They shift slowly but surelyA long-standing mystery about Mars' sand dunes may have just been sold by scientists. The strange thing about the dunes is that they look as if they were created by winds, but there are no winds on the surface of Mars.
Discovered in 1971 in pictures taken from space, above the surface, they look very similar to sand dunes on Earth, so they must have been created through the same processes as in our deserts. However, how could winds have been causing them when the Martian atmosphere is so thin and still?
Many consecutive missions to Mars have detected no change in the position and sizes of the dunes, unlike the dunes on Earth, which are shifting constantly.
Astronomers now found out that they really have been formed by the action of winds, only that winds on Mars are moving so slowly that they're virtually undetectable, meaning that the dunes were formed far more slowly than back on Earth, like in a movie seen in slow motion.
Eric Parteli at the University of Stuttgart and Hans Herrmann at the Swiss Federal Institute of Technology in Zürich has performed a series of computer simulations of the formation of two distinct types of dunes seen on Mars: the arrowhead-shaped "barchan" dunes, (formed when the wind blows mainly in one direction), and elongated "exotic" dunes (formed when the wind alternates between two directions).
They found out that if the Martian sand had been acted upon by to a 40-second gust of wind every five years, which is what robotic probes on Mars' surface have recorded, it would have taken the exotic dunes (derived from the barchan ones) a period of about 10,000 to 50,000 years to form.
The formation process has also been explained, and it's called "saltation," which means a small grain of sand is lifted by the wind, and driven along above the sand until it falls back, creating a splash of ejected grains. The ejected grains could then also be picked up by the wind leading to a rapid multiplication of the amount of sand that is being blown along.
Results also shown that the dunes on Mars do shift, only that they do it far too slowly for us to detect their movement, which is estimated at 1 meter (0.3 ft) every 4000 years.