WHEN you are in the business of sending spacecraft to other planets, it is probably wise to do everything you can to keep your space-probes sterile. NASA, America's space agency, certainly does so. After all, you would not want bugs from one planet to contaminate another where they might possibly thrive. But according to Curt Mileikowsky, of the Royal Institute of Technology in Stockholm, this may already have happened naturally billions of years ago when the solar system was young. For Dr Mileikowsky has taken a century-old idea called panspermia, and shown that it is plausible.
Panspermia is the theory that life does not start independently on each planet that has it (assuming that other planets do). Rather, it hops from place to place, “infecting” new worlds as it goes. Supported by experts in biology, geology and celestial mechanics, Dr Mileikowsky argued to the American Astronomical Society meeting in Atlanta that this is not as outlandish as it sounds.
Bungling space organisations apart, the only mode of travel open to microbes seems to be meteorites. Most of these are small bits of junk from the asteroid belt that have gone off course. But some are rocks that have been flung into space from the surfaces of planets as a result of those planets having been struck by even larger bits of rock—decent-sized asteroids or comets.
If there is life on such a planet, microscopic forms of it will probably live deep inside rocks, as they do on earth. The acceleration of lift-off would not kill something that size, and if a rock is large enough, the heat generated as it is thrown clear will be negligible except at its surface—where, if anything, melting may even produce an airtight skin to protect any microbes deeper down from the unpleasant vacuum of space.
Radiation, too, should not be a problem if a newly launched meteorite is at least the size of a soccer ball. An experimental colony of bacteria stranded in space by problems with the space shuttle survived for six years. Extrapolating from this result, Dr Mileikowsky thinks that in a meteorite with a diameter of 20cm it would take about 1m years for such a colony to be reduced to a millionth of its original size.
The exchange of rocky migrants that interests people most is between the earth and Mars. (Only a few years ago researchers claimed to have found fossilised traces of microbes in one such meteorite, though this is hotly disputed.) With luck, 1m years is enough time to reach the earth from Mars. In the past 4 billion years, Dr Mileikowsky has calculated, about 5 billion “landings” on earth from Mars have occurred in which the “vessel” size and travel time were just right. In the other direction, life could have made some 1 billion trips. That seems ample opportunity for mutual seeding of the two planets to have happened.
Indeed, Mauri Valtonen, of Turku University in Finland, said that although he doesn't feel particularly Martian, the inward route is actually the more probable one. Besides the larger number of emigrant rocks from its surface, Mars cooled faster than the earth after it formed, and so would have reached conditions favourable to life (a dense atmosphere, allowing for surface water) earlier.
That would make it more likely that earth was the recipient, not the giver, of life. Afterwards, of course, Mars became the cold, apparently lifeless, body that it is today. So for the past few billion years microbes have probably been travelling in one direction only—from earth to Mars.
An even more intriguing question—and the one that the original theory of panspermia tried to address—is whether life might have arrived on earth from another solar system. In present circumstances, according to Dr Mileikowsky's calculations, that idea is a non-starter. The chance that a suitable meteorite could make the journey fast enough is virtually zero. But the sun was probably born in a cluster with many other stars. This would mean that 4 billion years ago, the age of the earliest signs of life on earth, interstellar distances were much smaller. In that case, he calculates, the chance of life having crossed to the earth from another solar system, or vice versa, is about 1%. That is still pretty unlikely in most people's books. But it does make the seemingly universal two-leggedness of aliens in science fiction movies a tad more convincing.