Mars is a dynamic planet where the surface is constantly being reshaped by wind. Sand dunes are a prominent and vast feature, covering expansive regions and serving as significant geological structures. These wind-sculpted formations are active landscapes that illustrate the power of atmospheric forces on an alien world.
Confirmation and Appearance
The existence of Martian dunes was first confirmed by early orbital surveys, such as the Viking missions. Modern, high-resolution imagery from the HiRISE camera on the Mars Reconnaissance Orbiter confirms their variety and activity. Dunes are scattered across the planet, from equatorial craters to the vast sand sea known as the North Polar Erg, or Olympia Undae.
Similar to terrestrial deserts, Martian dunes form various characteristic shapes reflecting local wind conditions. Common types include crescent-shaped Barchan dunes, which indicate a single dominant wind direction. Other formations include long, parallel linear dunes and complex star dunes, sculpted by winds blowing from multiple directions. Many of these dunes are massive, with some towering up to 120 meters (400 feet) in height.
The Mechanics of Martian Dune Formation
The physics governing sand movement on Mars differs from Earth due to the planet’s thin atmosphere, which is less than one percent of Earth’s surface pressure. To initiate the bouncing motion of sand grains, known as saltation, Martian winds must reach speeds up to ten times greater than those required on Earth. Only intense wind gusts, potentially reaching 150 kilometers per hour, are strong enough to initially lift the sand.
Once a grain is launched, the lower gravity and reduced atmospheric drag allow it to travel in “giant saltation” trajectories. These grains fly much higher, sometimes up to five meters, and travel much farther, covering distances between 20 and 120 meters in a single hop. The impact of these high-velocity grains then kicks up new particles, sustaining the movement even when the wind speed drops significantly. This phenomenon allows sand transport to continue at wind speeds much lower than the initial threshold.
Composition and Coloration
The composition of Martian sand dunes differs from the familiar reddish dust that coats the rest of the planet. Martian sand grains are primarily composed of basaltic material, a dark, volcanic rock rich in minerals like pyroxene and olivine. The planet’s overall red hue comes from fine, iron-oxide dust that settles globally after being lofted by storms.
The dune-forming sand is often darker, sometimes appearing gray or blue in enhanced color images. This dark coloration occurs because the active sand grains are relatively fresh and less weathered than the omnipresent red dust. Constant wind action prevents the fine, red oxide dust from settling and adhering to the larger, darker sand grains, revealing the underlying material.
Dynamics: Size and Movement Rates
Martian dunes include large sand formations categorized as megadunes. Individual dunes range widely in size, from a few meters to over a hundred meters tall. The vast size of these formations is attributed to the planet’s long geological timescales and lower gravity.
Despite occasional high-speed saltation events, the dunes migrate at extremely slow rates compared to those on Earth. Crescent-shaped dunes have been measured creeping forward at an average speed of 0.5 to 0.6 meters per Earth year. This slow migration is roughly fifty times slower than some of the fastest terrestrial dunes. This is due to the low density of the atmosphere, which limits the overall amount of wind energy transferred to the sand. The infrequent nature of wind powerful enough to initiate saltation also contributes to the sluggish pace.