher R. Glein, Murthy S. Gudipati, Bryana L. Henderson, Hsiang-Wen Hsu, Sascha Kempf, Fabian Klenner, Georg Moragas-Klostermeyer, Brian Magee, Lenz Nölle, Mark Perry, René Reviol, Jürgen Schmidt, Ralf Srama, Ferdinand Stolz, Gabriel Tobie, Mario Trieloff & J. Hunter Waite. Macromolecular organic compounds from the depths of Enceladus. Naturevolume 558, pages564–568 (2018)

Super Earths: More Surface Area to Evolve Life, but More Gravity to Keep You From Leaving

Super-Earths have more surface area and may be more likely to evolve life merely for this reason. But ironically those planets have higher gravity that makes them harder to get off of - and that life is therefore more likely to be trapped. This is similar to the idea that planets closer to the galactic center might be more likely to have life because they're older, but less likely to have life because those planets are more subject to bombardments triggered by more-frequently close-passing stars, supernovas or gamma ray bursts. When the same cause (larger terrestrial planet) can plausibly have two contradictory effects and we can't decide which one will dominate, that shows how little quantitative knowledge we have and how little we really know with any confidence.

Last post about alien evolution: Method Proposed for Detecting Exotic Biochemistry

Finding Extraterrestrial Organics is Old News; Let's Look for Evidence of Life

There's a new mass spec study[1] of crystals from two meteorites, one of which in turn has material originating from two separate parent bodies. The objects were about 4.5 billion years old, i.e. dating to the birth of the solar system, and showed evidence of organics resulting from aqeous reactions. Some findings of interest: "...signatures of low-mass C5 to C10 hydrocarbons at around 70 to 200 atomic mass units." Not much benzene, suggesting that any aromatic rings are locked up in larger structures. We're finding organics everywhere we look it seems, including Ceres, and that includes even amino acids and nucleobases. Given how quickly after the Earth formed we started seeing evidence of self-replicating molecules (at least the ancestors of cells, if not cells themselves), this means that life originated quickly on Earth, and therefore was a highly probable event.

It's also relevant that polyaromatic hydrocarbons (PAHs - for instance, tar, graphite, anthracene in coal, and fullerenes) have been found in nebulas, as well as in Titan's atmosphere. Not only are they thought to be quite common in the universe, but possibly crucial to the origin of life (see PAH World Hypothesis.) PAH's are predicted to make up a large portion of the carbon at the surface of carbon planets. While carbon planet systems (unlike our own silicate system) were theorized only recently, it turns out that the Hypatia Stone, a bizarre meteorite found in the Egyptian desert, is loaded with PAHs and originated from outside our solar system - possibly as impact debris from just such a planet.[2] (It's becoming increasingly clear that objects from outside the solar system enter it frequently. First Wild-2 (which had amino acids in it), then Oamuamua, and now Hypatia. We've found these things on the Earth's surface without looking that hard for them! Given these observations, we should expect that interstellar mixing on relatively short geologic time scales is the rule.

This suggests several things and begs several questions.

- If a pile of complex molecules were delivered to Earth - say, a bunch of RNA that survived intact inside an impactor - that pushes back the question of the origin of life, but it also suggests it's very likely elsewhere.

- Have we looked for polymerized RNA or amino acids? Mass spec can detect and distinguish small fragments.[3]

- You might ask, why RNA? Why assume any similarity to Earth biochemistry? This raises the larger question of, if there is active extraterrestrial biochemistry in asteroids, how could we detect it? This is the question asked about desert varnish (which has been speculated as evidence of a shadow biosphere of non-DNA based life operating here on Earth under our noses.) If we did find alien biochemistry, how would we know what we were looking at, against the background of organics that we already know is there? While we haven't seen anything that obviously screams "alien biochemistry", that's the point - HOW does something look if it screams "alien biochemistry"? Are there general principles of such systems? You can't just look for macromolecules - if those are composed of the some monomers, they won't necessarily carry information (e.g. aliens trying to figure out our biochemistry from sequencing the fatty acids in our membrane phospholipids will not learn very much.) So it has to be a macromolecule with a limited number of discrete subunits. So far our samples have been limited t one biosphere. If we ever get enough complex organics from a sample return mission to be able to afford to destroy some of it in aqueous chemistry experiences, that will be a boon to astrobiology.

