THE LAVA PLANET
To peel away the clouds and map Venus's geological fea-tures, scientists used an imaging radar on a spacecraft named Magellan. Building on work by US and Soviet probes and Earth-based radar, Magellan inventoried a museum of volcanic features, completing its global survey in 1994.
Sixty percent of Venus's surface is a lava plain. The rest falls mainly into two continents: an equatorial one named Aphrodite Terra, and a northern one called Ishtar Terra. These stand a few miles higher than the plains. The remainder of the surface consists of individual mountains and broad-based volcanic peaks.
On the plain, Magellan revealed long channels that mark past lava flows, and found fissures and vents from which vast sheets of lava once poured, often in repeated eruptions. Scientists identified pancake-like lava domes, and found strange oval features called coronae that have no counterpart on Mercury, Earth, or Mars. The coronae may have formed when a rising blob of molten rock pushed up the surface, and subsided, leaving a distorted ring around a central depression.