Mars may have once had a huge ancient ocean, on what was a warm, wet planet very different from the harsh, frozen wasteland we see today.
That’s the conclusion of new topographical maps of Mars that suggest the planet once had an ancient northern ocean with rising sea levels.
The new finding is based on layers of sediment spotted around the edges of the ocean that is thought to have existed 3.5 billion years ago.
Benjamin Cardenas, assistant professor of geosciences at Penn State, said: "What immediately comes to mind as one the most significant points here is that the existence of an ocean of this size means a higher potential for life.
"It also tells us about the ancient climate and its evolution. Based on these findings, we know there had to have been a period when it was warm enough and the atmosphere was thick enough to support this much liquid water at one time."
Scientists have debated whether Mars had a northern ocean for a long time, Cardenas said.
Using topography data, the research team was able to show definitive evidence of a roughly 3.5 billion-year-old shoreline with substantial sedimentary accumulation, at least 3,000 feet thick, that covered hundreds of thousands of square miles.
"The big, novel thing that we did in this paper was think about Mars in terms of its stratigraphy and its sedimentary record," Cardenas said.
"On Earth, we chart the history of waterways by looking at sediment that is deposited over time. We call that stratigraphy, the idea that water transports sediment and you can measure the changes on Earth by understanding the way that sediment piles up. That's what we've done here – but it's Mars."
The team used software developed by the United States Geological Survey to map data from Nasa and the Mars Orbiter Laser Altimeter.
They discovered over 6,500 kilometres of fluvial ridges and grouped them into 20 systems to show that the ridges are likely eroded river deltas or submarine-channel belts, the remnants of an ancient Martian shoreline.
Elements of rock formations, such as ridge-system thicknesses, elevations, locations and possible sedimentary flow directions helped the team understand the evolution of the region’s paleogeography.
The area that was once ocean is now known as Aeolis Dorsa and contains the densest collection of fluvial ridges on the planet, Cardenas said.
He added: "The rocks in Aeolis Dorsa capture some fascinating information about what the ocean was like.
"It was dynamic. The sea level rose significantly. Rocks were being deposited along its basins at a fast rate. There was a lot of change happening here."
Cardenas explained that on Earth, the ancient sedimentary basins contain the stratigraphic records of evolving climate and life. If scientists want to find a record of life on Mars, an ocean as big as the one that once covered Aeolis Dorsa would be the most logical place to start.
"A major goal for the Mars Curiosity rover missions is to look for signs of life," Cardenas said. "It's always been looking for water, for traces of habitable life.
"This is the biggest one yet. It’s a giant body of water, fed by sediments coming from the highlands, presumably carrying nutrients. If there were tides on ancient Mars, they would have been here, gently bringing in and out water. This is exactly the type of place where ancient Martian life could have evolved."
Cardenas and his colleagues have mapped what they have determined are other ancient waterways on Mars.
"The stratigraphy that we're interpreting here is quite similar to stratigraphy on Earth," Cardenas said.
"Yes, it sounds like a big claim to say we’ve discovered records of large waterways on Mars, but in reality, this is relatively mundane stratigraphy. It's textbook geology once you recognise it for what it is. The interesting part, of course, is itss on Mars."
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