Polished, smooth rocks found by NASA's Curiosity Rover suggest the planet once had a swift moving streambed, with depths of up to 3 feet and water that moved at speeds of up to 1.6 miles per hour.
The pebbles were originally discovered by the rover in 2012, but new analysis of the data has given scientists more clues about the stream.
"We know it was a streambed because it takes a fast flow to move pebbles of this size, and they're rounded," says Dawn Sumner, a researcher at the University of California, Davis, one of the authors of the study. "The rounding requires that they're banged against each other and the sand a huge number of times to break the edges of the rocks. It's like how you polish rocks in a polisher, you hit them against each other over and over."
The pebbles are believed to be at least two billion years old. Researchers are unsure how long the stream was, but Sumner says in order for rounding to take place, the stream must have been "flowing for a long period of time over a long distance."
"You aren't going to get rounding with transient water or a flash flood," she says.
The team found hundreds of pebbles that suggested movement by water. Other discoveries have found what is known as a "mudstone," which likely once sat in standing water.
"The combination of finding some rocks that suggest flowing and some that suggest quiet water means that there were a lot of different environments around at one point," Sumner says.
Orbiting satellites have captured other evidence of ancient rivers and streams. In January, the European Space Agency found a "striking" river with "numerous tributaries that ran for nearly 1,000 miles and may have been as deep as 1,000 feet at some points. Sumner says discoveries such as those are important, but the Curiosity Rover is able to tell scientists things from the ground that photos from satellites cannot.
"One thing about finding an ancient riverbed is that it's hard to estimate how fast and how long the flow lasted," she says. "By finding these pebbles, we can't say how big the stream was, but we can estimate other things. They're all pieces of the puzzle of understanding the history of Mars."