Researchers have discovered what appears to be a dried up delta on Mars, where a rushing river may have met a vast ocean.
The ocean could have been large enough to cover Mars' northern hemisphere, extending over about a third of the planet, a California Institute of Technology (Caltech) press release, reported.
"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.
The area is flatter, and at a lower elevation than the rest of the planet. This is similar to set up of Earthly oceans.
The research team examined high-resolution photographs from the Mars Reconnaissance Orbiter (MRO) to study the ocean-like area.
The nearby Gale Crater, where NASA's Curiosity rover is exploring, is covered in river-like ridges.
The team believes the channels were created by gravel-like material being carried down flowing rivers.
Photographs from above revealed the ridges fan out, the team had three possible explanations for the terrain's features.
Caltech researchers thought: "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; the water could have flowed in the other direction, creating an alluvial fan, in which a single river channel branches into multiple smaller streams and creeks; or the channels are actually 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 such as an ocean."
The researchers examined the channel's stratigraphic layers through the photos to get an idea of how the system might have worked.
By measuring the slope of the channels, the team determined the water was most likely "spreading out instead of converging."
The ridges came to a steep slope towards what could have been the downstream area, suggesting they emptied into a larger body of water like an ocean, meaning the ridges were probably part of a delta as opposed to an alluvial fan.
"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," Roman DiBiase, a postdoctoral scholar at Caltech and lead author of the paper, said.
The appearance of Mars' northern hemisphere could be caused by a large crater that has since been erased, but even this explanation would require the surface of the red planet to be more active than it is now.
"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."
