The chances that Earth has sent out alien invaders of its own are better than you think.
The notion that life can hitchhike its way through space via comets and asteroids has captured the human imagination for over a century. The concept, called lithopanspermia, is probably best known from its appearances in science fiction: A space rock that delivers alien life to Earth is one of the genre's most beloved tropes.
But did alien rocks bring life to Earth, or was it the other way around? Rather, is it Earth rocks bringing life to alien planets?
"If it can survive the transfer, it's likely that life from Earth has already been brought to other planets," said Rachel Worth, a PhD candidate at Penn State University and lead author of a recent paper published in Astrobiology addressing this very question.
Meteorites bombard our planet constantly, and their impacts cause little chunks of microbe-infested Earth to go shooting off into space. Worth and her colleagues find "significant probability" that terrestrial rocks have made the trip to Jupiter's icy moon Europa and Saturn's Titan, our solar system's most promising candidates for hosting extraterrestrials.
Worth used computer simulations to track the fate of over 100,000 Earth rocks for 10 million years. While the modeling method she used is not new, recent advances in computer processing power allowed her to scale up the number of simulations by a factor of ten.
we can't rule out the possibility of life from Earth seeding an outer solar system moon
"We're taking advantage of improved computing technology to trace the fate of more asteroids than ever before," said Worth. "This is essential for detecting events such as the transfer of material from Earth to, say, Europa, which may occur very infrequently."
Infrequent, yes: But not necessarily never. Since the earliest traces of life on Earth 3.5 billion years ago, Worth predicts each of the six major moons of Jupiter and Saturn (including Europa and Titan) has received one to ten terrestrial impacts.
"At the end of the day, it's not the numbers themselves that matter, but the order of magnitude," Worth tells me. "If the numbers were much lower than one, we'd assume this event is unlikely to have occurred since the origin of life on Earth. But one to ten impacts over the last several billion years tells us we can't rule out the possibility of life from Earth seeding an outer solar system moon."
It's worth noting that Earth's most epic impacts also launch the most life-filled debris into space. The impact at Chicxulub, Mexico roughly 66 million years ago—widely believed to have caused the mass extinction of the dinosaurs—flung about 70 billion kilograms of rock into space. Some 20,000 kg of this rock could have reached Europa. Chances that a rock big enough to harbor life reached the icy moon are better than 50/50, Worth says.
All this sounds promising, but there's a big "if" here: Whether the Earth microbes that hitched a ride on the debris could actually survive the trip to another world.
Worth estimates an asteroid trip from Earth to an outer solar system moon would take at least three million years, and maybe closer to ten.
So basically we just need a microbe that can survive in extreme cold, intense radiation, and vacuum exposure. For millions of years. Oh, and it will have to handle the extreme force and blazing heat of a cosmic impact. Is there any living thing on planet Earth that could actually do that?
To find out, I spoke to Dr. Rocco Mancinelli, an exobiologist at NASA's Ames Research Center. Mancinelli has made a career of studying extremeophiles, or microbes that survive on the edge of what we consider habitable. He also authored the most comprehensive review paper to date on space microbiology.
I was pleasantly surprised to learn the prospects are not so bleak, after all. While a microbe would face numerous perils during a trans-solar system asteroid journey, there's solid scientific evidence that many of those challenges could be overcome.
For one, surviving the physical impact wouldn't be a problem. Scientists have smashed enough microbe-filled rocks together to be sure of that.
"The temperature is the critical thing with asteroid ejection and impact," Mancinelli said. "The heat will kill any bacteria on the surface, but if they're buried a few meters down they could survive it."
We know that bacteria buried in rock are protected from the radiation of space
Encasement in rock would also go a long way towards protecting our voyagers from the harmful radiation of space. Still, it would take a special type of bacteria to survive the freezing temperatures and utter lack of food or water during the long journey.
"If any microbe could do it, it's probably going to be a spore-former," said Mancinelli.
Spore formation is a sort of suspended animation in which a bacterium transforms itself into an armored package of DNA.
"When times get tough, these bacteria dehydrate themselves, making what's called a spore coat. You can think of it as putting on a suit of armor," said Mancinelli. "The spore is then highly desiccation- and radiation-resistant."
Bacterial spores have already performed some jaw-dropping feats. The current record for survival in space goes to a handful of Bacillus subtilis spores that toughed it out on the Long Duration Exposure Facility for six years. And several years back, scientists resuscitated Bacillus spores that had been buried in salt crystals for hundreds of millions of years.
So, to recap: Bacterial spores encased in rock can probably survive ejection, impact, and a lot of the outer-space dangers. Is that enough to argue lithopanspermia is actually underway?
According to Mancinelli, there's one last, major unknown: Radiation produced from the decay of elements within the asteroid itself.
"We know that bacteria buried in rock are protected from the radiation of space. But we know very little about the amounts of radiation generated within an asteroid over millions of years, and whether bacteria could survive that radiation. This to me is one of the most important knowledge gaps," said Mancinelli.
But even with some lingering unknowns, the possibility of lithopanspermia raises some big questions. If scientists one day find life buried beneath the icy surface of Europa or Titan, how certain will we be that it's truly extraterrestrial and not just an ancient transplant from Earth?
"That'll depend on how similar it is to Earth life. If it's composed of different basic building blocks, it's probably extraterrestrial," said Mancinelli. "If that life came from Earth millions or billions of years ago, it's likely to share our genetic code, but that genetic code should still be quite divergent." (If it's neither, it's probably contamination.)
There's one final footnote to the lithopanspermia story. Apparently, asteroid debris released from planet Earth falls back to the surface all the time, raising the possibility of "self-panspermia." Were Earth to suffer an Armageddon-style asteroid impact or nuclear winter, orbiting space bacteria might just be able to re-seed the surface. Our very own bacterial Oort cloud may be our planet's ultimate lifeline.
Perhaps we'll never know for sure if alien rocks brought life to Earth, or if Earth has spawned life on other planets. But that doesn't mean we'll stop imagining the possibilities. Whether you're an Octavia Butler fan already converted to Earthseed, or a Trekkie holding out hope for the improbable but romantic notion of a galaxy filled with humanoid aliens, the notion that life on our little world might be part of a much larger biological universe is well worth our imagination.