by Paul Gilster | Dec 16, 2022 | Culture and Society |
Interstellar flight poses no shortage of ethical questions. How to proceed if an intelligent species is discovered is a classic. If the species is primitive in terms of technology, do we announce ourselves to it, or observe from a distance, following some version of Star Trek‘s Prime Directive? One way into such issues is to ask how we would like to be treated ourselves if, say, a Type II civilization – stunningly more powerful than our own – were to show up entering the Solar System.
Even more theoretical, though, is the question of panspermia, and in particular the idea of propagating life by making panspermia a matter of policy. Directed panspermia, as we saw in the last post, is the idea of using technology to spread life deliberately, something that is not currently within our power but can be reasonably extrapolated as one path humans might choose within a century or two. The key question is why we would do this, and on the broadest level, the answer takes in what seems to be an all but universal assumption, that life in itself is good.
Image: Can life be spread by comets? Comet 2I/Borisov is only the second interstellar object known to have passed through our Solar System, but presumably there are vast numbers of such objects moving between the stars. In this image taken by the NASA/ESA Hubble Space Telescope, the comet appears in front of a distant background spiral galaxy (2MASX J10500165-0152029, also known as PGC 32442). The galaxy’s bright central core is smeared in the image because Hubble was tracking the comet. Borisov was approximately 326 million kilometres from Earth in this exposure. Its tail of ejected dust streaks off to the upper right. Credit: ESA/Hubble.
How Common is Life?
Let’s explore how this assumption plays out when weighed against the problems that directed panspermia could trigger. I turn to Christopher McKay, Paul Davies and Simon Worden, whose paper in the just published collection Interstellar Objects in Our Solar System examines the use of interstellar comets to spread life in the cosmos. An entry point into the issue is the fi factor in the Drake Equation, which yields the fraction of planets on which life appears.
We need to know whether life is present in any system to which we might send a probe to seed new life forms – major problems of contamination obviously arise and must be avoided. If we assume a galaxy crowded with life, we would not send such missions. Directed panspermia becomes an issue only when we are dealing with planets devoid of life. To the objection that everything seems to favor life elsewhere because we couldn’t possibly live in the only place in the universe where life exists, the answer must be that we have no understanding of how life began. Abiogenesis remains a mystery and the cosmos may indeed be empty.
We live in the fascinating window of time in which our civilization will begin to get answers on this, particularly as we probe into biomarkers in exoplanet atmospheres and conceivably discover other forms of life in venues like the gas giant moons. But we don’t have such answers yet, and it is sensible to point out, as the authors do, that the Principle of Mediocrity, which suggests that there is nothing special about our Solar System or Earth itself, is a philosophical argument, not one that has been proven by science. We have no idea if there is life elsewhere, even if many of us hope it is there.
Protecting existing life is paramount, and the authors point to the planetary protection issues we face in terraforming Mars, the latter being a local kind of directed panspermia. They cite the basic principle: “…planetary protection would dictate that life forms should not be introduced, either in a directed mode or through random processes, to any planet which already has life.”
I like the way McKay, Davies and Worden present these issues. In particular, assuming we picked out a likely planet in the habitable zone of its star, would there ever be a way to demonstrate that life does not exist on it? The answer is thorny, it being impossible to prove a negative. This gives rise to the possibilities the authors consider when evaluating whether directed panspermia could be used. From the paper:
1. Life might exist on a target planet in low abundance and be snuffed out by seeding.
2. Alien life might be abundant on a planet but present unfamiliar biosignatures yielding a false negative.
3. A comet might successfully seed a barren target planet but go on to contaminate others that already host life, either in the same planetary system or another. The long-term trajectory of a comet is almost impossible to predict.