- If there is such a thing as a simple space-borne organism - or even the remnants of aberrant von Neumann probes that have "gone to seed" after eons-long selection for fecundity over their exploration functions - it would make sense to be adapted to low gravity bodies that are cheapest to move back and forth between. If Earth's biosphere is just overgrown von Neumann probes, that might just be because we're a dead end at the bottom of a gravity well.

Previous post on alien evolution, First Interstellar Asteroid? It's Interstellar, But Not the First We've Seen


REFERENCES

[1] Queenie H. S. Chan, Michael E. Zolensky, Yoko Kebukawa, Marc Fries, Motoo Ito, Andrew Steele, Zia Rahman, Aiko Nakato, A. L. David Kilcoyne, Hiroki Suga, Yoshio Takahashi, Yasuo Takeichi and Kazuhiko Mase. Organic matter in extraterrestrial water-bearing salt crystals. Science Advances 10 Jan 2018: Vol. 4, no. 1, eaao3521. DOI: 10.1126/sciadv.aao3521

[2] Georgy A.Belyanin, Jan D.Kramers, Marco A.G.Andreoli, Francesco Greco, Arnold Gucsik, Tebogo V. Makhubela, Wojciec, J.Przybylowicz, Michael Wiedenbeck. Petrography of the carbonaceous, diamond-bearing stone “Hypatia” from southwest Egypt: A contribution to the debate on its origin. Geochimica et Cosmochimica Acta, Volume 223, 15 February 2018, Pages 462-492.

[3] Zhaojing Meng and Patrick A. Limbach. Mass Spectrometry of RNA: Linking the Genome to the Proteome. Brief Funct Genomic Proteomic. 2006 Mar; 5(1): 87–95.

Organics on Ceres Are From Ceres (not from other impacting bodies)

The organic material on Ceres, while intriguing, appears to be native, rather than delivered from other impactors. So says data from the Southwest Research Institute at the 2017 Astronomical Society meeting. The possibility of simple organic replicators on low-gravity bodies in the solar system ("space viruses", to be dramatic) an interesting one, and is one form (or one part) of the pan-spermia hypothesis that's been considered for over a century, going back at least to Arrhenius. (Space viruses might also be the only evidence we would ever see of alien life or even an alien singularity.) What this tells us is that the large majority of material on Ceres, and presumably on most large old asteroids, is native to those bodies since the dawn of the solar system.

What the findings mean for the "space virus" hypothesis is that we can be more confident that Ceres is not crawling with foreign space viruses - although if there is a replicator that can use the typical organics on large asteroids as building materials, that's not what you would usually see. That is to say, when an organism gets infected by a virus, the organism isn't infiltrated with foreign matter, but rather with a tiny bit of foreign matter that then rearranges the atoms in the organism into copies of itself.

Life's Origins at Four Billion Years Ago; Implications for Our Future

A group from the University of Tokyo (Tashiro et al, 2017) argues in a Nature paper that carbon isotope ratios in rocks in northern Labrador, Canada means that those rocks harbored life almost four billion years ago. This pushes back the early bound on origin of life almost two hundred million years, almost to the Hadean eon. To be sure this finding has not been universally accepted, but it's worth thinking about what it would mean. In particular, and perhaps not coincidentally, this is also right about when the Earth's surface transitioned from molten to solid. [Added later: it turns out the Moon may have been briefly "habitable", i.e. had an atmosphere and liquid water. Look for the same signature in rocks there?]

A recent paper reconstructing the last universal common ancestor (LUCA's) genome from a massive tree of millions of genes showed that it was pretty clearly a sulfur-vent organism. This is good news if you're looking for life on Europa or Enceladus, because that means that life on Earth didn't need the sun (and neither would any life that could evolve along vents under Europa's icy crust.) If you assume that the chance of life evolving by 3.5 billion years ago on Earth was 50%, and that the chance of life evolving is based on surface area, and all other things are equal (admittedly speculative when we don't even have all the information for our N=1) then there is a one in three chance of life on Europa. (If that probability correlates instead with the volume of water, then it was overwhelmingly more likely for life to evolve on Europa!)

