Scientists have used orbiters and rovers to find dried streams, lakes, and gullies on Mars that hint at its watery past, but their cavalry of robots has struggled to prove the Red Planet ever had an actual ocean.
A new study that leverages data from China's now-defunct rover provides some of the strongest evidence so far of a long gone vast body of water — one that wasn't just a temporary lake formed of melted ice, but a much larger sea. The findings lay bare what seems to be an ancient shoreline for an ocean that would have covered about one-third of the planet’s surface.
The new paper, published in the Proceedings of the National Academy of Sciences, suggests Earth's space neighbor had a warm and wet period that lasted for perhaps tens of millions of years.
Such an Earth-like environment would seemingly increase the odds that life could have existed there. Though no one knows whether Mars was ever inhabited, the presence of an ocean means this location was at least habitable, said Benjamin Cardinas, a sediment geologist at Penn State and one of the coauthors.
"Scientists who study the origin of life really do think that one of the main places that it might have started is along beaches not so different from what we think we saw here" on Mars, Cardinas told Mashable. "You've got shallow water, you've got air, you've got lands, and it's these interfaces where scientists who study this thing think that life probably cropped up on Earth in the first place."
The new data comes from the Zhurong rover, part of China's Tianwen-1 mission, which landed on Mars in May 2021. The six-wheeled rover was sent to investigate Utopia Planitia, a region far from NASA's Curiosity and Perseverance rovers. It's the same rubbly plain where the U.S. Viking 2 lander touched down in 1976.
Collaborations between Chinese and U.S. researchers can be difficult to achieve, due to the Wolf Amendment established in 2011. The federal law prevents NASA from working with China because of concerns that the space program could exploit U.S. technology to enhance its weapons. But some U.S. scientists contributed to the study without receiving any federal government funding. China, for its part, made the rover data public, a requirement of publishing the research.
Based on satellite images, scientists had previously hypothesized that Utopia Planitia, an area in Mars' northern hemisphere, once held water. But the idea remained debatable because they had lacked the underground evidence to substantiate it until now. The features that resembled coasts sat at different elevations, making it hard to determine whether water created them or something else, such as burbling lava, wind-blown sand dunes, or ancient rivers.
Zhurong did not survive the Martian winter as planned in December 2022. But it had traveled about one mile on the Red Planet over the course of a year before going kaput.
In that time, the rover collected a lot of data, including some from a special instrument called ground-penetrating radar. The tool works by sending radio waves underground to measure the signals that bounce back. This helps scientists identify and plot different layers of rock and soil beneath the surface.
The data revealed distinctive slopes of underground material, just like how waves build up sediment along Earth's coasts. These ancient beaches extended for about 4,300 feet — nearly a mile — and were buried 30 to 115 feet below the surface.
"To accumulate more than a kilometer of beach deposits on Earth takes a long time — hundreds of thousands of years to millions of years," Michael Manga, a UC Berkeley geoscientist and coauthor, told Mashable. "So if we say that the processes that operate on Earth also operated on Mars, at roughly the same kind of rates, it means the ocean was there for a decent amount of time."
From the new data, scientists can infer a larger water cycle for Mars. In order for beaches to creep nearly a mile into a body of water, there would need to be tides, standing water, and rivers feeding sediment into the ocean over a long period.
Manga, who has long-believed in the ocean hypothesis, found the Zhurong data deeply satisfying.
"Just the fact that you can go to Mars with a rover and move over the surface and look underground is kind of mind-boggling to me," he said. "But then to see something that has structure and that's coherent — and by that I mean similar over such a broad scale — was really super exciting."
The Perseverance rover has also detected sloping underground layers at its landing site in Jezero Crater, a former lakebed, but those could have been created by water or magma. A key difference between the two rovers' radar data is that the Jezero material had what's called "high permittivity" — holding more electrical charge. This could indicate the presence of volcanic rock. The material at Utopia Planitia, on the other hand, had "low permittivity" and is likely composed of a sand and pebble mixture, similar to what's found along many of Earth's shorelines.
That Zhurong and Perseverance had different findings is a reminder that environments can vary a lot globally. Perseverance is about 3,000 miles away from Zhurong, farther than the distance between New York and LA. A few months ago, research on carbon-rich minerals at Gale Crater, where Curiosity roams about 2,000 miles from Zhurong, found that the region would have been icy and salty — quite hostile for life to emerge, at least above ground.
"It's not necessarily surprising to me that you can look at different parts of Mars, and you'll find that the story is more complicated," Cardinas said. "Regions of Mars may have been fairly different at different times."
While the new research helps to confirm Mars had a surface ocean in its past, it also prompts new questions — namely, where did all of that water go? Did it freeze beneath the surface, collect into ice sheets at the poles, or escape into space? The answer could help scientists understand how planets evolve and whether such a change could occur on Earth.
Researchers may also want to further consider how gravity factors into beach formations, Manga said. Mars' gravity is 62 percent less than Earth's, and scientists don't yet know whether that could fundamentally alter how beaches work. That gap in knowledge could mean the team misinterpreted the shoreline features. But of all the possible explanations, the collaborators feel confident a stable and vast ocean is likely the best fit.
"It would be interesting if it turns out that Mars did have large oceans and never created life," Manga said. "That would say something about how difficult life is to initiate."
