The Venusian clouds kept astronomers from seeing the planet's surface. Without visible landmarks, they could not measure how fast Venus turns on its axis. It took cloud-penetrating radar, first aimed at Venus in the early 1960s, for scientists to discover that Venus rotates backward as compared to the other planets in the solar system. This means that the Sun rises in the west and sets in the east. And Venus' rotation is extremely slow, so the length of a "day" on Venus is longer than a year.
The planet's "retrograde" rotation could be the result of a collision between Venus and a large proto-planet billions of years ago, when the solar system was forming.
Early spacecraft discovered that Venus' atmosphere is thick and toxic. It is made primarily of carbon dioxide, with a surface pressure 90 times greater than on Earth -- equivalent to the pressure at a depth of more than a half-mile (900 meters) below the surface of the ocean. Clouds of sulfuric acid, blown by high-speed winds, top the atmosphere.
This dense atmosphere makes Venus the hottest planet in the solar system. The atmosphere absorbs the Sun's heat but doesn't allow it to escape. This "greenhouse effect" has heated Venus' surface to about 860 degrees Fahrenheit (450°C) — hot enough to melt lead.
The first views of the surface came from four Soviet landers, each of which survived for only a few minutes before succumbing to the heat and pressure. The images showed flat, angular rocks around the landers, but not much else.
The best view of Venus came from the Magellan spacecraft, which entered orbit around Venus in 1990. Its cloud-penetrating radar mapped 98 percent of the planet's surface. Magellan discovered mountains on Venus that are taller than any on Earth, as well as a valley that is the longest and deepest in the solar system. Thousands of volcanoes dot the Venusian surface. Magellan provided hints that some of them may still be active, but it did not detect any eruptions. Magellan's observations suggest that the planet's entire surface was repaved by global volcanic eruptions about 800 million years ago.
The only impact craters detected on Venus are large. The surface lacks smaller craters because small meteoroids burn up in the planet's thick atmosphere before they hit the ground.
Although Venus is completely dry today, a spacecraft in the 1980s found evidence that the planet could have been much wetter in the distant past. In fact, it might have had enough water to cover the entire planet to a depth of 50 feet or more.
Scientists think Venus' interior is a lot like Earth's, with an iron-nickel core surrounded by a rocky mantle. But unlike Earth, Venus does not have tectonic plates or a magnetic field.