When NASA's Viking spacecraft touched down on the surface of Mars in 1976, one of the geologic features it observed was massive mounds inside craters. More recently, the Curiosity Mars rover got up close with one of these mounds called Mount Sharp where it landed in 2012 inside the Gale Crater. It revealed that the base of the three-mile (4.8-km)-high mound was made from sediment carried by water, while the upper layers consisted of regolith deposited by wind. To find out just how such a mixed mound could be developed, researchers created a "crater layer cake" and they popped it in a wind tunnel.

The researchers, led by Mackenzie Day, a graduate student at the University of Texas at Austin Jackson School of Geosciences, said that although the composition of the mounds was known thanks to Curiosity, no one knew exactly how they formed over billions of years, but it was suspected that they were the result of wind erosion alone — something impossible here on Earth.

"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 said. "Wind could never do this on Earth because water acts so much faster, and tectonics act so much faster."

To test out the wind theory, Day and her team built what she calls a "crater layer cake," which was a miniature crater made from damp sand that measured 30 cm wide by 4 cm deep (about 12 x 1.5 inches). That crater was placed in a wind tunnel and, sure enough, the mounds emerged and, under continued wind force, began to erode as well. The researchers also verified their results with computer modeling.

Understanding this process helped the researchers understand when the surface of Mars shifted from a wet topography to a dry one, as the bottom of the mound was made in a wet climate, while the top was made during a dry one. They pinned this shift in Mars' climate to the Noachian period, an era that started approximately 3.7 billion years ago.

"Planet-scale desiccation would herald a period of aeolian construction as dried sediment becomes available for transport by the wind," write the researchers in a paper published March 30 in the journal, Geophysical Research Letters. Aeolian refers to wind action, named after Aeolus, Greek god of the wind.

"Exposed subaqueous deposits would be reworked by the wind within craters, and sand-transporting global winds would decelerate upon entering crater topographic basins, thereby supplying the external sediment required to fill craters with aeolian strata," they continue.

"Over time, with available sediment increasingly housed within craters and other sinks, and sediment supplies from subaqueous deposits exhausted, winds become increasingly undersaturated and the wind regime shifts from depositional to erosional."

Or, as Day says: "So the cool thing about our paper is we figured out the dynamics of how wind could actually do that."