29 August 2002

Planet formation: Core chronologies

The timescale of core formation in planetary bodies can be calculated using a number of isotopic decay phenomena, most importantly the relatively new ¹⁸²Hf–¹⁸²W (hafnium–tungsten) chronometer. Lithophilic ¹⁸²Hf is preferentially partitioned into silicates, then ¹⁸²Hf decays to ¹⁸²W with a half-life of 9 million years. Models based on this chronometer have been out of step with other indicators, suggesting that the Earth's core formed gradually, 60 million years after the formation of the Sun. But now two groups propose revised ¹⁸²Hf–¹⁸²W dates based on new isotope data from meteorites including samples from Mars and the asteroid Vesta. On the revised chronology, planet formation was rapid, taking just 10–30 million years.

Age and origin of the Moon
QINGZHU YIN, S. B. JACOBSEN, K. YAMASHITA, J. BLICHERT-TOFT, P. TÉLOUK & F. ALBARÈDE
Nature 418, 949–952 (2002); doi:10.1038/nature00995
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Core formation on Mars and differentiated asteroids
T. KLEINE, C. MÜNKER, K. MEZGER & H. PALME
Nature 418, 952–955 (2002); doi:10.1038/nature00982
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Planetary science: Birth of a Solar System
A. G. W. CAMERON
Radioisotope dating of meteorites suggests that planets formed in the Solar System over shorter timescales than had been thought. There are consequences for how the Moon formed, but is this the final word?
Nature 418, 924–925 (2002); doi:10.1038/418924a
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Planets were precocious