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Wednesday, July 7, 2010

Our moon is younger than we thought


OK, not that much younger. It turns out that the moon may only be 4.4 billion years old, rather than 4.5 billion years old. This age discrepancy was uncovered by Tais Dahl (then at the Niels Bohr Institute and currently at Harvard) and his thesis advisor, David Stevenson of Caltech.

The solar system formed 4.57 billion years ago as small planetoids collided with each other and coalesced into the planets, asteroids, moons and other stellar objects we find today. In the case of the moon and Earth, a massive collision with a Mars-sized object knocked the moon out of the ‘pre-Earth’. One school of thought held that that this collision occurred about 30 million years after the formation of the Earth and the rest of the solar system. New evidence puts that date closer to 150 million years.

The researchers used radiometric dating to come to their conclusions. Hafnium-182 182Hf, a substance that binds to silicates (rock, which is mostly in the surface, or mantle), decays to tungsten-182 (182W), an isotope that binds preferentially to metal (which is now predominantly in Earth’s core). As the half-life of 182Hf is 9 million years, in about 100 million years, only about 0.1% of the 182Hf is left. By looking at the amount of 182W in the rocky mantle versus in the metal core, scientists can tell how early the metallic core formed. For example, if the metal had remained in the Earth’s crust for the first 100 million years and then sunk into the core, it would have taken most of the 182W with it. Conversely, if the metal had sunk in the first few million years before much 182W was even around, the 182Hf would have decayed to 182W on the surface and remained there.

It turns out that most of the 182W is in the mantle. This indicates that at the time of the moon-forming collision, most of the 182Hf had already decayed. Clear as regolith?