Rover Finds the Shoreline of an Ancient Beach on Mars

Data from the Chinese rover Zhurong is adding to the pile of evidence for oceans on ancient Mars. For a year, this little craft traveled over nearly two kilometers of the Martian surface and made radar scans of buried natural structures that look like ocean shorelines.

Zhurong’s ground-penetrating radar (GPR) looked under the surface to a depth of 80 meters. There, the radar instrument found thick layers of material similar to beach deposits on Earth. The best way to create such formations is by wave action stirring up and depositing sediments along the shore of an ocean. If these findings stand, they’ll provide a deeper look into Mars’s ancient warm, wet past, and the existence of long-gone seas.

Figuring Out Mars Shorelines

“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,” 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. “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.”

At first, scientists considered lava flows or dunes to explain the structures Zhurong measured. But, their shapes say otherwise. “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. He was part of Hai’s team that recently published a paper about Zhurong’s findings. “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.”

Digging into the Past

Aside from their meteorological and geological value, the presence of these shoreline structures also implies that Mars’s ancient oceans were ice-free. “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. That tells us Mars was a warmer world—at least for a while. Rivers could well have flowed across the surface, contributing rocks and sediments along the shorelines. And, of course, there are structures that imply the presence of oceans. On Earth, oceans provide life habitats and there’s no reason to think that Mars oceans couldn’t have done that, too.

“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.”

Shoreline Evidence for Changes on Mars

As far back as Viking, scientists had images showing what looked like irregular shorelines and flow features on the surface. Those features implied bodies of water and flowing rivers. Other missions returned images and data showing ponded areas where smaller bodies of water existed. More recent missions returned images of regions scoured and changed by catastrophic floods. The shoreline features imply that oceans existed.

We know today that Mars’s surface no longer hosts bodies of water. In the past, much of it escaped to space along with Mars’ atmosphere. But some water also went underground and remains there as ice deposits. And, some combined with rocks to form new minerals. Other geological features seem to point to the existence of Martian oceans, like the shorelines Zhurong and Viking measured.

However, the irregular shape of those shorelines continued to intrigue planetary scientists. That’s because they didn’t exactly look like shorelines like we see along Earth’s oceans, which are level. In 2007, Manga came up with the idea that the shapes of the shorelines were altered by changes in the planet’s rotation. Why did that happen? Blame it on volcanoes in the Tharsis region. Some 4 billion years ago volcanic activity there built up a huge bulge. That eventually messed with the planet’s rotation. “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.

If the findings hold up, the buried shorelines tell a compelling story of the last days of oceans on Mars. Based on the team’s paper, that water appears to have lasted tens of millions of years. As it disappeared and the climate dried up, wind-blown regolith covered the shorelines that Zhurong measured.

For now, the Zhurong data provides a look into shoreline deposits that are pristine—but buried under the subsurface. “There has been a lot of shoreline work done,” said Cardenas, “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.”

For More Information

Ancient Beaches Testify to Long-ago Ocean on Mars
Ancient Ocean Coastal Deposits Imaged on Mars

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