Earth's crust was first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system, a new study in the journal Nature Geoscience has found. A time-line of the history of our planet places the formation of the Jack Hills zircon and a "cool early Earth" at 4.4 billion years, researchers said. With the help of a tiny fragment of zircon extracted from a remote rock outcrop in Australia, the picture of how our planet became habitable to life is coming into sharper focus. An international team of researchers led by University of Wisconsin-Madison geoscience Professor John Valley reveals data that confirm the Earth's crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. The work shows that the time when our planet was a fiery ball covered in a magma ocean came earlier. "This confirms our view of how the Earth cooled and became habitable. This may also help us understand how other habitable planets would form," said Valley. The study confirms that zircon crystals from Western Australia's Jack Hills region crystallised 4.4 billion years ago, building on earlier studies that used lead isotopes to date the Australian zircons and identify them as the oldest bits of the Earth's crust. The microscopic zircon crystal used by Valley and his group is now confirmed to be the oldest known material of any kind formed on Earth. The study, according to Valley, strengthens the theory of a "cool early Earth," where temperatures were low enough for liquid water, oceans and a hydrosphere not long after the planet's crust congealed from a sea of molten rock. "The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago," and possibly life not long after, said Valley. The study was conducted using a new technique called atom-probe tomography that, in conjunction with secondary ion mass spectrometry, permitted scientists to accurately establish the age and thermal history of the zircon by determining mass of individual atoms of lead in the sample. The clusters of lead atoms formed 1 billion years after crystallisation of the zircon, by which time the radioactive decay of uranium had formed the lead atoms that then diffused into clusters during reheating. "The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots," Valley said.
