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Consequently, the number of collisions between planetesimals, or meteorites, decreased.
Fewer items available for accretion meant that it took a long time to build up a large planet. Wetherill of the Carnegie Institution of Washington suggests that about 100 million years could pass between the formation of an object measuring 10 kilometers in diameter and an object the size of Earth.
This observation rejuvenated the theory of accretion postulated by Otto Schmidt.
The Russian geophysicist had suggested in 1944 that planets grew in size gradually, step by step.
Scientists used to believe the rocky planets, including Earth, Mercury, Venus and Mars, were created by the rapid gravitational collapse of a dust cloud, a deation giving rise to a dense orb.
In the early 1990s, however, work by one of us (Allègre) on lead isotopes led to a somewhat new interpretation.
Among the many clocks, those based on the decay of uranium 238 into lead 206 and of uranium 235 into lead 207 are special.
Geochronologists can determine the age of samples by analyzing only the daughter product--in this case, lead--of the radioactive parent, uranium.
As Patterson argued, some meteorites were indeed formed about 4.56 billion years ago, and their debris constituted Earth.
But Earth continued to grow through the bombardment of planetesimals until some 120 million to 150 million years later. Geological Survey in Denver two decades ago and is in agreement with Wetherills estimates.
Very few fragments have survived this geologic machine.