New insights into the Moon’s evolution

The Moon rocks from China’s Chang’e 5 lander have been analysed, adding to the story of the Moon’s thermal and chemical evolution.

More than 50 years ago, the Apollo program brought 382 kilograms of lunar material back to Earth, including Moon rocks, core samples, pebbles, sand and dust. These samples gave us a glimpse of the Moon’s geological history, including how it formed in the early Solar System.

But since then, no one has walked on the Moon to collect any rocks – only 300 extra grams of material have been returned for analysis by uncrewed Soviet landers in the 1970s.

Now, China’s Chang’e 5 lander has stepped in to fill the gap. On 17 December 2020, the mission returned 1.73 kilograms of lunar material to Earth from a specifically chosen landing site: Oceanus Procellarum, or ‘Ocean of Storms’. This volcanic plain is covered in basaltic rocks formed by lava gushing out of fissures on the Moon’s surface. The site was chosen in the hope of getting some ‘fresher’, younger rocks than those collected by previous missions.

The results of the first analysis of the Chang’e 5 samples are now in, with Chinese researchers, along with international colleagues, publishing three new papers in Nature.

So, what did the three new studies find?

Firstly, the rocks are younger than any lunar samples we’ve previously dated. Earlier radioisotope dating of samples suggested that the Moon was last volcanically active around 2.9 to 2.8 billion years ago, but these new samples formed a mere 2.03 billion years ago.

“This is the youngest crystallisation age ever reported for lunar basalts by radiometric method, extending the duration of lunar volcanism by 800 to 900 years,” the team, led by Qiu-Li Li from the Institute of Geology and Geophysics at the Chinese Academy of Sciences, notes in the first paper.

This means the Moon flowed with lava for much longer than previously thought, which in turn means that its interior was still evolving 2 billion years ago.

“This finding brings forward the time at which the Moon’s interior was still evolving from around 4 billion years ago to 2 billion years ago,” says Li.

Direct dating is not the only way that researchers can estimate the age of volcanism on the Moon – they can also count craters. A more heavily cratered region is older than one that has been smoothed over by recent lava flows, as it has had more time to accumulate craters from impacts.

Using this method, scientists already knew that the sparsely cratered Oceanus Procellarum was younger than the landing sites of the Apollo missions. But this new analysis has provided a direct confirmation of just how young the region is.

“The new age provides a pivotal calibration point for crater-counting chronology in the inner Solar System and sheds new light on the volcanic and thermal history of the Moon,” the team wrote.

The other two papers have also added to our knowledge of the Moon’s history. One analyses the water composition of the basalt samples, showing they contain less water than samples from other regions with older volcanic activity.

“The distribution of water in the Moon’s interior carries implications for the origin of the Moon, the crystallisation of the lunar magma ocean, and the duration of lunar volcanism,” this paper stated.

It suggests that the source of the younger basalts in the mantle may have become dehydrated by a long period of volcanic activity.

The final study also looked at the composition of the basalts, but focused on measuring the rocks’ heat-producing trace elements. Interestingly, the samples had lower levels than expected.

“The highly evolved origin of the 2-billion-year-old Chang’e-5 basalts implies that the lunar interior was substantially cooler at that time than at ~3.5 billion years ago when the variety of more primitive basalts sampled by Apollo were formed,” this paper noted.

“And this is not the end of the story,” adds Alexander Nemchin, a geochemist from Curtin University, who was not an author on these studies but was involved in dating the Chang’e 5 samples in a recent Science paper. “More studies are needed to establish a range of ages, chemical compositions, etc, of more fragments.”

This, he explains, will fill in the picture of the source of these rocks and how long it took to form the volcanic plain that the samples were taken from – which is 50 metres thick and 70,000 square kilometres in area.

“All planetary bodies in the Solar System are linked and have a somewhat similar history, so investigating Moon and other bodies helps to understand our own planet,” he says.

“We can use that knowledge to describe better what was happening on Earth in the past and even try to predict the future of the planet.”

While the samples are currently only available for investigation by China, Nemchin says this will soon change.

“I suspect that as early as the next year Chinese National Space Agency will develop rules for international access to the samples,” he concludes.

All three papers were led by authors from the Chinese Academy of Sciences in Beijing.

• Two billion-year-old volcanism on the Moon from Chang’e-5 basalts, led by Qiu-Li Li.
• A dry lunar mantle reservoir for young mare, led by Sen Hu.
• Non-KREEP origin for Chang’e-5 basalts in  the Procellarum KREEP Terrane, led by Heng-Ci Tian.