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Earth and Moon 60 million years older than thought

Jenny Winder, News Writer
Jun 11, 2014, 15:00 UTC

Sen—The cosmic crash that created the Earth and Moon happened around 60 million years earlier than previously thought, a new study shows.

The single object from which they were both produced was struck by a planet-sized object about 40 million years after the Solar System began to form, it suggests.

Geochemists from the University of Lorraine, in Nancy, France, have discovered that previous age estimates for both the Earth and the Moon are underestimates.

It becomes difficult to put a date on early Earth events because there is little “classical geology” dating from the time of the formation of the Earth, such as rock layers. So geochemists have to rely on other methods like measuring the changes in the proportions of different gases (isotopes) which survive from the early Earth.

Guillaume Avice and 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, and compared them to current isotopic ratios of xenon. This allowed them to calculate that the date of the Moon-forming impact is around 60 million years (+/- 20 m. y.) older than had been thought.

Previously, the time of formation of the Earth’ s atmosphere had been estimated at around 100 million years after the Solar System formation. This revision puts the age up to 40 million years after it started to form.

According to Avice, "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.

The dark areas are the familar lunar "seas", or lava plains, on the side of the Moon that faces Earth. Image credit: Paul Sutherland

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."

Meanwhile in other research astrophysicists at Penn State University, in Pennsylvania, think they have found the reason why fewer 'seas' or maria exist on the far side of the Moon.

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.