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Probing the Likelihood of Panspermia

I’m looking at a paper just accepted at The Astrophysical Journal on the subject of panspermia, the notion that life may be distributed through the galaxy by everything from interstellar dust to comets and debris from planetary impacts. We have no hard data on this — no one knows whether panspermia actually occurs from one planet to another, much less from one stellar system to another star. But we can investigate possibilities based on what we know of everything from the hardiness of organisms to the probabilities of ejecta moving on an interstellar trajectory.

In “Panspermia in a Milky Way-like Galaxy,” lead author Raphael Gobat (Pontificia Universidad Católica de Valparaíso, Chile) and colleagues draw together current approaches to the question and develop a modeling technique based on our assumptions about galactic habitability and simulations of galaxy structure.

Panspermia is an ancient concept. Indeed, the word first emerges in the work of Anaxagoras (born ca. 500–480 BC) and makes its way through Lucian of Samosata (born around 125 AD), through Kepler’s Somnium, to re-emerge in 19th Century microbiology. Accidental propagation of life’s building blocks was considered by Swedish chemist Svante Arrhenius in the early 20th Century. Fred Hoyle and Nalin Chandra Wickramasinghe developed the idea still further in the 1970s and 80s.

So how do we approach a subject that has remained controversial, likely because it does not appear necessary in explaining how life emerged on our own Earth? As the paper notes, modern work falls into three distinct categories, the first involving whether or not microorganisms can survive ejection from a planetary surface and re-entry onto another. Remarkably, hypervelocity impacts are not show-stoppers for the idea, suggesting that a small fraction of spores could survive impact and transit.

As to timescale and kinds of transfer mechanisms, most work seems to have focused on mass transfer between planets in the same stellar system, usually through lithopanspermia, which is the exchange of meteoroids. It’s true, however, that transit between different stars has been investigated, looking at radiation pressure on small grains of material. There are even a few studies on whether or not a stellar system might be intentionally seeded by means of technology. The term here is directed panspermia, a subject more often treated in science fiction than academic circles.

Although not entirely. While directed panspermia is off the table for Gobat and colleagues, we’ll take a look in a month or so at what does appear in the literature. Some interesting ideas have emerged, but they’re not for today.

What Gobat and co-authors have in mind is to apply a model of galactic habitability they have developed (citation below) in conjunction with the simulations of spiral galaxies based on hydrodynamics that are found in the McMaster Unbiased Galaxy Simulations (MUGS), a set of 16 simulated galaxies developed within the last decade. On the latter, the paper notes:

These simulations made use of the cosmological zoom method, which seeks to focus computational effort into a region of interest, while maintaining enough of the surrounding large-scale structure to produce a realistic assembly history. To accomplish this, the simulation was first carried out at low resolution using N-body physics only. Dark matter halos were then identified, and a sample of interesting objects selected. The particles making up, and surrounding, these halos were then traced back to their origin, and the simulation carried out again with the region of interest simulated at higher resolution.

Simulation and re-simulation allow the MUGS galaxies to reproduce the known metallicity gradients in observed galaxies and likewise reproduce their large-scale structure, including disks, halos and bulges. The authors use one of the simulated galaxies, a spiral galaxy similar to but not identical with the Milky Way, to investigate the probability and efficiency of panspermia as dependent on the galactic environment.

Image: This is Figure 1 from the paper. Caption: Mock UV J color images of the simulated galaxy g15784 (Stinson et al. 2010; Nickerson et al. 2013), for both edge-on (left) and face-on (right) orientations, using star and gas particles, and assuming Bruzual & Charlot (2003) stellar population models and a simple dust attenuation model (Li & Draine 2001) with a gas-to-dust ratio of 0.01 at solar metallicity. Additionally, we include line emission from star particles with ages ≤ 50 Myr, following case B recombination (Osterbrock & Ferland 2006) and metallicity-dependent line ratios (Anders & Fritze-v. Alvensleben 2003). All panels are 50 kpc across and have a resolution of 100 pc. Two spheroidal satellites can be seen above and below the galactic plane, respectively. Credit: Gobat et al.

