Geosciences graduate student Cory Hughes studies the geology of the Earth to better understand Mars.
Billions of years ago, water flowed across Mars. Most scientists agree the red planet had rivers. But did those rivers flow into an ocean? New research from the University of Arkansas found strong evidence in Mars’ geology of an ocean in the planet’s 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 better understand the geology of Mars’ ancient rivers, the researchers compared river rocks on Earth to rocks on Mars, including sandstone created by a river that flowed across Northwest Arkansas 300 million years ago.
Hughes studied Mars extensively before coming to Arkansas. He decided to pursue a Ph.D. at the U of A so he could work with John Shaw, associate professor of geosciences and an expert on Earth’s deltas. By learning more about our planet, Hughes could better understand Mars.
The results were published in the journal Geophysical Research Letters.
HOW RIVERS FLOW
Imagine a river with no human-built levees to constrain its course. The river would shift constantly, like a ribbon snaking across the landscape. Rivers carry sediment, or solid material like silt, clay and rocks. The sediment eats away at one side, causing the river to curve in that direction, while depositing sand and fine dirt on the opposite side.
The region that defines how far a river shifts from side to side over time is called the channel belt.
A river slows down as it approaches an ocean — a massive, comparatively still body of water. As a river’s flow velocity decreases, it can carry less sediment. The solid matter starts to fall out of suspension, creating a river delta. And with less sediment to erode its banks, the river’s movement from side-to-side decreases. In other words, the channel belt narrows as a river approaches an 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.”
ANCIENT HISTORY REVEALED
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.”