S.I. Rasool and C. De Bergh, 1970

Nature, volume 226, pages 1037-1039

Although Venus and the Earth are similar in size and mass, are adjacent in the solar system and were probably formed out of the same homogenous mix of gas and dust about 4.5 billion years ago, their atmospheres and surface conditions differ markedly. For example, the atmosphere of Venus is ~75 times more massive than that of the Earth and is largely composed of carbon dioxide, a gas which constitutes only 0.03 percent of the Earth’s atmosphere. The Venus atmosphere seems to be deficient in water, with respect to Earth, by as much as a factor of 104. The surface temperature of Venus is 700 K.

We believe that the chief differences can be explained by the single circumstance that venus was formed 30 percent closer to the Sun. If the Earth had formed only 6 to 10 million km nearer to the Sun, it may also have become a hot and sterile planet. As for Mars, it seems that is the relative smallness of its size and mass — a weaker internal activity — which has slowed its progress towards accumulating an Earth-type atmosphere and oceans.

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