Looking down from its orbit, the MRO has been able to image these veined, raised structures and found what appears to be an ancient river delta leading into the depressed portion of Aeolis Dorsa.
On Earth, river deltas form at the mouths of rivers connecting to seas and oceans. The Nile Delta is a classic example, where the water from the river flows into the Mediterranean Sea. Sediment has built up at the Nile's mouth to create fanned, multichannel features, leading to a sudden drop-off - i.e. the sea.
However, understanding which way the water would have been flowing in an ancient Mars "river delta candidate" can be challenging. Could the water actually have been flowing the other way? Perhaps this isn't a delta at all; it could be reversed - several streams and small river tributaries flowing into a larger channel.
PHOTOS: Mars Through Curiosity's Powerful MAHLI Camera
Because of HiRISE's precision, high-resolution 3D images could be created to map which direction the landscape is sloping, revealing the direction of water flow. Indeed, the water would have been flowing downhill, creating a fan-like delta and the drop-off at the end of this Mars delta could be the beginning of an ocean basin. Therefore this could be the strongest evidence yet of a Martian coastline.