Long-Lasting Rivers, Lakes Once Covered Mars, Researchers Say

Water once existed in long-lasting lakes on Mars. This image, taken by NASA's Curiosity Mars rover, shows a series of sedimentary deposits in the Gale Crater, including mudstone, in the lower left, formed in an ancient lake. NASA

In recent years, scientists have uncovered hints that water once flowed on Mars's surface, and a study published last week suggests there is still extremely briny liquid on the planet.

Now, in a significant step forward, researchers have produced convincing evidence that water once existed on the red planet's surface in long-lived rivers and lakes. The study, published Thursday in the journal Science, suggests that early Mars had a warmer, wetter and even habitable environment, says Alan Howard, a researcher at the University of Virginia who wasn't involved in the study.

The paper, authored by Caltech scientist John Grotzinger and scores of colleagues, put together reams of evidence collected by NASA's Curiosity Rover, including photographs and measurements of sediments in a large depression called the Gale crater. There, the rover found many layers of rock that could have been formed only in a long-lasting lake, Grotzinger says.

Specifically, the researchers uncovered laminated mudstone, which is formed when sediment settles to the bottom of a "quiescent" lake. The findings suggest the rivers and lakes existed there for thousands or even millions of years, according to the study.

This makes it even more likely than previously thought that life may have once had a toehold on Mars.

"You're giving life the best chance it has to survive," says Kevin Bohacs, a sedimentary geologist for Exxon Mobil who wasn't involved in the paper.

Various rocks formed by an ancient lake are seen in the Gale crater on Mars. J.P. Grotzinger et al./Science

"On Earth, wherever we have water, we've got life," says Marjorie Chan, a sedimentologist at the University of Utah who wrote a commentary in Science accompanying the study. "These things are so intertwined that we really can't help but wonder if we're going to find evidence of life on Mars. Because there is so much evidence of different kinds of water, and it being there for so long."

Besides being around for a long time, the water appears to be have been relatively neutral in pH—meaning it wasn't particularly acidic or basic and thus conducive to life. The scientists found evidence of carbonate, which cannot form in acidic environments that, at least on Earth, are less friendly to microorganisms, Howard says.

The research points to just this one body of water, but it almost certainly wasn't alone. "You aren't going to have just one lake in the middle of the desert," Bohacs says. The study suggests that the climate was much wetter and warmer, and probably produced precipitation, and that the lakes existed at least seasonally for many years. Bohacs says there's no evidence of freezing or glacial activity, which leaves marks in the geological record.

This illustration depicts the possible extent of an ancient lake inside the Gale crater. The existence of a lake there billions of years ago was confirmed from examination of mudstone in the crater's Yellowknife Bay area. For this illustration, the possible extent was estimated by mapping the ancient lake and stream deposits and recognizing that water flowed from the crater rim into the basin (arrows). The water would have pooled in the linear depression created below the crater rim. NASA/JPL-Caltech/MSSS

This conflicts with existing models that re-create that past climate of Mars. These generally conclude that the planet would've been too cold to support long-lasting lakes even more than 3 billion years ago, when this lake system existed in the Gale crater, Grotzinger says. The study will force other scientists to rethink these models, according to Chan.

"It seems like something is missing here," Grotzinger says.

Of course, just because conditions were much nicer those many years ago doesn't mean there was life. "We are very unsure of how easy or hard it is for life to develop, even if all the correct elements, compounds and environment are present," Howard notes.