Ancient beaches testify to long-ago ocean on Mars

(Press-News.org) A Chinese rover that landed on Mars in 2021 detected evidence of underground beach deposits in an area thought to have once been the site of an ancient sea, providing further evidence that the planet long ago had a large ocean.

The now-inactive rover, called Zhurong, operated for a year, between May 2021 and May 2022. It traveled 1.9 kilometers (1.2 miles) roughly perpendicular to escarpments thought to be an ancient shoreline from a time — 4 billion years ago — when Mars had a thicker atmosphere and a warmer climate. Along its path, the rover used ground penetrating radar (GPR) to probe up to 80 meters (260 feet) beneath the surface. This radar is used to detect underground objects, such as pipes and utilities, but also irregular features, such as boundaries between rock layers or unmarked graves.

The radar images showed thick layers of material along the entire path, all pointing upward toward the putative shoreline at about a 15-degree angle, nearly identical to the angle of beach deposits on Earth. Deposits of this thickness on Earth would have taken millions of years to form, suggesting that Mars had a long-lived body of water with wave action to distribute the sediments along a sloping shoreline.

The radar was also able to determine the size of the particles in these layers, which matched that of sand. Yet, the deposits don't resemble ancient, wind-blown dunes, which are common on Mars.

"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 University of California, Berkeley, professor of earth and planetary science. "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."

Manga is the contributing author of a paper about the Zhurong measurements to be published the week of Feb. 24 in the journal Proceedings of the National Academy of Sciences.

According to the paper's Chinese and American authors, beaches imply a large, ice-free ocean on Mars, even though Mars is too cold today for water to flow as a liquid. They also imply that there were rivers that dumped sediment into the ocean that was distributed by waves along the beaches.

"The presence of these deposits requires that a good swath of the planet, at least, was hydrologically active for a prolonged period in order to provide this growing shoreline with water, sediment and potentially nutrients," said co-author Benjamin Cardenas, an assistant professor of geosciences at The Pennsylvania State University (Penn State). "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."

"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 and a core member of the science team for the Tianwen-1 mission, which included China's first Mars rover, Zhurong.

Deuteronilus shoreline Images taken by the Viking spacecraft in the 1970s first led to speculation that an ocean once existed on Mars, likely during a time when the planet had a denser atmosphere that could retain heat and thus liquid water. The Viking images showed what looked like a shoreline around a large portion of Mars’ northern hemisphere and a depression that could be an ancient seabed.

Yet, the shoreline was so irregular, with ups and downs of up to 10 km, that planetary scientists doubted this scenario. Shorelines, like those on Earth, should be level. Other conundrums, such as what happened to the water, also cast doubt on this theory. The polar ice caps do not contain enough water to fill such an ocean.

Subsequent missions to Mars, however, provided evidence that, while a lot of the planet's water likely escaped to space along with Mars' atmosphere as the planet cooled, much probably also went underground, either as ice or combined with rocks to form new minerals.

In 2007, Manga and his colleagues proposed a theory to explain how today's uneven shoreline could have been created by an ocean. Based on computer modeling, they argued that the planet's huge volcanic region, Tharsis, which contains the solar system's largest volcanoes, altered the planet's rotation after it formed about 3.7 billion years ago, making the level shoreline uneven. He revised that theory in 2017, suggesting that Mars' rotation actually changed while the Tharsis bulge formed, starting about 4 billion years ago.

"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," he said.

With its ground penetrating radar, Zhurong had an opportunity to look for underground evidence of an ancient ocean.

"The southern Utopia Planitia, where Zhurong landed on May 15, 2021, is one of the largest impact basins on Mars and has long been hypothesized to have once contained an ancient ocean," Liu said. "Studying this area provides a unique opportunity to investigate whether large bodies of water ever existed in Mars’ northern lowlands and to understand the planet’s climate history."

Hai and Zhurong scientists reached out to Manga through Cardenas to help interpret the GPR data primarily because of Manga's long interest in Mars' oceans. Manga says that the Rover Penetrating Radar (RoPeR) detected radar reflections about 10 meters below the current surface that are classic indications of sloping, sandy beaches lining an ocean.

"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 coming and going up and down the beach," Manga said, noting that Mars has many features that look like ancient rivers. "So there must have been rivers transporting sediment to the ocean, though there's nothing in the immediate vicinity that would have disturbed these beach deposits."

In January 2025, other researchers reported evidence of ripples in sedimentary rocks at the bottom of Gale Crater, the landing site for NASA's Curiosity rover, suggesting the presence of long-gone bodies of liquid water with no ice covering the surface. The Perseverance rover has also found evidence of a river delta in Jezero crater, a mere 2,400 kilometers (1,500 miles) from Zhurong's landing site. But both of these craters are thought to have been lakes, not oceans.

"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.

The approximately 10 meters (30 feet) of material overlaying the beach deposits were likely deposited by dust storms, material thrown out by asteroid impacts or volcanic eruptions over the billions of years since the ocean disappeared. This turned out to be fortuitous, Cardenas said.

"The shoreline deposits imaged here are pristine, still in the subsurface," he said. "There has been a lot of shoreline work done, but it’s always a challenge to know how the last 3.5 billion years of erosion on Mars might have altered or completely erased evidence of an ocean. But not with these deposits. This is a very unique dataset."

Other co-authors of the paper are Liu's colleagues at Guangzhou University, Jianhui Li, Xu Meng, Diwen Duan and Haijing Lu; Jinhai Zhang, Bin Zhou and Guangyou Fang of the Chinese Academy of Sciences in Beijing; Fengshou Zhang of Tongji University in Shanghai; and Derek Elsworth of Penn State. Fang, who is with the Aerospace Information Research Institute, developed the Rover Penetrating Radar for Tianwen-1.

The Chinese team was supported by the Natural Science Foundation of China and the Guangdong Basic and Applied Basic Research Foundation. Manga was supported by the Earth4D program of the Canadian Institute for Advanced Research.

END