While analyzing an area inside Aeolis Dorsa, however, the researchers noticed a collection of inverted channels. On Mars, these raised ridges were likely caused by the deposition of rocks and other material by the flow of water down river channels. Long after the rivers ran dry, the rocky deposits remained behind. The softer sediment surrounding the river channels eventually weathered away, blasted by the Martian wind and other erosion processes. However, the deposited rocks, once found at the bottom of the rivers, were immune to this weathering, causing them to rise out of the landscape as ridges, tracing the paths of ancient riverbeds.
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.
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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.
"This is probably one of the most convincing pieces of evidence of a delta in an unconfined region - and a delta points to the existence of a large body of water in the northern hemisphere of Mars," said Roman DiBiase, Caltech postdoctoral scholar and lead author of the paper.
Although more work is needed, the researchers estimate that the "ocean" would have, at least, covered Aeolis Dorsa, all 100,000 square kilometers (39,000 square miles) of it. To compare with a terrestrial mass of water, this is larger than Lake Superior (82,414 square kilometers or 31,820 square miles).
In the year since Curiosity landed inside Gale Crater, the mission has uncovered incredible insights to the water history of the Red Planet inside a comparatively small impact crater. Imagine what a long-duration rover mission to a river delta would reveal. I for one would love to see a robotic mission clawing through the exposed ridges of ancient riverbed, hunting down evidence for the past habitability of an ancient Mars coastline.
Image credit: DiBiase et al./Journal of Geophysical Research/NASA/JPL-Caltech/USGS/NASA Landsat