Ancient delta is newest evidence for Martian ocean

Topographic map from Mars Global Surveyor showing part of the lowlands region in the northern hemisphere (blue) which is thought to have once been an ocean. Credit: NASA / MOLA
Topographic map from Mars Global Surveyor showing part of the lowlands region in the northern hemisphere (blue) which is thought to have once been an ocean. Credit: NASA / MOLA

Whether or not Mars once had an ocean has been a subject of much debate for many years. There is substantial evidence pointing toward the possibility, but no “smoking gun” yet. Now, a new discovery from scientists at the California Institute of Technology (Caltech) is fueling that debate again – an ancient delta that appears to have emptied into the hypothetical ocean in the northern hemisphere.

Based on topographical studies, it is thought that the former ocean occupied most of the northern hemisphere and about a third of the planet overall. There is a clear distinction between the relatively smooth lowlands in the northern hemisphere, where the ocean is thought to have been, and the rugged, cratered highlands in the southern hemisphere.

“Scientists have long hypothesized that the northern lowlands of Mars are a dried-up ocean bottom, but no one yet has found the smoking gun,” said Mike Lamb, an assistant professor of geology at Caltech and a coauthor of the new paper.

The ancient delta in the Aeolis Dorsa region, discovered by the Mars Reconnaissance Orbiter, is right on the boundary between the lowlands and highlands, which would have been the coastline of the ocean. By studying its topography, scientists were able to determine that the water spread outward along the channels in the delta, suggesting that the delta emptied into a larger body of water as is typical on Earth.

Other deltas found previously on Mars emptied into smaller lakes, usually defined by a geological boundary. But in the case of the Aeolis Dorsa delta, there is no such boundary, suggesting that it did indeed empty into a much larger body of water, ie. the northern ocean. The scientists also found that there is a sharp increase in the slope of the sedimentary beds near the downstream end of the channels, a common feature where a stream or river empties into a larger body of water. According to Roman DiBiase, a postdoctoral scholar at Caltech and lead author of the paper, “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.”

It may not be quite the smoking gun yet, but it is an exciting discovery which supports the theory that there was indeed once an ocean on Mars. Combined with other ever-growing evidence for past rain, rivers and lakes, it is becoming clear that the Red Planet also used to be a very wet world.

Paul Scott Anderson is a freelance space writer with a life-long passion for space exploration and astronomy. He currently writes for AmericaSpace, Universe Today and His own blog The Meridiani Journal is a chronicle of planetary exploration.

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