Radar dark parabola

In 1991 it was described as the “least understood” feature on Venus; its origin was first explained in 1992. To recreate the suspense of that first year, we give the currently accepted mechanism only at the end of this text. Until then, readers can work out their own theory.

Parabolas on Venus reach 10–50 crater diameters. Their opening arms face west and their apex east, and the crater lies at their focus. Some have anisotropic radar properties: their radar brightness differs when seen from the east or from the west, as with microdunes. This may be a trace of the wind.

Radar-dark parabolas are seen only around craters younger than 150 million years. Their layer, up to 10–15 m thick at first, is slowly carried away by the wind (Bondarenko and Head 2009). Around older craters the dark area therefore grows ever smaller (Bondarenko 2003).

The layered stone slabs visible on the Venera images taken at the surface may be the layers that make up radar-dark parabolas (Basilevsky et al. 2004) (see plate 2).

Their origin is linked to the winds of the upper atmosphere of Venus, which blow at 400 km/h at a height of 60–70 km. This is the height of the top of the cloud zones of Venus. These winds circle Venus in only four or five days. Seen from outside, Venus therefore seems to turn once every four days. In fact only the atmosphere turns – that is, the pattern of the clouds that we see. The surface below turns 50 times more slowly. (The Japanese probe Akatsuki made a film of this; Horinouchi et al. 2017.) This is called superrotation.

So although a solar day on Venus lasts 117 days (from noon to noon; Pollack and Young 1975), heat does not build up on the day side. The atmospheric flow spreads it evenly. The flow is westward (from the east), which matches the direction in which Venus rotates (opposite to Earth’s).

At impact, a hot vapour cloud rises. With great energy it carries finely ground solid debris and droplets of impact melt beyond the edge of the atmosphere. From there the particles continue on ballistic paths. When they fall back into the atmosphere, they soon lose speed. As they fall, the strong westward flow of the upper atmosphere sweeps them along. By the time the particles leave the wind zone and settle out of the atmosphere, they form a parabola-shaped blanket on the surface (Vervack and Melosh 1992).

On Earth, ash fall from volcanic eruptions or snowfall are analogues of fine debris blown sideways by the wind as it falls. On Earth, in the stratosphere above the clouds, at about 10 km, steady westerly or easterly winds blow (they alternate from time to time; Newman et al. 2016). The stratospheric flow on Earth is about as fast as the upper-atmosphere flow on Venus (one circuit in 4 days). But compared with the surface below, which turns once in 24 hours, it moves more slowly: it is a subrotation.