Mars – a planet often perceived as dusty, dry, and desert-clad – was once a water-rich world, according to a new study. This study reveals that Mars not only had lakes but also massive oceans.
Recent observations using ground-penetrating radar have uncovered underground features on the red planet consistent with ancient beaches from 4 billion years ago. This is some of the strongest evidence yet that Mars once hosted a northern sea.
The research team has named this ancient sea Deuteronilus. "We're finding places on Mars that used to look like ancient beaches and ancient river deltas," says geologist Benjamin Cardenas of The Pennsylvania State University. "We found evidence for wind, waves, and no shortage of sand – a proper, vacation-style beach."
The water history of Mars is a complex puzzle. At first glance, the planet appears as though it has never seen a drop of liquid. Its legendary global dust storms contribute to this perception.
It might seem easy to believe that Mars has always been a dry rock; however, an overwhelming body of evidence indicates that Mars once had liquid water flowing abundantly on its surface.
So, there's no longer any question that water existed on Mars. But there are still many questions. How much water was there? How long ago did it vanish? Where did it go, and how?
"Oceans are important on planets. They have a large effect on climate, shape the surface of planets, and are potentially habitable environments," says geophysicist Michael Manga of the University of California, Berkeley.
Hence the 'follow the water' theme of Mars exploration. "Most exciting to me was the chance to look beneath the surface at a place we think there could have been an ocean and to see what we think are beach deposits," Manga adds.
Using data from the Chinese National Space Administration's (CNSA) Zhurong Mars rover, a joint Chinese-American team led by engineer Jianhui Li and geologist Hai Liu of Guangzhou University has provided a deeper answer to how much water Mars had: enough to fill an ocean.
As it traveled along the Utopia Planitia, Zhurong used ground-penetrating radar (GPR) to measure rock up to 80 meters (260 feet) below the surface of Mars. This technology sends radio waves into the ground, which bounce back in different ways when they encounter materials of varying densities, allowing for the creation of a three-dimensional map of structures deep below the ground.
A previous study based on Zhurong data suggested a shoreline, but that interpretation was not confirmed. The GPR data revealed thick layers of material along Zhurong's route, sloped upwards towards the supposed shoreline at an angle of 15 degrees, similar to ancient buried shorelines on Earth.
"The structures don't look like sand dunes, impact craters, or lava flows," Manga explains. "That's when we started thinking about oceans."
The orientations of these features are parallel to what the old shoreline would have been. They have the right orientation and slope to support the idea of a long-lasting ocean accumulating sand-like beach deposits.
These features imply a large, liquid ocean, fed by rivers dumping sediment, as well as waves and tides. This suggests that Mars had a water cycle for millions of years, the duration required for such deposits to form on Earth. Such deposits would not form at the edges of a lake.
The new discovery bolsters the case for past habitable conditions on Mars for life as we know it and suggests potential sites to search for signs of ancient life on the red planet.
Coastal environments, where water, land, and atmosphere converge, are potentially habitable. Knowing the locations and timelines of these environments can guide exploration and interpretation of observations from satellites, Manga explains.
Shorelines are great locations to look for evidence of past life. It's thought that the earliest life on Earth began in similar settings, near the interface of air and shallow water.
Recent research by Manga and his colleagues suggests that much of Mars' water may have been absorbed into its interior, existing today as vast, unreachable liquid reservoirs. This new study could be another piece of the puzzle, indicating the presence of enough liquid water to fill these reservoirs during Mars's fascinating, mysterious past.
The next step is to investigate the idea of liquid oceans further and model these alien waves and tides.