4. Even if terrestrial life does not directly engage with alien life, it may be more successful in appropriating resources, thus driving indigenous biota to extinction by starvation
There are ways around these issues. Snuffing out life would not be likely if we seeded a protoplanetary disk rather than a fully formed world, which would also remove objection 2, for there would be no biosignatures to be had. A planet that turns out not to be barren might be saved from our seeding efforts by using some kind of ‘kill switch’ that is available to destroy the inoculated life. But all these issues loom large, so large that directed panspermia collapses unless we establish that numerous habitable but lifeless worlds do exist. If life is vanishingly rare, then a kind of galactic altruism can be invoked, seeing our species as gifted with the chance to spread life in the galaxy.
Off on a Comet
All this is dependent on advances in exoplanet characterization and research into life’s origins on Earth, but the questions are worth asking because we may, relatively soon as civilizations go, begin to learn tentative answers. It seems natural that the authors would turn to interstellar comets as a delivery vehicle of choice. Here’s a passage from the paper, examining how spores from a directed panspermia effort could be spread through passing comets by the injection of a biological inoculum into comets whose trajectories are hyperbolic or could otherwise be modified. Such objects need not impact another planet but could be effective simply passing through their stellar system:
These small particles are subsequently shed as the comet passes through systems that have, or will form into, suitable planets, such as protostellar molecular clouds, planet-forming nebulae around stars, and recently formed planetary systems. The comets themselves are unlikely to be gravitationally captured or collide as they move through star systems… but the small dust particles released by the comet—as observed in 2I/Borisov—will be captured. Particles measuring a few 10s to 100s microns in radius are large enough to hold many microorganisms but small enough to enter a planetary atmosphere without significant heating.
Image: This artist’s impression shows the first interstellar object discovered in the Solar System, `Oumuamua. Note the outgassing the artist inserts into the image as a subtle cloud being ejected from the side of the object facing the Sun. Credit: ESA/Hubble, NASA, ESO, M. Kornmesser.
The focus on comets is natural in the era of ‘Oumuamua and 2I/Borisov, and the expectation is widespread that we will be learning of interstellar objects in huge numbers moving through the Solar System as we expand our observing efforts. Why not hitch a ride? There is every expectation that the inoculum injected into a comet could survive the journey, to one day settle into a planetary atmosphere. Thus:
One meter of ice reduces the radiation dose by about five orders of magnitude… In a water-rich interstellar comet, internal radiation from long-lived radioactive elements (U, Th, K) would be expected to be less than crustal levels on the Earth. In such an environment, known terrestrial organisms might remain viable for tens to hundreds of millions of years. We can also take into account advances in gene editing and related technologies that might enable psychrophiles, which are able to very slowly metabolize and repair genetic damage at temperatures as low as-40°C…, to ”tick over,” although slowly, at still lower temperatures. That would enable them to remain viable for even longer durations.
The time scales for delivering an inoculum to an exoplanet are mind-boggling, on the order of 105 to 106 years just to pass near another stellar system. The authors point out that given the hyperbolic velocity of 2I/Borisov, it would take the comet approximately 40,000 years to travel the distance to Alpha Centauri, and 500 million years to travel the distance of the Milky Way’s radius. Indeed, the most likely previous encounter of ‘Oumuamua with another star occurred 1 million years ago.
Perhaps orbital interventions when seeding the comet could alter its trajectory toward specific stars, to avoid the random nature of the seeding program. And I think they would be necessary: Random trajectories might well take our comet into stellar systems with living worlds that we know nothing about. Thus the authors’ point #3 above.
The Rhythms of Panspermia
Clearly, directed panspermia by interstellar comet is for the patient at heart. And as far as I can see, it’s also something a civilization would do completely out of philosophical or altruistic motives, for there is no conceivable return from mounting such an effort beyond the satisfaction of having done it. I often address questions of value that extend beyond individual lifetimes, but here we are talking about not just individual but civilizational lifetimes. Is there anything in human culture that suggests an adherence to this kind of ultra long-range altruism? It’s a question I continue to mull over on my walks. I’d also appreciate pointers to science fiction treatments of this question.
