Billions of years ago, water moved across the surface of Mars, altering its landscape. While most scientists agree that ancient rivers carved channels into the Martian terrain, it remains unclear whether these rivers flowed into a larger ocean.
A recent study published in Geophysical Research Letters suggests that Mars displays clear geological signs indicating that these rivers once emptied into a massive body of water in its northern hemisphere. The finding not only adds weight to the long-debated “northern ocean hypothesis” but also provides clues for where Martian life might have once existed.
Rivers, Deltas, and Backwater Zones
For decades, scientists have studied images of dried riverbeds, channels, and sedimentary deposits scattered across the Martian surface. Many planetary scientists interpret these features as possible evidence for an ancient ocean that may have once covered the planet’s northern lowlands.
To test the ocean hypothesis, lead author Cory Hughes, a Ph.D. student at the University of Arkansas, and associate professor John Shaw focused on how rivers behave as they approach the ocean. On Earth, rivers slow down when they enter a large, still body of water. This decrease in velocity causes sediment to settle out and form into deltas. River channel belts also begin to narrow as they approach an ocean.
This narrowing occurs in what geologists call a backwater zone, which can extend for hundreds of miles inland. For example, the backwater zone of the Mississippi River begins near Baton Rouge, more than 200 miles upstream from the Gulf of Mexico.
Using orbital imagery, Hughes identified Martian river systems that show similar backwater narrowing. “This is a large-scale process taking place, which is why we’re able to see it from space on Mars,” Hughes said. He proposes that the presence of mature deltas and long backwater zones is strong evidence that ancient rivers once flowed into a large body of water.
Parallel in Arkansas
The team also examined other areas of Earth for comparison. The Wedington Sandstone in northwest Arkansas is home to an ancient river delta that formed approximately 300 million years ago. Over time, erosion washed away most remnants, leaving only the sandstone ridges that were once at the bottom of the river. These are referred to as inverted ridges or inverted channel belts and are formed through the process of topographic inversion.
By studying these formations, Hughes and Shaw refined their understanding of how inverted deltas form and what they should look like in satellite imagery. The surface of Mars contains similar inverted ridges as the surrounding softer rock eroded away. The parallels between these features and the Wedington Sandstone provide a rare Earth-based parallel for interpreting another planet.
“I literally came here to study this without knowing it was in the backyard,” Hughes said. “No better word can describe that besides serendipity.”
Extraterrestrial Implications
The discovery provides new evidence in support of a northern Martian ocean. If confirmed, it suggests that Mars once had not only rivers and lakes but also a functioning hydrological system on a planetary scale. This would drastically increase the chances that the planet could have supported microbial ecosystems in the past.
As Hughes explains, “We don’t know of any lifeforms on Earth, or anywhere in the universe, that don’t require liquid water. So the more liquid water we have on Mars, a simple argument could be made that you have a higher chance of life.”
The study also identifies new targets for future exploration. On Earth, deltas and backwater zones are known to trap sediment and preserve organic material, as well as other records of past life. Investigating similar sites on Mars could help guide the search for evidence of past habitability on the Red Planet.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds a Master of Business Administration and a Bachelor of Science in Business Administration, along with a certification in Data Analytics. His work combines analytical training with a focus on emerging science, aerospace, and astronomical research.