Credit: NASA, HIRISE, MRO, LPL (U. ARIZONA)

The surface of Mars bears the scars of numerous giant dust devils — vortices of warm air that swirl across the planet's barren surface. During their fleeting existence, the dust devils dance and weave through the craters, climbing crater walls and leaving dark trails in their wake. The Viking orbiters first identified martian dust devils in the 1970s and 1980s. They are generally much larger than their equivalents on Earth. Although their size and power can pose a significant threat to our visiting technology, they have also proved to be strangely useful. In March 2005, the martian rover Spirit saw her power levels jump. A passing dust devil is thought to have cleaned Spirit's solar panels by removing a layer of dirt that had accumulated, giving her a new lease of life.

Dust devils are whirlwinds, formed by vertical columns of rotating air. They can form almost anywhere, but most commonly they form in barren desert regions. They develop when air near the surface is heated and rises. More surface air rushes in to fill the gap left by the rising air. As it passes over and around uneven topography, the air may begin to rotate. The vertical motion intensifies the spinning, drawing in more air and forming a vortex. The vortex can whip up any loose dust and debris from the surface, creating a visible funnel. Dust devils provide an important mechanism for transporting dust from the surface into the atmosphere.

The surface of Mars is littered with abundant dark dust-devil tracks. Analysis of martian imagery has provided some insight into how the tracks may form. It is thought that the dust devil erodes the surface, removing the fine-grained fraction of surface dust, or causing it to infiltrate deeper into the substrate, exposing coarser-grained material below. The coarse-grained substrate has a lower albedo compared with the surface dust and appears darker. The dust devil therefore creates a dark trail in its wake. Although dust devils are abundant on Earth, observations of their darkened tracks are very rare. Because of their rarity, we do not know precisely how terrestrial dust-devil tracks form.

Dennis Reiss at the Westfälische Wilhelms-Universität, Münster, and colleagues carried out the first in situ analysis of rare darkened dust-devil tracks observed in the Turpan Desert, north-western China (Geophys. Res. Lett. doi: 10.1029/2010GL044016; 2010). Using high-resolution photographs, they measured the size of the grains in the dark tracks compared with the surrounding surface. The dark tracks were composed of very coarse-grained material that had been stripped of any finer grain sizes by the dust devil, creating a dark, low-albedo surface. Their analyses confirmed that dark dust-devil tracks on Earth can form in the same way as those on Mars.

The Turpan Desert analyses reveal that a layer of dust only ∼2 μm thick needs to be removed to create a dark dust-devil track, implying that it is not the thickness of the removed layer that creates the dark track. The researchers suggest that the formation of dark tracks is dependent on the albedo contrast that is formed. If the grain size of the dust that is removed from the surface is significantly smaller than the grain size of the underlying material, a strong albedo contrast is formed and a dark track appears. Thus, the lack of contrast between the grain size of the surface dust and the underlying surface, for example, in a desert where fine sand rests on yet more fine sand, could help to explain why dark dust-devil tracks are observed so rarely on Earth.