Astrobiology is based on the hope that life is widespread in the universe. There are two ways this might be the case. One is that life is easy to incubate and so will pop up wherever planets resemble Earth. The other is that life’s origin requires very rare and special conditions, but that once it gets going it spreads around the universe, a theory known as panspermia, meaning “seeds everywhere”.
The basic idea of panspermia goes back to antiquity, but it was placed on a modern footing by the Swedish chemist Svante Arrhenius in the early 20th century. In the 1970s it was refined by the British astronomer Fred Hoyle and his collaborator Chandra Wickramasinghe. The theory remains highly controversial in the original form, with naked microbes wafting across interstellar space, which we now know is saturated with deadly radiation.
But there is one convincing version of the theory. From time to time, Earth and Mars take a hit from a comet or asteroid with enough force to blast rocks around the solar system. Some terrestrial rocks will fall on Mars and vice versa – my university has half a dozen Mars rocks that landed as meteorites.
If Earth and Mars can trade rocks, surely they can trade life too? Shielded within a rock, a hardy microbe could easily withstand the harsh environment of outer space and so arrive at the other end still viable.
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