Radar waves are used to map Venus’s surface through its dense cloud layers.
Venus is permanently hidden beneath thick clouds rich in sulfuric acid, so ordinary visible-light cameras cannot see the ground from orbit. Radar, especially microwave radar, can penetrate the atmosphere and bounce signals from the surface. Instruments measure the returning echoes to reveal terrain, roughness, slopes, and elevation.
The most comprehensive mapping came from NASA’s Magellan spacecraft. Its synthetic-aperture radar mapped almost the entire planet during the early 1990s, producing images of volcanoes, impact craters, lava plains, rift zones, and other structures. Radar brightness does not represent ordinary color: bright areas generally return stronger echoes, while smooth surfaces may appear darker because they reflect energy away from the spacecraft.
Ultraviolet observations are useful for studying patterns in Venus’s upper clouds, but they do not map the ground. Gamma rays and visible light likewise cannot provide the global surface maps made by radar.