NASA’s announcement about evidence for flowing water on Mars is indeed very exciting – but I’d like to point out (as NASA has also been careful to do) that although water seems to be a necessary component for life, it does not follow that ‘where there’s water, there’s life’.
This might be the case on our own dear planet, but that logic cannot be extended to Mars. The only thing we know is that Life emerged once. We really can’t infer anything beyond that, because we don’t know how life started on Earth and whether the same process could happen again on another planet. It may turn out that life is a robust phenomenon, popping up when given even the slightest chance. Or life may have been a freak occurrence on one planet, which we’re only seeing because we’re among its products. We have no way of knowing until we travel to several other planets and investigate (by person or probe) thoroughly.
Personally, I’d be ecstatic to learn of life on Mars. But the history of research in exobiology and the Origin of Life often reads like a case study of grand hopes dashed. (if you’d like to read 70,000 words on the topic, here you go. Knock yourself out)
I think we all hold an instinctive expectation that life will be found on other planets; after all, our entire history as a species, for millions of years, has been one of continual exploration and movement from one place to another, and finding new and exciting life there. That all sort of stopped around the 19th century or so, when we finally ran out of unexplored territory. The planet was demystified, and we had no more world to conquer. And so we looked to the heavens – it is, I think, no coincidence that science fiction literature begins at this point in history – and we’re exploring other worlds, the final frontier. We know they’re completely different from ours, but somewhere embedded in our psyche is the assumption that we’ll find what we’ve always found when setting out into unexplored territory – life, opportunity, and exotic people. The idea that other planets are a different kind of thing, possibly barren lumps of rock too far away to be of any use, is something we sometimes find it hard to accept.