There may have once been an ocean on the red planet Mars, according to clues buried in its rock. Sounds interesting? Scientists are learning more about the watery past of Earth thanks to its own ancient rivers.
Water used to flow across Mars' surface billions of years ago. Although most scientists agree that rivers once flowed on the planet, it is still unclear if those rivers eventually emptied into the ocean.
There is strong geological evidence in a recent study, published in Geophysical Research Letters, from the University of Arkansas that Mars' northern hemisphere was once covered by an ocean.
Cory Hughes, a U of A geosciences Ph.D. student and the study's lead author, said, "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 research team compared rocks formed by rivers on Earth with those found on Mars in order to investigate the geology of the planet's ancient rivers. Sandstone from a river that ran through what is now Northwest Arkansas some 300 million years ago was part of their analysis.
Hughes had already spent years researching Mars before coming to the University of Arkansas. In order to work with John Shaw, an associate professor of geosciences and an expert on Earth's river deltas, he decided to complete his doctorate there. Hughes thought he could learn more about the history and surface processes of Mars by researching the geological systems of Earth.
Imagine a river flowing naturally without any man-made levees to direct it. A river like that would constantly change, slicing across the landscape like a ribbon. Sediment, or particles like silt, clay, and rock, is carried by rivers and progressively changes the environment. Sand and other finer materials are deposited on one bank while one bank is eroded by the flowing water.
The channel belt is the region that determines how much a river meanders over time.
A river's flow slows as it approaches the ocean because it comes across a large, comparatively motionless body of water. Its capacity to transport sediment is diminished by this decrease in velocity, which leads to the settling of particles and the formation of a delta.
The river moves less side-to-side when there is less material eroding the riverbanks. In other words, as the river gets closer to the ocean, the channel belt gets smaller.
The backwater zone is the area where the riverbed falls below sea level and the channel belt narrows.
A river that empties into the ocean has a lengthy backwater zone. For instance, the Mississippi River's backwater zone starts 230 miles from the coast, close to Baton Rouge.
Geological evidence of ancient river backwater zones was discovered by Hughes while observing Mars from orbit.
"This is a large-scale process taking place, which is why we're able to see it from space on Mars," Hughes said.
Large rivers once flowed on Mars and emptied into an ocean before the planet's surface dried up billions of years ago, as evidenced by the existence of deltas with lengthy backwater zones.
"These are very mature deltas," Hughes said, adding, "This is a strong point in favor of an ancient ocean, or at the very least a large sea."
The coarsest grain is drawn to the bottom of the riverbed by gravity as the river flows. That coarse sediment is buried if the river eventually stops flowing. The sediment eventually turns into sandstone as a result of pressure and heat.
That rock will be forced to the surface of the Earth by shifting tectonic plates, and everything except the coarse channel bed will be eroded by wind and rain, leaving behind a ridge where a channel once existed. We call this process topographic inversion. An inverted channel belt or inverted ridge occurs when the top of a ridge is made up of sandstone that was once at the bottom of a river.
Since Mars lacks tectonic plates, the inverted ridges were probably created by the erosion of finer deposits surrounding the sandstone. The evidence of long-gone rivers can be seen in those inverted ridges.
Shaw invited Hughes to see the Wedington Sandstone, a rock formation located throughout Northwest Arkansas, shortly after Hughes' arrival. A 300-million-year-old river that formerly flowed from present-day Indiana to a sea that encircled central Arkansas created a branching network of inverted ridges, which the two discovered were the stone cliffs.
For thirty to forty years, scientists have been aware of the topographic inversion process. However, Hughes and Shaw found the only known instance of an inverted river delta on Earth in Northwest Arkansas.
"I literally came here to study this without knowing it was in the backyard," Hughes said. "No better word can describe that besides serendipity."
