Meteorites are Key to the Story of Earth's Many Layers

How did Earth form its layers? The answer may lie in the chemical make-up of meteorites. A new study of meteorites reveals a bit more about how our planet's crust formed billions of years ago.

In order to learn a bit more about our planet's history, the researchers measured the amount of two rare elements, hafnium and lutetium, in the mineral zircon in a meteorite that originated in the early solar system.

"Meteorites that contain zircons are rare," said Yuri Amelin, one of the researchers, in a news release. "We had been looking for an old meteorite with large zircons, about 50 microns long, that contained enough hafnium for precise analysis. By chance we found one for sale from a dealer. It was just what we wanted. We believe it originated from the asteroid Vesta, following a large impact that sent rock fragments on a course to Earth."

The heat and pressure in Earth's interior mixes the chemical composition of its layers over billions of years. As denser rocks sink and less dense minerals rise toward the surface, a process called differentiation occurs. Determining how and when the layers formed relies on knowing the composition of the original material that formed into the Earth before differentiation. Meteorites, in particular, are remnants of the original pool of material that helped form our planet.

In this case, the researchers accurately measured the ratio of isotopes hafnium-176 and hafnium-177 in the meteorite. This gave them a starting point for Earth's composition. They then compared the results with the oldest rocks on Earth, and found that the chemical composition had already been altered. This suggests that a crust had already formed on the surface of Earth about 4.5 billion years ago.

The findings are published in the journal Proceedings of the National Academy of Sciences.

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