Panspermia appears to be more likely in the central regions of the galactic bulge, as we might assume due to the high density of stars there, a factor which counterbalances their lower habitability in this model. Panspermia is found to be much less likely as we move out into the central disk. In the model of habitability as developed by Gopat and Sungwook Hong in 2016, habitability increases as we depart from galactic center, while the new paper shows that the likelihood of panspermia works inversely, being more likely toward the bulge.

In a sense, we decouple habitability from panspermia. The paper uses the term ‘particles’ to refer not to individual stars, but to ensembles of stars with a range of masses but the same metallicity. This reflects, say the authors, the resolution limits of the simulations, which cannot track individual stars through time. From the paper, noting the narrow dynamic range of habitability vs. panspermia [the italics are mine]:

In dense regions [of the simulated galaxy], many source particles can contribute to panspermia, whereas in the outer disk and halo the panspermia probability is typically dominated by one or, at most, a few source star particles. Unlike natural habitability, whose value varies by only ∼ 5% throughout the galaxy, the panspermia probability has a wide dynamic range of several orders of magnitudes..

The models used here have a number of limitations, but it’s interesting that they point to panspermia as being considerably less efficient at seeding planets than the evolution of life on the planets themselves. At best, the authors find the probability of panspermia to be no more than 3% of all the star particles in their simulation. This may be an overly generous figure, and the paper acknowledges that it cannot be more precisely quantified other than to say that when it comes to efficiency, local evolution wins going away. Higher resolution galaxy simulations will offer more realistic insights.

We have a result, as the authors acknowledge, that is more qualitative than quantitative, a measure of how much we have to learn about galaxies themselves, and about the Milky Way in particular. The sample galaxy, for example, has a higher bulge-to-disk ratio than the Milky Way. But more significantly, the capture fraction of spores by target planets and the likelihood that life actually does develop on planets considered habitable are subjects with no concrete data to firm up the conclusions.

We can anticipate that future simulations will take into account a rotating evolving galaxy as opposed to the single simulation ‘snapshot’ the paper offers. Nonetheless, this modeling of organic compounds being transferred between stars points to the orders of magnitude difference in the likelihood of panspermia between the inner and the outer disk, a useful finding. Given that so few of the star particles the simulation generates have high panspermia probability, the process may occur but under conditions that make it much less effective than prebiotic evolution.

The paper is Gobat et al., “Panspermia in a Milky Way-like Galaxy,” accepted at the Astrophysical Journal (preprint). The paper on galactic habitability is Gobat & Hong, “Evolution of galaxy habitability,” Astronomy & Astrophysics Vol. 592, A96 (04 August 2016). Abstract.


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  • Michael Fidler November 16, 2021, 11:54

    Good place to look in our local neighborhood would be the Jupiter satellites. Jupiter is getting pelted by chunks coming into our solar system and our own Oort cloud. Plenty of panspermia on fast flying snowballs from beyond hitting Europa and other icy worlds. Titan and Mars would be good second best targets for finding the traces left from panspermia…

    • Michael November 16, 2021, 16:39

      Ceres would ideal, low gravity and nearbyish.

  • John walker November 16, 2021, 12:15

    I have serious doubts about the viability of the panspermia idea.

    My doubts boil down to two problems: the sterilizing effects of long term exposure to GCR and the sterilization from the shock and thermal effects of atmospheric entry and impact.

    The two are bound to each other. On the one hand it has been argued that microscopic particles would not suffer the impact stresses that create shocked strata and impact melting. Spores either free floating or embedded in fine particulate could settle into a gravity well like a feather in the air. Let’s ignore the likelihood of high relative velocities of spore particle and prospective seed world which would likely lead to vaporization in the atmosphere. The lack of a large protective layer around the spores will permit the rapid sterilization of the gene carrying material through the dna fragmentation caused by GCR. The rapidity of this process(years to decades) precludes interplanetary microparticle panspermia not to mention interstellar panspermia since both require transit times many orders of magnitude higher than the much shorter time needed to obliterate life from such small objects.

