This image reveals the dark sediments and worn path of what was once a river delta, connecting a river with its lake terminus. Though this river bed has been dry for eons, it's proof that rivers once ran on Mars.
Proof of rivers and lakes like this are not unheard of on Mars, but this finding by the ESA's Mars Express orbiter is still a rare and impressive find. It's located in the planet's Southern Highlands, specifically in the Eberswalde crater. The river delta is quite small by Earth standards - it's got nothing on the famous Nile, Amazon, and Mississippi deltas, but then, few rivers do - but its location is rather more important than its size. It's some of the clearest proof that liquid water did indeed flow on the surface of Mars in not insignificant quantities, albeit eons ago.
ESA astronomers offer some more detail on this ancient river:
Within the visible part of Eberswalde, the delta and its feeder channels are well preserved , as seen near the top right of the crater. The delta covers an area of 115 square kilometres. Small, meandering feeder channels are visible towards the top of the crater, which would have filled it to form a lake.
After the deposition of the delta sediments in the crater's ancient lake, fresher sediments accumulated to cover up a major part of both the channels and the delta. These secondary sediments, presumably deposited by the wind, were later eroded in the delta area, exposing an inverted relief of the delta structure.
The Eberswalde crater - along with the Holden crater, which it partially overlaps - is one of the most anticipated areas of Martian exploration, and indeed it was on the shortlist of landing sites for NASA's upcoming Mars Science Laboratory mission. Beyond its obvious importance as a probable site of past liquid water, it also appears to have some of the most diverse mineral deposits in all of Mars. However, it was ultimately decided to send the rover to Gale crater, which has even more intriguing features along the same lines. So this ancient river will have to remain as is, at least for a little while longer.