During the formation of ancient quartz, pockets of atmospheric gases were trapped inside, freezing a chemical fingerprint of the early atmospheric conditions. By looking at the ancient ratios of xenon isotopes and comparing them with today's, geochemists Guillaume Avice and Bernard Marty were able to precisely zero-in on the cataclysmic Earth impact that eventually formed the moon.
In the solar system's early history, planetary collisions were common and approximately 4.5 billion years ago the Earth was hit by a hypothetical Mars-sized object - nicknamed "Theia" - unleashing huge quantities of energy, turning the planet into a searing globe of magma. The ejecta from this impact formed the moon, although the exact formation processes are not fully understood.
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But through the analysis of xenon, at least we can now define when the impact occurred.
"It is not possible to give an exact date for the formation of the Earth," said Avice in a press release. "What this work does is to show that the Earth is older than we thought, by around 60 million years.