A growing body of evidence from disparate sources is reinforcing the long-debated hypothesis that a vast ocean once covered the northern hemisphere of Mars. By studying the planet’s topography, ancient river deltas, and subsurface geology, separate teams of researchers have uncovered compelling new data that points to a dynamic, water-rich past. These findings, pieced together from orbital mapping and rover-based radar, are painting a more detailed picture of a world that was once far more Earth-like, with a large, stable body of liquid water that may have created a habitable environment billions of years ago.
The latest research adds crucial new layers to the theory of a primordial Martian sea, often called Oceanus Borealis. While the idea has been theorized for decades, based on the stark elevation difference between the smooth northern lowlands and the cratered southern highlands, conclusive proof has remained elusive. Now, by analyzing the subtle geological clues left behind by ancient rivers and coastlines, scientists are moving closer to a consensus. These studies provide some of the strongest support yet that Mars once hosted not just rivers and lakes, but a true ocean, a finding with profound implications for the enduring search for life beyond Earth.
Reading the Rivers in the Rock
One of the most compelling new lines of evidence comes from geoscientists who looked to earthly geology to understand features on Mars. A research team from the University of Arkansas focused on the telltale signs left by ancient rivers as they empty into a larger body of water. Their work, published in the journal Geophysical Research Letters, compared observations of Martian riverbeds to well-understood formations on our own planet, providing a powerful analogy for interpreting the Red Planet’s past.
From Arkansas to Mars
The researchers drew comparisons between Mars’s ancient river systems and the Wedington Sandstone, a rock formation in northwest Arkansas that preserves a 300-million-year-old river channel. On Mars, many ancient riverbeds are visible today as “inverted ridges,” where the sediment that once filled the channel was more resistant to erosion than the surrounding plains. Over billions of years, the surrounding land eroded away, leaving the old riverbed as a raised, snaking ridge. By studying how these inverted ridges are formed and how they behave on Earth, scientists can more accurately interpret the Martian landscape. This comparative approach allows them to identify subtle features that reveal the environment in which these rivers flowed and, crucially, what they flowed into.
The Telltale Deltas
A river’s behavior changes dramatically when it meets a large, still body of water like an ocean or a sea. Its velocity slows, causing it to drop the sediment it carries and reducing its tendency to meander from side to side. This creates a distinct “backwater zone” and narrows the river’s channel belt—the region that defines its side-to-side movement over time. Examining Martian deltas from orbit, the University of Arkansas team found exactly this evidence. According to Cory Hughes, a geosciences Ph.D. student and the study’s lead author, the features are indicative of very mature deltas. “This is a strong point in favor of an ancient ocean, or at the very least a large sea,” Hughes stated, highlighting that this geological pattern is a large-scale process visible from space.
Mapping an Ancient Shoreline
In a separate study, researchers focused on a broad region of Mars known as Aeolis Dorsa, which lies at the boundary between the planet’s northern lowlands and southern highlands. This area has long been a candidate for a former coastline. New, detailed topographical maps of this region have revealed what appears to be the remnants of an extensive shoreline shaped by significant sea-level rise. The research identified over 4,000 miles of what are known as fluvial ridges—the elevated remains of former water channels. These ridges, grouped into 20 distinct systems, are interpreted as the relics of eroded river deltas or submarine channel belts that once fed into a massive ocean. According to Benjamin Cardenas, an assistant professor of geosciences who led the research, the rocks in this area suggest the ancient ocean was highly dynamic. “The sea level rose significantly,” Cardenas explained, noting that the evidence points to a period of rapid and substantial environmental change.
Peering Beneath the Plains
While orbital data can reveal surface features, some of the most groundbreaking new evidence has come from looking beneath the ground. Data from the ground-penetrating radar on China’s Zhurong rover provided a first-ever glimpse of the subsurface structure in Utopia Planitia, a vast basin within the northern lowlands. The rover’s radar, which can penetrate tens of meters below the surface, detected extensive, buried layers of sediment that dip in a consistent direction. This pattern, imaged over a traverse of more than a kilometer, strongly resembles the subsurface structures created by sediments deposited along a coastline on Earth. The researchers concluded that these dipping reflectors were likely laid down by the action of waves and tides in a persistent, stable ocean. The finding discounts other possible origins, such as sediment from localized floods or volcanic activity, and provides powerful subsurface proof to support the surface-level observations of an ancient sea.
A Dynamic and Water-Rich Past
The convergence of these independent research efforts—from analyzing river deltas, mapping ancient shorelines, and probing the subsurface—presents a cohesive and compelling case for a Mars that was once partially covered by an ocean. This body of water, likely filling the Vastitas Borealis basin, would have been immense, potentially covering nearly a third of the planet’s surface to a depth of several kilometers. Some estimates suggest its total volume could have been comparable to Earth’s Arctic Ocean. This vision of early Mars, approximately 3.8 to 4.1 billion years ago, is one of a planet with a much thicker atmosphere and a warmer climate capable of sustaining a complex water cycle, including glaciers, rivers, and a northern ocean. The evidence from Aeolis Dorsa suggests this was not a static environment but one that experienced significant sea-level changes, hinting at a complex and active climate system.
The Enduring Question of Life
Confirming the existence of a long-lived Martian ocean has profound implications for the possibility of ancient life. Every known life form requires liquid water to survive, making its historical presence the single most important factor in determining a planet’s habitability. As Cory Hughes noted, “The more liquid water we have on Mars, a simple argument could be made that you have a higher chance of life.” While the presence of an ocean does not prove that life existed, it establishes that a key prerequisite was met for a significant period of the planet’s history. These ancient coastal and sedimentary environments are now considered prime targets for future missions. Just as sedimentary basins on Earth hold the fossil record of our planet’s biological and climatic evolution, locations like Aeolis Dorsa may hold the ultimate clues to whether we have ever been alone in the universe.