    The other option which posits large rock bodies protecting the seed material from the meter scale penetration of GCR, suffers from the drawback of high thermal and pressure stresses that entry and impact impart upon the impactor. The larger the impactor, the less likely that atmospheric heating kills the seed material. However, more massive objects have proportionally higher impact temperatures and pressures.

    Perhaps there is a Goldilocks combination of variables that could enable survival of viable seed material over millenia in space and also in the end contact phase. Extremely low relative velocity and moderate impactor size. But, that keyhole will be very small. High numbers of carriers and long time scales might serve to increase the probability of such a confluence of factors. I’ve yet to see a panspermia proposal that includes a comprehensive quantitative breakdown of the biohazards I listed. Until such time as I find one, I remain sceptical of panspermia as a viable idea.

  • Alex Tolley November 16, 2021, 12:55

    Some thoughts:

    1. If there model is reasonably correct with panspermia only offering a 3% of the probability of the source of life on a planet, there is little to predict about the distribution of life in the galaxy.

    2. A previous study on colonization based on stellar movement and close encounters showed colony waves along the spiral arms. I would expect this to be similar with panspermia.

    3. The model says nothing about the evolutionary fitness of the life from different sources. If equally fit, then local abiogenesis and panspermia will result in a wide variety of biology across the stars. If very different, then there might be just one or a few biologies surviving in the galaxy. Chance would play a part in the distribution of different biologies.

    4. If abiogenesis is rarer than teh authors envision, then there should be many exoplanets in the HZ that look as though they should bear life, but do not and remain sterile.

    5. Panspermia is only going to deliver bacterial/archaea spores and viruses. If they are delivered by radiation propelled dust it might be worth building some outward facing dust traps to look for the arrival of interstellar life.

    6. If our stellar probes sample life in a volume of stars around our sun and the life appears to have an identical biology to terrestrial life, could we determine is we are the source or recipient of life? AFAICS, the only method would be to study the oldest stars and planets to determine when life started on those worlds. If earlier than Earth, then we might be the recipients, if later, then we might be the source. [This is a project for the far future.]

  • Ron S. November 16, 2021, 15:09

    I have not read the paper but I already have to wonder how they justify the following: “Unlike natural habitability, whose value varies by only ∼ 5% throughout the galaxy, the panspermia probability has a wide dynamic range of several orders of magnitudes.”

    What is “natural habitability” and what does that have to do with abiogenesis, which is the appropriate comparison? Natural habitability is required for both panspermia and abiogenesis.

    This is not to say that I believe panspermia is terribly likely — I don’t think it is, for several reasons — but it can still be a significant factor. Even it is at play in a small fraction of planets where we will (we hope) discover biosignatures it is important.

    That said, I find panspermia to be of little interest in comparison to abiogenesis. Panspermia is far easier to investigate, so that’s what may do. Worse, when we find life (e.g. on Earth) or simply biosignatures there is no good way to determine whether their origin is by panspermia or abiogenesis.

    In light of that, I wonder whether papers like this are useful or merely curiosities.

    • Robin Datta November 16, 2021, 21:25

      If enough markers (triplet genetic code, chirality, etc.) match, it would strongly suggest panspermia. Indeed if some organism from kilometers-deep in Earth’s rocks is found to be at variance, the question would be panspermia vs. parallel abiogenesis.

  • wdk November 16, 2021, 22:20

    Not necessarily a proponent of panspermia myself, but still mystified by how we can be a singular example of whatever caused us. So given the
    inspiration of the pros and cons of the paper…

    When martian rocks were first found on Earth, they were not immediately recognized as martian. But as time went on, we began to wonder if they really arrived as empty shells. The sampling we have is just what we can sort out under the best conditions: off ice packs and Saharan sands. Over millions of years, martian bombardment of the inner solar system could have been significant for introducing ,,, What?
    Viruses perhaps? Maybe more.