There is an interesting candidate for directed panspermia close to the Sun: Epsilon Eridani. Here we have a youthful system, thought to be less than a billion years old, with two debris belts and two planets thus far discovered, one a gas giant, the other a sub-Neptune. If there is a terrestrial-class world in the habitable zone here, it would be a potential target for a life-bearing mission. So too might a Titan-class world, which raises the interesting question of whether different types of habitability should be considered. We may well find exotic life not just on Titan but also under the ice of Europa, giving us three starkly different possibilities. Would a directed panspermia effort be restricted to terrestrial class worlds like Earth?
Whatever our ethical concerns may be, directed panspermia is technologically feasible for a civilization advanced enough to manipulate comets, and thus we come back to the possibility, discussed decades ago by Francis Crick and Leslie Orgel, that our own Solar System may have been seeded for life by another civilization. If this is true, we might find evidence of complex biological materials in comet dust. We would also, as the authors point out, expect life to be phylogenetically related throughout the Solar System, whether under Europan ice or on the surface of Mars or indeed Earth.
Always complicating such discussions is the possibility of natural panspermia establishing life widely through ejecta from early impacts, so we are in complex chains of causation here. We’re also in the dense thicket of human ethics and aspiration. Let’s assume, as the authors do, that directed panspermia is out for any world that already has life. But if life is truly rare, would humanity have the sense of obligation to embark on a program whose results would never be visible to its creators? We cherish life, but where do we find the imperative to spread it into a barren cosmos?
I’ll close with a lengthy passage from Olaf Stapledon, a frequent touchstone of mine, who discussed “the forlorn task of disseminating among the stars the seeds of a new humanity” in Last and First Men (1930):
For this purpose we shall make use of the pressure of radiation from the sun, and chiefly the extravagantly potent radiation that will later be available. We are hoping to devise extremely minute electro-magnetic “wave-systems,” akin to normal protons and electrons, which will be individually capable of sailing forward upon the hurricane of solar radiation at a speed not wholly incomparable with the speed of light itself. This is a difficult task. But, further, these units must be so cunningly inter-related that, in favourable conditions, they may tend to combine to form spores of life, and to develop, not indeed into human beings, but into lowly organisms with a definite evolutionary bias toward the essentials of human nature. These objects we shall project from beyond our atmosphere in immense quantities at certain points of our planet’s orbit, so that solar radiation may carry them toward the most promising regions of the galaxy. The chance that any of them will survive to reach their destination is small, and still smaller the chance that any of them will find a suitable environment. But if any of this human seed should fall upon good ground, it will embark, we hope, upon a somewhat rapid biological evolution, and produce in due season whatever complex organic forms are possible in its environment. It will have a very real physiological bias toward the evolution of intelligence. Indeed it will have a much greater bias in that direction than occurred on the Earth in those sub-vital atomic groupings from which terrestrial life eventually sprang.
The paper is McKay, Davies & Worden, “Directed Panspermia Using Interstellar Comets,” Astrobiology Vol. 22 No. 12 (6 December 2022), 1443-1451. Full text.
by Paul Gilster | Nov 16, 2021 | Astrobiology and SETI |
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.
by Paul Gilster | Sep 1, 2015 | Astrobiology and SETI |
Would panspermia, the idea that primitive life can spread from star to star, be theoretically observable? Henry Lin and Abraham Loeb (both associated with the Harvard-Smithsonian Center for Astrophysics) believe the answer is yes. In a paper accepted for publication in Astrophysical Journal Letters, the duo make the case that panspermia would create statistical correlations regarding the distribution of life. Detecting biosignatures in the atmospheres of exoplanets may eventually allow us to apply statistical tests in search of these clustering patterns. If panspermia occurs, the paper argues, we can in principle detect it.
“In our theory,” says Lin, “clusters of life form, grow, and overlap like bubbles in a pot of boiling water.” The paper argues that future surveys like TESS (Transiting Exoplanet Survey Satellite) could be an early step in building the statistical database needed. TESS could detect hundreds of terrestrial-class explanets, some of whose atmospheres will be subject to study by ground-based observatories and instruments like the James Webb Space Telescope. Next-generation instruments will do more, allowing us to look for detailed spectral signatures like the ‘red edge’ of chlorophyll or, conceivably, the pollution of a technological society.
