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 has revealed xenon isotopes. These isotopes indicate that the Earth and Moon are 60 million years older than was previously believed.
The scientists measured changes in the proportions of different gases (known as isotopes) which have survived in some rocks from when Earth was very young.
Bernard Marty and Guillaume Avice, two of the key scientists in the research, analyzed xenon gas, found in South African and Australian quartz, which was found to be 3.4 and 2.7 billion years old respectively.
This is because the gas sealed in the xenon is in a ‘time capsule’, which allows scientists to compare the current isotopic ratios of xenon with those that existed billions of years ago. This allowed them to estimate when Earth began to form.
“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. Our results mean that both the earth and the moon are older than we had thought,” said Avice.
The absence of these dark lava plains, due to a difference in crustal thickness between the side of the Moon we see and the hidden side, is a consequence of how the Moon originally formed. The outer layers of the Earth and the impact object were flung into space and eventually formed the Moon.
“Shortly after the giant impact, the Earth and the Moon were very hot,” said Steinn Sigurdsson, co-author of the study.
The Moon was closer to Earth than it is now, and quickly assumed a tidally locked position, and, being smaller than Earth, cooled more quickly. The far side cooled, while the Earth-facing side was kept molten creating a temperature gradient between the two halves.
Large meteoroids struck the nearside of the Moon and punched through the crust, releasing the lakes of basaltic lava that formed the nearside maria. When meteoroids struck the far side of the Moon, in most cases the crust was too thick and no magmatic basalt welled up, creating almost no maria.