Earth and Moon cores similar, adds support to impact theory

William Atkins
Tuesday, 03 July 2007 06:56

Science - Space

Scientists from Oxford University (England), University of California (Los Angeles, U.S.A.), and Swiss Federal Institute of Technology (Zurich, Switzerland) compared silicon isotopes from Earth rocks, along with rocky materials from meteorites and other materials from our solar system.

From volcanoes, mountains, and ocean floors, to about a depth of 2,900 kilometers (1,800 miles) into the Earth (not quite half way to the center of the Earth--what is called the mantle and crust), its materials are made of silicate, a compound consisting of silicon, oxygen, and other elements.

At that halfway mark and extending to the center of the Earth—the rest of the inner part of the Earth—is the dense metallic iron material that makes up the Earth’s core.

The team’s results show that the heavier isotopes from silicate samples taken from the Earth consisted of increased amounts of the heavier isotopes of silicon. They found that Mars, the asteroid Vesta, and various chrondites (primitive meteorites that never produced ainner cores) do not contain such an arrangement, even though they have an iron core. Mars is much smaller than the Earth (about one-eighth the size), so did not have enough mass to generate the pressure necessary to form the same core as found in the Earth.

On the other hand, the researchers found that the Moon did show a similar composition of the silicon isotopic composition as the Earth. However, it, too, is much smaller than the Earth—about one-fiftieth as large as the Earth and about one percent of the Earth’s mass—making it even less likely to have been able to generate enough pressure to form an Earth-like iron core.

However, such a core does exist at the center of the Moon. How did it get there?

The researchers contend that the Moon was created during a giant impact by a planet-size object (probably around the size of the planet Mars) that hit during the early development of Earth. The impact was large enough that the materials, which eventually formed the Moon, mixed with the materials from the Earth, which already had a heavy silicon isotopic composition.

They state within their paper in the journal Nature: “The similar isotopic composition of the bulk silicate Earth and the Moon is consistent with the recent proposal that there was large-scale isotopic equilibration during the giant impact.”

This research—the first of its kind using isotopes in this manner—brings important insights into the creation of Mars, the Earth, and the Moon—and how life evolved on the Earth and whether or not it might have existed at some time on Mars.

The researchers include R. Bastian Georg and Alex N. Halliday, both from Oxford University’s Earth Sciences Department, Edwin A. Schauble, from the University of California, Los Angeles, and Ben C. Reynolds, from the Department of Earth Sciences at the Swiss Federal Institute of Technology.

Their result, titled “Silicon in the Earth’s core”, is found in the June 28, 2007 issue of the journal Nature.