[Added several days later: someone has finally run the numbers. A model of RNA polymer formation by Pearce et al suggests that the first RNA world molecules were most likely to have formed in small surface pools rather than sulfur vents - but even earlier, 4.17 billion years ago. If a wet-dry cycle is needed, this suggests ocean worlds like Europa are less likely than once-wet places with exposed land like Mars. The lesson of this paper is that you need puddles, not bone-dry deserts or world-spanning oceans. In this model, a world with puddles and organics seems all but certain to develop into an RNA world. A paper by Cardenas et al from the Geological Society of America Bulletin strongly suggests that 3.5 billion years ago, Mars was exactly the kind of place to have puddles. The logical argument is that life, or at least an RNA world, also developed very quickly there, and we should look for similar deposits to the ones found by Tashiro et al. If Pearce's argument does not produce findings like Tashiro's on Mars, we at least can start looking for differences in the early environments of the two.]

Two things to keep in mind about the LUCA paper: 1) LUCA is the last universal common ancestor. There could be a long lineage before it; and 2) the smaller and simpler a system, the more profound the changes possible in that system. If at one point Earth was an RNA World, molecular clock techniques developed based on modern DNA metabolism would probably be pretty bad at retrodicting LUCA. That two hundred million year gap map be exactly that. All that carbon might be free-floating ribosomes, or peri-biotic viroids.

Even more importantly, this has implications for the likelihood of the evolution of life. This discovery should worry you if you consider the Great Filter. The idea is that it seems very likely that life would evolve anywhere there's liquid water. Yet the universe is not obviously filled with intelligent life. Something is therefore stopping the progression from the evolution of life, to that life spreading from its home planet. (This is typically assumed to be some natural event and need not be some science fiction plot of an alien menace stamping out intelligence wherever it appears.) And every time that the origin of life is pushed back a bit further - that gives greater cause to worry, because where probabilistic events are concerned, the faster something happened, the more likely it was. If this paper is correct, then life on Earth appeared essentially as soon as the surface cooled from magma to solid. [Added several days later:

The real question is whether the Great Filter is behind us (we're freaks that got more complicated than algae) or in front of us (every intelligence is powerful but short-sighted and wrecks its own ecology before it can escape its home planet.) Therefore, a very reassuring discovery would be simple life - the local flavor of blue-green algae - under the ice Europa of Enceladus,* and in the ancient mud of dried Martian riverbeds, and baked into Venusian bedrock. That would mean that somehow, we got past the gate - still no guarantees, but we already passed the filter. This would mean that if we do manage to get out of the solar system, we'll find a lot of alien bacterial mats, but no alien minds. Boring? That idea is actually quite reassuring.

On the other hand, a bad discovery would be mass fossil beds of complex multicellular things (like the radioactive squid in Europa Report), especially ones with extrasomatic adaptations (tools.) We have had a number of landers on Mars and Venus, and none of them captured any obvious macroscale life. But a positive finding by SETI would be even more harrowing, especially because it's unlikely that there would be only one other intelligence that happens to be even within a million years of our technology - even if they're within 1% as old as we are, that's a gap of 40 million years in either direction! In such a situation we would have to include they must be legion. In such a situation, we would have to reason: we can hear them, but for some reason they never get away from their home planet - and we are unlikely to be any different.

*If indeed we believe that Enceladus only formed in the Cretaceous, then there is much less likely to be life there than Europa, and we should focus on Europa.

Previous post about alien evolution, Vast Cool and Unsympathetic: Other Worlds Detecting Earth


REFERENCES
Benjamin T. Cardenas, David Mohrig, Timothy A. Goudge. Fluvial stratigraphy of valley fills at Aeolis Dorsa, Mars: Evidence for base-level fluctuations controlled by a downstream water body. GSA Bulletin, 2017; DOI: 10.1130/B31567.1

Pearce BKD, Pudritz RE, Semenov DA, Henning TK. Origin of the RNA world: The fate of nucleobases in warm little ponds. 10.1073/pnas.1710339114 PNAS October 2, 2017

Tashiro T, Ishida A, Masako Hori M, Motoko Igisu M, Mizuho Koike M, Pauline Méjean P, Naoto Takahata N, Yuji Sano Y, Komiya T. Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada. Nature 549, 516–518 (28 September 2017) doi:10.1038/nature24019