Mars may once have hosted a warm, ocean-bearing environment, according to new data collected by a rover that explored an area believed to be an ancient shoreline.
The evidence points to buried “beach” deposits, shaped by waves over a long period – highlighting the possibility of large bodies of liquid water in the planet’s distant past.
Looking beneath the surface
The findings come from the Chinese Zhurong rover, which landed in Utopia Planitia in May 2021 and operated for about one year.
Using ground penetrating radar (GPR), the rover investigated up to 80 meters (260 feet) below the red planet’s surface.
Throughout its 1.9-kilometer (1.2-mile) traverse, Zhurong’s radar detected thick, sloping layers consistent with material deposited along a beach – where waves carry sediment upslope at a relatively uniform angle.
“The structures don’t look like sand dunes. They don’t look like an impact crater. They don’t look like lava flows. That’s when we started thinking about oceans,” said Michael Manga, a professor of earth and planetary science at the University of California, Berkeley.
“The orientation of these features are parallel to what the old shoreline would have been. They have both the right orientation and the right slope to support the idea that there was an ocean for a long period of time to accumulate the sand-like beach.”
Mars’ beaches were similar to Earth
These layered deposits, described in a paper published in the journal Proceedings of the National Academy of Sciences, fit the scenario of wave-driven accumulation of sand.
On Earth, similarly thick beach sediments form over millions of years, suggesting that the Martian ocean – if it existed – must have persisted for quite some time.
Furthermore, the radar indicates that these layers are composed of sand-sized particles rather than fine dust or coarser rock fragments, which also supports their interpretation as ancient beach deposits rather than wind-blown dunes.
Implications of beaches on Mars
“This strengthens the case for past habitability in this region on Mars,” said Hai Liu, a professor with the School of Civil Engineering and Transportation at Guangzhou University, who is part of the Tianwen-1 science team that managed Zhurong.
Other co-authors note that the presence of ancient beaches on Mars implies that rivers once carried sediment into an open body of water, and that liquid water – free of ice – existed there.
“Shorelines are great locations to look for evidence of past life. It’s thought that the earliest life on Earth began at locations like this, near the interface of air and shallow water,” added Benjamin Cardenas, an assistant professor of geosciences at the Pennsylvania State University.
Mars is now cold and dry, with a thin atmosphere that makes stable liquid water unlikely on the surface. However, numerous studies indicate that the planet once had a denser atmosphere capable of supporting liquid water on a large scale.
While some of that water likely escaped to space over billions of years, the rest may remain frozen beneath the Martian surface or locked in mineral formations.
The shoreline controversy
For decades, planetary scientists have debated whether a massive ocean once occupied Mars’ northern lowlands. Images from NASA’s Viking orbiters in the 1970s showed what looked like a shoreline wrapping around much of the northern hemisphere.
Yet the proposed shoreline appeared far from level, varying in altitude by up to 10 kilometers – something that doesn’t match the relatively flat shorelines found on Earth.
One proposed solution is that volcanic activity reshaped Mars’ spin axis and topography long after an ocean had formed.
“Because the spin axis of Mars has changed, the shape of Mars has changed. And so what used to be flat is no longer flat,” Manga explained.
The study suggests that the Tharsis volcanic region – home to the solar system’s largest volcanoes – altered the planet’s rotation while forming, resulting in warped elevation levels. These dynamic processes may have distorted any once-level shoreline.
Beaches buried on Mars
Zhurong’s Rover Penetrating Radar (RoPeR) has offered a fresh perspective by detecting features hidden beneath the surface, where they have remained relatively untouched by eons of erosion or asteroid impacts.
“The sand that’s on those beaches is coming in from the rivers, and then it’s being transported by currents in the ocean and continually being transported up and down the beaches by the waves,” Manga said.
In other words, the rover uncovered structures one would expect to find along a coastline, still preserved after billions of years.
Overlying the beach deposits are around 10 meters (30 feet) of later material, likely derived from dust storms, volcanic ash, and debris from meteorite impacts.
This overburden fortuitously kept the beach layers intact, preventing weathering or erosional processes from completely removing the evidence of ancient shorelines.
Mystery of water on Mars
Zhurong’s discovery aligns with other indications of past water on Mars.
NASA’s Curiosity rover found sedimentary “wave ripples” in Gale Crater, suggesting an ancient lake free of ice, while the Perseverance rover revealed a fossilized river delta in Jezero Crater. Yet these findings pointed to lakes, not seas.
Zhurong’s data, by contrast, offers a vision of a far more expansive body of water – a genuine ocean or sea.
“To make ripples by waves, you need to have an ice-free lake. Now we’re saying we have an ice-free ocean. And rather than ripples, we’re seeing beaches,” Manga said, highlighting the difference in scale.
Though Zhurong is no longer active, the results of its one-year mission add compelling new evidence to the idea that Mars was once home to a significant ocean – complete with beaches shaped by waves.
Future missions that explore these buried shoreline deposits may shed even more light on how Mars transitioned from a planet with long-lived bodies of water to the arid world we know today.
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Featured image: A hypothetical picture of Mars 3.6 billion years ago, when an ocean may have covered nearly half the planet. The blue areas show the depth of the ocean filled to the shoreline level of the ancient, now-gone sea, dubbed Deuteronilus. The orange star represents the landing site of the Chinese rover Zhurong. The yellow star is the site of NASA’s Perseverance rover, which landed a few months before Zhurong. Credit: Robert Citron
The full study was published in the Proceedings of the National Academy of Sciences (PNAS).
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