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Comet Impacts: Triggers for Life?

With Rosetta’s continuing mission at Comet 67P/Churyumov–Gerasimenko, now post-perihelion but continuing to gather data, comets and their role in the history of the Solar System stay very much on my mind. Their role as delivery mechanisms for volatiles to an infant Earth is widely investigated, as is the idea that comet impacts may be linked to some of the great extinction events. But perhaps nothing is as provocative as the idea that comets had a role in actually starting life on our planet, with obvious implications for the likelihood of life elsewhere.

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Image: This series of images of Comet 67P/Churyumov–Gerasimenko was captured by Rosetta’s OSIRIS narrow-angle camera on 12 August 2015, just a few hours before the comet reached the closest point to the Sun along its 6.5-year orbit, or perihelion. The image at left was taken at 14:07 GMT, the middle image at 17:35 GMT, and the final image at 23:31 GMT. The images were taken from a distance of about 330 km from the comet. The comet’s activity, at its peak intensity around perihelion and in the weeks that follow, is clearly visible. In particular, a significant outburst can be seen in the image captured at 17:35 GMT. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.

And now we have the interesting news out of the recent Goldschmidt geochemistry conference in Prague, where we learned about experiments designed to duplicate early cometary impacts in a laboratory environment that replicates the Earth as it was four billion years ago. The study was performed by two Japanese scientists, Haruna Sugahara (Japan Agency for Marine-Earth Science and Technology, Yokahama) and Koichi Mimura (Nagoya University).

We learn that the result of simulated comet impact is the production of peptides up to three units long. Peptides are two or more amino acids linked in a chain, making them small proteins. Tripeptides — what Sugahara and Mimura found in their experiment — consist of three amino acids bound by two peptide bonds.

The researchers used a propellant gun to simulate the cometary impact, working with frozen mixtures of amino acids, water ice and silicates cooled to a brisk 77 K. Gas chromatography could then be used to analyze the result, showing that a significant number of the amino acids had joined into peptides after impact. According to this European Association of Geochemistry news release, the amount of peptides produced in these simulated events was roughly the same as would have been produced by terrestrial processes including lightning storms.

Because proteins are made up of polypeptides, the mechanisms that can form them are key to life. Bear in mind that NASA’s Stardust mission has already found amino acids like glycine in Comet Wild 2, while the Deep Impact collision in 2005 revealed organic particles inside the comet, adding to the notion that early comet strikes be a factor on Earth. Says Sugahara:

“Our experiment showed that the cold conditions of comets at the time of the impacts were key to this synthesis, as the type of peptide formed this way are more likely to evolve to longer peptides. This finding indicates that comet impacts almost certainly played an important role in delivering the seeds of life to the early Earth. It also opens the likelihood that we will have seen “similar chemical evolution in other extraterrestrial bodies, starting with cometary-derived peptides.”

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Image: This spectacular image of comet Tempel 1 was taken 67 seconds after it obliterated Deep Impact’s impactor spacecraft. The image was taken by the high-resolution camera on the mission’s flyby craft. Scattered light from the collision saturated the camera’s detector, creating the bright splash seen here. Linear spokes of light radiate away from the impact site, while reflected sunlight illuminates most of the comet surface. The image reveals topographic features, including ridges, scalloped edges and possibly impact craters formed long ago.
Credit: NASA/JPL-Caltech/UMD.

What this work highlights is the fact that we’re still learning about the ingredients that go into comets — Sugahara and Mimura, after all, worked with only some of these constituents. Thus the value of missions like Rosetta, as we continue to plumb the depths of cometary interiors. A model in which comets contributed some necessary substances while terrestrial processes like lightning created others is still very much in the mix. Both cometary and asteroid impacts during the Late Heavy Bombardment would have delivered a wide range of substances to the surface.

