Revisiting Panspermia: did life come from outside of Earth?
About 20 years ago, the eminent scientist Joe Kirschvink from Caltech suggested in the Carl Sagan Memorial Lecture at the American Geophysical Union Meeting in San Francisco that life on Earth may have originated on Mars. Ten years later, another prominent scientist, Steve Benner, from the Foundation for Applied Molecular Evolution, made a case for a possible Martian origin of terrestrial life. So, where are we standing today on panspermia, the hypothesis that life can be transported from one planet to another?
The idea of panspermia has been around since ancient Greek times but regained attention in modern times with Svante Arrhenius in 1908, who suggested that bacterial spores could be pushed by solar wind from star to star and end up on habitable planets seeding them with life.
Arrhenius’ original idea has been discredited mainly because several lines of evidence speak against it. First, given the vastness of space and the tiny space planets occupy, it seems to be statistically extremely unlikely that a spore could leave one solar system, against the gravitational attraction of that system, and eventually arrive in another to seed a planet. Even if that would occur, the spore would likely fall into the central star of that solar system and burn up. Also, while spores are the dormant form of certain bacteria and have a protective coating, they would not survive the time needed to travel from one solar system to another, given the damage from cosmic radiation. It’s important to keep in mind that the famous and most studied Martian meteorite, ALH84001, needed several million years to get just from Mars to Earth!
Another eminent scientist promoting panspermia was the physicist Fred Hoyle, but his idea that disease agents, such as flu viruses, came from space is implausible from a biological viewpoint. Disease agents co-evolve with their host, and since there are no humans on other planets, I don’t see how a life form from outer space could be so well-adapted to infect us. However, comets and asteroids brought the building blocks of life, such as amino acids, to early Earth. They likely played an important role in the origin of life on our planet and possibly on other terrestrial planets.
But like the building blocks of life, couldn't microbial organisms be transferred from one planet to another? Bacteria without a thick shielding layer are too vulnerable in space, even if the transfer would only occur from one planet to another in the same solar system. But what if a dormant bacterium or spore would be protected within a meteorite underneath multiple layers of rock or sediment? There have been many studies since the hypotheses of Kirschvink and Benner, which indicated that this is a reasonable scenario. If microbes were within a meteorite, a significant fraction of that microbial load would likely survive a transfer from Mars to Earth. There is also supporting evidence from studies on Martian meteorite ALH84001 because its interior was never heated above 40C—well below sterilization temperature—neither during ejection from Mars nor during its entry into Earth´s atmosphere.
Clearly, the proposed transfer of life between Mars and Earth would require that life on Mars originated there in the first place. This seems plausible given that early Mars was warmer, wetter, and very Earth-like. Life on Mars could have evolved earlier than on Earth, because it became habitable faster. It cooled down from its hot birth quickly since it was smaller and further away from the Sun. On the contrary, Earth was hotter and, in addition, had to “digest” the collision with the planetary impactor that created our Moon. In the aftermath of that collision, magma oceans dominated Earth's surface for quite some time before a habitable surface developed on our home planet.
A transfer of meteorites and life could, in principle, go both ways, from Mars to Earth or vice versa, and start the origin of life on another planet. However, the trajectory toward Earth is much more likely because Earth has a larger gravitational attraction, and the Sun is in the center of the Solar System. Any ejected material from Mars would most likely fly toward the Sun. By chance, our planet might be in the right location to intercept Martian rocks.
So, could we all be Martians? Given the lack of knowledge we have about the origin of life, we can’t say. But it is possible. And it is a hypothesis that should be tested in future missions to Mars. If we design a life-detection mission right away, we should be able to analyze Martian life—if it exists—and its organic building blocks to find the answer we’re searching for.