Moreover, SETI searches at radio or optical wavelengths could produce detections that eventually allow us to test for clustering, the point being that life that arises by spreading through panspermia should exhibit more clustering than life that arises spontaneously. The statistical models that Lin and Loeb develop in the paper have observable consequences that could begin to turn up as we expand our investigations into astrobiology. Think of Lin’s ‘bubbles’ of life that grow and overlap, or consider the spread of life from host to host in terms of the spread of an epidemic. We may eventually have the data to confirm the idea. From the paper:
In a favorable scenario, our solar system could be on the edge of a bubble, in which case a survey of nearby stars would reveal that ? 1/2 of the sky is inhabited while the other half is uninhabited. In this favorable scenario, ? 25 targets confirmed to have biosignatures (supplemented with 25 null detections) would correspond to a 5? deviation from the Poisson case [a probability distribution], and would constitute a smoking gun detection of panspermia.
Image: The center of the Milky Way as seen from the mountains of West Virginia. Is there life out there, and if so, does it arise spontaneously, or spread from star to star? Credit: Forest Wander.
The transition between an uninhabited to an inhabited galaxy occurs much faster through panspermia than through a gradual buildup of life arising spontaneously in random areas. There is even a Fermi implication here — if life started at roughly the same time everywhere, then we would expect fewer advanced civilizations at the present time than if life started at random times throughout the universe (the authors note that the Drake equation is based on the assumption that life arises independently everywhere, which contradicts efficient panspermia).
The paper continues:
A more generic placement would increase the number of required detections by a factor of a few, though an unusual bubble configuration could potentially reduce the number of required detections. It should be noted that the local environment of our solar system does not reflect the local environment ? 4 Gyr ago when life arose on earth, so the discovery of a bubble surrounding earth should be interpreted as the solar system “drifting” into a bubble which has already formed, or perhaps the earth seeding its environment with life.
The paper notes that any species capable of panspermia will have enormous fitness advantages as it can move from one stellar host to another. Lin and Loeb believe that if panspermia is not viable and Earth is the only inhabited planet, interstellar travel may lead to colonization of the galaxy. In this case panspermia models may still be relevant: A culture using starships may provide the opportunity for primitive forms of life including disease and viruses to spread efficiently, with the same processes of growth and diffusion occurring throughout space.
Can primitive life, then, spread on its own, or does it need intelligent life to create the conditions for its growth outward? Either way, we see life expanding in all directions, producing what this CfA news release calls “a series of life-bearing oases dotting the galactic landscape.” If such patterns exist, finding them will depend upon how quickly life spreads, for any ‘bubbles’ or ‘oases’ could be lost in the regular flow of stellar motion and redistribution about the galaxy. But whatever the biological mechanisms of panspermia might be, it is in principle detectable.
The paper is Lin and Loeb, “Statistical Signatures of Panspermia in Exoplanet Surveys,” accepted for publication at Astrophysical Journal Letters (preprint).
by Paul Gilster | Feb 12, 2010 | Astrobiology and SETI |
Panspermia, the idea that life might travel through space to seed other planets and even other star systems, is a fascinating topic for conjecture, and our understanding of the survival of various forms of life in extreme environments only adds to its appeal. But just as SETI has an active counterpart that seeks to send rather than simply receive interstellar messages, so panspermia has its own advocates for a new kind of mission: To seed the stars from Earth. A group called SOLIS (Society for Life in Space) has sprung up around the notion. Its goal:
To propagate our family of organic Life throughout the Milky Way Galaxy and beyond. We propose to seed young planetary systems in star-forming interstellar clouds. We shall design and launch directed panspermia missions carrying the microbial representatives of Life by the year 2050.