“This is a new piece of work which adds significantly to the exciting field of the origin of complex molecules on the Earth,” says Mark Burchell (University of Kent), who goes on to say:

“It has long been known that ices under shock can generate and break bonds in complex organics. The detection of amino acids on comet 81P/Wild 2 by the NASA Stardust mission in the last decade, and the now regular exciting news from the Rosetta mission to comet 67P/Churyumov-Gerasimenko indicates that comets are a rich source of materials. Two key parts to this story are how complex molecules are initially generated on comets and then how they survive/evolve when the comet hits a planet like the Earth. Both of these steps can involve shocks which deliver energy to the icy body. For example, Zita Martins and colleagues recently showed how complex organic compounds can be synthesized on icy bodies via shocks. Now, building on earlier work, Dr. Sugahara and Dr. Mimura have shown how amino acids on icy bodies can be turned into short peptide sequences, another key step along the path to life.”

The paper is Sugahara and Mimura, “Peptide synthesis triggered by comet impacts: A possible method for peptide delivery to the early Earth and icy satellites,” Icarus Vol. 257 (1 September 2015), pp. 103-112 (abstract).

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Comments on this entry are closed.

  • Alex Tolley August 24, 2015, 17:03

    Just as liquid water has proven more widespread that once thought, now we learn that some pre-biotic chemistry is also more widespread than we thought, the latest suggesting that short peptides are possibly created in comets.

    Unfortunately we still don’t have much evidence about how life started:- metabolism via proteins, replication via DNA, or a combination via RNA, and what conditions were necessary). As a result, we seem to be arriving at an ever wider base of conditions that might have allowed life to start, but so far evidence of only one genesis, here on Earth. If this proves to reflect reality, then the bottleneck is going to prove to be some condition[s] necessary for life to emerge from a rich pre-biotic chemistry. It should also mean that pre-biotic chemistry should be found very widely, perhaps given us hints of how far it can develop before life emerges.

  • andy August 24, 2015, 18:27

    I’m with PZ Myers on this one. While this is interesting chemistry, any link to the origin of life is really tenuous.

    It is not enough to postulate that there was, once upon a time, a cold soup of organic subunits in the ocean, that just sort of congealed into life — it doesn’t work. The authors made no calculations about the concentrations of their tripeptides in the prebiotic ocean (it would have been an exceedingly thin, dilute soup), don’t consider that these compounds were probably degrading as fast as comet-smashing was making them, and that their mechanism does not say anything at all about what reactions early precursors to life would have been using to synthesize peptides.

    There are far more reasonable environments for abiogenesis than the brief, temporary violence of comet collisions.

    For some reason, journalists and the public love these scenarios of cosmic forces colliding in big explosions to create life — it’s as if they desperately want Michael Bay to be in charge of biology. It’s not how it would have worked. If you want to look for answers to the origin of life, you need to look at conditions that generate a gentler flux of energy that is a better analog to metabolism. It’s about flow, not bang.

    Also worth checking out the references about abiogenesis he gives at the end of his blog post.

  • Eniac August 24, 2015, 23:06

    This finding indicates that comet impacts almost certainly played an important role in delivering the seeds of life to the early Earth.

    Almost certainly? On what grounds? Later we read this:

    According to this European Association of Geochemistry news release, the amount of peptides produced in these simulated events was roughly the same as would have been produced by terrestrial processes including lightning storms.

    Now, according to Occam, which of 1) comet impacts or 2) lightning would “almost certainly” be the main source of tripeptides on the early Earth?

    Not that it matters, because tripeptides do not make life. Not even a little bit.

    I completely concur with andy and, judging from andy’s quotation, with PZ Myers as well.

