We could spread life to the Milky Way with comets. But should we?
Here's a thorny problem: What if life doesn't always appear on planets that can support it? What if we find more and more exoplanets and determine that some of them are habitable? What if we also determine that life hasn't appeared on them yet?
Could we send life-bringing comets to those planets and seed them with terrestrial life? And if we could do that, should we?
This is the question that a new research article in the journal Astrobiology explores. The paper is "Directed Panspermia Using Interstellar Comets." The authors are Christopher P. McKay, Paul C.W. Davies, and Simon P. Worden. They're from NASA's Ames Research Center, the Beyond Center for Fundamental Concepts in Science at Arizona State University, and Breakthrough Initiatives, respectively.
The idea that life spreads throughout the universe is called panspermia. (Ancient Greek: pan meaning all and sperma meaning seed.) It's not a new idea. The Greek philosopher Anaxagoras first proposed it back in the 5th century BCE. He wasn't explicit about it but mentioned that seeds might be part of the universe. Historians have put two and two together to divine what he meant. Basically, according to panspermia, life exists throughout the universe and was spread by asteroids, comets, and even space dust—seeds, according to Anaxagoras.
Some researchers have proposed that powerful impacts on planets like Earth or Mars could eject microbe-carrying rocks into space. Mars has lower gravity than Earth, and we know that impacts on Mars have sent rock into space. We've found over 270 of them on Earth. Since it happened on Mars, it would've happened in other cases and in other solar systems.
We also suspect Mars was inhabited by microbes billions of years ago. This is speculative, but Martian microbes could've hitched a ride on impact-ejected rocks and been sufficiently sheltered from the hazards in space to endure a long journey. Eventually, the rock could've crashed into another body, and if the heat from the impact didn't wipe out any surviving microbes and if the body they crashed into was hospitable, life could theoretically spread this way. Multiply that idea by the immense number of collisions in solar systems throughout the Milky Way, and the panspermia idea starts to take shape.
That's accidental, or natural, panspermia. But if a civilization did this on purpose, it's called directed panspermia. That's the subject of the paper, and the civilization is ours.
We don't know how life originated on Earth. We know some of the necessary conditions, but our knowledge is full of gaps. So by extension, we don't know how it could originate on other worlds. "We have a scant idea of the appropriate geological/chemical setting for non-life to transform into life, with most of the popular scenarios being largely unproven and no consensus has emerged," the authors point out.
We know life exists, obviously, and we suspect it may exist elsewhere. But we don't know if all planets that can support life have life on them. "It may well be that a favorable location for life's origin is very different from a planetary environment in which life might be successfully sustained over the long term," the authors write.
Panspermia doesn't address the question of how life started. It asks us to consider how life might spread from world to world in the Milky Way rather than appear on each world separately. The Milky Way galaxy contains around 200 billion stars. 200 billion stars is an awful lot of solar systems, planets, asteroid belts, Kuiper Belts, and Oort Clouds. If panspermia does occur, it has lots of opportunities.
Our solar system is a puzzle, and each planet and moon is a piece of it. Mars likely had life in the past, but not anymore, unless it's somewhere underground, sheltered from the inhospitable surface. There's intriguing evidence that some of the icy moons, like Enceladus and Europa, have hospitable oceans under thick ice caps. And distant, frigid, Titan is the only body besides Earth with liquid on its surface, though it's not water. Then there's itself, a planet that "ripples with life," in Carl Sagan's words. Could panspermia be the thread that somehow connects all these pieces?
One of our solar system's puzzles is life on Earth and how rapidly it appeared. The young Earth was barely habitable when life appeared. Cellular life might date back as far as 3.95 billion years ago. At that time, Earth had just emerged from the Hadean eon, when our young, barely recognizable planet was wrapped in a thick, carbon dioxide atmosphere and ruled by super-heated conditions.
Some scientists wonder how endogenous life could have appeared so soon after the Hadean. Though there's a lack of clarity, this thinking does support the idea of panspermia, at least potentially. Earth and other young planets might be able to support life seeded by panspermia before their own life could appear.
Modern thinkers have fleshed out the panspermia idea in detail. We may soon be able to characterize all of the exoplanets in a 100-light-year sphere centered on our solar system. There are nascent proposals to send spacecraft carrying terrestrial life to any planets that could harbor it. These are largely thought experiments, but time has a way of passing, and someday humanity may have to wrestle with the idea more realistically.
The authors point out that this idea is physically possible (With lots of caveats.) But what about the expense? What about spacecraft reliability?
Nature already produces objects capable of long interstellar voyages: comets. They've become part of the discussion around directed panspermia and form the bulk of this research article. "In this article, we build on the foregoing research and propose the concept of directed panspermia using interstellar comets as opposed to dedicated spacecraft," the authors explain.
