Two planets collected most of the rocky material that was left in the inner nebula after the gas and volatile elements were swept away by early solar activity. They finished up with nearly an equal share. Here I investigate why these twins that have almost the same mass and density have turned into such different worlds.

Venus

Venus, rising in the morning or setting in the evening skies, is the most brilliant object in the sky, after the Sun and the Moon, and has been admired since antiquity. Because it is an apparent twin of the Earth, it has always been of interest as the only similar planet in the solar system. When Venus was found to have an atmosphere, it did not take much imagination to make it a hotter version of the Earth. In popular literature, it was clothed with thick tropical forests and swamps, that were populated with various monsters. Dinosaur-like creatures were favorites of science-fiction writers [1].

Early observers thought that the planet was either spinning rapidly, similar to the 24-hour period of the Earth, or perhaps on a monthly period. But Venus was discovered from radar observations to be unique among the planets. Although the atmosphere of Venus rotates in about four days, the planet itself has a retrograde rotation that takes 243 days [2] (Table 12.1). Venus orbits around the Sun in 225 days and so the day on Venus is longer than the year.