Clues hidden in Martian rock suggest the red planet may have once hosted an ocean. Earth’s own ancient rivers are helping scientists uncover its watery past.
Billions of years ago, water once coursed across the surface of Mars. Scientists widely agree that the planet once hosted rivers, but an enduring question remains: did those rivers empty into an ocean? A recent study from the University of Arkansas presents compelling geological evidence suggesting that an ocean once covered Mars’ northern hemisphere.
“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,” said Cory Hughes, a U of A geosciences Ph.D. student and the study’s lead author.
To explore the geology of Mars’ ancient rivers, the research team compared river-formed rocks on Earth with those observed on Mars. Their analysis included sandstone from a river that flowed through what is now Northwest Arkansas around 300 million years ago.
Before arriving at the University of Arkansas, Hughes had already dedicated years to studying Mars. He chose to pursue his doctorate there to collaborate with John Shaw, an associate professor of geosciences and a specialist in Earth’s river deltas. Hughes believed that by studying Earth’s geological systems, he could gain clearer insight into the history and surface processes of Mars.
The team’s findings were published in Geophysical Research Letters.
How rivers shape landscapes
Picture a natural river without human-made levees guiding its path. Such a river would continuously shift, weaving back and forth like a ribbon across the terrain. Rivers transport sediment—particles such as silt, clay, and rock—which gradually reshape their surroundings. The moving water erodes one bank while depositing sand and finer materials on the opposite side.
The area defining how much a river meanders over time is known as the channel belt.
As a river nears the ocean, its flow slows because it encounters a vast, relatively still body of water. This drop in velocity reduces its ability to carry sediment, causing particles to settle and form a delta. With less material eroding the riverbanks, the river’s side-to-side movement decreases. In essence, the channel belt becomes narrower as the river approaches the ocean.
This section where the channel belt narrows, and the riverbed drops below sea level, is called the backwater zone.
The backwater zone of a river flowing into an ocean is long. In the case of the Mississippi River, for example, the backwater zone begins near Baton Rouge, 230 miles from the coastline.
Looking at Mars from orbit, Hughes found geological evidence of ancient river backwater zones.
“This is a large-scale process taking place, which is why we’re able to see it from space on Mars,” Hughes said.
The presence of deltas with long backwater zones provides strong evidence that large rivers once flowed on Mars and emptied into an ocean before the surface of the planet dried up billions of years ago.
“These are very mature deltas,” Hughes said. “This is a strong point in favor of an ancient ocean, or at the very least a large sea.”
The process of topographic inversion
How can scientists know the contours of a river that dried up billions of years ago?
As rivers flow, gravity pulls the coarsest grain to the bottom of the riverbed. If the river eventually dries up, that coarse sediment is buried. Over time, due to heat and pressure, the sediment becomes sandstone.
On Earth, shifting tectonic plates will push that rock to the surface, and then wind and rain will erode everything but the coarse channel bed, leaving behind a ridge where a channel used to be. This process is known as topographic inversion. When a ridge top is comprised of sandstone that used to be at the bottom of a river, it is called an inverted channel belt or an inverted ridge.
Mars does not have tectonic plates, so its inverted ridges were likely formed when finer deposits around the sandstone eroded. Those inverted ridges provide the evidence of long-vanished rivers.
Not long after Hughes arrived, Shaw invited him to visit the Wedington Sandstone, a rock formation found across Northwest Arkansas. The two realized the stone cliffs were part of a branching network of inverted ridges formed by a 300-million-year-old river that once ran from modern-day Indiana to a sea that covered central Arkansas.
Scientists have known about the process of topographic inversion for 30 to 40 years. But in Northwest Arkansas, Hughes and Shaw discovered the only known example of an inverted river delta on planet Earth.
“I literally came here to study this without knowing it was in the backyard,” Hughes said. “No better word can describe that besides serendipity.”
Reference: “Stratigraphic Evidence of Backwater Morphodynamics and Lowland River Deltas in the Northern Hemisphere of Mars” by C. M. Hughes, J. B. Shaw, A. M. Fernandes and T. E. Swanson, 20 June 2025, Geophysical Research Letters.
DOI: 10.1029/2024GL112957
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1 Comment
There have been many indicators of the northern ocean before such as shorelines, but this is persuasive evidence of deep waters.