Researchers from Massachusetts Institute of Technology (MIT) may have solved the long-held mystery behind why the surface of most asteroids appear redder than meteorites, asteroidal remnants that fall to Earth. (Photo : NASA)

A possible delta where a mighty river may have once flowed into a vast ocean on Mars' surface has been discovered by researchers at the California Institute of Technology (Caltech).

This ocean, if it existed, could have covered much of Mars' northern hemisphere, stretching over as much as a third of the planet, according to the study published in the Journal of Geophysical Research. At the very least, the researchers say, the water would have covered the entire Aerolis Dorsa region, which spans about 100,000 square kilometers (60,000 square miles).

"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," Mike Lamb, an assistant professor of geology at Caltech and a coauthor of the paper, said in a press release.

While the new findings may not be "proof" per se, they provide some of the strongest support yet of one, according Roman DiBiase, a postdoctoral scholar at Caltech and lead author of the paper.

In order to come to their conclusion regarding the delta, the Caltech team used new high-resolution images from the Mars Reconnaissance Orbiter (MRO) to study a 100-square-kilometer area located on the possible former coastline.

Previous satellite images have shown that this area is covered in ridge-like features called inverted channels that form when coarse material, like gravel and cobbles, are carried along a river and deposited at the bottom.

After the river dries up, the finer materials erode away, leaving behind the coarser remains. This sediment then appears as today's ridge-like features, tracing the former river system.

When looked at from above, the inverted channels appear to fan out, a configuration that suggests a number of origins.

First, the channels could have once been a drainage system in which streams and creeks flowed down a mountain and converged to form a larger river.

Second, the water could have flowed in the opposite direction, creating an alluvial fan, in which a single river channel branches into multiple smaller streams and creeks.

Finally, the channels could be part of a delta, which is similar to an alluvial fan except that the smaller streams and creeks empty into a larger body of water.

To figure out which of these scenarios was most plausible, the researchers turned to satellite images taken by the HiRISE camera on the MRO.

By taking pictures from different points in its orbit, the spacecraft was able to make stereo images that allowed the scientists to determine the topography of the Martian surface.

Using this information, the Caltech researchers analyzed the stratigraphic layers of the inverted channels, piecing together the history of how sediments were deposited along these ancient rivers and streams.

In addition, the team was able to determine the slopes of the channels back when water was still coursing through them, which in turn can reveal the direction of water flow. By doing so, they were able to narrow their options down to an alluvial fan or delta since, in this case, the water was spreading out instead of converging.

Next, the researchers were able to remove alluvial fans from their choices after they found evidence for an abrupt increase in slope of the sedimentary beds near the downstream end of the channels, suggesting that it once emptied into a large body of water.

Thus, the scientists were left with a delta as their most likely answer.

Should this be the case, it would not mark the first time scientists have discovered Martian deltas; however, most are found inside a geological boundary, like a crater. Water therefore would have most likely flowed into a lake enclosed by such a boundary and so did not provide evidence for an ocean.

This new delta, on the other hand, isn't located near any confining boundary, which suggests that the water likely emptied into a large body of water such as an ocean.

"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," DiBiase says.

Other plausible explanations remain, such as a large confining boundary that has since been erased. For this reason, the researchers plan to continue exploring the boundary between the southern highlands and northern lowlands - the hypothetical ocean coastline - and analyze other sedimentary deposits in order to determine if they yield more evidence of an ocean.

"In our work and that of others - including the Curiosity rover - scientists are finding a rich sedimentary record on Mars that is revealing its past environments, which include rain, flowing water, rivers, deltas, and potentially oceans," Lamb says. "Both the ancient environments on Mars and the planet's sedimentary archive of these environments are turning out to be surprisingly Earth-like."