Kicking off the new year with a bang, China made history in early January by landing the first-ever spacecraft on the far side of the moon. Now, results from that mission suggest another bombshell: the first signs of lunar mantle material available for scientific study. Yutu-2, the rover partner to the Chang’e-4 lander, used reflected radiation to analyse the minerals within its landing site inside the moon’s Von Kármán crater. In doing so, it spotted layers rich in two mineral types that aren’t a match for a typical lunar crust. The study authors argue that it is likely these mineral patches represent upper mantle material, according to work appearing today in the journal Nature. If confirmed, mantle rock from the moon would give researchers a game-changing look at our celestial companion’s inner workings, perhaps helping to solve long-standing mysteries about the moon’s formation and evolution. (Find out why geologists now think the moon may be more tectonically active than previously realized.) “If this really is a bit of the mantle, then that is so cool,” says Sara Russell, a professor of planetary sciences at the Natural History Museum in London.
What we know about the moon, from how it came to be to how it’s behaving today, is often due to the samples brought back by the Apollo missions, which all landed on the moon’s near side. These rocks are invaluable, but they only tell a tiny part of the story, since they are all lunar surface material from a very small section of the moon. The moon’s mantle, a solid layer beneath the crust that was once hotter and more toothpaste-like, remains far more mysterious, and scientists have been yearning for samples of it for decades. A genuine lunar mantle sample would represent a “critical piece of the jigsaw” found at long last, Russell says. Some researchers, however, have expressed uncertainty about how conclusive the study’s claim is at this stage. “This is an exciting step forward,” says Clive Neal, an expert in lunar geology at the University of Notre Dame, but alternative explanations need to be explored further before he considers it a slam dunk.
Digging up the past
The Chang’e-4 mission is unquestionably unprecedented. All prior successful lunar landings, by the U.S. and the Soviet Union, landed on the moon’s near side, partly because of logistical problems landing on the far side. China is now the first and only country with a rover exploring the far side. Its landing spot in Von Kármán crater is in the middle of the colossal South Pole-Aitken basin, a 1,550-mile-wide ancient crater that is among the largest impact structures in the solar system. That makes it a plausible place to hunt for the lunar mantle, which is one of the reasons why the China National Space Administration was keen to land there. (Find out why multiple space agencies are racing to explore the moon’s south pole.) “Numerical models of the impact event indicate it should have blasted through the lunar crust to excavate mantle material located below,” says Bill Bottke, a planetary scientist at the Southwest Research Institute. “If you’re going to find mantle material anywhere, the South Pole-Aitken basin is the best place to go,” Russell adds.
Scientists suspect that the moon was covered in a magma ocean during its very early days. As it progressively cooled and solidified, denser minerals remained in the ocean’s depths while less dense minerals floated to the surface. This meant the geochemical compositions of the eventual mantle and crust layers turned out to be distinct from each other. Driving around Von Kármán, the Yutu-2 rover found material that, according to its spectrometer, looked to contain at least two minerals: low-calcium pyroxene and olivine. These would be a good match for what models predict the moon’s upper mantle would contain. The researchers suspect that when the entire basin formed, it scattered mantle material all across the lunar surface. Later on, a less powerful impact formed the smaller Finsen crater, and the team speculates that this event jettisoned previously excavated mantle material into the nearby Von Kármán crater.
A tale of two worlds
If the sample is truly mantle material, its composition won’t solely record the moon’s magmatic and thermal evolution, says lead study author Chunlai Li from the Chinese Academy of Sciences. Some of Earth’s early history is locked up in the lunar underworld, too. (Also find out about a moon rock that may be the oldest known piece of Earth.) The most widely accepted theory is that the moon was born when a massive protoplanet slammed into young Earth, sending plenty of terrestrial building blocks into orbit around it. Earth has destroyed much of its oldest geological history through plate tectonics, but there is every chance that echoes of it have been chronicled within the underbelly of the far less self-destructive moon. The team cannot say for sure that the spectrometer is unequivocally seeing mantle material. Study co-author Bin Liu, a researcher at the Chinese Academy of Sciences, says that the mission will need to gather much more spectral data to help validate these early interpretations.
Bottke favours the idea that the material is the exposed early mantle, but he agrees that other possibilities cannot be ruled out yet. One alternative explanation, he says, is that the material is the solidified melt from the impact that created the basin in the first place. Or it could be a feature of the solidified lava that is known to exist within the crater, Neal says. Daniel Moriarty, a lunar geologist at NASA’s Goddard Space Flight Center, is also cautious. Japan’s spectrometer-armed Kaguya orbiter spied volcanic glass within Von Kármán crater and other craters nearby, he notes. To a spectrometer, olivine and volcanic glass can look quite similar, which means that Yutu-2 could have found volcanic glass, not mantle minerals. There also isn’t much evidence for olivine within Finsen crater, the supposed source of the mantle material seen in Von Kármán crater, he says. He suspects that a fresher crater in the middle of Von Kármán is the far more likely source of the features Yutu-2 is looking at.
One giant leap for robot-kind
Both Neal and Russell say that this study demonstrates why it’s important to fund more missions that will return lunar samples to Earth, where they can be analysed thoroughly in a lab. Fortunately, China is already on the case. It’s upcoming Chang’e-5 mission will send near-side lunar samples back to terra firma, and its Chang’e-6 mission aims to do the same with samples from the lunar south pole. Until that happens, scientists will probably not agree on whether Chang’e-4 has really found mantle material. What they do agree on, however, is that the new study is nothing less than a pioneering feat. “Although I’m fairly sceptical of some of their interpretations of their measurements, I think the fact that they even made these measurements is an incredible achievement,” Moriarty says. “That’s really done a lot to advance lunar science and exploration.
Credit: Robin George Andrews for National Geographic, 15 May 2019.