  • Marko Amnell August 25, 2015, 8:47

    There have been doubts expressed about the suggestion that comets added a significant amount of organic compounds to the early Earth. J. Oró, A. Lazcano and P. Ehrenfreund write in “Comets, and the Origin and Evolution of Life” (in _Comets and the Origin and Evolution of Life_, eds. P. Thomas, R.D. Hicks, C. Chyba and C. McKay, Springer, 2006):

    >”The idea that comets and meteorites made major contributions of organic material and volatiles to the primitive Earth has been challenged by Miller
    (1991a, b), who has argued that incoming extraterrestrial organic compounds deposited in the planet would be removed as the hydrosphere underwent periodic ≈ 350◦C passages through hydrothermal submarine vents every 10,000,000 years. If the rate of volatile influx is increased to compensate for this destructive process, then we are faced with an overabundance of carbon. This has led Miller (1991a, b) to conclude that the amount of extraterrestrial organic material added by comets and meteorites to the primitive Earth was small compared to terrestrial-based synthesis. This criticism is shared by Zhao and Bada (1991), who, on the basis of the concentration of the extraterrestrial amino acids α-amino isobutyric acid (AIB) and racemic isovaline, associated with the K/T boundary clays (Zhao and Bada, 1989), have estimated the efficiency of impact delivery of extraterrestrial organics to the Earth. Their results are not very encouraging: according to their calculations, the estimated concentration of AIB in the primitive oceans would be approximately 2 nM, which suggest that exogenous delivery was an inefficient process when compared to abiotic synthesis occurring on the Earth (Zhao and Bada, 1991).” (pp. 11f)

  • Mark Zambelli August 25, 2015, 11:16

    Personally I’m not that fond of panspermia so the message I’ve taken from this is that we have perhaps found another source for providing some of the numerous precursor molecules associated with earthly biology (provided that synthesis occurs faster than hydrolysis to allow any kind of build-up).

    But is this directly related to abiogenesis or more a way of providing enriched chemistry it’s clearly too soon to tell. I think it makes a lot more sense to be moving away from the existing ‘primordial soup’ idea by including hydrothermal vents as a source of the energy gradients required by life so this notion of ‘life with a bang’ doesn’t seem right to me. I would be unsurprised to find the comet findings operate as a backdrop to abiogenesis rather than a key player.

    For anyone interested… the SETI Institute’s youtube channel have had a couple of talks this year on the hydrothermal perspective. The first one I watched was by Bruce Damer and Dave Deamer entitled “A New Model for the Origin of Life” https://youtu.be/nk_R55O24t4 … and the second, most recent, was by Kathleen Campbell entitled “Extreme Environments: Hydrothermal Settings for Early Life on Earth (and Mars?)” https://youtu.be/eu_f9IZZCvQ

  • swage August 26, 2015, 8:22

    With all due respect to Mr. Tolley i’d like to specify that albeit we do have evidence for at least one genesis in the form of all life on Earth, including ourselves, not necessarily evidence for a genesis spatially occurring on Earth.

    It may seem like a pretty straightforward assumption, but in the absence of any locatable evidence how life came into existence on this planet it is worth insisting on this distinction, less we might take something for granted which really isn’t.

    In the face of chemosynthesis it may not be pre-biotic at all. Despite all the raised eyebrows Mr. Wickramasinghe’s cometary models (admittedly among many other proposals) caused, looking at this data we have to admit that he was much closer to the real thing than any dirty snowball could reach. He is, after all, still an expert in cosmic dust and spectral analysis.

    That is scientifically spoken, of course, not a proof of anything. Its not hard evidence, but probability wise… it again leaves all doors open. I really would expect to encounter a closed door occasionally, unless…

  • Alex Tolley August 26, 2015, 11:59

    @swage – if you are positing a panspernia hypothesis, it is true we cannot dismiss this. However, as an explanation of biogenesis, it is unsatisfactory as we have to seek an explanation removed from our possible examination.

    If you haven’t done so, I recommend you watch the 2 SETI talks on biogenesis linked to by Mark above as good examples of current thinking. I also recommend Hazen’s Genesis: The Scientific Quest for Life’s Origins review of the theories of biogenesis.

    As regards Wickramasinghe, I’m afraid that I no longer accept him as a credible authority regarding life and extra-terrestrial bodies. He sees life in almost everything these days.

    I personally would be very excited if we found evidence of fossil life elsewhere in the solar system. Even better would be extant life, as that would be a compelling playground for biologists, especially for comparing details of information storage and metabolism, let alone structures. I would expect increased efforts at sample missions to retrieve this life and culture it in secure biological containment facilities. Detecting life on exoplanets would also be exciting, although it would be out of reach for at least a century, although again, what a potential driver for an fast interstellar probe.

