Today, the Moon is dark and quiet. But 2 billion years ago, the Moon was hot and bothered, spewing out vast pools of lava, based on an analysis of rocks returned last year by a Chinese spacecraft. And that presents a puzzle for lunar researchers. Three studies published today suggest not only that the Moon was volcanically active long after it was thought to have cooled off, but also that it was erupting without the benefit of water and hot radioactive elements, which make it easier to produce magma under the surface.
The papers raise questions about how small planetary bodies like the Moon can “support volcanic eruptions in the late stage of life,” says Qing-Zhu Yin, a geochemist at the University of California, Davis, who was not involved in the research.
On 1 December 2020, China’s Chang’e-5 spacecraft touched down near Mons Rümker, an inactive volcano that sits within the Oceanus Procellarum, a vast “sea,” or mare, of dark, hardened lava called basalt. The lander drilled and scooped 1.73 kilograms of soil and rock and launched it back to Earth. The capsule containing the samples landed in Inner Mongolia on 17 December, delivering the first Moon rocks and soil to Earth since 1976. Since July, more than 30 scientists at Chinese universities and research institutions have been studying some 1% of the samples.
On 7 October, a team reported a 1.97-billion-year-old age for the Chang’e-5 basalts in Science. Today, that dating was corroborated by a new study, led by Xianhua Li of the Institute of Geology and Geophysics (IGG), part of the Chinese Academy of Sciences. An analysis of the radioactive decay of isotopes in 47 hardened lava samples put the age of the rocks at 2.03 billion years old, 1 billion years younger than samples from the U.S. Apollo missions, and 800 million years younger than the youngest lunar meteorites previously dated, Li and his team report today in Nature.
The findings will help improve age predictions for surfaces on the Moon and other rocky planets that are based on a technique called crater counting, which assumes that surfaces scarred with fewer asteroid impact craters are younger than those that have sustained more. Samples from the Apollo and Soviet Luna missions helped calibrate that method by providing firm dates. But those samples were almost all older than 3 billion years—making the dating of younger surfaces highly uncertain. The new paper provides “a key data point” in calibrating the crater counting method, Yin says.
But the age is also a surprise to scientists who thought that lunar volcanism tapered off rapidly 3 billion years ago, as the Moon’s interior cooled. A second paper in Nature, in which researchers measured the chemical composition of the lunar samples, adds to the mystery. Based on orbital measurements, Oceanus Procellarum is thought to be enriched in radioactive potassium, uranium, and thorium—heat sources that would help melt magma in the Moon’s mantle and drive eruptions. But the laboratory analysis found the mantle source of the Chang’e-5 samples was likely depleted of these elements. The chemical composition is quite different from Apollo and Luna samples, “representing a new type of lunar basalt” that deserves further studies, says Audrey Bouvier, a geochemist with the University of Bayreuth who is not involved with the research.
In a third Nature paper, researchers found the Chang’e-5 samples were exceptionally dry. That was a surprise because water lowers the melting point for rock and in its absence, eruptions are more difficult. Researchers estimated the sample’s parent magma to contain between just 1 and 5 parts per million of water. That’s less than 5% of the water content of the mantle rocks underneath the Apollo sites, says Mahesh Anand, a planetary scientist from the Open University in Milton Keynes, U.K., and co-author of the paper.
Yin says the papers present “a real conundrum.” How could melting have occurred so late in the Moon’s history without significant amounts of water or radioactive elements? Possible mechanisms, he suggests, include tidal heating from Earth, or a thick layer of lunar soil that acted as a blanket to retain heat. Another possible, but not obvious, energy source would be the impact of a large asteroid or comet, Li says. But such impact-driven volcanism typically occurs on the opposite side of the moon or planet from the impact point, a spot called the antipode. “The thing is, scientists haven’t found clear evidence for this connection at the antipode yet,” Li says.
The China National Space Administration (CNSA) today released a long list of researchers who would receive a second batch of Chang’e-5 samples—another 1% of the entire collection. Although researchers outside China are keen to analyze samples in their own labs, the samples so far have only been distributed to labs in China. At a press conference at IGG today, a CNSA official said the agency is still formulating rules for managing international loans of Chang’e-5 samples.
Still, Anand, who has worked with samples from all six Apollo missions, says the new results are just “the tip of the iceberg” for Chang’e-5 research. “If anything, the Apollo era has taught us that 50 years on we’re still looking at the same samples and finding new things.”