Violent moon-forming impact occurred 60 million years earlier



Through the analysis of isotopes trapped inside ancient quartz crystals, geochemists have realized that the Earth-shattering cosmic impact that laid waste to our young Earth and formed the moon happened 60 million years earlier than thought.

Speaking Tuesday at the Goldschmidt Geochemistry Conference in Sacramento, Calif., researchers from the University of Lorraine in Nancy, France, discussed their analysis of xenon gas isotopes trapped inside South African and Australian quartz. These geological “time capsules” were formed on the primordial Earth 3.4 and 2.7 billion years ago, respectively.

The key problem with dating the early evolution of our planet is the lack of “classical geology” that we can tap into. This means that there is very little pristine material or layers of rock that has remained unchanged for billions of years. So geochemists have stepped in to use the technique of isotopic analysis to gauge the chemical conditions of early Earth.

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.

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.

“The composition of the gases we are looking at changes according the conditions they are found in, which of course depend on the major events in Earth’s history. The gas sealed in these quartz samples has been handed down to us in a sort of ‘time capsule.’ We are using standard methods to compute the age of the Earth, but having access to these ancient samples gives us new data, and allows us to refine the measurement.”

It was thought that the Earth’s atmosphere formed around 100 million years after the formation of the solar system. However, the primordial atmosphere would not have survived the massive Theia impact event. By studying the xenon isotope ratios from 3.4 and 2.7 billion years ago and comparing those ratios with modern Earth, Avice and Marty have been able to look back in time to find that the Earth’s atmosphere likely started to form only 40 million years after the solar system’s formation, meaning the Theia impact occurred approximately 60 million years earlier than previous estimates.

“The xenon gas signals allow us to calculate when the atmosphere was being formed, which was probably at the time the Earth collided with a planet-sized body, leading to the formation of the moon,” said Avice. “Our results mean that both the Earth and the moon are older than we had thought.”

“This might seem a small difference, but it is important,” added Marty. “These differences set time boundaries on how the planets evolved, especially through the major collisions in deep time which shaped the solar system”.