  • Mark Zambelli August 26, 2015, 20:09

    (Thankyou, Alex, for that additional link… I’ll be sure to add that to my kindle-app’s reading list)

  • swage August 27, 2015, 6:15

    Thanks for your reply Mr. Tolley. It is appreciated. I agree that as an explanation for biogenesis it is unsatisfactory. I’d go as far as to say, strictly spoken, it is no explanation for biogenesis at all. That is as unsatisfactory as it gets, i guess. I am not sure if it would necessarily remove the explanation from our reach, but that is a possibility and at the very least it would complicate the matter quite a bit.

    That being said, there may also be evidence for a biogenesis on Earth and we find it tomorrow. I realize ultimately this leads us to Occam’s razor and singling out the most simple answer, but its not exactly proof.

    If approached as spatial problem, is it really simpler to suggest the rise of chemistry to life instead of “outside”? The distinction between “How did life start?” and “Where did life start?” is a very fine one (systemic and spatial).

    Regarding the credibility issue… well there have been incidents, most prominently the Archaeopteryx incident, which certainly are problematic (although, if you follow his line of thinking you can at least understand why he misled himself). This is understood, but i don’t think it ultimately is a question of reputation at all, which seems to be an unfortunate trend when it comes to these things.

  • J. Jason Wentworth August 28, 2015, 0:00

    Chemically speaking, life seems to be unnecessary; it’s like an excessively-complicated Rube Goldberg device for accomplishing a very simple task (such as lighting a hurricane candle). All of the valence and energy balance requirements are satisfied by the much simpler mixtures of organic molecules that we find in interstellar clouds, asteroids, comets, and meteoroids. Also:

    Chemists have remarked on the fact that nothing else even approaching the complexity of living chemistry exists in nature–there’s chemistry (inorganic & organic), and there’s life, with a vast gulf separating the two in terms of molecular complexity. Ruminating on this–and on the apparent inability of biochemists to produce even the simplest living cell “from scratch” in the laboratory–I would not be surprised (but I would be disappointed) if it turned out that life came into existence only *once* in our galaxy (or perhaps even in the entire universe), here on Earth. And I’m not alone; astronomer James S. Trefil expressed the very same misgivings about the hoped-for universal abundance of life in his book “Space, Time, and Infinity.”

  • Eniac August 28, 2015, 23:39

    J Jason Wentworth: You make a great point, one which I also like to make often. On one thing, though, I would like to add a comment: “Chemically speaking, life seems to be unnecessary”. Chemically, maybe. Statistically speaking, though, life is inevitable: Despite it’s staggering complexity, it is the most parsimonious way for us to be here today and wonder about it. This last one is a fact that we do know, and we have to make our probability estimates contingent on it. Life is extremely unlikely, but it is a whole lot more plausible than, say, a Boltzmann Brain, for producing sentience.

    Of course, if you think it through, the universe itself has to be the most parsimonious way for life to arise, which means that it is likely just large enough to give rise to a single instance. The universe is an equation carefully constructed and calibrated to produce billions of billions of planets on which there is a chance for us to arise. Not calibrated by someone, purposefully, but by the simple fact that if it were different, we would not exist.

    Reasoning thus, I concur with your hunch that ours may well be the only life existing.

  • J. Jason Wentworth August 31, 2015, 4:06

    Thank you. The possibility that we may be alone (in *this* universe, of baryonic matter) doesn’t dampen my enthusiasm for searching for intelligent interstellar signals and–closer to home–probes or other artifacts at all. As Patrick Moore said about so many things (including this), “We just don’t know.” More than once in science, phenomena which were confidently thought to *not* exist (stones falling from the sky were in this category for a long time, even when scientists were given recovered meteorites!) weren’t found and investigated for decades or even centuries, because scientists wouldn’t look for things that–they were certain–weren’t there to be found.