Not far from the spot where Opportunity landed on Mars sits a large basin that looks a lot like the salt flats seen on our own planet. According to a new study out this week in Geology, this location may have been home to one of the final lakes to exist on Mars before the Red Planet turned into the dry desert world we see today.

The chloride salt deposit located in the Meridiani region measures about 46 square kilometers (18 square miles), or the size of a small city. A team led by Brian Hynek from the University of Colorado-Boulder wanted to know how salty the lake would have been and how recently it existed – which could tell us important clues about the habitable history of Mars. So, they combined mineral analyses from Mars orbiters with digital mapping of the terrain in the area. 

The lake, they found, would have been no older than 3.6 billion years, putting it at the tail end of the period when liquid water would have flowed across the Martian surface. (The solar system itself formed about 4.6 billion years ago.) "This was a long-lived lake, and we were able to put a very good time boundary on its maximum age," Hynek says in statement. "We can be pretty certain that this is one of the last instances of a sizeable lake on Mars."

The researchers say that the lake here would have been created when a valley formed in the surrounding highlands and water began to pool. Eventually, the water level rose high enough to spill over the drainage divide. What water remained would have evaporated, leaving behind salts at the bottom of the lakebed. As a result, what we see today looks like the Bonneville Salt Flats in Nevada and other places where former lakes evaporated.

Based on the depth and thickness of the salt left behind, the team estimated that the onetime lake would have been only about 8% as salty as the Earth's oceans. While there's plenty that we don't know (including the acidity of the ancient lake), they say that its low salinity means it could have been habitable at one time. If that's true, then this could have been one of the last potentially habitable, watery places on the surface of Mars.