Unlocked mystery fills key gaps in solar system history
October 10 2021 11:33 PM

All lunar fans should be overjoyed by the fact that moon rocks brought to Earth by a Chinese mission fill key gaps in solar system history. China’s new moon-rock treasure trove may be a billion years younger than the material the Apollo programme brought home decades ago, according to new research. It was in December last year that China’s Chang’e 5 spacecraft returned with nearly 2kg of moon rock. The new study reports the rare catch, a first since 45 years when the-then Soviet Union’s Luna 24 mission brought back lunar samples, is perfect to fill a critical hole in scientists’ knowledge. Two tiny pieces of the newly returned lunar rock have now been dated to about 1.97 billion years old, give or take 50 million years.
One of the striking findings from the latest samples is that the moon’s volcanoes were alive and active considerably longer than scientists thought. “All our experience tells us that the moon should be cold and dead 2 billion years ago. But it is not, and the question is, ‘Why?’” said Alexander Nemchin, a professor of Geology at Australia’s Curtin University and author of the analysis published last Thursday in the journal Science.
“It is the perfect sample to close a 2-billion-year gap,” Brad Jolliff, a planetary scientist at Washington University in St. Louis in Missouri and a co-author on the new research, which was led by a team based in Beijing, said in a statement. That gap stretches from about 3 billion years ago, when most of the rocks in the returned Apollo samples formed, to about a billion years ago, when some young dateable impact craters formed.
Scientists hoping to untangle the 4.5-billion-year history of the solar system have combined those time-stamped samples with a technique to identify relative ages called crater dating. “Planetary scientists know that the more craters on a surface, the older it is; the fewer craters, the younger the surface. That’s a nice relative determination,” Jolliff said. “But to put absolute age dates on that, one has to have samples from those surfaces.” Using that approach, scientists could look at the source of Apollo samples, note their age as determined in the lab and calculate how many craters are present. Then, planetary scientists could use that approximate date for other just-as-battered surfaces — not only on the moon itself but across the solar system, on worlds that scientists have never been able to get into the lab. And without any lunar samples from between 3 billion and 1 billion years ago, scientists’ crater-dating timeline had a huge gap in it — a long stretch during which scientists couldn’t put even approximate ages to surfaces.
Chang’e 5 has helped fill that hole. “In this study, we got a very precise age right around 2 billion years, plus or minus 50 million years,” Jolliff said. “It’s a phenomenal result. In terms of planetary time, that’s a very precise determination.” 
The age of these samples is also important because they are a type of rock called basalt, which forms during volcanic eruptions — and scientists previously only had evidence of lava flowing on the moon up until about 3 billion years ago. They did expect to find some of the moon’s youngest basalts in the region because the lunar crust is thin there and relatively rich in elements that produce heat. But the researchers are still unsure how the rock remained molten until such a late date. With more findings expected on the way, it is only a matter of time when more questions will be resolved about the great lunar puzzle.

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