In December 2020, China’s Chang'e 5 mission touched down on Oceanus Procellarum — a region of the Moon that was once a vast plain of molten lava. The site had been targeted by scientists for decades: Curiously, its surface is somewhat sparse with impact craters, suggested that its last lava flood had occurred quite recently (in lunar terms). Determining its age was one of the mission’s top priorities.
In all, the Chang'e 5 lander scooped and drilled 3.8 pounds (1.7 kilograms) of lunar material, with its ascent stage delivering them to the grasslands of Inner Mongolia on Dec. 16, 2020 — the first Moon rocks returned to Earth since 1976. The stash was then collected and parceled out to several research groups. Now, nearly ten months later, scientists are beginning to report what they have found.
The first major scientific paper detailing mission findings was published today in Science. According to the team’s analysis, the samples confirm the relative youthfulness of the landing site’s volcanic rocks: 1.96 billion years old, give or take a few tens of millions of years. This is about a billion years younger than any of the volcanic lunar samples returned by the Apollo missions and the Soviet robotic Luna missions.
The find indicates that volcanoes were erupting on the Moon as recently as 2 billion years ago — which throws a wrench into our understanding of how bodies like planets and moons form. Scientists think that when such bodies are young, radioactive uranium and thorium sink deep into their interiors. These slowly decay and release heat, which, in a large body, can keep the mantle molten for billions of years. But models suggest that an object as small as the Moon should lose all its heat quickly.
“We always said that, OK, 3 billion-year-old basalts is fair enough, probably it can be sustained by this radioactive decay,” says Alexander Nemchin, a geochemist at Curtin University in Perth, Australia, and one of the team’s leaders. But 2 billion years is too young for current models, he says — “so now we've got a problem.”
Nevertheless, the result is exactly what scientists hoped for when they chose the probe’s landing site, says study co-author Brad Jolliff, a planetary scientist and mineralogist at Washington University in St. Louis. “This actually shows that the main science goal was met — and that’s pretty awesome.”