    As stated, and without invoking miracles, we can assume abiogenesis locally (Earth) or we can imagine some integration with a wider network of material sources for organic chemistry, possibly with some initial laboratory bubbling. Getting a glimpse of something like Ultima Thule with the New Horizons flyby, it would appear that there is a large surrounding reservoir from which organic chemistry could spew.
    The Oort cloud could well be a remnant of something shared by the sun’s formative star cluster. If you need a meter of shielding from GCR, then Oort Cloud objects certainly have that. Though I imagine reaction rates are slow now. but in their early formation days they had some half lifing isotopes too. What could they cook? Enceladus could be representative of a significant class of objects related to this controversy.

    We do tend to look on extremophiles as local extreme adaptations rather than a means by which life might have spread. In addition, one could well wonder just how hardy tardigrades could be when it comes to interplanetary hops.

    If we were to suppose that the medium for transmitting life basics were tied with the star forming regions rather than planets ejecting pods or pellets of life forms, I wonder if precursor chemistry would suffice to start life processes very early on terrestrial planets.
    Or, maybe as suggested, the initial steps were undertaken on worlds
    farther out from the sun.

    Alternatives to consider might be gaseous worlds which are longtime cauldrons for chemistry. But most of them are like checking into Hotel California as the Eagles song relates. In this case, maybe Hotel Neptune. You have enormous volumes and materials for all manner of chemistry, but just no easy way to release it back into space. Unless the life that resulted took care of that matter itself.

    Rather than the Milky Way itself, perhaps a better environment would be the early days in a satellite globular cluster with a high star density,

    • Robin Datta November 17, 2021, 18:24

      The difference between organic molecules in meteorites, asteroids, interstellar nebular clouds and molecular cell biology is akin to the difference between vacuum tube and integrated circuit microprocessors/CPUs.

      • wdk November 19, 2021, 9:03

        Point taken. Considering it, I would offer that if we point a radio telescope and identify H2O and hydrocarbons, it is much akin to LGMs pointing scopes at us and identifying oceans of H2O here.
        So far so good. Concentration of life in ET environments could be less dense, down to zero – or substantial enough to constitute life. At closer resolution.

        If viruses have DNA strands, is it impossible that viruses could be constructed, retained or stored in some space environments? Whether pro or con, you probably can address that one better than I can.

  • Afterthought November 16, 2021, 22:54

    Obstacles to the acceptance to panspermia: no evidence of life anywhere besides Earth.

    That there is something lacking in a home for life that does not allow it to be an incubator for life.

    • Gary Wilson November 17, 2021, 13:45

      As of now Afterthought saying there is “no evidence of life anywhere besides Earth” is a bit like walking outside your residence, looking up at the sky and saying “I see no evidence of aircraft, therefore there are no aircraft.” No offense intended but we have no idea whether life exists beyond Earth. Absence of evidence is not evidence of absence as has been said many times on here. Best wishes.

    • Zanstel November 24, 2021, 5:01

      Out capabilities to see if there is life or not outside our planet is very very limited.

      It could exists life even in our solar system beyond Earth and we couldn’t know it.

      On the underground of Mars or on the subsurface oceans of some moons.

      That’s probably our best chance to test the panspermia theory. If we find life there, and it’s based on a common origin, panspermia theory will be almost sure.

      In fact, I think that low gravity bodies has a greater chance of being seeds of panspermia. For example, Enceladus is launching jets directly into space.

      At the same time, if the ice crust is not too deep, it’s easier to a ice rock to penetrate in a high speed impact directly into the deep sea.

      If panspermia is right, it’s very probable that moons are the main route for send and receive life first in each solar system, and later spreads into the planets like Earth.

  • Jeff Wright November 17, 2021, 4:58

    Now, I have often thought vorticity may be the key to DNA production. The ’86 MN tornado had a double-helix shape…why not a smoker plume with tides sloshing back and forth with vortex shedding? This would be the organic chemistry version of the Unilever soap nozzle TED talk. But why not go back further, and imagine a Titan like moon stripped by a passing black hole…its carbon rich accretion disk cooked…and hit by GCRs as the cosmos’ CRISPR snippers..disk mini-swirls like those in a hurricane eyewall as per Fujita…then centrifuged out…curling back in with microgravity. Black-holes as the smokers of space…alembics of the deep…

    • wdk November 17, 2021, 11:08

      Well, for a vortex shedding phenomenon down here on Terra, it
      often helps to have an airfoil ( e.g., a cylinder) in a flow. And whether tongue in cheek above or not, you can see evidence of vortex shedding all over the place: our atmosphere and the gas giants. I suspect others have examined organic chemistry in circum-stellar disks, but the hazard for life likely increases with raining down on a hot pavement like a terrestrial planet. Looking at it on a galactic scale, why not follow the trail of gas and dust clouds. And say hello to Fred Hoyle’s friend the cloud while we are out there.

