Editor’s note: On Nov. 26, 2018, NASA successfully landed the InSight rover on Mars’ surface. This mission will study the interior of Mars and give us a better understanding of things like the planet’s seismic activity, heat flow and early geological evolution.

The ancient valley networks of Mars have long captivated astronomers and the public. But as telescopes became more powerful, they brought a new question into sharper focus: Did the water that carved them come from above or below the planet’s surface?

By comparing the patterns of rivers in different climates here on Earth, a new Science Advances study published June 27 by University of Chicago and ETH Zurich researchers adds to a growing body of evidence that river valleys on Mars were created by water flowing across the surface—the result of rain or snowmelt, not by slow leaks from underground. The answer provides clues to what Mars’ climate would have looked like eons ago when the rivers were flowing.

Visible all over Mars are distinctive branching channels made by water flowing through valleys. There are two possible explanations for these networks, based on how water tends to flow on Earth, and each paints a different picture of Mars’ early days.

They might have formed gradually by a slow trickle of groundwater, possibly in a colder climate with only a modest amount of water. “You can see these on Earth, for example, in the Florida Panhandle—the groundwater slowly forms these beautiful networks of channels by moving very fine grains of sediment,” said study coauthor Edwin Kite, an assistant professor in the Department of Geophysical Sciences at the University of Chicago who studies solar system and exoplanet habitability.

The other explanation is more familiar to Earthlings: surface runoff from rainfall or melting snow that erodes chasms into the ground, like flash flooding that carves arroyos in the desert Southwest. “That generally requires more water, and it usually means a warmer climate,” Kite said.

Examining the size of the pebbles carried by the water would be a huge clue to the mystery. “That’s the first thing civil engineers on Earth look at to determine the frequency and force of floods in a region,” Kite said. But since they don’t have many sets of pebbles from Mars and can’t measure pebble size from orbit, scientists needed to get more creative.

Hansjoerg Seybold, an ETH Zurich scientist and the study’s first author, had worked on a team studying the branching patterns of rivers here on Earth, which determined that you could distinguish whether a riverbed was carved by above-ground flows or slow leaks called seeps. A slow seep creates branches that tend to fork off at a 72-degree angle. More violent surface flows have sharper angles.