Dark delta sediments deposited in a crater on Mars are evidence that it was once a giant lake during the red planet's watery past as seen in these images from the European Space Agency's (ESA) Mars Express satellite.
Located in Mars' southern highlands, the delta was spotted in the Eberswalde crater, which is 65 kilometers (40 miles) in diameter with a semi-circular shape. It is believed to have formed when an asteroid struck Mars over 3.7 billion years ago.
The delta spans a region of 115 square kilometers (44 square miles) with feeder channels near the top of the crater, which would have filled the lake.
ESA's Mars orbiter has been assisting NASA to decide on the best landing site for the next-generation Mars rover, Curiosity, which will launch late this year to arrive in mid-2012.
Eberswalde crater was one of four shortlisted destinations, along with Holden crater, as both have features suggestive of past liquid water and a habitable environment.
However, Gale crater was selected as the Mars Science Laboratory mission's landing site due to its high mineral and structural diversity associated with water.
Mars Science Laboratory is part of NASA's Mars Exploration Program, a long-term robotic exploration effort. The rover will assess the red planet's habitability, ie whether it was or is an environment able to support microbial life.