The violent impact that created our moon may have occurred millions of years earlier than previously thought, according to new research.
Mounting evidence suggests that the moon came into being after the Earth got slammed by a Mars-sized object named Theia in the early days of the solar system. The impact was so great that much of the Earth became a magma ocean, and part of it vaporized. Dust from the Earth and Theia went shooting off into space, and then coalesced to form the moon.
Previously, it was suggested that this massive impact occurred 100 million years after the solar system formed. However, after smashing open ancient quartz crystals that date as far back as 2.7 billion and 3.4 billion years, researchers at the University of Lorraine in Nancy, France, say that the impact probably occurred 40 million years after the start of the solar system.
No, quartz doesn't have anything to do with the formation of the moon, but it can contain tiny amounts of the Earth's atmosphere that get trapped in what are called "inclusions" in the crystals.
If you know how old the atmosphere is, then you know how long ago the massive impact with Theia occurred, said Guillaume Avice, a geochemist at the University of Lorraine. That's because the impact would have removed the Earth's previous atmosphere entirely.
Unfortunately, remnants of the paleoatmosphere are hard to come by. They can only be found in very old rocks that have managed to avoid heat.
"When you heat the rock you diffuse the signal," said Avice, who presented the research at the Goldschmidt Geochemistry Conference in Sacramento this week. "To preserve a billion-year-old signal, you have to keep the rock cold."
After finding suitable crystal candidates, Avice and his colleagues smashed them in a vacuum to access the atmospheric "time capsules" trapped inside. Not every crystal smashing yielded results, but those that did allowed the researchers to measure the isotopic ratio of xenon in the atmosphere billions of years ago, and compare it to what is found in the atmosphere today.
Avice explained that using a xenon isotopic signature to date the formation of the Earth's atmosphere is not new. What is new is the signature they found in these ancient rocks. By looking at what the atmosphere looked like 3.4 billion years ago and 2.7 billion years ago, Avice and his team were able to build more precise models of the actual age of our atmosphere, and therefore the time of the Theia impact.
"When we corrected the model, we can compute new ages for the atmosphere and the moon, and that age becomes 40 million years after the solar system formation," Avice said.