The dark, narrow, thread-like stripes that can be seen in the images are the traces of small whirlwinds – known as dust devils – which are caused by atmospheric turbulence. Dust devils on Mars form in a similar way to their counterparts on Earth – as the Martian soil is warmed by sunlight during the day, the heated air rises, creating an updraught. Under certain conditions, this updraught then begins to rotate, causing a vertical vortex. Differences in atmospheric pressure create a suction effect, lifting any loose material from the surface. On Mars, this loose material is dust; on Earth it may also come in the form of sand or snow. When a dust devil moves across the Martian surface, it lifts a thin layer of light-coloured sand from the ground, exposing the darker underlying material. These narrow dark tracks mark the paths taken by dust devils.
The trails left by dust devils on the Martian surface can be up to several hundred metres wide and several kilometres long. They tend to form in the afternoon on warm spring and summer days, when the surface is illuminated by sunlight for a longer time, and thus heats up more. Nevertheless, dust devils have also been observed during the winter months. Dust devil tracks may run straight or follow curved or winding paths, as well as intersecting or overlapping with one another. Their lifespan is relatively short, as they tend to be covered over again by light-coloured sand that is whipped up by dust storms, so that they disappear within a matter of days or months.
Dust devil traces can be found almost everywhere on Mars. They are particularly well documented on the plains of Amazonis Planitia and Argyre Planitia, as well as on Hellas Planitia, the lowland plain in Hellas, the largest impact basin on Mars. However, they can also be seen in the Proctor and Russell impact craters near the area shown here, as well as in many other regions of Mars. Although first-hand sightings of dust devils are far more common on our planet, they leave few recorded traces. They are typically observed in summer in dry desert landscapes, such as in the southwestern United States, Africa, Australia and China.
Large temperature differences due to intense warming during the day mean that the dust devils on Mars can be much bigger than those on Earth. On Mars, they can reach a height of eight kilometres, effectively carrying dust into the Martian atmosphere. Estimates suggest that overall, dust devils on Mars can lift as much material as a global dust storm, thus contributing significantly to an increased amount of dust in the atmosphere.