The article is motivated by specific events from the last few years. In 2017, the interstellar object "Oumuamua passed through our solar system. Two years later, the interstellar comet 2I/Borisov also briefly visited our solar system. They were the first two observed interstellar objects (ISOs) to pass through our system, making Borisov the first comet we've seen doing so. These occurrences beg the two-pronged question, how many more ISOs have/will make the same journey?
The discovery of two ISOs in such a short period of time is a result of our technological advancements and the large number of telescopes that watch the skies. There were certainly many others in the solar system's long history, and there will be many more in the future. They are likely common, and they present an opportunity, according to the authors.
"Interstellar comets enable low-cost directed panspermia that is potentially wide in scope in terms of the number of possible probes and range ultimately covered," they write. A 2021 study predicted that a total of about 6.9 objects like 2I/Borisov per year should pass within one AU of the sun. When the Vera Rubin Observatory comes online sometime in 2023, we'll start finding these ISOs, maybe five per year.
Comet Borisov is particularly interesting to the authors. Its size is uncertain, but estimates put it as large as 16 km in diameter. That makes it large enough to shelter an inoculant from radiation. It lost mass during its journey through the inner solar system, but that's actually good. The dust it left behind "… can be a mechanism for disseminating inoculum," they explain.
The authors explain how comets like Borisov could be used to spread life throughout the Milky Way. Panspermia by ISO would be a combination of natural and directed panspermia. "It combines them effectively, using interstellar comets as transporters of opportunity by adding biological inoculum to the comet without attempting to change its trajectory," they explain.
The ideal inoculum would be a collection of life forms that could successfully seed different habitats on different exoplanets. "Inoculum for planets with liquid water on the surface, such as Earth and early Mars, may be set to develop rapidly into diverse and complex life evolving in tandem with the planetary environment," the authors write. For moons like Enceladus, Earthly methanogens might be the most suitable inoculum.
The inoculum needn't be restricted to single-celled organisms. Small multicellular organisms might make the most sense, at least in some instances. The hardy tardigrades appear in the paper because they can survive the vacuum and radiation in space.
If humanity ever undertakes a program of directed panspermia, genetically-modified organisms might play a role. That technology will probably be necessary because there could be a wide variety of habitable worlds that are nothing like Earth. We have them in our solar system, and the best example might be Titan. It's the only body other than Earth with surface liquid. "But as synthetic biotechnology advances, we might be able to construct life forms that could thrive on Titan and in other non-water habitats discovered on exoplanets in the future," the authors explain.
This could all be a wasted effort, and the authors acknowledge that. Is it really as simple as plopping some life forms onto a planet? "A serious argument that might be advanced against panspermia stems from the point of view that life is a planetary phenomenon forming a complex globally distributed web of interdependent organisms that exchange material and information," they write. "Therefore, merely dropping a few microbes on a habitable but otherwise barren planet would not successfully seed it."
In that case, the inoculum would have to be a much more custom design. It would have to be a web of its own, designed for a specific environment, that could successfully implement the relationships between life forms that characterize biospheres like Earth's. That's a difficult proposition. "Determining the minimal subset of organisms required is a formidable challenge that may require significant advances in our understanding of the web of life."
But things get really tricky when we imagine a future where all of this starts to become possible. We'll be exploring some of Jupiter's icy moons soon, and a mission to Titan will likely become a reality. What will we find? If they're barren but look like they can support life, will we be tempted?
Some familiar, uncomfortable questions arise. The more powerful our technologies become, the more far-reaching the consequences of using them. Technological advancements like genetic modification and climate engineering engender powerful responses as people consider how they could go wrong. These concerns would "… carry over with a vengeance to the purposeful dissemination of life across the galaxy," the authors write.
"Now them crazy scientists want to plow ahead and start messing with the entire galaxy," some will think, and we can see the headlines and opinion pieces in our mind's eyes. But we're nowhere near doing any of this so we can pull ourselves back from the ledge and think soberly about it.
A critical question around directed panspermia poses itself before we even get to the questions of 'can we' or 'should we." We simply don't know how many planets that could support life actually have life. "Astrobiological optimists tend to assume that habitable planets are very likely to be inhabited," the authors write. But that's only an assumption.
With all we don't know, it's entirely possible that only a tiny fraction of habitable planets and moons have life on them. Maybe there are a billion or more planets and moons that can support life, like blank canvasses, but natural panspermia hasn't reached them yet. "… it is entirely possible that only an exceedingly small fraction of all habitable planets actually hosts life," the authors point out.
What if panspermia is a natural part of the universe, and since we're a natural part of the universe, we have a role to play in spreading life? Maybe we even have the duty to do so. Maybe Earth was seeded by directed panspermia. Maybe a long-dead civilization faced what we're facing now and decided to go for it.
