Mars May Have Had a Planet-Wide System of Underground Lakes

Today, Mars is a dry, dusty planet with a small smattering of frozen water at its poles. But the Red Planet’s geology indicates that sometime in the past, the place was a watery world; wet enough, in fact, to potentially host life. Now, a new study of images from two Martian probes has found the first geological evidence that the subsurface of Mars was once pocked with underground lakes, and it’s even possible that the entire planet had an interlinked groundwater system.

In previous studies, models indicated that Mars likely had a planetary groundwater system or regional aquifers at one time, reports Yasmin Tayag at Inverse, but this is the first study to offer evidence. Looking at 24 images of deep, enclosed craters in the planet’s northern hemisphere captured by the European Space Agency's Mars Express probe and NASA’s Mars Reconnaissance Orbiter, the team found features on the crater floors that they believe could only be carved by rising and falling groundwater, including deltas, channels etched into crater walls, terraces caused by standing water, and valleys made by water funneling out of the craters.

Because those features tend to occur at roughly the same depth within the crater, 13,000 to 16,000 feet below the arbitrary Martian “sea level,” the researchers believe these craters were once interconnected. It’s also possible that the groundwater was connected to a Martian that which some researchers believe existed on the planet 3 to 4 billion years ago. These findings appear in the Journal of Geophysical Research: Planets.

“Early Mars was a watery world, but as the planet’s climate changed this water retreated below the surface to form pools and ‘groundwater’,” lead author Francesco Salese of Utrecht University says in a press release. “We traced this water in our study, as its scale and role is a matter of debate, and we found the first geological evidence of a planet-wide groundwater system on Mars.”

The massive subsurface water system on Mars is something very different from what has been discovered on Earth. That’s because, Salese tells Tayag at Inverse, Mars is a “one-plate planet,” meaning its crust is composed of one solid piece, unlike Earth which has seven major tectonic plates and many minor plates. That means the groundwater system on Mars could be interconnected without being cut off from other regions by plate boundaries.

The other major finding from the study is the presence of certain clays, carbonates and silicates in five of the craters studied, minerals that are linked to the emergence of life on Earth. These basins were deep enough that they would have been soaked in Martian groundwater for a long time—long enough to host the emergence of life or to allow life to persist, even as the rest of the planet dried out. That puts them at the top of the list of sites to search for evidence that life once existed on the fourth planet from the sun.

“Findings like this are hugely important; they help us to identify the regions of Mars that are the most promising for finding signs of past life,” says Dmitri Titov, ESA's Mars Express project scientist, says in the release.

Not everyone views the images from the crater floors in the same light, however. Jack Mustard, a geological scientist at Brown University not involved in the study, tells Frank Jordans at the Associated Press that he doesn’t see the same evidence of underground water that the authors see in the data.

Even if these images don’t ultimately prove that aquifers once underlaid the Martian surface, there is a lot of evidence showing that lakes and rivers of liquid water once flowed through the Martian landscape and a significant amount of water may still exist on the planet. Last year, the Mars Reconnaissance Orbiter discovered eight sites where huge deposits of underground ice are likely present. In 2011, researchers hypothesized that salty liquid water was making dark lines on the edges of craters called recurring slope linea, though another recent study raises the possibility the streaks could just be flowing sand. If there are puddles of salty liquid water under the surface of Mars, though, another study from last year calculated that there should be enough dissolved oxygen to support organisms as complex as sponges.