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.
When I suggested this in the early 1990s I received nothing but derision. It was objected that microbes wouldn’t survive being kicked off a planet, or the fiery plunge through the atmosphere. However, it has been shown that the Mars meteorites generally do not show signs of shock heating, only the outer layer of a meteorite becomes incandescent, and it all happens so fast that the interior doesn’t get hot. Today, these objections have largely melted away.
The bombardment of the planets by comets and asteroids was far more severe in the past, especially before about 3.8 billion years ago, after which it tailed off somewhat, although it has never ceased entirely. Evidence suggests that until about 3.5 billion years ago Mars was warm and wet and far more earthlike than it is today. As we know there was life on Earth at that time, it seems inevitable that the transfer of viable organisms from Earth to Mars would have occurred, thus seeding the red planet with Earth life. Of course, the same mechanism works in reverse; indeed, it is easier to knock rocks off Mars because of its lower gravity and thinner atmosphere.
All of which raises the intriguing question of whether life on Earth may have started on Mars and come here in impact ejecta, implying that we are all the descendants of Martians. Mars does have a few favourable aspects as an incubator of life; certainly early Mars was no less congenial than early Earth for biology to get started. But whichever way around it was, it seems that if we ever find traces of life on Mars, chances are it will just be good old terrestrial life.
Fascinating though that may be scientifically, it would fail to answer the much deeper question of whether life is easy or not to start. Ideally we would like to find a second genesis of life on Mars, thus answering the question in the affirmative.
The spread of life between near-neighbour planets could be common throughout the universe. But what about longer journeys?
Calculations show that some Earth ejecta will reach the outer moons of the solar system, such as Europa, although the probability of a successful transfer of life isn’t promising. It will also happen from time to time that ejected Earth rocks will be flung out of the solar system altogether by the gravitational field of Jupiter. But now the numbers are very unfavourable: the chances of a terrestrial rock ever hitting another earthlike planet beyond the solar system are tiny, and even the hardiest microbe would be unlikely to survive a journey of millions of years.
On the other hand, our sun was born amid a cluster of closely-spaced stars, so if life was established quickly somewhere in the cluster, there is a possibility that it could have spread rapidly between the nascent planetary systems.
If life on Earth did arrive from elsewhere, the problem of how and where it first arose gets shifted off into unknown territory. Whether kicking the can down the road – or across the galaxy – amounts to good science is debatable.