So says the SOLIS Web site and so says society coordinator Michael Mautner, who is a research professor in chemistry at Virginia Commonwealth University. Mautner has in the past worked with solar sail expert Gregory Matloff on propulsion systems that would make it possible to seed new solar systems and has written up the idea for the Journal of the British Interplanetary Society. Now he offers a new paper for the Journal of Cosmology that focuses on what he believes to be our obligation to proceed with directed panspermia, ensuring that life does not come to an end.
Panspermia as Obligation
In a short article at Physorg.com, Mautner states his premise:
“We have a moral obligation to plan for the propagation of life, and even the transfer of human life to other solar systems which can be transformed via microbial activity, thereby preparing these worlds to develop and sustain complex life. Securing that future for life can give our human existence a cosmic purpose.”
The idea is that once we have identified planets with conditions suitable for life (and protoplanetary situations where life might one day develop), we should send organisms to seed these worlds as a way of accelerating local processes of evolution. Even the arrival of such a payload onto a comet or asteroid in a distant planetary system could pave the way for its eventual transportation to a habitable planet by local panspermia, in much the same way that material from Mars has occasionally made its way to Earth.
From accretion disks and interstellar clouds to planets identified by Kepler as being in the habitable zone of their stars, the list of targets should be extensive. The propulsion challenge is less of a problem than you might think, for Mautner is in no hurry to get there. Solar sail methods might take hundreds of thousands or even millions of years to deliver their payload, but the idea is long-term survival of life. Capsules containing about 100,000 microorganisms each and weighing 0.1 micrograms would be the delivery mechanism.
Ethics Among the Stars
All of which leads us to the ethical dilemma. How do we choose our targets so as not to disturb already existing life? Mautner considers this in his paper (internal references omitted for brevity):
Can panspermia missions perturb existing extraterrestrial life? At present, there is no conclusive scientific evidence for extraterrestrial life; though admittedly not all scientists share this opinion… Every living cell needs thousands of complex components as DNA, proteins and membranes, and the probability of these components coming together to originate life may be very small even on billions of planets…
If we still detect extraterrestrial life, we can avoid these targets. In any case, we can target new solar systems where life could not have evolved yet. We may seed a few hundred new solar systems, that will secure the future of our family of gene/protein life but will leave all the other hundred billion stars in the galaxy and their possible indigenous life unperturbed.
Yes, we can target locations where life is not likely to have already evolved, but how accurate can our assessments be given the constraints of current observational technology? Moreover, even that approach leads to potential problems. Panspermia assumes movement of life’s building blocks and even life itself through space. Seed a planetary system with life and it could be millions of years before that life moved from an asteroid in the system to a planet in the habitable zone, one that in the interval had developed life forms of its own. We can never be sure we are not displacing local life.
Mautner thinks even this scenario is not a showstopper:
If there is local life there that is fundamentally different, it will not be affected; if it is gene/protein life, it may be enriched and we can induce higher evolution. The new biospheres may prepare the way for human colonization if interstellar human travel becomes possible.
Which Life Survives?
But I’m thinking that sending cyanobacteria to other star systems to consume toxins and pump out oxygen is a dangerous form of meddling because it assumes that forms of life related to our biosphere are the ones that should survive. Ian O’Neill has an amusing but pointed take on this in a recent post:
If our life takes hold of a planet where another life had the opportunity to evolve into an interstellar civilization in a couple of billions of years time, wouldn’t we be in violation of some kind of cosmic anti-monopoly regulation (or at least in violation of the Prime Directive)?
And there’s another thing to ponder: What if “life” is the universal equivalent of some kind of infection. Is life rare because the universe has a very strong immune system? Firing our genetic code far and wide could be considered to be biological pollution.
I’m all for spreading the human influence around the galaxy, but I think this can only be considered if we physically go to these alien worlds, to evaluate these places in person before we start setting up home. Blindly sending life from Earth to habitable worlds and planet-forming accretion disks seems a little reckless, especially as we have no clue about the consequences if we started impregnating unsuspecting planets.
