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.

Terraforming Mars Just Got a Lot Harder

The act of terraforming a planet, even one as comparatively hospitable as Mars, would never be an easy task. But whenever discussing mankind's future on the Red Planet, it seemed to be the inevitable conclusion. We would colonize Mars, and then change it in Earth's image. It would be a daunting task of engineering, and the ethical dilemma of uprooting any potential native life was not going to go away either, but it almost seemed a foregone conclusion that we would eventually terraform Mars.

Mars is probably going to stay red in the future.

According to data from the MAVEN space probe currently orbiting Mars and studying the Martian atmosphere, however, there may be a big problem with future terraforming endeavors. The infant Mars had a thick atmosphere, but as time went on, that atmosphere was lost. It was assumed that most of the carbon dioxide that originally made up the atmosphere had frozen out into the soil, but that is not the case. Once Mars lost its magnetic field, the solar wind, more active when the Sun was young, pummeled the planet, and Mars, already much smaller and with much weaker gravity than Earth, had no way to hold onto its atmosphere. It took maybe around 500 million years, and by around 3.7 billion years ago, the Mars we know today had taken shape. 

Why is this bad for terraforming? Well, if the carbon dioxide had settled into the Martian soil, warming the planet back up again would release it, building up the atmosphere and setting up a greenhouse effect (a good thing in this case), which would heat the planet up even more, releasing even more carbon dioxide and so on. But the carbon dioxide from the original Martian atmosphere is just gone, lost forever to the cosmos. There is still some on the surface, frozen in the ice caps, but likely not enough to work with. This news does not necessarily mean Mars cannot be terraformed, but in order to do so, it's going to take more than just warming the place up.