A new study shows that the timing of the giant impact between the Earth's ancestor and a planet-sized body occurred around 40 million years after the start of solar system formation.

"It is not possible to give an exact date for the formation of the Earth. What this work does is to show that the earth is older than we thought, by around 60 million years," said lead researcher Guillaume Avice from University of Lorraine in Nancy, France.

Previously, the time of formation of the Earth's atmosphere was estimated at around 100 million years after the solar system formation.

To reach this conclusion, geochemists discovered an isotopic signal which indicates that previous age estimates for both the Earth and the Moon are underestimates.

To put a date on early earth events, one of the standard methods is measuring the changes in the proportions of different gases (isotopes) which survive from the early earth.

Avice and colleague Bernard Marty analysed xenon gas found in South African and Australian quartz, which had been dated to 3.4 and 2.7 billion years respectively.

The gas sealed in this quartz is preserved as in a "time capsule".

Recalibrating dating techniques using the ancient gas allowed them to refine the estimate of when the Earth began to form.

This allows them to calculate that the moon-forming impact is around 60 million years older than what was thought.

"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," Avice concluded.

The researchers presented their findings at the Goldschmidt Geochemistry Conference in Sacramento, California, this week.


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