Why does the Earth's core appear to be so light?

The old tales may be true: There is brimstone in the underworld, and lots of it. Brimstone, the biblical name for sulfur, is often found near hot springs and volcanic fissures on Earth’s surface (above). But scientists studying the formation of Earth’s core have shown that the lightweight nonmetal might also be present there in vast quantities, answering a question that has long troubled earth scientists: How could Earth’s core—predominantly made of the heavy elements iron and nickel—appear as light as it does when analyzed using seismic waves?

Researchers report online today and in the July issue of Geochemical Perspectives Letters that the answer may be sulfur trapped deep within Earth. To come up with their results, the team compared the proportions of copper isotopes in ancient meteorites—the presumed building blocks of our planet—with the proportions of copper isotopes in rocks originating from the mantle—the deep layer of viscous rock beneath Earth’s crust.
Copper often binds to sulfur, and researchers suspected that the presence of one element would indicate the presence of the other. In their experiment, the researchers found that an average sample of copper from the mantle weighed about 0.025% more than samples taken from the meteorites. That meant that lighter isotopes of copper were not present in the mantle in the quantities expected. So where did they go? The team’s analyses suggest that geochemical processes deep within Earth—including those involved in our planet’s separation into core, mantle, and crust—sent a large amount of relatively light copper downward to the core-mantle boundary, where it joined immense amounts of sulfur, oxygen, and iron to form a kilometers-thick layer of material.