      • Paul Gilster November 17, 2021, 13:55

        Great book, The Black Cloud. Totally mesmerized my childhood self.

  • Henry Cordova November 17, 2021, 11:41

    Panspermia is a lot like divine intervention. Even if its true, it doesn’t really answer any questions. It merely postpones them.

    • NS November 17, 2021, 21:24

      But if (say) life appeared on Earth via panspermia and not through abiogenesis, we’re wasting our time trying to understand abiogenesis based on early Earth conditions. That would be useful to know. If abiogenesis is rare, and life mostly appears on different worlds via panspermia, that would also be useful to know. So I don’t see investigating panspermia as “question postponing”.

    • Alex Tolley November 17, 2021, 22:25

      It may not answer the question of abiogenesis by side stepping it, but it does open up new questions and hypotheses.

      As for abiogenesis, suppose a lab experiment using conditions that never occurred on Earth succeeds in generating molecules and structures that are a key to life on Earth and significantly moves the science forward on abiogenesis. That might suggest that abiogenesis occurred on a world that had/has those conditions and we could look for those.

      Even though I won’t be alive at that time, I would hope that panspermia is rare and that life has lots of separate abiogenesis events around the galaxy, offering a cornucopia of different biologies, rather than a common type that is implicitly assumed in most SciFi shows like Star Trek.

  • Floyd Mathews November 17, 2021, 14:15

    I agree with Henry Cordoba’s comment above that even if panspermia exists, how did it originate, if not by abiogenesis somewhere else?

    Also, given that it appears that all life forms we know of on Earth are based on various DNA molecules, if panspermia had occurred here in the past, shouldn’t we be able to find some non-DNA life forms or fossils right here on Earth?

    • Henry Cordova November 17, 2021, 21:41

      That’s “Cordova”, con “v” de vaca, no “b” de burro.

      The standard answer to your question is that alternate biochemistries that arrive on a world after life (whether from abiogenesis or panspermia) becomes established cannot compete with the dominant or original form. If it is superior, the other may simply be wiped out. And it would be impossible to determine DNA life from any alternative simply from a fossil. Morphologically, I suspect critters all look pretty much the same at the tissue and organ level–parallel evolution.

      But we really don’t know that for sure, do we?

      It would be interesting to speculate on a what an ecosystem with two or more different but parallel biochemistries might be like. Would they be able to eat or parasitize each other? Would they be mutually toxic? Would they be so different they could just ignore eaxh other? Perhaps DNA is the only possible genetic material. Perhaps not. One of the first things we’ll want to determine about alien life is whether or not it uses DNA in the same way we do, or if some other molecule has taken the role.

      • Floyd mathews November 18, 2021, 12:57

        Perdon Senor Cordova, mi erroneo de mi Castellano. Muchas gracias por su explicacion.

    • Alex Tolley November 17, 2021, 21:58

      if panspermia had occurred here in the past, shouldn’t we be able to find some non-DNA life forms or fossils right here on Earth?

      This is the concept behind Paul Davies’ group looking for evidence of a “shadow biosphere”. You may recall the Wolfe-Simon [false] claim of finding arsenic-based life.

      Unless that other life is so different that it does not compete for the same resources as known terrestrial life, it would likely not remain around, outcompeted by another form – i.e. our life. I can imagine such life may retreat to a refuge, e.g. in the lithosphere or where extreme conditions are outside the limits of extremophiles. Our biology almost certainly outcompeted earlier forms of our biology, e.g. 4-base DNA instead of a proposed 2-base.

