Among the many questions regarding life elsewhere in the universe, most attention tends to focus on Mars. Is there life there? Is there life there now, after shooting robot after microbe-infested robot there from Earth for decades? This is an important question, and not only because we might have inadvertently polluted Mars with terrestrial germs that are even now burrowing into the aquifers there and starting colonies of buried immigrants. A recent article in Acta Astronautica by Gerda Horneck asks this question, and provides a very interesting discussion of whether or not there are enough Martian ‘oases’ of relative warmth and moisture to get a biosphere rolling there from accidental contamination from Earth.
If we can actually start a viable invasion of bacteria just by landing a few little machines on Mars, this raises some major questions about how easily Earth or Mars could have seeded the other with life in the geologic past. If a few thousand stray E. coli and S. aureus on the landing pads of Viking can get a biosphere started, pretty much any rock blasted by impact from one world to the other could also do it, at any point in early planetary history. We already know there are Martian meteorites on Earth. The converse is almost certainly true, too. In Mars’ early history it was warmer and wetter than it is now. Life could have started on Mars or on Earth, and then gotten itself transplanted ballistically to the other world.
Unfortunately, we’re nowhere close to answering this question. Even if we used Mars as an experiment, by watching to see if contamination from our robots gets a foothold there, the experiment would take decades if not centuries to play out. But an interesting experiment it would be. We already know that individual microbes can survive as spores inside fluid inclusions in rocks for a long time. A very long time; 250 million years. That’s enough time for the Sun to make one orbit around the center of the Milky Way galaxy. So, if life can survive on a rock in deep space for a quarter billion years, and if landing rocks can get biospheres started, one planet could get life started throughout a galaxy in a couple billion years. Maybe.
Panspermia is an old, largely discredited concept – namely that life on Earth started elsewhere in the universe and came here, or was put here, but some process or some weird creatures with questionable motives. As an hypothesis to explain the emergence of life on Earth, Panspermia is pretty useless. The concept doesn’t really answer any questions, it only opens new ones, and largely just shifts the burden of proof. But in principle Panspermia is a viable concept, given the reality of planetary impacts and rugged bacteria sheltered inside space-rocks. It’s just very very unlikely, and without some kind of convincing evidence it’s just not worth entertaining seriously.
The odds are literally astronomically against a single rock getting from a planet in one star system to a habitable planet in another star system. However, given sufficient time even low probability events can realistically occur. It’s actually quite easy to move a rock from planet to planet in one solar system, and we have examples. The open question remains; can bugs realistically survive the journey, and how easily can they get busy upon arrival?
Finding active colonies of terrestrial bacteria in the Martian regolith would go a long way toward addressing that question. Even more interesting would be an experiment where a germy Earth rock is dropped from Mars orbit onto the planet, and the impact site watched for a few decades to see if anything germy gets started there. But I doubt that one would get past the NASA proposal review panel.