The Earth gets clocked

How old is the Earth's core? You might think such a fundamental question would have long since been settled, yet various geological "clocks" give conflicting birthdays as far apart as 50 million years. That's a big discrepancy and a big puzzle for earth scientists.

One clock relies on the rate at which hafnium-182 radioactively decays into tungsten-182. By that reckoning, the core was formed 30 million years after the origin of the solar system, or about 4.54 billion years ago. But a second clock, based on the decay of two isotopes of uranium into lead, dates the core to 80 million years after the solar system's birth. Now the geochemists Bernard J. Wood of Macquarie University in Sydney, Australia, and Alex N. Halliday of the University of Oxford think they have resolved the inconsistency.

Wood and Halliday maintain that the hafnium-tungsten clock is correct. But, they point out, about 45 million years after the birth of the solar system a Mars-size object hit the young planet. It added its metallic core to Earth's and spun out debris that coalesced into the Moon.

The investigators think the collision sparked a dramatic change in Earth's chemistry, causing lead in the mantle to join the core. That event reset the uranium-lead clock, and so by its estimate the core looks much younger than it really is. (Nature 437:1345-48, 2005)

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