Mars surface water persisted until 757 million years ago, Zhurong data suggest
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A reanalysis of data from China's Zhurong rover suggests Mars may have hosted surface water as recently as 757 million years ago, hundreds of millions of years later than previously thought. The presence of selenite crystals, identified in rock images from Utopia Planitia, indicates that concentrated salty water once pooled there. The findings, published in Nature Astronomy, challenge the long-held assumption that the planet's surface froze solid around 3 billion years ago.
The Rover Mission
Zhurong landed in Utopia Planitia, a vast plain in Mars' northern hemisphere, in May 2021 and operated for 93 days before a dust storm ended its mission in 2022. During that time, its cameras photographed bright, flat rocks poking through the sand, and its instruments beamed back readings hinting at water-bearing minerals. The exact mineral compositions, however, remained elusive from those initial images alone.
Selenite Crystal Evidence
A team led by Jiacheng Liu at the University of Hong Kong re-examined Zhurong's data, focusing on tiny crystal shapes just beneath the rock surfaces — some curling like cabbage leaves, others branching like Christmas trees. Combined laser and infrared spectroscopy measurements identified the structures as selenite, a large-crystal form of gypsum that only grows by crystallizing directly from concentrated salty water. The thickness of the mineral layer suggests it formed under a water column 6.25 to 25 meters deep, while crater-density dating places the ground at roughly 757 million years old.
Implications for Water History
The findings indicate that surface water could have persisted on Mars hundreds of millions of years later than most models predict, likely fed by underground brine melted by volcanic heat and then concentrated as it froze from the top down. Because selenite crystals can seal tiny pockets of the liquid they formed in, they may preserve a chemical snapshot of the ancient environment. This makes the study's patch of Utopia Planitia a strong candidate for inspection by future rovers equipped to study crystal-encased environments.
What's Next
The study's authors suggest that Utopia Planitia should be a priority target for future rover missions capable of analyzing sealed crystal pockets. However, verifying the mineral composition and age directly would require a new lander with different instrumentation, and it remains unclear when such a mission might be feasible.
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Mars surface water persisted until 757 million years ago, Zhurong data suggest
