Network of drainage valleys in the Arda Valles region of Mars

Dry today, the Arda Valles region of Mars once boasted rivers that might have been filled by rain. Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

Planetary science

Mars’s river valleys whisper of a rainy past

Waterways branch in patterns similar to those of the US Southwest.

Fast-flowing waterways on ancient Mars carved river valleys much like those on modern Earth.

Although Mars is cold and dry today, channels on its surface look as if running water shaped them, leading researchers to think the planet was warm and wet in the past. But scientists have struggled to determine whether that water fell from the sky as rain or seeped upward from the ground.

To discern the water’s source, Hansjoerg Seybold at the Swiss Federal Institute of Technology (ETH) Zurich and his colleagues analysed the geometry of Martian valley channels. The channels branch off at relatively narrow angles, as do waterways in arid landscapes on Earth, such as the US Southwest. More-humid landscapes with a lot of groundwater — the Amazon rainforest, for example — host river channels that branch at wider angles.

The discovery bolsters the idea that the Martian channels were carved by surface runoff rather than by water percolating from below.