I grew up in a universe teeming with life. Alien forms lived on and traveled throughout the planets. Sometimes they could even be found on neutron stars or giant rings constructed around a sun, or, shockingly, returned from the earth’s own upper atmosphere. When people meet me for the first time and realize I am an astronomer, the second thing they ask is often: What do you think about the possibility of life on other planets? (The first thing they ask, of course, is: What happened to Pluto? To which I have a book-long answer to hand them).
For years, my stock answer to this question has involved pointing out that, while I do have opinions, they are not any more informed than anyone else’s. When there is no science to aliens the only scientifically defensible position is to be an alien agnostic.
These days, though, I have been become a convert. I consider myself no longer an alien agnostic, but a true believer. A range of scientific discoveries over the past decade makes the possible of life outside of the earth appear -- to me, at least – not just possible, but actually inevitable.
Inevitable! That’s a pretty strong word from a former agnostic, so let me clarify what I think is inevitable. I believe that, within my lifetime, we will have solid evidence of currently living microbial life forms on Mars. That’s not quite telepathic natives yearning for earthlings, but it’s a lot closer to the universe of my childhood reading than I would have ever guessed we would come.
There are three key insights that make microbial life on Mars seem inevitable. First, one of the most profound things that we’ve learned about life in the past decade is that, on the earth at least, it can eke out an existence almost everywhere: in the deepest parts of the ocean, far underground, up in the atmosphere, in arsenic pools, inside nuclear containment vessels, on your kitchen counter even after a generous application of disinfectant. Life is very difficult to get rid of once established. While the surface of Mars is currently quite inhospitable to anything we can imagine, sterilizing an entire planet is likely impossible. If life ever had been established on Mars it would certainly still be hiding in little cracks today, perhaps in near-surface hot springs or deep underground caves or somewhere else we haven’t yet looked.
The second key scientific insight is that life could well have become established on Mars earlier in Martian history. The armada of orbiters and rovers and telescope circling, driving, and staring at Mars over the past decade have general come to an agreement that the cold dry bleak Mars of today -- which seems an unlikely spot for the development of life -- was warmer and wetter billions of year ago.
But simply saying that Mars could have evolved life and then would have kept it if it had does not mean that it did evolve life. It certainly does not make it inevitable. We have absolutely zero understanding of the probability of life developing, even if it has ideal conditions.
This is where that the third and most important insight comes in. Planets don’t live in isolation. We have found pieces of Mars and of the Moon on the surface of the earth. These pieces were blasted off of their home planets by a meteor impact, which caused them to lift into space, travel in orbit for a while, and then plunge to the earth. The reverse must also have happened: chunks of the earth which have been blasted off by meteorite impacts have inevitably landed on Mars (and elsewhere) throughout solar system history.
Experiments now show that microbes can live dormant in space for years and years – longer than the shortest travel times between the planets. We can now say with pretty good certainty that microbe-containing rocks have been blasted off the face of the earth and that some of these have landed on Mars with the microbes still alive. We can also be pretty sure that this same process occurred back in the time when Mars was warmer and wetter. If enough terrestrial microbes landed on Mars, some would have found a way to survive.
And if they did? They’re still there. Somewhere. It is inevitable.
It’s still not quite hive-minded colonies of insectoids accidentally destroying humanity. The inevitable aliens that I believe we will find will simply turn out to be colonists sent from the earth long ago. Or maybe not. Some scientists have speculated that Mars might have been an even more hospitable place than the early earth for life to have evolved. Perhaps early in earth’s history the first Martian microbial colonists landed on a lifeless earth and, finally, four billion years later we finally return to pay our respects to the now destroyed mother world. I think I read that book when I was a kid, too.