Radar feature

On radar images, landforms are told apart by the way they reflect radio waves. This is how they become “radar features”.

Radar can use many radio wavelengths. Venus has so far been studied at 12 cm. (Longer waves would also reach layers below the surface.) A 12 cm signal is reflected best by objects of about that size; from smaller ones it glances off. The more signal comes back, the stronger the radar “echo”, and the brighter the pixel in the image. Rough lavas and the ejecta blankets around craters look bright in this way.

A surface that is smooth to the radar sends no echo back, so it stays dark in the images. Examples are old, eroded or dust-covered areas, but also some fresh, smooth lava flows (pahoehoe). A liquid surface would also look dark, but Venus has none. Nor does it have ice, which looks bright to radar. In dry, loose and porous material (such as tuff) the radar can penetrate, and no echo returns.

The electrical conductivity and moisture of the surface material also affect how it reflects radar waves. Moist material returns more signal, because the radar cannot penetrate as deep.

Radar can also make topographic maps, in altimetry mode. The transmitter then sends a signal straight down. The arrival time of the echo gives the distance it travelled, down and back.

The glowing surface: once, despite the thick cloud layers, the surface has been observed in visible light. The 2020 photograph on the cover of the book was taken by a solar probe on its way past the night side of Venus, with a long exposure. Light from the hot rock surface, at the boundary of red and near-infrared, glows through the clouds. It does so only above the low-lying areas. The higher ground is cooler and stays dark – here Ovda Regio, in the middle of the map (Wood et al. 2022).