Massive, wind carved mounds more than a mile high have been found on Mars. Having formed more than billions of years, these mounds help pinpoint when water on the Red Planet dried up during a global climate change event.
Led by Mackenzie Day, a graduate student at the University of Texas at Austin Jackson School of Geosciences, the recent study highlights how the Martian landscape has been shaped by wind - a force that, on Earth, is overpowered by other processes.
"On Mars, there are no plate-tectonics, and there's no liquid water, so you don't have anything to overprint that signature, and over billions of years you get these mounds, which speaks to how much geomorphic change you can really instigate with just wind," Day explained. "Wind could never do this on Earth because water acts so much faster, and tectonics act so much faster."
The mounds, first spotted during NASA's Viking program in the 1970s, are found at the bottom of craters. Mount Sharp, for example, is a mound more than three miles high, located in the Gale Crater.
Since August 2012, the Mars rover Curiosity has been exploring the Gale Crater as part of NASA's Mars Science Laboratory mission. Recent analyses from the rover revealed that the thickest mounds are comprised of sedimentary rock.
While the base of these mounds were formed by sediments carried by water that used to flow into the crater, the tops are made of sediments deposited by wind. However, how the mounds formed inside craters that were once full of sediments remains a mystery.
"There's been a theory out there that these mounds formed from billions of years of wind erosion, but no one had ever tested that before," Day added. "So the cool thing about our paper is we figured out the dynamics of how wind could actually do that."
To test this theory, researchers designed a wind tunnel experiment, in which they built a miniature crater 30 centimeters wide and four centimeters deep, filled it with damp sand, and racked the elevation and distribution of sediment in the crater until all of it had blown away.
In the end, researchers found the model's sediment was eroded into forms similar to those observed in Martian craters today, with a crescent-shaped moat that deepened and widened around the edges of the crater until all that was left was a mound.
"We went from a filled crater layer cake to this mounded shape that we see today," Day said.
Researchers then built a computer model that simulated how the wind flowed through the crater at different stages of erosion in hopes of learning more about the wind dynamics on the Red Planet.
Perhaps the most important discovery is that the formation of these mounds is directly linked to climate change on Mars, as the bottoms were built during wet times and the tops were shaped by dry conditions. This climatic shift is known as the Noachian period, which began about 3.7 billion years ago.
"This sequence signals the change from a dominance of depositional processes by water during a wetter time, to wind reworking of these water-laid sediments with the onset of aridity, followed by wind erosion once these sediment supplies have been exhausted," co-author Gary Kocurek said. "Overall, we are seeing the complete remaking of the sedimentary cycle on Mars to the one that characterizes the planet today."
Their findings were recently published in the journal Geophysical Research Letters.
