Valleys in Melas Chasma. Caption throwback (2008): The interpretation is that the valleys formed by precipitation in the late Hesperian epoch and the water that formed them collected in the basin to produce a lake where sediment was deposited. This location is therefore particularly interesting to scientists because it shows water activity that resulted from rain in a relatively young region near the equator of Mars.
HiRISE Picture of the Day archive. [More at links]