Mars may have once had an ocean, new river evidence suggests

The strongest new evidence comes from features called backwater zones, areas where rivers slow as they meet a standing body of water and deposit sediment. On Earth, such zones mark the transition into seas or oceans.

Cory Hughes, a geosciences Ph.D. student at the University of Arkansas, and colleagues found similar patterns in Martian river deltas. Their analysis of Aeolis Dorsa, a region with extensive fluvial ridges, shows narrowing consistent with backwater morphodynamics. This suggests that rivers once drained into a vast body of water in the northern lowlands of Mars.

The team measured nine Martian river channel belts and found backwater lengths ranging from 5–17 km (3–10 miles), a scale comparable to lowland river deltas on Earth.

Backwater morphodynamics describe how river hydraulics and sediment deposition change as flow meets still water. The Mississippi River offers a familiar example. Its backwater influence extends hundreds of kilometers inland, with Baton Rouge located about 370 km (230 miles) from the Gulf of Mexico.

The same narrowing signature was identified in Martian riverbeds. The presence of mature deltas with extended backwater zones provides evidence that large rivers once emptied into a northern ocean or sea.

The study argues that backwater morphodynamics are universal and can be expected wherever river systems meet seas, including on Earth and even Titan, Saturn’s largest moon.

Mars preserves river traces in a way Earth cannot. Without tectonic plate activity, ancient landscapes remain intact. One key feature is the inverted channel belt, formed when coarse river sediments harden into sandstone and surrounding softer material erodes away.

Aeolis Dorsa contains extensive inverted ridges, recording where ancient rivers once flowed. Their distribution and scale point to long-lived deltas at the edge of a standing ocean.

To ground their Martian findings, Hughes and his advisor, John Shaw, studied the Wedington Sandstone in northwest Arkansas. This rock formation preserves a branching network of inverted ridges created by a 300-million-year-old river system that drained into an inland sea.

The Wedington outcrop is described by the researchers as the only known example of an inverted river delta on Earth. It provided a unique analogue to interpret Martian landscapes and supported the interpretation of deltas at the Aeolis Dorsa site.

If Mars once had an ocean, it would have covered much of the northern hemisphere, with depths of hundreds of meters. Such a body of water would dramatically expand the range of potentially habitable environments that once existed on the planet.

Water remains the single strongest requirement for life as we know it. More evidence of liquid water on Mars strengthens the argument that conditions may once have supported microbial life.

By applying hydrodynamic principles, scientists can also reconstruct aspects of paleogeography, including river slopes and grain sizes, offering new insight into Mars’ climate history.

The possibility of a northern ocean has been debated for more than three decades. Viking mission images in the late 1970s suggested possible shorelines, and later orbiters identified valleys and deltas consistent with sustained water flow.

Skepticism persisted, as some features could be explained by wind erosion or catastrophic floods. The new study adds weight by showing that Martian river systems display the same hydraulic behavior as Earth rivers entering oceans.

References:

¹ Did Mars Once Have an Ocean? New Research Suggests Yes – Arkansas News – September 30, 2025

² Stratigraphic Evidence of Backwater Morphodynamics and Lowland River Deltas in the Northern Hemisphere of Mars – C. M. Hughes et al. – Geophysical Research Letters – June 20, 2025 – https://doi.org/10.1029/2024GL112957 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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