The more you learn about life on the Earth and the incredible things it’s done to adapt to strange environments—for instance, even here in Tucson, look at the Palo Verde trees. If you take a seed off a Palo Verde tree and put it in some nutrients and add water, it will not grow. Why not? Because its seed coating is so tough that water can’t penetrate. The only way a Palo Verde tree can sprout is if the seed, which has landed on the ground, runs along a stream and wears off the outer part of the seed coating. Then it will sprout. So it only sprouts when it’s wet. How does a tree know how to do that? That is really clever. And that’s just one out of a billion example of how life adapts.
Life came almost instantly on the Earth after it cooled. Mars would have cooled first, and water would’ve been stable on the surface. There’s probably lots of water on Mars. Certainly we see remnants of it today. Why wouldn’t have life formed immediately there? And if it did, why can’t it adapt as ferociously as life has on the Earth to every little niche and cranny, including, as the water dried up, underground? It would’ve gone right underground with the water, wouldn’t it?
I don’t know if the ice in the northern plains of Mars is where life took hold or not, but it’s a good chance. That’s where water comes up to the surface. There’s ice all under the surface, probably all around Mars. This is the one place you can get to it. So we’re hopeful. What are the chances? I try to keep myself from getting too excited. Basically we’re throwing a dart at a map, and what are the chances that that’s the place? I am feeling lucky though.
There’s no evidence for it, so scientifically we have to say that the jury is out. Based on my studies of the origin of life and the chemistry involved with basic processes I’d say it is very likely that biology is going on somewhere else in this universe. We haven’t figured out exactly how the origin of life started, but we’ve got a pretty good framework for how it could get going. Nothing seems like a show-stopper right now. There are some critical steps we need to work out, like how you make up a polyimide molecule, but all the basic building blocks are there and they’re very common. So I would say yes, there’s a high probability. There’s just no evidence for it, or against it.
I’ve wavered with this, being pessimistic and optimistic. At this point I’m pessimistic, and the reason is that there are so many things that have to go just right, for an Earth-like planet as we find it today. A lot of things have to come together just right, just so: Quite narrow ranges of physical parameters and initial conditions for Earth to be comfortable for life, particularly advanced life.
But then, you know, it’s a pretty ill-defined question. How close do conditions have to be for us to say that some alien planet is an Earth-like planet? Conditions may be equally exciting and interesting on other planets, other worlds, but totally different than on ours.