As we await results from Kepler and more from CoRoT, we still have no realistic assessment of the number of terrestrial planets around stars in our galaxy, nor do we have spectroscopic data that can tell us whether or not such worlds bear life. Is the meaning of life wrapped up in self-propagation, as Mautner’s paper suggests? If so, then pushing life from our biosphere outward is simply fulfilling our basic purpose.
But perhaps there is more to life, including the ethical responsibility to let life take its own directions in those niches where it has already taken hold. I’m not persuaded by a panbiotic ethics that doesn’t take into account the huge gaps in our knowledge about how and where life may form.
The paper is Mautner, “Seeding the Universe with Life: Securing Our Cosmological Future,” Journal of Cosmology Vol 5, (January, 2010), pp. 982-994 (available online).
by Paul Gilster | May 5, 2008 | Astrobiology and SETI |
Our recent look at panspermia concepts was largely devoted to the transmission of life via microbes or spores here in our own Solar System. The even richer question of how life might pass from star to star is far more problematic, but as a follow-up to that earlier story, I want to look at work that graduate student Jess Johnson did with Jonathan Langton and advisor Greg Laughlin at the University of California, Santa Cruz. Their work suggests that while life might readily survive an interstellar journey, it is unlikely to wander close enough to seed another system.
Ponder the era here on Earth known as the Late Heavy Bombardment (LHB). After the period of planetary accretion ended some 4.4 billion years ago, life apparently began. But 3.8 to 4 billion years ago, the LHB saw the planet again pummeled, causing debris to be ejected into space. Looking specifically at the mass that is ejected at 16.7 kilometers per second in the direction of the Earth’s motion (this is Solar System escape velocity), Johnson, Langton and Laughlin found that a substantial amount of rock (about 5 X 1021 grams) would have been blasted free of the Sun.
Remember, this is a period after life has started, so biological material could presumably be involved in any materials lifted into space. But what could survive the 20,000 g’s the ejecta would have experienced, and then cope with vacuum, radiation, cosmic ray strikes and ultimate re-entry and collision upon arrival? Bacillus subtilis is a common bacteria that needs no oxygen to survive, uses carbon and nitrogen as nutrients and forms spores when it lacks the nutrients to thrive. The dormancy period we’re talking about runs into the tens of millions of years, obviously long enough for an interstellar journey — even our glacially slow (by interstellar standards) Voyager spacecraft could make it to the Centauri stars in 75,000 years or so if they were pointed in that direction.
Here’s a striking fact: A viable sample of Bacillis has been found in the stomach of a mosquito encased in amber that has been dated at 25 million years old. Moreover, Bacillus passes all the other tests, able to survive impact pressures upon arrival, capable of enduring 33,800 g’s and, shielded by a sufficient outer encasement of rock, more than able to withstand the radiation hazards of the journey. In deriving the amount of ejected materials (the 5 X 1021 grams mentioned above), the Santa Cruz team chose only those fragments of rock greater than one metre in diameter to ensure the necessary shielding.
So everything looks promising for interstellar panspermia except the possibility that such life-bearing rocks may make their way to another stellar system. Producing calculations on the odds of capture, the trio found a result discouraging for interstellar panspermia theorists:
The results of our work found that, although there are microrganisms that are easily capable of surviving all of the challenges of interstellar travel, the probability of capture by another planetary system is vanishingly small. It should be noted that this in no way negates the possibilty of transport between worlds in our own system, a situation that seems quite possible.
A poster on this work (though without the later results) can be found here.
Related: An upcoming paper by William Napier and Janaki Wickramasinghe (Cardiff Centre for Astrobiology) in Monthly Notices of the Royal Astronomical Society discusses the Solar System’s movement through the plane of the galaxy, suggesting that the chances of comet collision go up every 35 to 40 million years. The potential for disaster on Earth is obvious, but the paper argues that such impacts help life to spread. Says Chandra Wickramasinghe, the Centre’s director, “This is a seminal paper which places the comet-life interaction on a firm basis, and shows a mechanism by which life can be dispersed on a galactic scale.” Wickramasinghe collaborated with Fred Hoyle in the 1981 book Evolution From Space. More in this news release.
