Wind streak

Wind streaks are traces of wind-swept areas on the surface, often near a topographic obstacle.

A wind streak can form where the wind removes sediment and sweeps the surface “clean”. On Earth, wind streaks are rare outside deserts because of the vegetation. On Mars they can be seen in many places, where the wind scours the dusty surface. On Venus, wind streaks are visible on radar images. Depending on the place, they are brighter or darker than their surroundings.

On radar images, brightness shows how smooth the surface is at the centimetre scale: coarse-grained material is radar-bright. On Mars, grain size also affects brightness on optical images: there, coarse-grained material is dark. Where the wind removes the fine grains, the remaining lag cover may change in brightness in this way.

Greeley et al. (1994) found 5830 wind streaks on Magellan images. They are on average 20 km long and 1 km wide, and shaped like drops, fans or long streaks. Near craters, parallel streaks are more common; these are called zebra-striped. Streaks near craters point west, following the global wind flow. Others point towards the equator. Streaks have also been found behind small shield volcanoes. There, the pattern of the wrinkle ridge plains shows through where the streak lies.

Wind streaks are usually deflation (blow-out) features. But the wind can also drop the dust and sand it has picked up in another zone. These deposition zones are called echo dunes and shadow dunes (in front of and behind the obstacle), or wind tails when they are smaller. Such obstacle-bound forms are common around topographic obstacles, and they stretch far out on the lee side.

During stronger windstorms, wind scouring an open surface also causes deflation on flat ground. The shock waves around impacts can be seen this way, too. In an otherwise calm region they may be the most important aeolian process.

Wind-cut grooves and residual ridges (yardangs) form in wind channels. So these forms can be seen as the topographically expressed versions of wind streaks. Seasonal or occasional winds, by contrast, can make wind streaks that last only a few weeks. Later winds then blow them away, or falling dust covers them. Such changing forms have been watched in detail on Mars since the 19th century. On Mars, however, global dust storms every few years rewrite the dust cover of the surface.

For Venus there is as yet no long series of surface observations, and its dust conditions are unknown. The surface images, however, show a dust-free, rather rocky or rubbly surface. Measurements show that the winds at the surface of Venus are weak. Dust devils are common on Earth and Mars. If they also move in the hot atmosphere of Venus, the radar images may not yet be sharp enough to show their winding tracks.