On Earth, deep time is an open book. By measuring trace radioactive compounds in rocks that decay with metronomic regularity, dating experts have learned when oceans opened, volcanoes erupted, and mass extinctions struck. But the story is muddled elsewhere in the Solar System because records are sparse. Scientists estimate ages on the Moon and the rocky planets from the number of craters that pock their surfaces. They have fixed dates from just nine places, all on the Moon: the six Apollo and three Soviet Luna sites from which samples were returned to laboratories on Earth.
China’s Chang’e-5 mission, set to launch on 24 November, aims to make it 10, by returning the first Moon rocks since the last Luna mission in 1976. Getting a firm date from another location will improve the shaky crater counting scheme, says Kentaro Terada, a cosmochemist at Osaka University. It will also sharpen the picture of the Moon’s history. A fresh sample date “is the most important and exciting new finding [that will come] from the Chang’e-5 samples,” Terada says. Getting it will require a tour-de-force, round-trip space flight that has not been attempted for more than 40 years.
Chang’e-5’s target is Mons Rümker, a 70-kilometer-wide volcanic mound on the Moon’s near side, which may have erupted as recently as about 1.3 billion years ago. It is “the youngest mare basalt on the Moon,” says Xiao Long, a planetary geoscientist at the China University of Geosciences, referring to the dark lava also seen in the Moon’s maria, or seas. Brett Denevi, a planetary geologist at Johns Hopkins University’s Applied Physics Laboratory and science chair of a NASA lunar analysis group, says China has picked a spot where it can have a big scientific impact. “Understanding the age of those samples and all of the Solar System–wide implications that flow from that result will be a big leap forward for planetary science,” she says.
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The crater counting method for determining age relies on the notion that surfaces scarred with fewer craters are younger than those that have accumulated more. Regions dated with Apollo and Luna samples have helped calibrate the method. But except for one young outlier, all of those dates cluster between 3.2 billion and 3.9 billion years, leaving the method unanchored, and highly uncertain, for surfaces younger than 3 billion years old, Terada says. “Chang’e-5 samples will provide another data point,” he says.
Getting a firm date for Mons Rümker will also shed light on how lunar volcanism changed over time. Evidence suggests numerous eruptions in the first billion years of the Moon’s existence blanketed the surface with volcanic basalts, forming the dark maria, before tapering off about 3 billion years ago. If Mons Rümker material proves to be just 1.3 billion years old, it will raise questions about how the interior of a small planetary body remained hot enough to erupt so long after formation, says Romain Tartese, a planetary scientist at the University of Manchester.
Retrieving the samples will require a complex deep-space ballet. After launch from the Wenchang launch center in southern China, Chang’e-5 will arrive at the Moon about 3 days later, where an orbiter will release a lander. Over the course of 14 days, the lander’s robotic arm will scoop up surface samples and a drill will retrieve cores down to 2 meters. Scientists are hoping for 2 kilograms of material. (NASA’s Apollo program brought back more than 380 kilograms; three Soviet robotic Luna missions returned 301 grams.) An ascent vehicle will ferry the samples to the orbiter, where they will be packed into a re-entry capsule for return to Earth and a touchdown in the grasslands of Inner Mongolia. Xiao says international investigators will have access to the samples, but U.S. scientists may not because of limits on cooperation with China set by the U.S. Congress.
Chang’e-5 is the latest in a set of increasingly ambitious Moon missions from the China National Space Administration, all named after Chang’e, a Chinese Moon goddess. A pair of orbiters, launched in 2007 and 2010, focused on mapping and remote observations. The lander-rover Chang’e-3 mission, in 2013, carried the first ground-penetrating radar to the lunar surface. In 2019, Chang’e-4, another lander-rover, was the first spacecraft to soft-land on the far side of the Moon. Three more Chang’e missions and a robotic scientific research station are planned by 2035.
Results from Chang’e-4, still trundling along after having traveled nearly 600 meters, are raising questions for later missions. The craft landed in the South Pole–Aitken basin, the Moon’s largest, deepest, and oldest impact crater, at perhaps 4 billion years. Scientists have calculated that the impacting body likely burrowed 70 kilometers into the Moon and churned material from the mantle up to the surface. In a study published in 2019 in Nature, one group of Chinese scientists said the rover’s instruments had detected mantle minerals, but other groups, including Xiao’s, have challenged that interpretation. Patrick Pinet, a planetary geophysicist at France’s Astrophysics and Planetology Research Institute, says researchers are debating why such an enormous impact apparently did not exhume mantle material—or whether the mantle composition is somehow unexpected.
Zou Yongliao, a geochemist at the Chinese Academy of Sciences’s National Space Science Center, says China is making the South Pole the focus of its near-term lunar plans. And although the target site has not been revealed for Chang’e-6, another sample return mission, planetary scientists are rooting for South Pole–Aitken. A basin sample would provide clues to the mantle puzzle. It would also anchor the older end of the crater-counting curve, says Carolyn van der Bogert, a planetary geologist at the University of Münster, and “illuminate the early history of the Moon.”
sciencemag.org, 19 November 2020