Recent evidence suggests that there is still some liquid water present on the planet Mars, but billions of years ago there was likely tons of it. According to a recent study from the University of Pennsylvania, a team of researchers has observed the shapes of stones photographed by the Curiosity rover in 2012 and made an estimate of how far they once traveled at the bottom of a flowing river.

The study, published in the journal Nature Communications, describes the first method for quantitatively guessing the distance traveled by river pebbles solely based on their shapes. In some cases, researchers estimated that pebbles on the riverbed they examined traveled nearly 30 miles before the water dried up.

Studying rivers on Mars is useful for a number of reasons. First, it can help biologists pin down when and where the conditions on the red Planet allowed for the highest chances of extraterrestrial life to exist. It also can help geologists determine where there may be valuable resources on Mars, like gold.

As stones on a riverbed are pushed along by the rushing current, they are smashed into larger rocks and chipped away at until they have smooth, round sides. The researchers teased out the geometric patterns for how rocks of a similar size affect each other’s shape when they collide.

They tested rolling limestone fragments in a barrel and recorded the changes to mass and shape. The changes in shape followed a pattern predicted in the early calculations of the experiment.

The team then brought their experiment to the field to a mountain river in Puerto Rico. They observed chunks of jagged rock falling from a cliff into the water, and then traveled downstream collecting samples and comparing their shapes every few hundred meters.

Their data and models allowed them to gauge the distance traveled by pebbles in Martian riverbeds by comparing the shapes of rocks found at different points.