Although Venus is called the Earth twin, its surface conditions appear to be very different from those of the Earth. The planet is surrounded by thick clouds of sulfuric acid, preventing its observation. The surface of Venus is extremely hot and dry. The temperature of the planet's surface is about 870 degrees F (465 degrees C), the temperature is higher than that of any other planet.

Most astronomers believe that Venus's high surface temperature can be explained by what is known as the greenhouse effect. A greenhouse lets in radiant energy from the sun, but it prevents much of the heat from escaping. The thick clouds and dense atmosphere of Venus work in much the same way. The sun's radiant energy readily filters into the planet's atmosphere. But the large droplets of sulfuric acid present in Venus's clouds -- and the great quantity of carbon dioxide in the atmosphere -- seem to trap much of the solar energy at the planet's surface.