We've known for a while now that there's water ice on Mars, and we've seen lots of tantalizing suggestions that liquid water may have once existed on the surface in huge rivers and oceans. Today, NASA announced that for the first time, they may have actually spotted some flowing, liquid water, and here are the pics to prove it.
The picture above shows Newton Crater on Mars. If you look about halfway down the crater walls, you'll see a bunch of dark streaks extending outward. These streaks appear early in the Martian spring, extend down the walls of canyons and steep valleys through the summer, and then recede in the fall and winder. Each streak is up to five meters wide, they can extend for hundreds of meters, and they only show up on slopes that face towards the equator.
So far, the best explanation for all this involves flowing, liquid water. It's too cold on Mars for fresh water to remain liquid, but if the water was salty (about as salty as ocean water here on Earth), it would lower the freezing point enough that during the Martian summer, a briny liquid could flow at or very near the surface of sun-warmed slopes. You'd get the highest flow rates during the summer, and then the water would refreeze during the winter, which seems to be what the imagery shows.
As exciting at this discovery is, there are still lots of questions that need to be answered. Where this water actually comes from is an important one that could influence future target sites for future unmanned or manned Martian missions. And of course, there's the potential for life: here on Earth, briny water makes a great habitat for microbes. Ultimately, these new images may spawn a whole new series of riddles, but that's just part of the charm of Mars: far from being a dead and dusty planet, it's turning out to be a dynamic world with an evolving surface that may provide insights to the processes that have shaped our own Earth.