Venus is usually noted for its proximity to and near identical size with Earth. This was noted shortly after the invention of the telescope hundreds of years ago, yet that is about all we knew of Venus until a few decades ago. Even its unusual retrograde and very slow rotation (243 days) were unknown until radio telescope technology had advanced sufficiently in the mid-1960's.Venus has been the most visited planet, with 9 orbiters, 7 atmospheric probes or hard landers, 2 atmospheric balloons, 10 soft landers, and 6 flybys since 1962. Despite this, geologic studies of Venus were hampered by our inability to see through the dense clouds. Until about 10 years ago, there was only a crude sense of what the overall surface looked like: the Pioneer-Venus mission provided us with a coarse topographic map, and Earth-based radio telescopes gave us images with detail of about 1 kilometer (roughly that which we get when viewing the moon from a small visual telescope). Even these images were limited by a near-orbital resonance with Earth, which brings nearly the same face of Venus into view when it is closest to Earth. It was not until the early 1980's, when the Soviet Venera 15 and Venera 16 spacecraft sent back images of the northern quarter of Venus, that the spectacular nature of the venusian surface was clearly revealed. The Magellan mission mapped nearly the entire planet, and in more detail than the earlier missions; scientists are still interpreting the data and mapping the geology.
These spacecraft revealed a global network of plains on Venus, interrupted by islands and continents of tesserae terrain's: high, complex surfaces that are heavily fractured and sometimes bordered by mountainous fold or thrust belts. These tesserae-bounding mountain belts (and also the ridge belts that are found in the low-lying plains) show that crustal convergence has occurred on Venus. The Veneras also revealed the interesting coronae-very broad, low domes that appear to be the surface expressions of rising plumes of hot mantle material. Fracture belts, where the crust appears to be pulling apart, encircle Venus (see figure). Large shield volcanoes and extensive lava flows are often astride these fracture zones. The observed convergent landforms indicate metamorphism of rocks probably occurs, but in the absence of significant erosion these rocks are much less likely to be eventually exposed at the surface (as is common in highly folded mountainous regions on Earth). In the areas of high terrain on Venus, the gravity field is high. This correlation between terrain height and the gravity field is only sometimes true for Earth landforms. Explanations of this phenomenon argue for geologic activity in the past, and some even require that there be some current activity. Having relatively few impact craters further tells us that Venus' surface is no more than about 500 million years old, about the average age of Earth's surface. Though still old, in the absence of any near-surface liquid, erosion on Venus is most likely to be chemical in nature, and therefore less effective than erosion landforms experience on Earth. For this reason, Venus' landforms will still look pristine even though they may be comparable in age to similar features on Earth.