China's new moon-rock treasure trove may be a billion years younger than the material the Apollo program brought home decades ago, according to new research.
All told, in December, China's Chang'e 5 spacecraft managed to return 3.81 pounds (1.73 kilograms) of moon rock from a region called Oceanus Procellarum to scientists on Earth. Since then, scientists with access to the precious material have begun a bevy of experiments to understand the rocks and the secrets of the solar system that they might hold.
And the spacecraft seems to have snagged the perfect sample to fill a critical hole in scientists' knowledge, a new study reports: 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.
"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.
With even just these two tiny rocks — each only a few millimeters in size (0.1 to 0.15 inches) — 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.
The scientists 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.
The results are described in a paper published on Thursday (Oct. 7) in the journal Science.