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In Brief

5 January 2011

Did magma rain on the early Earth?

IF MOLTEN rock once rained onto Earth’s surface, it might explain puzzling differences between the chemical compositions of our planet and the moon.

After a Mars-sized body struck the infant Earth, a common atmosphere of rock vapour enveloped the resulting magma-covered Earth and its disc of orbiting magma. This atmosphere should have thoroughly mixed material from Earth and the disc, which later formed the moon.

But while the Earth and the moon have the same relative abundance of different oxygen isotopes, some measurements suggest that moon rocks are iron rich and magnesium poor compared with those on Earth.

Magma rain could explain the discrepancy. A team led by Kaveh Pahlevan at Yale University calculate that as rock vapour rose from Earth’s hot, roiling surface, the magnesium oxide it contained would have condensed into droplets and rained back to the surface more readily than the more volatile iron oxide, which could then have mixed into the moon-forming disc (Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2010.10.036).

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