Mars' valleys formed by precipitation

Valley network branching angles suggest that Mars' ancient hydrology was not dominated by groundwater flow.

The controversy about the origin of Mars' channel networks is almost as old as their discovery 150 years ago. Over the last few decades, new Mars probes have revealed more detailed structures in Martian drainage networks, and new studies suggest that Mars once had large volumes of water. But how this water flowed, and how it could have carved these valley networks, remains unclear.

Our new analysis of Mars' valley networks suggests that the channel networks on Mars tend to have branching angles that resemble those created by surficial flows on Earth. This result is based on a recent observation which has shown that climate leaves a characteristic imprint in the branching geometry of stream networks. Networks growing in humid regions have an average branching angle of 72°, which is characteristic of network growth by groundwater sapping. Networks in arid regions, where overland flow erosion is more dominant, show much smaller branching angles.

The narrow branching angles of Mars' valleys support the inference that Mars once had an active hydrologic cycle and that its valley networks were formed primarily by surface runoff erosion with groundwater seepage playing only a minor role.