An ancient riverbed has been discovered on Mars , indicating that water once flowed on the red planet.
Based on images taken by the Mars Express spacecraft, experts from the European Space Agency (ESA) have found a river system that stretches out for nearly 435 miles.
The river system lies just south of Mars’ equator and was shaped by both flowing water and rock impacts.
ESA has named the ancient valley system ‘Nirgal Vallis’, and predicts that it is between 3.5 and 4 billion years old.
ESA explained: “Nirgal Vallis is a typical example of a feature known as an amphitheatre-headed valley.
“As the name suggests, rather than ending bluntly or sharply, the ends of these tributaries have the characteristic semi-circular, rounded shape of an Ancient Greek amphitheatre.
“Such valleys also typically have steep walls, smooth floors and if sliced through at a cross-section, adopt a ‘U’ shape.”
The images indicate that the valleys are about 200 metres deep and 1.2 miles wide.
Similar valleys are often seen on Earth, including those in the Atacama Desert and on the islands of Hawaii, and ESA believes that Nirgal Vallis was formed in a similar way.
It explained: “As there appear to be no branching, tree-like tributaries feeding into the main valley of Nirgal Vallis, it is likely that water was replenished on ancient Mars by a mix of precipitation and overland flow from the surrounding terrain.”
ESA also believes that the system may have roots in a process known as groundwater sapping, which occurs when water seeps laterally through material in layers beneath the surface.
It added: “We see this kind of mechanism on Earth in environments where surface material is very fine and loose and thus difficult for water to penetrate – largely silty, sandy, unconsolidated, and fine-grained environments, where lower layers of the surface are permeable and friendlier to water than those above.”
