Mile-high Mars mounds built by wind: study

The massive mounds of more than a mile high on Mars were carved by wind over billions of years, according to a new study that may help pin down when water on the red planet dried up during a climate change event.
The findings show the importance of wind in shaping the Martian landscape, a force that, on Earth, is overpowered by other processes, said Mackenzie Day, a graduate student at The University of Texas at Austin Jackson School of Geosciences.
"On Mars there are no plate-tectonics, and there is 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 said.

"Wind could never do this on Earth because water acts so much faster, and tectonics act so much faster," she said.
First spotted during NASA's Viking programme in the 1970s, the mounds are at the bottom of craters.
Recent analysis by the Mars rover Curiosity of Mount Sharp, a mound over three miles high inside Gale Crater, has shown that the thickest ones are made of sedimentary rock, with bottoms made of sediments carried by water that used to flow into the crater and tops made of sediments deposited by wind.

"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. We figured out the dynamics of how wind could actually do that," Day said.

To test whether wind could create a mound, researchers built a miniature crater 30 centimetres wide and 4 centimetres deep, filled it with damp sand, and placed it in a wind tunnel.
They tracked the elevation and the distribution of sand in the crater until all of it had blown away. The model's sediment was eroded into forms similar to those observed in Martian craters, forming a crescent-shaped moat that deepened and widened around the edges of the crater.
Eventually all that was left of the sediment was a mound - which, in time, also eroded away.
"We went from a filled crater layer cake to this mounded shape that we see today," Day said.

To understand the wind dynamics, researchers also built a computer model that simulated how the wind flowed through the crater at different stages of erosion.
The mounds' structure helps link their formation to climate change on Mars, with the bottom being built during a wet time, and the top built and mound shaped in a dry time, researchers said.
The research helps home in on Mars' Noachian period, a geologic era that began about 3.7 billion years ago, a period when Mars started to change from a wet world to a dry one.
Scientists were able to link the climate change to the Noachian by studying the location of more than 30 mounds and finding that sedimentary mounds were only present on terrain that was exposed during that period.

The research was published in the journal Geophysical Research Letters.