Discovery of ancient riverbeds suggests Mars once wetter than thought

The Guardian

09-07-2025

Thousands of miles of ancient riverbeds have been discovered in the heavily cratered southern highlands of Mars, suggesting the red planet was once a far wetter world than scientists thought.
Researchers spotted geological traces of nearly 10,000 miles (16,000km) of ancient watercourses, believed to be more than 3bn years old, in high resolution images of the rugged landscape captured by Mars orbiters.
While some of the riverbeds are relatively short, others form networks that stretch for more than 100 miles. The widespread rivers were probably replenished by regular rain or snowfall in the region, researchers said.
'Water has been found on Mars countless times before, but what's really interesting here is that this is an area where for a long time we've thought there wasn't any evidence for water,' said Adam Losekoot, a PhD student at the Open University. 'What we found is that the area did have water and it was very distributed,' he added. 'The only water source that could have sustained these rivers over such a vast area would have to be some kind of regional precipitation.'
The most dramatic signs of ancient water on Mars are the huge valley networks and canyons, thought to have been carved by water flowing across the terrain. But some areas of the planet have few valleys, leading scientists to question how wet the regions once were.
One region that particularly puzzled researchers was Noachis Terra, or Land of Noah, one of the oldest landscapes on Mars. According to computer models of the ancient Martian climate, the region should have had substantial rain or snowfall, sculpting the terrain as the water flowed.
Faced with a lack of evidence for ancient riverbeds, Losekoot and his colleagues turned to high-resolution images of Noachis Terra captured by instruments onboard Nasa's Mars Reconnaissance Orbiter (MRO) and Mars Global Surveyor. The images covered nearly 4m square miles of the planet's southern highlands, a land area much larger than Australia.
The images revealed scores of geological features called fluvial sinuous ridges, also known as inverted channels. These form when tracks of sediment carried by ancient rivers harden over time, and are later exposed when the softer ground around them erodes. While some tracks are relatively narrow, others are more than a mile wide.
'We have lots of little ridge segments, and they are usually a couple of hundred metres wide and about 3.5km long, but there are some that are much, much larger than that,' Losekoot said.
In one image from the MRO the pattern of fluvial sinuous ridges reveals a network of meandering tributaries and spots where the ancient riverbanks burst. Two rivers can be seen crossing into a crater, where water probably flowed in and filled it up before breaching the other side.
The findings, to be presented on Thursday at the Royal Astronomical Society's national meeting in Durham, suggest an enduring presence of surface water in the Noachis Terra region of Mars about 3.7bn years ago.
In its warmer, wetter past, the planet held vast bodies of water. Mars became the arid world we know today when its magnetic field waned, allowing the solar wind to erode its atmosphere and the water to escape into space. But some water may remain, unseen. Beyond Mars's polar ice caps, an international team reported in April, a vast reservoir of water could lie hidden deep beneath the Martian surface.