VENUS

Structure
Atmosphere
Rotation 

Venus

Radius: 6050 km (3780 miles)

Mass: About the same as that of the Earth. Venus is often called Earth's sister planet. She is, however, a rather grim sister!

Composition: Probably similar to Earth.

Atmosphere

96% carbon dioxide

Permanent cloud cover; from 30 km (19 miles) up to 60 km (37 miles)--composition: sulfuric acid.

Pressure at surface: 90 times Earth's atmospheric pressure; that is, 90 kg per square centimeter.

The dense atmosphere of carbon dioxide causes Venus to suffer from an extreme greenhouse effect. 

The light from the Sun filters through the atmosphere and is absorbed by the planet, causing its surface to heat up. The heating causes the planet to emit infrared radiation which, in principle, would cause the planet to cool. But on Venus the carbon dioxide atmosphere is so dense that it is very effective in preventing much of this radiation from escaping into space. 

Surface temperature about 750 K (about 900 F).

There is a permanent rain of sulfuric acid. But it is so hot at the surface that the rain never reaches the ground!

One day on Venus is 243 Earth days. However, Venus rotates backwards with respect to the Earth! On Venus the Sun rises in the west and sets 118 days later in the east.

Venus' surface was mapped in detail by the United States space probe Magellan. The mapping was done by analyzing radar waves reflected off the surface. We expected Venus to have features very similar to those of Earth; in fact, the surface of Venus is quite unlike the Earth's. This is a great unsolved puzzle.

Venus is much less mountainous than the Earth. There are only two large mountainous regions: Ishtar and Aphrodite. (Ishtar is the Babylonian goddess of love; Aphrodite is the ancient Greek name for Venus.) The Ishtar region contains the highest volcanic peak called Maxwell Montes that rises to 11 kilometers (6.8 miles) above the plains. It is not known whether the volcanos on Venus are active. (Gula Mons, Maat Mons.)

A remarkable observation is that the craters on Venus are uniformly distributed over its surface; there are also fewer craters than expected. One possible explanation for this fact is that the craters formed relatively recently, and the older craters have been erased. This hypothesis suggests that, somehow, Venus re-surfaces itself on a time-scale of several hundred million years. But how this planet-wide re-surfacing can happen, given the apparent scarcity of plate tectonic activity, is far from clear.