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Lunar far side samples bolster theory that the moon was once covered in magma

A team of geologists at the Chinese Academy of Geological Sciences, the Institute of Space Sciences and the Shandong Institute of Geological Sciences, all in China, has found evidence in soil samples collected from the far side of the moon that bolsters a theory that the moon was once covered by an ocean of magma. In their study in the journal Science, the group analyzed a moon soil sample returned to Earth by China's Chang'e-6 mission.

In 2024, the China National Space Administration launched a spacecraft that carried a lander to the surface of the far side of the moon. The mission was the first to collect samples from the far side of the moon and return them to the Earth. For this new study, the research team obtained 2 grams of the soil for testing.

The researchers used multiple methods to determine its composition and then compared the results to those of analyses of soil samples from the near side of the moon.

The researchers found that the basalt in the sample was similar to basalt found in samples collected from the near side of the moon—the ratios of isotopes were similar. They also found similarities in age. These findings bolster theories that during the moon's early years, it was covered by an ocean of magma.

The magma ocean model suggests the moon experienced a massive melting event during its infancy, which led to the creation of a magma ocean. Then, as the magma cooled and crystallized, the densest material sank to form the mantle, while the least dense material floated to the top, leaving it on the surface when the ocean solidified.

Types of materials mixed and formed what has come to be known as the KREEP layer, named after its primary components. Moon samples collected from both the far and near sides were KREEP samples. Because of such strong similarities between them, the odds are good that they were formed in the same way, suggesting the magma ocean stretched across to the moon's far side and likely covered the entire moon, adding credence to the ocean magma theory.

The researchers note that there were some differences in the isotopes, which they suggest are likely due to differences in evolutionary history after the magma ocean solidified.