Heavy metal rain may explain Earth-Moon differences

New experiments show that the asteroids that slammed into Earth and the Moon more than four billion years ago were vaporized into a mist of iron. The findings, published in Nature Geoscience, suggest that the iron mist thrown up by these high velocity impacts was fast enough to escape the Moon’s gravity, but stayed gravitationally stuck on the more massive Earth. And these results may help explain why the chemistry of the Earth and the Moon differ.

When and how Earth’s metallic core formed is uncertain. Clues come from known differences in the preferences of certain elements to end up in either the silicate mantle or the metal core. In a mixture of silicate rock and iron metal, some elements, such as gold and platinum, tend to prefer to associate with the metal, while others, such as hafnium, prefer the silicate.

As Earth’s iron-rich core formed, it “sucked” the metal-loving elements out of the planet’s rocky mantle. However, measurements of the silicate mantle by James Day have previously shown that there are more of them left near the surface of the Earth than would be expected. This has often been attributed to a late veneer of asteroids that delivered an extra dose of metal-loving elements to the rocky mantle.