Imagine the Moon as a giant layered cake, where the “core,” the deepest layer, is a dense chocolate center. The Moon‘s mantle, on the other hand, represents the middle layers made of minerals, similar to layers of different flavors in a cake. Finally, the outer layer, the crust, is comparable to the icing.

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Just as a cake can contain hidden elements in its middle layers, such as fruit, scientists have been wondering for years if there is a specific component, garnet, hidden deep within the middle layers of the mantle. Unable to open the Moon‘s layers to check, they relied on an indirect method, such as measuring how sound waves travel.

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Like chefs baking a cake with fruit filling, researchers at Ehime University in Japan decided, in a study published in the journal “Earth and Planetary Letters,” to create rock samples that mimic the Moon, incorporating their own garnet filling into the middle layers (the mantle).

They then took these rocks to the high-tech Spring-8 facility in Hyogo Prefecture, Japan, where they could simulate the harsh conditions inside the Moon (high pressure and temperature) and measure how fast sound waves travel.

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Spring-8 is a large-scale synchrotron radiation facility. This radiation is a type of extremely bright X-ray produced when high-speed electrons are forced to travel in a curved path by a magnetic field. This powerful X-ray source allows scientists to examine materials at the atomic and molecular levels with very high precision.

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Using this method, the researchers found that sound waves travel through the small rock stuffed with garnet at speeds matching data from the actual Moon. This suggests that the middle layers of the real Moon (the mantle) likely also contain garnet.

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The researchers concluded that the Moon‘s mantle might contain garnet, much like discovering hidden fruit filling in a cake, which helps them better understand the Moon’s structure and history, including how it formed and what it is made of.