Moon May Be 85 Million Years Younger than Thought

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Earth’s only natural satellite formed 4.425 billion years ago — around 85 million years later than previous estimates, according to a new modeling study by researchers from the German Aerospace Center (DLR), the Technische Universität Berlin and the Institut für Planetologie at the University of Münster.

When the Moon formed into a sphere approximately 1,700 km in radius 4.425 billion years ago, its interior heated up considerably due to the energy released when it accreted. The rock melted and an ocean of magma, possibly more than 1,000 km deep, formed. Later, light rocks crystallized, which rose to the surface and formed a first crust on the Moon. This crust insulated the Moon from space, and the magma ocean beneath it cooled down slowly. Around 200 million years would pass before the Moon completely solidified. Image credit: NASA’s Goddard Space Flight Center.

When the Moon formed into a sphere approximately 1,700 km in radius 4.425 billion years ago, its interior heated up considerably due to the energy released when it accreted. The rock melted and an ocean of magma, possibly more than 1,000 km deep, formed. Later, light rocks crystallized, which rose to the surface and formed a first crust on the Moon. This crust insulated the Moon from space, and the magma ocean beneath it cooled down slowly. Around 200 million years would pass before the Moon completely solidified. Image credit: NASA’s Goddard Space Flight Center.

According to the giant impact hypothesis, the Moon was created out of the debris left over from a catastrophic collision between the proto-Earth and a Mars-sized protoplanet called Theia.

This collision produced a lunar magma ocean and initiated the last event of core segregation on Earth. However, the timing of these events remains uncertain.

“The results of our latest modeling suggest that the young Earth was hit by a protoplanet some 140 million years after the birth of the Solar System 4.567 billion years ago,” said lead author Dr. Maxime Maurice, a researcher at the DLR and the Technische Universität Berlin.

“According to our calculations, this happened 4.425 billion years ago — with an uncertainty of 25 million years — and the Moon was born.”

“The Moon was formed in a short time, probably in just a few thousand years,” added co-author Dr. Doris Breuer, head of the Planetary Physics Department at the DLR Institute of Planetary Research.

The scientists determined when the Moon was formed using a new, indirect method.

“Our calculations show that this most likely happened at the very end of Earth’s formation,” said co-author Dr. Sabrina Schwinger, also from the DLR.

The lunar magma ocean quickly began to solidify and formed a crust of floating, lightweight crystals at the surface — its ‘interface’ with the cold space.

But under this insulating crust, which slowed down the further cooling and solidification of the magma ocean, the Moon remained molten for a long time.

Until now, scientists were unable to determine how long it took for the magma ocean to crystallize completely, which is why they could not conclude when the Moon originally formed.

To calculate the lifetime of the Moon’s magma ocean, the authors used a new computer model, which for the first time comprehensively considered the processes involved in the solidification of the magma.

“The results from the model show that the Moon’s magma ocean was long-lived and took almost 200 million years to completely solidify into mantle rock,” Dr. Maurice said.

“The time scale is much longer than calculated in previous studies,” said co-author Dr. Nicola Tosi, also from the DLR.

“Older models gave a solidification period of only 35 million years.”

To determine the age of the Moon, the team calculated how the composition of the magnesium- and iron-rich silicate minerals that formed during the solidification of the magma ocean changed over time.

The researchers discovered a drastic change in the composition of the remaining magma ocean as solidification progressed.

This finding is significant because it allowed them to link the formation of different types of rock on the Moon to a certain stage in the evolution of its magma ocean.

“By comparing the measured composition of the Moon’s rocks with the predicted composition of the magma ocean from our model, we were able to trace the evolution of the ocean back to its starting point, the time at which the Moon was formed,” Dr. Schwinger said.

The results show that the Moon was formed 4.425 billion years ago.

This age is in remarkable agreement with an age previously determined for the formation of Earth’s metallic core with the uranium-lead method, the point at which the formation of the Earth was completed.

“This is the first time that the age of the Moon can be directly linked to an event that occurred at the very end of the Earth’s formation, namely the formation of the core,” said senior author Dr. Thorsten Kleine, a researcher in the Institut für Planetologie at the University of Münster.

The results were published in the journal Science Advances.

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M. Maurice et al. 2020. A long-lived magma ocean on a young Moon. Science Advances 6 (28): eaba8949; doi: 10.1126/sciadv.aba8949

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