China's lunar rover makes unexpected discovery on far side of the moon

Studying the South Pole-Aitken basin reveals the secrets of the lunar mantle.

The Chang'e 4 lunar lander sits pretty on the moon's far side in this image taken by the Jade Rabbit 2 rover.

As the first mission to land on the far side of the moon, China's Chang'e 4 couldn't help but make some enchanting discoveries. Its analysis of the the lunar crust, however, has seen the mission also make an unexpected one.

In research published in the journal Nature on May 16, scientists from the National Astronomical Observatories of Chinese Academy of Sciences revealed that the composition of the lunar surface at the South Pole-Aitken Basin is a little different to what was expected.

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One core theory posits that the moon wasn't always quite as cold and dead as it is today. Instead it likely began as a giant, molten marble full of magma oceans. These oceans gradually cooled, depositing heavy minerals such as the green-colored olivine or the low-calcium pyroxene deeper into the lunar mantle. Less dense minerals floated to the top, giving the moon a series of obvious geological layers like a cosmic onion. The crust, the uppermost layer, is composed mostly of aluminum silicate or plagioclase. 

"Understanding the composition of the lunar mantle is critical for testing whether a magma ocean ever existed, as postulated," said co-author Li Chunlai, in a press release. "It also helps advance our understanding of the thermal and magmatic evolution of the moon."

Understanding the composition of the mantle gives planetary scientists more insight into how the interiors of other planetary bodies -- including Earth -- might evolve. 

The Chang'e 4 lander landed in the Von Kármán Crater, which lies on the floor of the South Pole-Aitken Basin, back in January. It then dispatched a rover, Yutu-2, equipped with a spectrometer that measures reflected light. By studying the light reflected from the surface as the rover rolled along Von Kármán, the scientists were able to detect minerals and determine their chemical composition. Rather than seeing a lot of plagioclase, the rover detected a dominance of olivine and pyroxene.

Because these elements are expected much deeper in the mantle, the authors suggest they were ejected by a meteor striking the lunar surface. The rover is exploring close to the 72-kilometer Finsen Crater, so the minerals may have been sprayed across the surface during that crater's creation.

Although NASA's Apollo missions landed humans on the moon and Russia made a concerted effort to retrieve lunar samples during the 1970s, no study of the lunar mantle had previously occurred. That makes China's mission particularly important, but because of the complexities of studying moon minerals on a planetary body hundreds of thousands of miles away, further work will be necessary to gather a more complete understanding of the mantle's composition.