A late arrival for platinum and gold?

For decades, geologists have wondered why Earth's mantle, between the planet's crust and core, contains as much platinum, gold, and certain other elements as it does. New research suggests that this mysterious overabundance may be due to alien invasions.

These substances are siderophilic, or iron-loving. During Earth's early formation, they should have been much more attracted to the planet's iron core than to its mantle, which is dominated by silicates. The bulk of siderophiles do in fact reside in the core, yet even the lesser amounts in the mantle strike researchers as too high.

Some theorists account for the mantle's composition by proposing that siderophilic elements lose their affinity for iron at ultrahigh pressures deep below Earth's surface. That would have enabled them to remain in the mantle during the planet's early formation. Studies of moderately iron-loving elements under high pressure, such as nickel and cobalt, are consistent with that theory.

New experiments on two highly siderophilic elements, platinum and palladium, counter that evidence, however. In the July 27 NATURE, Astrid Holzheid of the University of Cologne in Germany and her colleagues report that these elements retain their preferences for the iron core even at high pressures.

She says the results support the other leading theory about mantle composition--that the seemingly anomalous siderophiles arrived in meteorites after Earth's formation.

In an accompanying commentary, Richard J. Walker of the University of Maryland in College Park agrees that the new data support a meteoric origin of some siderophiles, but he suggests a full explanation may fuse the two leading theories.