Mars may have once had a vast ocean that covered as much as a third of the planet.
Researchers at the California Institute of Technology
have found evidence of an ancient delta where a Martian river may have once emptied into the ocean.
"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," says Mike Lamb, an assistant professor of geology at Caltech and a coauthor of the paper describing the results. The paper was published online in the July 12 issue of the Journal of Geophysical Research
Scientists say the evidence is not extensive enough to prove the ocean existed, but note that most of Mars’ northern hemisphere is flat and at a lower elevation than the southern hemisphere. It is similar to ocean basins found on Earth.
Using images from the Mars Reconnaissance Orbiter (MRO), Caltech scientists studied a 100-square-kilometer area on what might have been the former coastline. This area is located in a larger zone known as Aeolis Dorsa, which is covered in ridge-like features called inverted channels.
On Earth, inverted channels are the remnants of rivers or streams. After the river dries, silt and other finer materials erode away, leaving the larger rocks behind.
In the images analyzed by Caltech scientists, the inverted channels on Mars appear to fan out, which the scientists say could have been caused by three things: The channels could have been a drainage system down a mountain where they converged and created a larger river; it could have been caused by an alluvial water drainage fan; or the channels could be part of a river delta.
To determine which scenario they were looking at on Mars, scientists took pictures of the area from different points in the MRO’s orbit, creating stereo images. The camera on the MRO can distinguish changes in elevation at a resolution of one meter.
They determined that the water that once flowed through the channels was spreading out rather than converging, meaning they were part of an alluvial fan or delta. Researchers also noticed an “an abrupt increase in slope of the sedimentary beds near the downstream end of the channels,” which is commonly seen when a stream empties into a large body of water.
Deltas have been observed on Mars before, but most are inside a geological boundary like a crater. The newly discovered delta showed no confining boundary.
"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, a postdoctoral scholar at Caltech and lead author of the paper.
But scientists are quick to point out that there could be other explanations. For example, there could have been a confining boundary that later disappeared due to geological activity, but that would lead to the conclusion that the Martian surface was more geologically active than previously thought.
Further analysis of the hypothetical coastline will be needed before determining if there was a large 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 said. "Both the ancient environments on Mars and the planet's sedimentary archive of these environments are turning out to be surprisingly Earth-like."