NASA reports that the Martian rover Opportunity is sitting on an ancient shoreline. The texture of ripples on the rocks indicates that they used to be under water; it must have been at least a couple of inches deep, in order to create the observed patterns. (Did you know that Opportunity has her own blog? Two of them, actually.)
Let’s indulge ourselves in thinking just about the scientific implications, instead of the icky politics. We instantly jump to speculations about life on Mars; the evidence is thin, but the temptation is irresistible.
I am by no means an expert on exobiology in general or Mars in particular, but it’s clear that sorting this out is going to be both complicated and fascinating. We don’t know all that much about the origin of life, to be honest. The famous Miller-Urey experiment showed that amino acids could be spontaneously generated inside a test tube filled with methane, ammonia, hydrogen, and water, if it was continuously zapped with electrical shocks (to simulate lightning). Amino acids are the building blocks of proteins, so this is certainly a step in the right direction.
But these days scientists think that the atmosphere of Earth long ago didn’t actually have the right compounds. Never fear, though; it seems as if the conditions for making amino acids happen naturally in outer space! Comets in particular seem to be thick with organic materials, and meteorites that have fallen to earth turn out to occasionally have actual amino acids in them. You might worry that the delicate organic materials would get destroyed when objects crashed into the Earth, but there’s some experimental evidence that they actually survive intact. In other words, it’s quite plausible that interplanetary chemistry played an important role in the first steps toward the development of life here on Earth.
I bring this up because 1) it’s intrinsically amazing, and 2) it’s going to make it very hard to sort out the life-on-Mars story. We might find all sorts of organic molecules on Mars, not because they developed there by themselves, but because they were brought by comets. We might even find evidence of Earth-like life, again not because it arose by itself, but because it was carried from Earth by our own spacecraft, or perhaps by rocks ejected from volcanoes.
None of this makes the effort to understand the status of life on Mars any less interesting; all of the possibilities are fascinating, for different reasons. But it will be a long time before we can say anything with confidence. Unless there is an entire civilization hiding underneath the Martian soil, waiting for the right moment to spring out and attack. Someone should make a movie about that.