That's a lot of maybes, but that's the nature of these questions. Another maybe is that this may be how it goes for civilizations. Maybe civilizations never become the advanced types laid out in the Kardashev Scale. Maybe they reach a point, one that's quickly approaching for humanity, where the Great Filter looms over all our affairs. Maybe, when civilizations reach that point, all they can do is try to spread life. And the decision might have to be made long before we understand exactly what's going on with life in the galaxy.
That's a whole bunch of maybes strung together on an uncertain trajectory. But there's another string of maybes and what-ifs that leads to caution when we follow it, and the authors outline these concerns.
What if we send life to a planet thinking it's uninhabited, but it's only at the very beginning of hosting life? In that case, our good intentions might end in disaster, as that planet's life is snuffed out by Terran life that outcompetes it.
What if we base our panspermia decisions on biosignatures, but our understanding of biosignatures is too biased towards Earth life? That could also end in disaster as our robust, genetically engineered microbes committed a uni-cellular atrocity and wiped out the planet's existing life.
Or, our life-bearing comet might find an appropriate uninhabited target and successfully seed it with microbe-carrying dust. But what if it didn't stop there and seeded other planets that were already inhabited? That's another disaster, as our good intentions manifest as an invasion or even a weapon.
The situation quickly grows complex. But it leads us back to questions about exactly what happened on Earth.
Early Earth life bubbled along for a long time before photosynthesis appeared. That changed everything, as oxygen became more concentrated in the atmosphere and complex life appeared and took over the planet. What if the genetic ability to perform photosynthesis was seeded through panspermia, either directed or natural? What if life on Earth never made the energy-exploiting jump to photosynthesis without a boost from panspermia?
We've got a lot more to learn about comets before any of this even approaches practicality. In 2019, the ESA selected the Comet Interceptor mission from among several candidate missions. They hope to launch it in 2029. The Interceptor will sit and wait at the sun-Earth L2 point for a suitable Long Period Comet (LPC) to approach the inner solar system. By 2029, we'll have more powerful telescopes that can identify a good comet long before it reaches the inner solar system.
When one is found, the Comet Interceptor will deploy two smaller probes to intercept the comet. The mission is purely scientific. LPCs are pristine objects, holders of clues to the origins of our solar system. The probes will study the comet in detail and create a rich 3D model of the comet and the region that surrounds it as it moves through space.
There will likely be many more comet-exploring missions in the near future. We'll keep learning about them and which ones might serve as vehicles for panspermia. As time marches on, we'll get closer to executing some sort of panspermia strategy. Maybe circumstances will force our hand.
The authors say that the idea of directed panspermia has migrated from the outright absurd to something that needs to be thought of more seriously, and the discovery of interstellar comets is responsible. "Until recently, the idea that humans could literally sow the seeds of a cosmic transformation having multi-million-year downstream consequences would have been regarded as absurd," they write. "But the discovery of interstellar comets has changed all that."
In their article, the authors outline their vision of a biological universe. Panspermia's objective "… is to enhance the richness and diversity of life in the universe," they say. We don't have the technology to do it, but future generations will. "Although we currently lack the technology to harness these comets as biological delivery vehicles, there is no difficulty in understanding what is needed to do so and in refining the strategy to achieve the goal of seeding the galaxy with life suitably constructed to thrive in a variety of exoplanetary environments."
That's a positive outlook, but there's a haunting aspect to panspermia, too. All stars burn out and fade away, and no world remains hospitable forever. Maybe natural panspermia leaves too much to chance, and we have a duty to spread life wherever we can because every instance of life faces extinction.
From that vantage point, is there really any difference between directed and natural panspermia? Maybe we're agents of nature, and we'll know it's the right thing to do when we know it's the right thing to do. Maybe the Great Filter will force our hand, and we'll take a bold step. Humanity's boldest step and defining act might be spreading life elsewhere, hoping it can find hospitable cradles throughout the galaxy. The cycle can continue, and life can persist.
Panspermia could be our grand gesture and a tip of the hat to life before we fade away. If it were a scene in a science fiction film, the setting would be a dying, resource-depleted Earth, with its biosphere in tatters and its aging sun bathing it all in an eerie light. The last few hundred thousand bedraggled humans would gather what resources they could and build one last spacecraft. They'd watch the flare of a rocket one last time, loaded with inoculant and headed for a rendezvous with a suitable interstellar comet that's passing through our inner solar system.
That may sound melodramatic, but is there something more dramatic than birth and death on a galactic scale?
More information: Christopher P. McKay et al, Directed Panspermia Using Interstellar Comets, Astrobiology (2022). DOI: 10.1089/ast.2021.0188
Journal information: Astrobiology
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