Venus has always been mysterious. The earliest observers had two names for it, the Morning Star and the Evening Star. Telescopes showed only a disk of brilliant white, thick clouds hiding all trace of a surface beneath. More recently radar mappers, the Russian landers and other spacecraft studies have lifted Venus's veil to science, but today it remains mysterious.
Venus is almost a twin of Earth in terms of size, mass and composition. It's only slightly smaller, formed in the same neighborhood of the solar system, and has a large iron core and rocky silicate mantle. Like Earth, its crust is largely basalt (though on Earth the basalt is almost all hidden by the ocean).
The Role of Heat in Venus Geology
The key to the geology of Venus seems to be its heat. Start with its thick, choking atmosphere of nearly pure carbon dioxide. Air pressure on Venus's surface is about 90 times Earth's, the same as it would be a full kilometer under the ocean. This greenhouse gas is so effective at trapping the sun's heat that the ground of Venus is literally hot as a furnace, reaching around 450 degrees C (or 725 K) at the equator, year round. There is no water anywhere except a little high in the atmosphere. The white clouds are composed of sulfuric acid droplets.
The crust of Venus appears to be almost entirely volcanic and basaltic. There is nothing there like Earth's continents—no granitic rocks at all, high in silicon and oxygen (except possibly the high plateau called Ishtar Terra). Venus instead has large, bizarre fractured structures called coronae ("crowns") and tesserae ("mosaic chips"). There is no large-scale motion of the crust, no plate tectonics. On Earth, plate tectonics is driven by surface cooling, which makes the cold plates denser than the soft rock layer beneath them. On Venus the surface cannot cool and the crust cannot overturn.
Nevertheless, the presence of lava everywhere we look on Venus means that deep heat from the underlying mantle and core can melt rocks and cause that magma to erupt. The eruptions appear random, unlike the organized lines and arcs of volcanoes on Earth. Is this all that ever happens? The way to answer that question is to study the geologic history of the planet in detail.
Venus's Geologic History
The backbone of planetary dating is crater counts. We have a good idea, from studying the other planets, of how old a surface is based on the number of craters it has. On all of Venus there are about 1000 craters, an unusually small number. The crater statistics tell us that although the planet is 4.6 billion years old, nothing on its surface is older than roughly 0.5 billion years. Almost all of Venus's history is a blank slate.
It looks as if half a billion years ago, all of Venus's surface was replaced. Within perhaps one-tenth that time, maybe 50 million years, almost all of the features we see were emplaced: highly deformed tesserae first, then large plains of lava heavily wrinkled in huge ridges. The lava flows that have occurred since that early period are fairly smooth and undisturbed. It is the picture of a thin, soft crust quickly growing harder and thicker as the mantle roiled beneath it. That's one working hypothesis.
It's easy to picture a gigantic planetary cycle in which Venus "boils over" and replaces its surface with a new lava crust. It's harder to prove such a model, or to say how fast it happens. Only with more information can we make progress. But there are no plans yet to return to Venus's surface (Venus Express, which arrived at Venus in 2006, and Planet-C, which failed to orbit Venus in December 2010, are both orbiter missions). For now we must work with the data we have—plus powerful computer simulations. Those suggest that Venus's wrinkled face may be due to hot flashes from above and below, as I explain next.
Venus crater-corona image derived from Magellan Image Browser (now at the Map-a-Planet site)