Researchers have used the shape of rounded Martian pebbles to extrapolate how far they must have traveled down an ancient riverbed on the Red Planet. The analysis suggests they moved approximately 30 miles (50 kilometers), indicating that Mars once had an extensive river system.

The shape of some Martian pebbles suggests these rocks once rolled dozens of miles down a river, hinting that ancient Martian waterways were stable and not merely ephemeral streams, researchers say.

NASA’s Mars rover Curiosity discovered the small, round stones near its landing site in Gale Crater on the Red Planet in 2013. Researchers previously determined that these stones resemble those found in rivers on Earth, which become round as they slide, roll and hop down riverbeds and scrape other rocks.

Now, a new study suggests the Martian rocks rolled in the river for quite a while — a finding that should help scientists reconstruct what ancient Mars was like and shed light on the Red Planet’s past potential to support life, study team members said.