Scientists have always suspected that the lunar mantle — a layer of rock between the moon’s outer crust and inner core — consists of heavy, iron-rich minerals. China’s exploration mission on the far side of the moon may have just provided the first evidence toward reimagining this theory as fact.

Data collected from China’s Yutu-2 lunar rover after it landed in the South Pole-Aitken (SPA) basin — the moon’s widest and deepest crater — have confirmed the presence of olivine and low-calcium pyroxene, materials believed to have originated from the lunar mantle, according to an article by scientists from the Chinese Lunar Exploration Program published Thursday in the scientific journal Nature. When a meteorite hit the SPA basin and created the Finsen crater within it, the impact may have “drilled holes” and sent materials from the upper lunar mantle to the surface of basin, the article suggests.

“This study provides direct evidence of the components of the lunar mantle,” Li Chunlai, deputy director of the National Astronomical Observatories and a lead author of the paper, told Sixth Tone on Thursday.

Despite over a hundred attempts by various countries to probe the moon, the structure and composition of the lunar mantle have remained a mystery. One hypothesis posited by U.S. scientists in the 1970s suggested that olivine and pyroxene — known components of other planetary mantles — were the main materials of a dense, ultraferrous lunar mantle — but a paucity of olivine detected on the near side of the moon had cast this theory into doubt. Missions such as the Apollo space program, which previously sampled the moon’s surface, had provided limited information about the mantle.

China’s Chang’E-4 mission is the first in the world to explore the far side of the moon. On Jan. 3, the Chang’E-4 lander touched down in the Von Kármán crater in the SPA basin, and the Yutu-2 rover began its exploration of the area. On its zigzagging route along the surface, the rover collected spectroscopy images that revealed evidence of abundant olivine and pyroxene — geological phenomena “significantly different” from what had been observed on the moon’s near side.

The Chinese scientists chose the SPA basin as the Yutu-2 rover’s landing spot specifically because they hoped the massive crater would hold the key to unlocking the secrets of the lunar mantle. With a diameter of 2,500 kilometers — greater than the distance from New York City to Houston — the 13-kilometer-deep basin was formed some 4 billion years ago and today is pockmarked itself with lesser impact craters. While the lunar mantle is dozens of kilometers below the surface, there is a good chance that these past impact events penetrated well below the crust and deep into the moon’s interior.

The Finsen impact crater — the youngest crater in the SPA basin, located to the northeast of the Yutu-2’s landing position — is like a “deep drill” into the basin, the scientists say: Its impact brought subterranean materials to the lunar surface.

Now resting over its fifth lunar night, the Yutu-2 will, when it resumes exploring, begin vetting its initial conclusions by analyzing the distribution, geological setting, characteristics, and origin of the possible mantle materials, according to the paper. The Chinese Lunar Exploration Program says it is now assessing scenarios for returning the collected samples to Earth.

Editor: David Paulk.

(Header image: A photo of the Yutu-2 lunar rover, taken by a camera on the Chang’E-4 lander. Chinese Lunar Exploration Program)