Panspermia: plausible or busted?
In my last post I reported on the results of ESA’s STONE-6 mission, which tested the concept of whether fossil evidence of life from Mars could survive being dropped from space onto Earth. The STONE-6 mission was a success, and demonstrates that if Martian sedimentary rocks ever made it to Earth, fossil or chemical traces of ancient Martian life inside them could remain intact. It would be a fantastic discovery, to say the least, if Martian meteorites carrying traces of alien life could be found. But so far all Martian meteorites found on Earth are basalts; a form of volcanic rock not particularly conducive to recording any fossils.
The results of STONE-6 show that fossils can make it here. What about living cells? The STONE-6 experiment didn’t really address this question. Researchers inoculated the surfaces of STONE-6 rock samples with bacterial cultures before launch, but I doubt anyone was surprised that the inferno of re-entry would convert all exposed microbes to ash. That’s exactly what happened. But could living microbes have survived inside the STONE-6 samples? Probably, if the experiment had been carried out that way.
Bacteria are hardy things. Tiny naturally-occurring devices capable of self-replication using external sources of energy, bacteria are natural nanomachines. Bacteria are constructed around a core engine of twisted RNA molecules that form a working machine to construct more twisted RNA molecules. Assembler modules called ribosomes take in feedstock amino acids and build products according to instructions encoded into DNA, which is basically the microbial hard drive. The machine works by using enzymes to trigger sluggish chemical reactions that release a tiny electrical current, which is used to power cellular functions. The electrical energy to run a microbe can come from solar power or chemical power.
Bacteria on Earth can survive nearly everywhere that liquid water is possible, including at geologic pressures deep in the planetary crust, in searing hot seafloor geysers, in briny fluids far saltier than seawater, in battery acid, and in radiation that would turn a human into soup. The bacterium Deinococcus radiodurans can survive exposure to 1,000,000 rads of hard ionizing radiation. The lethal dose for a human is 1,000 rads. D. radiodurans can also survive in acid and in space. Clearly this bug has seen some hard times, to evolve such uncanny resilience. An organism like that could probably survive in a deeply frozen state inside a meteor for quite some time. But for exactly how long?
Time is the key here. Moving between the stars takes a long time, using a human lifespan as a yardstick. Meteors move through space at a wide range of velocities; from about 10 km/s at the lower end of the scale, up to several tens of km/s for objects whipping in a hyperbolic orbit around the Sun. Assuming a conservative 10 km/s velocity, a meteor in interstellar space would need about 30,000 years to cross a light year of distance (about 9.46 trillion km). That seems like a long time, but on a geologic timescale that’s nothing. Cruising at 10 km/s, a meteor from Earth could reach Alpha Centauri in only 131,000 years, which is less than the current age of the human species.
Can frozen bacteria survive in space for that long inside a rock? That particular question has never been tested, but bacterial survival over geologic time has been demonstrated on Earth. In 2000 a study by biologist Russell Vreeland (Vreeland R. D. et al., 2000, Nature v. 407, pp. 897-900) demonstrated the survival of individual bacterial spores inside fluid inclusions within rock salt crystals that were 250 million years old. The spores, resussitated after a quarter-billion years floating in salt water, turned out to belong to a fairly mundane type of salt-loving species similar to those found today in the Dead Sea and the Great Salt Lake.
A quarter billion years is a long time to survive, but the Vreeland bacteria did so by closing up operations and becoming inactive spores. For a quarter billion years they just sat there, not metabolizing, not doing anything. The machines turned themselves off, until they were once again exposed to nutrients in Vreeland’s lab. Compared to a salt crystal, the desiccating cold of deep space is a much more stabilizing environment, except for the radiation. But inside a rock the radiation exposure would be diminished, and in any event creatures like D. radiodurans suggest that radiation can be managed.
How far can a meteor travel in a quarter billion years? At 10 km/s, about 8,000 light years. Within 8,000 light years of the Sun are several hundred thousand stars (or more… maybe Phil Plait can give a more precise number). For panspermia to work, a rock containing intact microbes has to cross space and get captured by another star’s gravity, then it has to swing around until it strikes a planet capable of supporting life, then it has to survive re-entry, then the bugs inside the rock have to escape into a suitable environment after they crash-land. Those are a lot of ifs, adding up to some very long odds.
But unlikely is not the same thing as impossible. Surprisingly unlikely events can occur, given sufficient time. If an impact-assisted swarm of rocks lifts off of Earth and heads into space, a majority of those rocks will either crash back into Earth or go into orbit around the Sun. Some will remain in permanent orbit and never encounter a planet. A few will swing close enough to the Sun to achieve a hyperbolic escape orbit (or fall into the Sun), or perhaps only enough boost to take them to the outer solar system. Earth has probably sent lots of rocks to every other planet in the solar system, over the last four billion years. Earth contamination is probably all over the solar system, already. If there were another hospitable planet circling the Sun, it’s very likely that we’d find life there, and it would derive from here.
Only a rare few rocks from Earth will leave Solar orbit. But again, over four billion years it’s more likely than not that a few have done so. The odds of any such rocks landing on a habitable planet around a distant star are exceedingly remote, but the physical possibility exists. Also keep in mind that a life-bearing rock need not cross 8,000 light years to find a good world. There are lots of stars within 10 light years of the Sun. Each time a successful seeding occurs the effective propagation range is extended, with multiple worlds providing specks of biological material to the interstellar medium. Each new transplant increases the odds of more transplants, until eventually – over the course of billions of years – an entire galaxy of planets could be seeded with life from only a few starter worlds. Geologic time is available, so it’s just a question of running the odds for long enough.
Panspermia is an interesting notion, because it appears to be physically possible, and given an astronomical time scale might even be inevitable, at least in principle. That doesn’t mean it happened here. Despite the physical possibility, the odds are absurdly long, and it would be utterly foolish to adopt panspermic propagation as the working hypothesis for how life started on Earth.
So, what’s the verdict? Not busted. But don’t expect that it happened here.