How Recently Did Mars Have Liquid Water?

Science is very hard. You think you know something, you think you've got things all figured out, but then you get some more information, and that previous assumption just goes flying out the window. Case in point: last year, I wrote about how the MAVEN space probe had studied the Martian atmosphere and determined that the pleasant Mars with a substantial atmosphere and abundant liquid water was gone nearly 4 billion years ago, and for most of Mars' history, it's looked pretty much the same as it does now. It was simple, it was elegant, and while it wasn't really good news, it fit the data we had.

You can probably see where this is going. As it turns out, there are valleys and basins on the Martian surface that were formed by liquid water, but formed a billion years after the Martian atmosphere was lost to space, and the planet's surface became too cold to host liquid water. We're not talking about an insignificant amount of water either, there was enough water in one of these lakes to fill Lake Erie and Ontario with water to spare. The valleys running into these basins are not as complex as the older river valleys, indicating a slower flow. This probably indicates the lakes were filled not with rain, but with runoff from fallen snow.

This is obviously a bit of great news. If water could stay liquid on the Martian surface 2 billion years ago, that means that it was potentially habitable for the same amount of time. That is a much longer timeframe then we previously thought. 500 million years is not a huge amount of time for life to evolve, but 2 billion years is a lot more generous. Of course, it also raises into question the findings of MAVEN. Was the atmosphere gone by that point? Where did all the snow come from? Mars just got a lot more interesting.