Traces of the ancient ocean discovered on Mars

A recently released set of topographic maps provides new evidence for an ancient North Ocean on Mars. The maps offer the strongest argument yet that the planet once experienced sea-level rise that was accompanied by an expansive warm and humid climate, not the harsh, frozen landscape that exists today.

“What immediately comes to mind as one of the most important points is that the existence of an ocean this size means a higher potential for life,” said Benjamin Cardenas, an assistant professor of geosciences at Penn State and lead author of the recently published study in the Journal of Geophysical Research: Planets. “It also tells us something about the ancient climate and how it evolved. From these results we know there must have been a time when it was warm enough and the atmosphere was thick enough to hold that much liquid water at once.”

There has long been debate in the scientific community as to whether Mars had an ocean at low altitudes in its northern hemisphere, Cardenas explained. Using topography data, the research team was able to show clear evidence of a roughly 3.5-billion-year-old coastline with significant accumulations of sediment at least 900 meters thick, covering hundreds of thousands of square kilometers.

“The big, new thing we did in this work was think about Mars in terms of its stratigraphy and its sedimentary record,” Cardenas said. “On Earth, we record the history of waterways by looking at sediments deposited over time. We call this stratigraphy, the idea that water transports sediment, and you can measure changes on Earth by understanding how sediment accumulates. We did that here – but it’s Mars.”

The team used software developed by the United States Geological Survey to map data from the National Aeronautics and Space Administration (NASA) and the Mars Orbiter Laser Altimeter. They discovered over 6,500 kilometers of river 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 coast.

Elements of rock formations such as Data such as ridge system thickness, elevations, locations and possible sediment flow directions helped the team to 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 river ridges on the planet, Cardenas explained.

“The rocks in Aeolis Dorsa contain some fascinating information about what the ocean was like,” he said. “It was dynamic. The sea level rose significantly. Rocks were deposited along its basins at a rapid rate. A lot has changed here.”

Cardenas explained that the ancient sedimentary basins on Earth hold the stratigraphic record of evolving climate and life. If scientists want to find records of life on Mars, an ocean as large as that which once covered Aeolis Dorsa would be the most logical place to start.

“A primary goal of the Mars Curiosity rover missions is to search for signs of life,” Cardenas said. “It has always been looking for water, for traces of habitable life. This is the biggest yet. It is a vast body of water fed by sediments brought down from the highlands, believed to carry nutrients. If there were tides on ancient Mars, they would have been here, gently carrying water in and out. This is exactly the kind of place where ancient Martian life might have evolved.”

Cardenas and his colleagues have found that other ancient waterways on Mars are maps. A forthcoming study in the Journal of Sedimentary Research shows that various outcrops visited by the Curiosity rover were likely sediment layers from old river bars. Another paper published in Nature Geoscience applies an acoustic imaging technique to probe the subseafloor stratigraphy of the Gulf of Mexico for a model of Martian-like basin erosion. The researchers determined that the landforms called river ridges, found far across Mars, are likely ancient river deposits eroded by large basins similar to Aeolis Dorsa.

“The stratigraphy we’re interpreting here is quite similar to the stratigraphy on Earth,” Cardenas said. “Yes, it sounds like a big claim that we have discovered records of major waterways on Mars, but in reality this is a relatively mundane stratigraphy. It’s textbook geology once you see it for what it is. The interesting part, of course, is that it’s on Mars.”

The other co-author of the JGR: Planets paper is Michael P. Lamb, Professor of Geology at Caltech. The work was funded by the National Aeronautics and Space Administration (NASA).