A continent-like geological formation discovered beneath the Martian terrain suggests that an expansive ocean once submerged as much as one-third of the planet, significantly altering prevailing perspectives on its historical hydrological conditions.
This artist’s impression shows how Mars may have looked about 4 billion years ago. Image credit: M. Kornmesser / ESO.
While there is a widespread consensus that liquid water once existed on the Martian surface, the extent of this hydration—whether confined to localized lakes and river systems or vast, enduring oceans—remains a subject of ongoing inquiry.
Previous lunar exploration endeavors have unearthed geological formations resembling ancient shorelines; however, these features are often indistinct and situated at disparate elevations across the planetary surface.
For these to be definitive indicators of a stable oceanic body, they would logically be expected to align at a uniform elevation, analogous to the consistent sea levels observed on Earth.
“Should Mars indeed have harbored an ocean, it would have desiccated a considerable epoch ago—potentially several billion years in the past, exceeding half the planet’s entire lifespan,” stated Professor Michael Lamb of Caltech.
“Scarcely any terrestrial geological records from such an ancient period survive; any Martian remnants from that era have likely been subjected to extensive degradation from billions of years of aeolian erosion, volcanic activity, and other disruptive geological processes that obscure subtle topographic signatures.”
“Our objective was to identify a more robust topographic marker than shorelines that could provide evidence for the existence of an ocean.”
Illustration taken from orbiter data identifying the coastal shelf region on Mars; analogous features on Earth are signatures of our global oceans, and only form over long periods of time. Image credit: A. Zaki.
Professor Lamb, in collaboration with Dr. Abdallah Zaki from Caltech and the University of Texas at Austin, initially turned to Earth’s geological record to ascertain which landforms are intrinsically linked to the presence of oceanic bodies on our own planet.
Employing sophisticated computational modeling techniques, they simulated the desiccation of Earth’s oceans to observe the resulting topographic remnants.
These simulations revealed that a cardinal feature left by extensive oceans is a substantial, level terrestrial expanse, potentially extending hundreds of kilometers in width, conforming to the perimeter where land interfaces with the sea—akin to the residual ring around a drained bathtub. This feature is recognized as the continental shelf.
While terrestrial sea levels, and consequently shoreline positions, have experienced considerable fluctuations throughout geological time, the continental shelf represents a significant landform characterized by considerable temporal stability.
Subsequently, the research team meticulously examined topographic data of Mars, acquired by orbital spacecraft, and identified a comparable band of terrain that compellingly suggests the presence of an ocean in the planet’s northern hemisphere, encompassing a third of its global surface area.
The formation of such a prominent landform necessitates extended periods of geological development and is notably absent in the vicinity of extensive lake systems, implying that any ocean present must have maintained stability for potentially millions of years.
Furthermore, the scientists observed evidence of river deltas—triangular sediment deposits formed where rivers discharge into larger bodies of water—which appeared to align with the inferred ‘bathtub ring’ shelf feature.
“This shelf represents a novel observation that effectively consolidates evidence pertaining to the configuration of an ancient coastal zone,” commented Dr. Zaki.
“Prior investigations had not specifically sought out this particular geological marker. It provides a significant corroborating piece of evidence supporting the hypothesis of a northern ocean on Mars, though extensive follow-up investigations are essential, involving ground-based rover examinations of deposits and further in-depth analysis of satellite imagery.”
The findings have been published in the esteemed scientific journal Nature.
_____
A.S. Zaki & M.P. Lamb. Identifying the topographic signature of early Martian oceans. Nature, published online April 15, 2026; doi: 10.1038/s41586-026-10381-2
