Research Leads to New View of the Earth’s Core

This model shows planetesimals (objects formed from dust, rock and other materials that can be anywhere in size from several meters to hundreds of kilometers) accreting to a growing Earth 4.56 billion years ago. The cutaway reveals the simultaneous formation of the Earth’s core as dense, iron-rich metallic material descending through a planetary magma ocean. 

Image courtesy of Antoine Pitrou/Institut de Physique du Globe de Parise Physique.

Earth formed about 4.56 billion years ago over a period of several tens of millions of years through the accretion of planetary embryos and planetesimals. The energy delivered by progressively larger impacts maintained Earth’s outer layer and an extensively molten magma ocean. Gravitational separation of metal and silicate within the magma ocean results in the planet characterized by a metallic core and a silicate mantle.

The formation of Earth’s core left behind geophysical and geochemical signatures in the core and mantle that remain to this day. In the past, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately rather than looking for a joint solution.

By combining experimental petrology, geochemistry, mineral physics and seismology, the team found that core formation occurred in a hot (liquid) moderately deep magma ocean not exceeding 1,800-kilometer depth, under conditions more oxidized than present-day Earth.

“This new model is at odds with the current belief that core formation occurred under reduction conditions,” Ryerson said. “Instead we found that Earth’s magma ocean started out oxidized and has become reduced through time by oxygen incorporation into the core.”

They found the oxygen concentrations in the core are higher than previously thought and silicon concentrations are lower than previous estimates.

Other collaborators include Institut de Physique du Globe de Paris, École Polytechnique Fédérale de Lausanne and University College London.

The research appears in the online edition of the Sept. 21-25 edition of theProceedings of the National Academy of Sciences(link is external).

Contacts and sources:
Anne Stark