Scientists discover mounds on Mars are hiding the secrets of planet's past

Mounds and hills on Mars contain layers of clay minerals that formed when flowing water interacted with the rocks, a new study has found.

One of the Red Planet’s features is known as the Martian dichotomy, which describes the sharp contrast between its southern and northern hemispheres. Around one-third of the planet's northern hemisphere is covered by the northern lowlands, while the southern highlands cover around two-thirds of the Red Planet.

Planetary scientists have long established that Mars once hosted ancient river valleys and oceans. Today it is a cold and dry world with a very thin atmosphere, which resulted in its water evaporating. In August 2024, a reservoir of liquid water was found for the first time on the planet, deep in its rocky outer crust. There is also water frozen at the Martian poles and evidence of vapour in the atmosphere.

It is believed that a large body of water once existed in the northern hemisphere billions of years ago. Researchers led by Joe McNeil, of the Natural History Museum in London have found further evidence supporting the idea of a northern Martian sea in the form of more than 15,000 mounds and hills that measure up to 1,640 feet (500 meters) in height. They contain clay minerals.

It is believed ancient Mars once had a large body of water in the northern hemisphereFlickr

“This research shows us that Mars’ climate was dramatically different in the distant past,” McNeil said in a statement. “The mounds are rich in clay minerals, meaning liquid water must have been present at the surface in large quantities nearly four years ago.”

The team studied a region roughly the size of the UK that is filled with thousands of these mysterious mounds, which are the only remnants of a highland area that has been eroded aay by water and wind. They are located in Chryse Planitia, the landing site of NASA’s Viking 1 mission in 1976. This area is also considered a potential ancient impact site.

High-resolution images and spectral composition data from NASA’s Mars Reconnaissance Orbiter were used by McNeil’s team, along with the European Space Agency’s Mars Express and ExoMars Trace Gas Orbiter. They found that the mesas and buttes on Mars were made from layered deposits. Among these layers were clay minerals which form when liquid water interacts with rock over millions of years.

Below the clay layers are older layers of rock with no clay detected, and above the clay layers are younger layers of rock that also contain no clay. This suggests the clay layers are from Mars’ wet era (the Noachian era) which spanned between 4.2 and 3.7 billion years ago.

"[This] shows that there must have been a lot of water present on the surface for a long time," McNeil explained. "It's possible that this might have come from an ancient northern ocean on Mars, but this is an idea that's still controversial."

He added: "The mounds preserve a near-complete history of water in this region within accessible, continuous rocky outcrops," said McNeil.

The findings were published this month in the journal Nature Geoscience.

It comes as recordings of earthquakes on Mars — or “marsquakes” — might have finally solved the mystery surrounding why one half of the planet is drastically different from the other.

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