The Red Planet is notoriously dry and dusty, but its scarred surface shows that that wasn't always the case. A new set of photos from the European Space Agency's Mars Express orbiter demonstrates some pretty clear evidence of an ancient river network that once wound across the Martian landscape.

While modern day Mars is drier than any Earthly desert, it's believed that roughly 4 billion years ago the Red Planet was much bluer, with a northern ocean bigger than the Atlantic. Over the years, the many eyes on Mars have spotted signs of ancient shorelines, lakes, flood plains, rivers and glaciers. That makes the newest European Space Agency (ESA) shots not particularly surprising, but no less beautiful.

These images were snapped on the southern highlands of Mars, a region pockmarked with craters and rich with evidence of old waterways. In this case, that takes the form of a branching network of valleys, snaking across the landscape to form shapes that are instantly recognizable as the handiwork of water.

Judging by the topography, water seems to have flowed downhill from north to south, which is right to left in these shots. The valleys left behind are up to 2 km wide and as deep as 200 m in places. That's particularly clear in the topographic view, where red is the highest ground, and it turns yellow, then green and blue the lower you go.

The structures resemble drainage systems seen here on Earth, suggesting they were formed as excess water ran off from stronger rivers and made its own way downhill.

While the bulk of that water has likely been lost to space, there are indications that some of it is still locked away underground, in the form of ice sheets or even liquid lakes. These stashes could be vital resources for eventual human colonists.

Source: ESA

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