The oldest rocks ever discovered in our solar system have been dated back about 4.57 billion years, meaning Earth obviously finished forming later than that. However, determining exactly when that happened can be difficult. New research presented by French geochemists from the University of Lorraine at the Goldschmidt Geochemistry Conference in Sacramento, California has revealed xenon isotopes. These isotopes indicate that the Earth and Moon are 60 million years older than was previously believed.
There aren’t many geological clues from the time our planet originated, just because so much of it has been destroyed over time. Layers of rock which provide a considerable amount of reliable information from other points in the planet’s history just don’t exist that far back. Instead, researchers need to use other techniques, such as analyzing the isotopes of different gasses in rock samples.
A quartz sample from Australia dated back to 2.7 billion years, while quartz from South Africa was found to be 3.4 billion years old. The xenon gas that was trapped inside is reflective of the conditions at the time, which researchers Guillaume Avice and Bernard Marty compared to xenon today. This allowed the researchers to tweak the technique used for dating rock samples, making it more accurate. In doing so, it pushes back the estimated date of the hypothesized impact that formed the moon by about 60 million years (+/- 20 million years).
"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,” Avice explained. “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 is believed that when the solar system was coming together, a planet about the size of Mars slammed into the proto-Earth, and the debris that was expelled eventually coalesced into the moon. This had to have happened before the atmosphere formed, otherwise it would have been obliterated in the collision. While many have traditionally believed that the atmosphere formed about 100 million years after the solar system formed, these results suggest that it could have appeared only 40 million years after the fact.
“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 continued. “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. These differences set time boundaries on how the planets evolved, especially through the major collisions in deep time which shaped the solar system,” Marty concluded.