Continued growth by accretion leads to larger and larger objects. The energy released during accretionary impacts would be sufficient to cause vaporization and extensive melting, transforming the original primitive material that had been produced by direct condensation in the nebula. Theoretical studies of this phase of the planet-forming process suggest that several bodies the size of the Moon or Mars must have formed in addition to the planets found today. Collisions of these giant planetesimals—sometimes called planetary embryos—with the planets would have had dramatic effects and could have produced some of the anomalies seen today in the solar system; for example, the strangely high density of Mercury and the extremely slow and retrograde rotation of Venus. A collision of Earth and a planetary embryo about the size of Mars could have formed the Moon. Somewhat smaller impacts on Mars in the late phases of accretion may have been responsible for the present thinness of the Martian atmosphere.
Studies of isotopes formed from the decay of radioactive parent elements with short half-lives, in both lunar samples and meteorites, have demonstrated that the formation of the inner planets, including Earth, and the Moon was essentially complete within 50 million years after the interstellar cloud region collapsed. The bombardment of planetary and satellite surfaces by debris left over from the main accretionary stage continued intensively for another 600 million years, but these impacts contributed only a few percent of the mass of any given object.
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