Venus is the planet most similar to the Earth in mass, radius and solar distance. Current theories of the early evolution of the solar system suggest that Earth and Venus each formed by the accretion of planetesimals—small rocky or rock‐metal objects—that collectively constituted a well‐mixed sample of material condensed from the inner solar nebula. The bulk compositions of the two planets should thus be similar. The rates of internal heat generation and the energy available to drive interior convection should also be similar. An important difference between the two planets, however, is the character of their atmospheres. The mass of the dominantly CO2 atmosphere of Venus is two orders of magnitude greater, as a fraction of planet mass, than that of Earth's atmosphere, and the surface temperature is 450 K higher, a consequence of continuous global cloud cover and a runaway greenhouse effect. The mass of H2O in a vertical column of unit area is four to five orders of magnitude less for Venus's atmosphere than for the atmosphere and hydrosphere on Earth. As a result, the surface of Venus lacks a water cycle, and the processes of weathering, erosion and sediment transport that dominate terrestrial landforms are comparatively unimportant.

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