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NASA’s Mars Reconnaissance Orbiter found traces of water that comes and goes on Mars—aka flowing water
For The Daily Beast:
We seem to discover water on Mars about once a year. Well, that’s not quite true: we’ve known Mars has water for quite a while. However, there are a lot of mysteries still to solve about how that water behaves and where it’s located. In particular, we’d like to know if water sometimes flows on the surface of the planet, which would tell us a lot about the cycles both above and below ground. And of course water is essential for life as we know it—finding flowing water, even transient flows, would make Mars seem a little more Earth-like.
The problem is that any liquid water evaporates quickly in the bone-dry Martian desert, and other processes can leave traces that mimic dried-up flows. When so little water is involved in the first place, it leaves us looking for the Martian equivalent of water spots on a long-dry drinking glass. And those spots are chemical traces—salt and other minerals once dissolved in the water—which must be identified by robotic spacecraft from orbit.