Ancient Ocean Evidence Discovered on Mars via Coastal Shelf Findings

Researchers from Caltech and the University of Texas at Austin have identified geological features on Mars that provide significant evidence for an ancient ocean. The findings, published in the journal Nature on April 15, 2026, describe the discovery of a wide and deep coastal shelf that may have once bordered an ocean covering approximately one-third of the planet’s surface.

The study was led by Michael Lamb, a Caltech professor of geology, and Abdallah Zaki, a postdoctoral researcher at the University of Texas at Austin Jackson School of Geosciences. The team sought a more reliable topographic indicator of an ancient ocean than shorelines, which are often subtle and found at inconsistent elevations across the Martian surface.

The “Bathtub Ring” Topographic Signature

To identify the coastal shelf, the researchers utilized computer simulations to “dry up” Earth’s oceans, observing which topographic features remained. On Earth, the most prominent signature of a global ocean is not a shoreline, but rather a band of low slope and curvature values consisting of coastal plains and the continental shelf, typically ranging from -410 meters to -15 meters in elevation.

Applying this methodology to Mars, the team observed a similarly flat zone located between approximately -1,800 meters and -3,800 meters in elevation. This circumglobal band of flat topography serves as a bathtub ring that suggests the presence of a stable, long-lived ocean.

The researchers noted that while local flat regions can be caused by other processes, such as lava flows, the global nature of this band makes a coastal shelf the most plausible explanation. This topographic feature is further supported by other observations within the same zone, including:

• River delta deposits, which are triangular sediment plains created where rivers enter an ocean.
• Coastal deposits and thick sequences of layered rock.
• Aqueously altered minerals.

Implications for Martian History and Life

The discovery addresses a long-standing debate in planetary science regarding whether Martian water was limited to isolated lakes and streams or if it existed as a vast, long-lasting ocean. The identified shelf suggests the latter, pointing to a period billions of years ago when liquid water was far more prevalent.

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According to Michael Lamb, if such an ocean existed, it likely dried up several billion years ago, potentially more than half the age of the planet. Over eons, these features have been eroded by volcanic activity and wind, making the discovery of the shelf a critical piece of evidence that survived these disturbances.

The shelf is a new observation that ties together evidence of what the coastal zone would have looked like. Nobody had really looked for it before.

Abdallah Zaki

The identification of these coastal regions provides new targets for future exploration. Because coastal sediments on Earth often preserve a historic record of fossils, researchers believe that sedimentary deposits from the Martian coastlines could potentially preserve signatures of ancient life.

Future Research and Verification

While the discovery is described as the best evidence to date for an ancient Martian ocean, the researchers emphasize that further verification is required. Future missions will need to utilize rovers to examine these deposits directly and conduct more detailed analyses of satellite data.

The research highlights the importance of using comparative planetology—applying known geological patterns from Earth to other planetary bodies—to uncover hidden histories of water and habitability in the solar system.