The Venus Transit 2004

... Brief InfoSheet D4

Venus, the Earth's sister planet ?

Until the 1960s, Venus was often considered a "twin sister" of the Earth, because Venus is the nearest planet to us, and because the two planets seem superficially to share many characteristics, for instance:

• Venus and the Earth are members of the group which is called "the terrestrial planets" and they were born in the same rotating cloud of gas and dust 4.6 billion years ago.
• Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).
• Both planets have atmospheres with dense clouds.

Until the era of space exploration with its missions to Venus in the early 1960s, the last noted point was a strong argument that under its dense clouds Venus might be very Earth-like and might even have life. Some people thought the surface of Venus might be covered with large jungles similar to those which existed on the Earth during the Carboniferous and Permian periods with prehistoric reptiles and insects.

However, we now know after years of intensive study of the numerous data transmitted by space probes that Venus is radically different from the Earth in many important ways. It is not a twin sister or even an ordinary sister of our planet - it is only a cousin!

The pressure of Venus' atmosphere at the surface is 90 atmospheres, that is, about the same as the pressure at a depth of 1 km under the Earth's oceans. The atmosphere is composed mostly of carbon dioxide. There are several layers of cloud many kilometres thick which completely obscure our view of the surface.

The dense atmosphere of Venus produces a runaway "greenhouse effect" that raises its surface temperature to about 470 °C. This is hot enough to melt lead! Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

Venus probably once had large amounts of water like the Earth but it all boiled away. It is now quite dry and the surface of Venus is a glowing hot stony desert, with continents, plains, canyons, mountains, impact craters and volcanoes.

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This topic is discussed in a more detailed way in the associated Extended InfoSheet.

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