If you've paid any attention to the news in the past couple of weeks, you've heard that scientists made a compelling case for the existence of briny water on Mars. But the liquid water that's (probably) present on Mars today only shows up periodically, wetting surrounding dirt like a sponge. In a study published Thursday in Science, researchers present evidence that the planet once held long-lasting rivers and lakes, too.

According to evidence from NASA's Curiosity rover, Mars may once have been much warmer than it is today, with surface conditions that would have allowed rivers and lakes to form and remain -- potentially long enough for life to evolve.

The study focuses on Mars's Gale Crater, an ancient depression that the rover has spent lots of time observing. According to the authors of the study, the layers of sediment found in the crater could only be formed by the presence of water -- and plenty of it. While individual lakes may have come and gone after a few hundred or thousand years, there could have been bodies of liquid water present for millions of years in total.

These rivers would have deposited sediment that slowly built up the base of Mount Sharp, a peak that now stands some three miles high. After the water was gone, wind likely continued to deposit sediment to form the brunt of the peak.

"During the traverse of Gale, we have noticed patterns in the geology where we saw evidence of ancient fast-moving streams with coarser gravel as well as places where streams appear to have emptied out into bodies of standing water," study author Ashwin Vasavada of the Mars Science Laboratory team said in a statement. "The prediction was that we should start seeing water-deposited, fine-grained rocks closer to Mount Sharp. Now that we've arrived, we're seeing finely laminated mudstones in abundance."

The effect is exactly like what we'd see in an ancient Earth lake, the researchers say. Newsweek reports that there are signs the water was fairly neutral as well -- not too acidic or basic for life like ours to form. It's not proof that there were ever microbes on Mars, but it's an indication that something Earth-like could have once managed to survive there.

Current atmospheric models of Mars's former climate suggest that it would have never been quite warm enough for this scenario. But the researchers believe that the geological evidence for a Mars that was once warm and wet -- one with a much heavier atmosphere than it has now -- is compelling enough for these assumptions to be revisited.

"We have tended to think of Mars as being simple," John Grotzinger, Caltech's Fletcher Jones Professor of Geology, chair of the division of planetary and geological sciences, and lead author of the paper said in a statement. "We once thought of the earth as being simple, too. But the more you look into it, questions come up because you're beginning to fathom the real complexity of what we see on Mars. This is a good time to go back to reevaluate all our assumptions. Something is missing somewhere."

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