Study Suggests It Rained on Ancient Mars

Scientists studying the Martian landscape said yes, a river ran through it — and not just one. The ancient red planet also seems to have experienced rain, they say.

The rivers may have cut the deep valleys in the Martian highlands near the equator, and also left calling cards elsewhere. Three Mars spacecraft spotted signs of fan-shaped river deltas inside ancient craters which some valleys clearly flow into.

"We can see layered sediments where these valleys open into impact craters," said Ernst Hauber, a geologist at the DLR (German space agency) Institute of Planetary Research in Berlin-Adlershof. "The shape of certain sediments is typical for deltas formed in standing water."

Rivers carry sediment downstream until the currents become too weak and let the material fall to the river bottom. The flow almost drops to zero at places where rivers empty into a larger body of water, such as a lake-filled crater.

Hauber and other researchers focused on possible ancient river valleys crisscrossing the Xanthe Terra highland region. They examined crater images taken by the European Mars Express, NASA's Mars Global Surveyor, and NASA's Mars Reconnaissance Orbiter.

One small 3-mile (5-km) crater contains possible proof in the form of a fan-shaped delta, where the Nanedi river flows into the crater from the south. Sediment has almost completely filled the crater up to 164 feet (50 meters) deep in an area covering slightly less than 9 square miles (23 sq km).

Researchers also counted the number of craters to roughly determine the age of the planetary surface in the area. Their crater count revealed that water flowed through the valleys sometime between 3.8 and 4 billion years ago. Additional work by Maarten Kleinhans, a geologist at the University of Utrecht in the Netherlands, showed that the crater sediment deposits formed in no less than a few hundred thousand years.

The valleys flowing into and out of the craters allow researchers to be "fairly certain that there were lakes on Mars," Hauber noted.

An ongoing debate is whether rain and snowmelt or groundwater may have played a bigger role in creating the valleys. The most recent findings presented at the European Planetary Science Congress in Muenster, Germany provide more evidence for the former.

"Our findings also point in this direction and we are convinced that both processes have played an important role in Xanthe Terra," Hauber said.

Other research has suggested that water flowed in the plains around the Valles Marineris region until even more recently, around 3.7 billion to 3 billion years ago. But no evidence so far has shown flowing surface water still exists on Mars.