Researchers from the California Institute of Technology (Caltech) have identified geological features on Mars that could hint at the presence of an ancient ocean, one that may have covered up to one-third of the planet’s surface.
The research was conducted by Abdallah Zaki, a former postdoctoral scholar at the institute, and Professor of Geology Michael Lamb. They were focused on a central question: whether water on Mars was restricted to lakes and rivers, or if it was capable of forming a stable, long-lasting ocean.
Previously, Mars missions had recorded “potential coastlines”; however, the variation in their elevations casts doubt on the stable ocean hypothesis. On Earth, sea level evens out globally, so it was expected that a true coastline should exist at roughly one consistent elevation.
According to Michael Lamb, if an ocean did exist on Mars, it vanished billions of years ago—during an era when the planet was already undergoing intense erosion due to wind, volcanic activity, and other geological processes. Therefore, the researchers sought more durable evidence than fragmented shoreline markers.
Sedimentological and mineralogical proof comes from sedimentary rocks, clay stratigraphy, and deltas of open basins situated along and within the discovered shelf. Figures a and b illustrate polar stereographic projections of the identified shelf, revealing 14,386 topographic highs interpreted as formations resulting from the retreat of the “Martian dichotomy boundary.” These structures consist of layered deposits up to 500 meters thick in the southern sector of Chryse Planitia. Furthermore, widely distributed clay layers and deltas of open basins are noted, which are associated with the presumed ancient aquatic system.
Initially, the team modeled Earth, digitally “removing” its oceans to understand what landforms persist after their disappearance. It turned out that the most resilient indicator is a broad and relatively flat expanse of land—a continental shelf that forms a distinct “ring” surrounding the former ocean.
Subsequently, they analyzed data from Martian orbiters and detected a similar structure in the northern hemisphere of Mars. This band of topography covers a significant portion of the planet and, in scale, corresponds to a potential ancient ocean that could have persisted for millions of years.
Deltas from ancient rivers, aligning with the suspected shelf line, provided additional corroboration. Abdallah Zaki notes that such a feature had not previously been considered a primary argument for an ocean, but now it integrates disparate pieces of evidence into a cohesive picture.
The authors suggest that if their hypothesis is validated, prime targets for future missions should be coastal sediments, which might have preserved traces of the ancient environment—much like Earth’s sedimentary rocks retain fossilized evidence of past life.
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