They discover the shores of an ancient ocean on Mars

“What immediately comes to mind as one of the most significant points here is that the existence of an ocean of this size means greater potential for life,” Benjamin Cardenas, an assistant professor of geosciences at Penn State and lead author of the study recently published in the Journal of Geophysical Research: Planets.

“It also tells us about ancient climate and its evolution. Based on these findings, we know there had to be a period when it was warm enough and the atmosphere was thick enough to support this much liquid water at one time.”

There has long been a debate in the scientific community about whether Mars had an ocean in its low-elevation northern hemisphere, Cárdenas explained. Using topographical data, the research team was able to show definitive evidence of a coastline approximately 3.5 billion years old with substantial sedimentary accumulation, at least 900 meters thick, covering hundreds of thousands of square kilometres.

“The most important and novel thing we did in this article was to think about Mars in terms of its stratigraphy and its sedimentary record,” Cardenas said. “On Earth, we trace the history of waterways by looking at the sediments that are deposited over time. We call that stratigraphy, the idea that water carries sediment and you can measure changes in the Earth by understanding how where the sediments accumulate. 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 the National Aeronautics and Space Administration (NASA) and the Mars Orbiter laser altimeter. They discovered more than 6,500 kilometers of river ridges and grouped them into 20 systems to show that the ridges are likely eroded river deltas or undersea channel belts, the remnants of an ancient Martian coastline.

Elements of the rock formations, such as the thickness of the ridge system, elevations, locations and possible directions of sediment flow, helped the team understand the evolution of the paleogeography of the region. The area that was once ocean is now known as Aeolis Dorsa and contains the densest collection of river ridges on the planet, Cárdenas explained.

“The rocks at Aeolis Dorsa capture fascinating information about what the ocean was like,” he said. “It was dynamic. Sea levels were rising significantly. Rocks were being deposited along its basins at an accelerating rate. A lot of change was happening here.”

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

“A primary goal of the Curiosity rover’s missions is to search for signs of life,” Cardenas said. “It’s always been looking for water, traces of habitable life. This is the biggest one yet. It’s a giant body of water, fed by sediment coming from the highlands, presumably carrying nutrients. If there were tides on ancient Mars, they would have been here, gently bringing water in and out. This is exactly the kind of place where ancient Martian life could have evolved.”

Cárdenas and his colleagues have mapped what they have determined to be other ancient waterways on Mars. An upcoming study in the Journal of Sedimentary Research shows that several outcrops visited by the Curiosity rover were likely sedimentary strata from ancient river bars.

Another paper published in Nature Geoscience applies an acoustic imaging technique used to view the stratigraphy beneath the Gulf of Mexico seafloor to a Mars-like basin erosion model. The researchers determined that landforms called river ridges, which are widely found on Mars, are likely ancient river deposits eroded from large basins similar to Aeolis Dorsa.

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