Oxygen in lunar rocks suggests the moon formed in huge collision

How did the moon form? A new analysis of lunar rocks could have the answer.

Our leading hypothesis about the moon’s birth suggests that a Mars-sized object called Theia hit Earth 4.5 billion years ago, which blasted a cloud of debris into orbit around our planet that eventually fused to become the moon.

If this idea is accurate, we would expect the rocks on the moon to look like a mixture of Earth and Theia, but samples brought back by the Apollo astronauts show that oxygen isotope levels in both lunar and Earth rock are remarkably similar. So what happened to the bits of Theia?

Erick Cano at the University of New Mexico and his colleagues conducted a new analysis of lunar rocks. They were able to take more precise measurements from individual rocks than had been done previously, while also considering both the type of rock and where they were found – and discovered huge variation.

“While the Earth rocks occupy a very narrow range of oxygen isotope compositions, the lunar samples demonstrated nearly three times that variability,” says Cano.

This had been overlooked in earlier studies, he says, in part because the Apollo rocks are in limited supply and researchers rarely get to analyse such a wide range of samples. What’s more, “typically, all the lunar samples are lumped together as a whole and the average value of all the lunar samples measured in the study is taken as the value for the moon”.

The team found that the lunar rocks from the deepest parts of the moon showed the most variation in oxygen isotope levels, compared with rocks from Earth. Cano says that these rocks could be most representative of Theia’s original composition.

“Since the giant impact hypothesis was formulated there have been various challenges doubting it,” says Simon Kelley at the University of Edinburgh, UK. “These findings are different because they appear to support and validate it.”

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