The research by an international team of scientists confirmed the Earth's first crust had formed around 4.5 billion years ago.
"Meteorites that contain zircons are rare," said Yuri Amelin, from The Australian National University (ANU) Research School of Earth Sciences, according to a press 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 the Earth's interior mixes the chemical composition of its layers over billions of years, as denser rocks sink and less dense minerals rise towards the surface, a process known as differentiation.
"Meteorites are remnants of the original pool of material that formed all the planets," Amelin said, according to the press release. "But they have not had planetary-scale forces changing their composition throughout their five billion years orbiting the sun."
The team accurately measured the ratio of the isotopes hafnium-176 and hafnium-177 in the meteorite, to give a starting point for the Earth's composition, according to the press release. The team was then able to compare the results with the oldest rocks on Earth, and found that the chemical composition had already been altered, proving that a crust had already formed on the surface of the Earth around 4.5 billion years ago.
The research is published in Proceedings of the National Academy of Sciences.
