Curiosity: An Ancient Stream Bed on Mars...

The Curiosity rover made a major discovery this week: a section of ancient rock that is unambiguously an ancient stream bed.  This sort of “alluvial fan” structure is very common on Earth – so common that geologists even know how to estimate the speed and depth of water flow from the stream bed's composition.

What's the significance of finding an ancient stream bed?  Mainly this: liquid water is one of the prerequisites for life-as-we-know-it to exist.  It's the only prerequisite not previously proven to exist on Mars.  Now we know that all the prerequisites have been met.  That doesn't mean we know that life did exist on Mars (or that it still does) – it just means that now we know for sure that it is possible that it did.  And that, of course, is just life-as-we-know-it.  It's entirely possible that some completely different form of life existed (or exists) on Mars.  The challenge there will be to figure out how to look for it.  That has been the subject of many a fine science fiction yarn...