Researchers have discovered strongest evidence yet for an ancient delta on Mars where a river might once have emptied into a vast ocean.
This ocean, if it existed, could have covered much of Mars's northern hemisphere-stretching over as much as a third of the planet, researchers said.
The findings provide some of the strongest support yet, said Roman DiBiase, a postdoctoral scholar at the California Institute of Technology (Caltech) and lead author of study.
Most of the northern hemisphere of Mars is flat and at a lower elevation than the southern hemisphere, and thus appears similar to the ocean basins found on Earth. The border between the lowlands and the highlands would have been the coastline for the hypothetical ocean, researchers said.
They used new high-resolution images from the Mars Reconnaissance Orbiter (MRO) to study a 100-square-kilometre area that sits right on this possible former coastline.
Previous satellite images have shown that this area - part of a larger region called Aeolis Dorsa, which is about 1,000 kilometres away from Gale Crater where the Curiosity rover is now roaming - is covered in ridge-like features called inverted channels.
Researchers said these inverted channels form when coarse materials like large gravel and cobbles are carried along rivers and deposited at their bottoms, building up over time.
After the river dries up, the finer material-such as smaller grains of clay, silt, and sand-around the river erodes away, leaving behind the coarser stuff. This remaining sediment appears as today's ridge-like features, tracing the former river system.
Researchers used satellite images taken by the HiRISE camera on MRO. The spacecraft was able to make stereo images that have allowed scientists to determine the topography of the martian surface.
Using this data, the team was able to determine the slopes of the channels back when water was coursing through them.
Researchers also found evidence for an abrupt increase in slope of the sedimentary beds near the downstream end of the channels. That sort of steep slope is most common when a stream empties into a large body of water-suggesting that the channels are part of a delta and not an alluvial fan.
The newly discovered delta is not inside a crater or other confining boundary, suggesting that the water likely emptied into a large body of water like 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.
This large body of water could be the ocean that has been hypothesised to have covered a third of the planet. At the very least, the researchers say, the water would have covered the entire Aerolis Dorsa region, which spans about 100,000 square kilometres.