Scientists have identified thousands of hills and mounds on Mars that contain layers of clay minerals, which formed when running water interacted with rocks nearly four billion years ago.
“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 billion years ago.”
The study focused on a region about the size of the United Kingdom, located in Chryse Planitia, a vast lowland area shaped by an ancient impact. Using high-resolution images and spectral data from NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express and ExoMars Trace Gas Orbiter, researchers identified over 15,000 hills and mounds, some towering up to 1,640 feet (500 meters). These features are composed of layered deposits, with up to 1,150 feet (350 meters) of clay minerals embedded within the layers, formed when liquid water soaked into the rocks for millions of years.
“[This] shows that there must have been a lot of water present on the surface for a long time,” said McNeil. “It’s possible that this might have come from an ancient northern ocean on Mars, but this is an idea that’s still controversial.”
Mars displays striking geological differences between its hemispheres: ancient highlands dominate the south, while the north features eroded plains. Chryse Planitia, situated in the north, is geologically linked to Mawrth Vallis, a southern highland area. These contrasting terrains, combined with the discovery of clay-rich mounds, provide compelling evidence for the possibility of an ancient northern ocean.
Immediately beneath the clay layers are older rock layers devoid of clay, while younger layers above also lack clay. This pattern suggests the clay minerals were deposited during a specific wet period in Mars’ Noachian era, a time when liquid water was abundant on the planet.
The clay-bearing region is geologically connected to Oxia Planum, the target site for the European Space Agency’s Rosalind Franklin rover, set to launch in 2028. The rover’s mission will include searching for evidence of past life on Mars.

“The mounds preserve a near-complete history of water in this region within accessible, continuous rocky outcrops,” McNeil explained. “The European Space Agency’s upcoming Rosalind Franklin rover will explore nearby and could allow us to answer whether Mars ever had an ocean and, if it did, whether life could have existed there.”
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This groundbreaking research not only deepens our understanding of Mars’ climate history but also underscores the importance of future missions in answering one of humanity’s most profound questions: Was there ever life on Mars?
The discovery, led by Joe McNeil of the Natural History Museum in London, was recently published in Nature Geoscience.