Today's Mars Photo was taken by the European Science Agency (ESA)'s Mars Express. This photo of Mars is evidence that a massive lake once existed within Eberswalde Crater, which measures 65 km in diameter.
The delta is thought to have formed 3.7 billion years ago during the Hesperian Epoch, when an asteroid struck the Martian surface. Prior to this discovery it was thought that Mars was a barren, frozen planet, during the Hesperian Epoch, but the existence of this lake proves that there was liquid water present on the planet.
To see the enlarged photo click on the image. See a related, more in-depth article at The Daily Galaxy