For years, researchers have pointed to ancient shoreline shapes in Mars’ northern lowlands as evidence of a past ocean.
Some questions remained. How long did that ocean actually last? Did it dry up within a few thousand years, or did it persist across geological time?
Now a chemical stain along the rim of a massive Martian basin has given scientists a number. The evidence comes from a mineral that builds up exactly where water meets a shallow shore – rewriting the timeline.
Mars’ mineral shoreline
Utopia Planitia is a 2,050-mile-wide (3,300-kilometer-wide) depression in Mars’ northern hemisphere, where China’s Zhurong rover landed in 2021. Dr. Yan Li at Peking University (PKU) and colleagues found a ring of manganese oxides along its edge.
Concentrations climb with elevation, rising from about 2.7% by weight at the lowest spots to 7.4% some 30 feet (9 meters) higher. The manganese oxides stop abruptly at a sharp upper limit, the same pattern that is seen around shallow lakes on Earth.
That ring has a formal name – the “bathtub ring” – and it shows up around shallow waters on Earth, where dissolved manganese oxidizes at the boundary between water and air. The same chemistry left the same kind of stain on Mars.
How AI helped
Identifying manganese minerals from infrared light is harder than it sounds – the oxides form thin, irregular coatings that scatter light in ways that traditional methods cannot read reliably.
The team built a custom neural network called SCANet, trained on 13,742 infrared light readings from lab-made samples designed to mimic conditions in Martian soil.
The AI work came together through a collaboration with researchers at Beihang University (BUAA), whose engineers worked on the deep learning architecture.
Then came the big data run: more than 5.7 million measurements from Zhurong and from European and American orbiters.
Predictions from the network matched independent chemical readings taken by the rover’s laser instrument.
Ring reveals ocean age
A ring is a snapshot, but the team wanted a clock – and manganese chemistry supplied one. Dissolved manganese converts to solid oxide minerals at a known rate in shallow, oxygen-rich water.
Combine that rate with the depth range over which the ring sits, and the math yields a time interval. The result places the ocean’s lifespan at 0.8 to 1.5 million years. This is the first hard number that anyone had put on it.
Earlier work had identified shoreline-like deposits and buried channels in the basin. Researchers had long suspected that an ancient ocean once filled Utopia Planitia on Mars. Until this study, however, none could say how long it had lingered.
Mars enters a dry age
The ring formed during the Hesperian – Mars’ middle geological period – roughly 3.7 to 3.4 billion years ago. That window sat between Mars’ wet youth and the cold, dry Amazonian era that followed.
The transition between those two eras, around 3 billion years ago, appears to have shut Mars’ ocean down. Then came the burial.
Volcanic eruptions from Elysium Mons likely pushed lava across the basin’s lower stretches, covering much of the manganese-rich seafloor.
Cold-climate activity near the north pole appears to have erased surface features farther up, leaving the ring’s clearest expression south of about 45 degrees latitude.
The mapping also revises the depth of this ocean on Mars, placing it at 490 to 1,310 feet (150 to 400 meters), which is shallower than older estimates suggested.
Enough time for life
A million-year stretch of stable ocean on Mars exceeds the minimum researchers believe is needed for the basic chemistry of life to get started.
The same window overlaps with when the earliest microbes are believed to have appeared on Earth, around 3.4 billion years ago.
Atmospheric modeling of early Mars suggests episodes of rising oxygen during this era, exactly the conditions the manganese chemistry would have needed.
The coincidence doesn’t prove that Mars hosted life, but it does mean the basin had time.
Manganese itself is interesting to astrobiologists for another reason. On Earth, microbes drive much of the oxidation chemistry that builds these very minerals.
This makes manganese-rich zones useful targets in any future search for signs of past life on Mars.
Future mission targets
Beyond the historical question, the manganese ring carries practical weight. These deposits can help split water molecules to release oxygen.
That chemistry could give future crewed missions a way to generate breathable air directly from the Martian surface.
Concentrated manganese near the basin’s rim also gives rover planners specific targets – places where traces of early biology might still sit.
Mars is often described as a planet of brief episodes of water. The manganese ring suggests otherwise.
In Utopia Planitia, a stable ocean lasted around a million years, and the mineral record of its retreat is still readable from orbit.
The study is published in Nature Communications.
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