Researchers from the University of Arkansas have uncovered compelling geological evidence suggesting that a vast ocean once covered the northern hemisphere of Mars billions of years ago. The study focuses on the existence of ancient river deltas.

According to researchers, river deltas exhibit features typically found only when large rivers flow into massive, relatively still bodies of water.

Ph.D. student Cory Hughes led the research team. They compared the geology of ancient Martian river systems to river formations on Earth, including a 300-million-year-old river delta found in Northwest Arkansas.

The Key is How Rivers Flow

A major key to the discovery lies in the concept of a backwater zone, or the section of a river where its flow begins to slow down as it approaches a large body of water. As the river’s velocity decreases, it loses its ability to carry sediment. As a result, materials settle and create a river delta. As the river slows, the channel belt narrows dramatically.

On Earth, this backwater zone can extend hundreds of miles inland. For example, the Mississippi River’s backwater zone begins 230 miles from its coastline. Looking at Mars from orbit, researchers found geological signs of ancient river deltas that displayed these telltale long backwater zones. 

“This is a large-scale process taking place, which is why we’re able to see it from space on Mars,” said Hughes. The presence of “very mature deltas” provides “a strong point in favor of an ancient ocean, or at the very least a large sea.” 

Implications for Life on Mars

The potential confirmation of a large, persistent Martian ocean significantly enhances the theory that Mars may have hosted life at some point in its history.

“We don’t know of any lifeforms on Earth, or anywhere in the universe, that don’t require liquid water,” Hughes emphasized. “So the more liquid water we have on Mars, a simple argument could be made that you have a higher chance of life.”

The research leveraged expertise from Earth-based formations. Hughes moved to the U of A to work with John Shaw, associate professor of geosciences and an expert on Earth’s deltas, to better understand Mars’s inverted ridges—topographical remnants of ancient, dried-up riverbeds.

In Northwest Arkansas, they identified the only known example of an inverted river delta on Earth, allowing them to precisely model the conditions necessary to create the Martian features. Hughes said, “I literally came here to study this without knowing it was in the backyard. No better word can describe that besides serendipity.”

Evidence of life on Mars is getting stronger. Earlier last month, the Mars rover Perseverance found the “strongest sign of potential life yet.”