In the 1960s and 1970s, the United States and the Soviet Union conducted unprecedented lunar exploration programs that remain unmatched to this day. The American crewed Apollo landings were complemented by uncrewed Soviet landings, with both yielding revolutionary bounties of scientific data, such as the hundreds of kilograms of lunar rock and soil that the missions returned to Earth. Oddly missing from all those samples, however, was any material that indisputably originated from the moon’s rocky mantle. Lying just beneath the cratered, desolate crust, the moon’s upper mantle is thought to be the frozen remnant of a vast magma ocean that existed more than 4 billion years ago. Directly studying samples of the mantle could unlock previously hidden chapters of lunar history, potentially reshaping our broader understanding of planetary formation and evolution. Now, a Chinese mission has discovered signs of mantle material at the moon’s surface, effectively setting an “X” on lunar maps for future explorers seeking this not-so-buried geological treasure.
China’s Chang’e-4 mission touched down near the south pole on the lunar far side on January 3, 2019, the first spacecraft ever to land intact on this largely unexplored region of the moon. Consisting of a lander and rover, the mission is still going strong today, with the rover—called Yutu-2—continuing its journey across the surface. On board are a variety of instruments, and today in Nature scientists from the Chinese Academy of Sciences in Beijing report the mission’s first scientific results, suggesting lunar mantle material has at last been located.
“We found that the material of the Chang’e-4 landing site is mainly composed of olivine and low-calcium pyroxene,” says Dawei Liu, one of the paper’s co-authors. “This mineral combination is the candidate mantle-derived material.”
Chang’e-4 rests inside the South Pole–Aitken (SPA) basin, which, at 2,500 kilometers across, is one of the solar system’s oldest and largest known impact craters. Specifically, the mission touched down in the 186-kilometer–wide Von Kármán crater within this larger basin. Von Kármán was produced billions of years ago by the impact of a large comet or asteroid; such collisions can excavate mantle material from deep underground, allowing it to be scattered across the surface by subsequent impacts. “These results seem to reveal that lunar mantle materials might be indeed exposed at the surface of the moon,” says Patrick Pinet from the French National Center for Scientific Research (CNRS) in Paris, who wrote an accompanying commentary on the findings.
The mantle material was discovered using the Visible and Near Infrared Spectrometer on Yutu-2, which can ascertain the chemical composition of rocks by studying their reflected light. Both olivine and pyroxene are believed to be among the first minerals that froze out from the moon’s magma ocean as it cooled, falling to its solid base deeper in the mantle. Because previous surveys from orbit have revealed much of Von Kármán’s floor to be composed of lava from volcanic eruptions rather than excavated mantle, the paper’s authors suspect the material detected by Yutu-2 was actually blasted into Von Kármán from the upper mantle beneath another nearby impact structure, the 72-kilometer–wide Finsen crater.
There are some caveats, however. Not everyone is convinced that Yutu-2 has definitively detected material from the lunar mantle. Mark Wieczorek from the Observatory of the Côte d’Azur in Nice, France, notes that the colossal SPA impact itself could have led to the formation of material that, while mantle-like, would be quite different from the moon’s actual mantle itself. And predictions had suggested a different composition would have resulted from the magma ocean’s crystallization—perhaps a sign of unexpected diversity in the lunar mantle’s makeup, if not a clue that Yutu-2 has not glimpsed any mantle material at all. “Though the Chang’e-4 data are exciting, the true origin of these rocks will probably only be determined by collecting new samples in this basin and returning them to Earth,” he says.
If confirmed, this first detection of mantle material on the lunar surface would provide a new window into the structure of the moon. Ananya Mallik from the University of Rhode Island notes that of the 1,737-kilometer lunar radius, about 300 kilometers form a dense metallic core at its center and 40 kilometers comprise the crust. “So, the remaining 1,397 kilometers is the mantle—which is huge!” she says, with seismic data from Apollo-era investigations providing the bulk of that prior knowledge. “Thus, understanding what the mantle is made of is a huge piece in the puzzle to unravel the lunar interior structure and composition.”
Results like these are a boon for China’s lunar exploration program too, helping justify its cost and bolstering arguments for future missions, even crewed forays to the surface, says Andrew Jones, a journalist covering the Chinese space program. “The apparent detection of lunar-mantle–derived material and the mission as a whole demonstrates that the country can plan and execute cutting-edge science missions and make new contributions in terms of human knowledge and understanding,” he says. “We are now starting to see the first science results.”
Yutu-2 is continuing to operate nominally on the surface, having driven nearly 200 meters, and it could survive for many months to come. In late 2019, China also hopes to launch another mission to the moon, called Chang’e-5, which will be the country’s first-ever sample return mission from the lunar surface. Alas, that mission is targeted for the moon’s near side, far from Von Kármán and Yutu-2’s remarkable potential discovery. For now, exploration of the Moon’s deep interior—and deeper past—will have to wait.