Chang’e-4, the first mission to land on the far side of the Moon, is helping researchers understand what the interior of our natural satellite is like. The lander touched down in the South Pole-Aitken (SPA) basin, one of the largest known impact craters in the Solar System.
This location is crucial to the mission. The 2,500-kilometer (1,550-mile) crater is expected to contain material from the Moon’s interior. The ancient impact ripped the crust and brought to the surface rocks from the mantle. To study the compositions of these rocks, the lander deployed Yutu-2 (Jade Rabbit), a rover equipped with a visible and infrared spectrometer.
The lander and rover are situated in the Von Kármán crater within the SPA, which is moderately flat although there are some deeper impact craters around. The rover's first observations were made on the rocks in the flat region and they were quite peculiar. Researchers were expecting to find an abundance of the mineral olivine, the main component of the Earth’s upper mantle. Surprisingly, in the flat plains of the SPA, the mineral is not common at all.
"The absence of abundant olivine in the SPA interior remains a conundrum," Li Chunlai, a professor at the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), said in a statement. "Could the predictions of an olivine-rich lunar mantle be incorrect?"
The first findings were only part of the picture. The team performed some spectral analysis of material from smaller and deeper impacts within the SPA and managed to find more olivine. Based on this, the researchers suggest that olivine and another mineral called pyroxene make up the mantle of the Moon in roughly equal composition. The findings are reported in Nature.
"Understanding the composition of the lunar mantle is critical for testing whether a magma ocean ever existed, as postulated," explained Li. "It also helps advance our understanding of the thermal and magmatic evolution of the Moon."
The more olivine-rich rocks are believed to have come from the nearby Finsen impact crater, which is 72 kilometers (45 miles) in diameter and much deeper than the SPA. Yutu-2 will continue to study these rocks to fully understand their origin and abundance, and will even assess the possibility of collecting them and bringing them back to Earth in a future mission.