A single ocean once covered much of the northern half of Mars, supplied with water from a belt of rain-fed rivers, new research suggests.
Scientists have produced a map showing that Martian valley networks are more than twice as extensive as had previously been thought, indicating that they were carved by rivers.
They are concentrated in a belt circling the planet's equator and mid-southern latitudes. Experts believe they mark the paths of rivers that once flowed from the planet's southern highlands into a huge ocean.
The evidence suggests that billions of years ago much of Mars had an "arid continental climate" similar to drier areas of the Earth. Rain would have fallen regularly, swelling the rivers and topping up the ocean basin. Such a wet period early in the planet's history would have greatly increased the chances of life.
Until now the only global map of Martian valley networks was drawn by hand from satellite images in the 1990s.
The new map, created by a computerised analysis of satellite data, shows that some regions of Mars had valley networks almost as dense as those on Earth, says a report in the Journal of Geophysical Research – Planets.
"All the evidence gathered by analysing the valley network on the new map points to a particular climate scenario on early Mars," said Professor Wei Luo, of Northern Illinois University in the US, who led the research.