    • wdk November 19, 2021, 8:50

      In the absence of direct evidence of panspermia, DNA discussions are much like tracing language family roots. We speak languages with ancient roots but written evidence for where they came from is sparse.

      Indo-European, for example, suggests connections with certain land masses in the Eastern Hemisphere. And yet we speak it over in this one where I hail from. And I am unaware of tours overseas to any ancient
      IndoEuropean speaking villages.

      Little green men, had they arrived last week, would be puzzled by the
      eastern hemisphere associations – unless they had been monitoring the planet for a few centuries or millenia watching how language groups could leap continents.

      Alternatively, if DNA had a widespread predominance as a means for propagating life, seeing it on another celestial body would argue for something, such as that it would not be a particular feature of terrestrial life, but more widespread than that.

      Photosynthesis is something that has been discussed in a similar manner on this forum. Without being aware of ET forests based on
      photosynthesis, we have judged red dwarf star environs as habitable zones based on whether photosynthesis can be achieved. So, either detecting off-world photosynthesis or DNA would tell us something
      about their cosmic role.

      But since these process agents or chemistries are fragile, perhaps the precursors should be studied more carefully. The things that make might make PS and DNA compounds more likely. That seems like a recurrent theme of science planning, especially with respect to ancient
      solar system objects such as comets.

      • Alex Tolley November 19, 2021, 12:58

        I would have thought that if LGMs had landed here last week that they would see the sky fully of aircraft crossing continents and therefore have little doubt about the resulting pattern of languages. They might well understand why English has become the dominant language in many domains of activity.

        If DNA was one of several early variants, there is no doubting that natural selection has made it the dominant (and only) one today. Similarly with the genetic code (although small variations do exist). As Dawkins has said, even if there were no fossils of extinct species, the genomic sequences would be able to show that the tree of life is real and point to the evolution of forms.

        While we can understand how organisms have populated the globe, there is no certainty that they can cross space to populate other worlds. There seems to be evidence that they can cross interpanetary space, although as yet there is no evidence that they have done so, let alone with any success. No need to remind ourselves that “space, is big, really big….” to wonder if directed panspermia, deliberate or unintentional (Thomas Gold’s space traveler garbage theory) is more likely than random events.

        Without interstellar probes taking and analyzing biological samples, we have little hope of determining whther life is similar to terrestrial life. If we get very lucky, an interstellar visitor like I2/Borisov might have samples of life within it to test locally, as might incoming dust grains.

        • wdk November 23, 2021, 12:30

          Invoking LGMs does have inherent problems. It is hard to agree on just what they might be in the dark about. The stock greeting of “Take me to your leader”, does suggests some naivete though. For example, on landing would LGMs know that people fly in the aircraft or are they just another local phenomenon to be investigated? Centuries from now, perhaps, the shoe could be on the other foot. Could the Earth have been seeded intentionally? Or unintentionally as, if I recall, Francis Crick had suggested. It seems like in either case, someone would have checked back on us by now. Odd that they would be that remiss.
          Or else that would not be the explanation and we are left with the same quandary about transmissability of life or its triggers.

  • Larry Kennedy November 17, 2021, 19:41

    I’m really skeptical over how you can declare how the efficiency of panspermia compares with the efficiency of local biogenesis when the latter is if anything an even bigger personal wild guess than the present paper.

  • wdk December 3, 2021, 10:35

    Before this topic sails off into the archives, wanted to draw attention again to the Murchison meteorite again: a 1969 100-kg carbonaceous chondrite observed to hit in Western Australia. While likely a heterogeneous combination, some of the composite material from isotope decays suggests nearly seven billion years since formation. Included in the organic compounds are amino acids of either “chirality”, right or left and of about the same abundance.

    Any further comment on my part is inference, if I haven’t gone off the rail already. But this suggests to me that Murchison could have originated in a cloud supplying newly forming stars: some earlier and some later ( e.g., 6 or 7 billion years ago and 4.8 billion). A little tricky, that time scale. Still, with the building block chemistry detected already, it is hard to determine how much more remains in the interstellar icebergs. A head start? Or do we still have to invoke abiogenesis as 99
    out of 100 “exobiologists” are likely to assume?