In the 1970s, NASA's Viking program observed enormously high mounds in Martian craters. It has taken until now for us to understand how wind can form these giants.
The one really scientifically inaccurate part of "The Martian" is the dust storm at the start threatening to blow over an ascent vehicle. The thin Martian atmosphere is incapable of anything so dramatic, but like the proverbial bird sharpening its beak, given enough time even such weak forces can make mountains.
"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 said in a statement. "So the cool thing about our paper is we figured out the dynamics of how wind could actually do that."
Gale Crater's central peak is called Mount Sharp. More than 5 kilometers (3 miles) high, base to tip, Mount Sharp surpasses the largest mountains on most Earth continents, but Mars' low gravity allows such towering peaks it can be called a mound. Studying the mountain was a major reason the crater was chosen as the landing site for Curiosity, and Day used the information the rover has returned to explain the mound's formation.
Day and colleagues built a physical model of Gale Crater 30 centimeters (12 inches) wide and 4 centimeters (1.6 inches) deep. Once packed with wet sand and exposed to a variety of air speeds in a wind tunnel, the model eroded into something similar to what can be seen in Gale and other craters.
The tunnel's wind only came from one direction, and the short experiment didn't dry out the model the way several billion years have done to Mars. Nevertheless, the pattern produced of a central mound surrounded by a moat near the crater's rim is a familiar one from Mars.
Top line: Martian craters at different stages of erosion. Bottom line: Day's model with time in the wind tunnel. Red is maximum elevation. Mackenzie Day
Crucially the process only worked because the sand was initially damp, creating a solid base. This fits with Curiosity's observations that Mount Sharp's bottom is made from sedimentary rocks from a time when the crater was a huge lake.
Studies of the Martian surface show that mounds only exist in places that probably held water during Mars' Noachian period 3.7 billion years ago. The findings offer a chance to learn more about what Mars was like during its wet period, possibly shedding light on the best places to look for remnant signs of life.
"On Mars there are no plate-tectonics, and there's no liquid water, so you don't have anything to overprint that signature," said Day. "Wind could never do this on Earth because water acts so much faster, and tectonics act so much faster."
The future may be grim for Mount Sharp, however. Left in the wind tunnel long enough, the model mounds eroded away.