A first look at the rocks from the far side of the moon creates a volcanic mystery

The first samples from the far side of the moon contain signs of surprising volcanic activity near the lunar south pole.

Two separate analyzes of lunar rocks brought back to Earth by China’s Chang’e-6 spacecraft show the rocks formed from magma cooling relatively recently, about 2.8 billion years ago, according to papers published Nov. 15 in Science AND Nature. The measurements may help solve the mystery of why the far side of the Moon is so different from its near side, but they also raise new questions about the history of lunar volcanism.

The two faces of the moon are like night and day, with different topography, chemical composition, crater density, and evidence of volcanism. Large pools of solidified lava called mare cover almost a third of the nearby side. But only about 2 percent of the outer rim shows signs of lava flow.

“The enigmatic asymmetry between the near side and the far side of the moon … is a long-unsolved puzzle,” write geochemist Qiu-Li Li of the Chinese Academy of Sciences in Beijing and colleagues in Nature.

Until recently, all the rocks that humans had brought back from the moon were close-ups. Samples from the Apollo and Luna missions in the 1960s and 1970s suggested that the moon was most volcanically active about 4 billion years ago and had largely cooled about 3 billion years ago. Rocks from China’s Chang’e-5 mission show more recent volcanism, about 2 billion years ago (SN: 10/7/21).

But the remote volcanic history was a complete mystery – until China’s Chang’e-6 mission returned the first samples ever collected from the region in June (SN: 6/5/24). The spacecraft returned to Earth with almost two kilograms of lunar soil from the South Pole-Aitken Basin, the oldest and largest crater and the source of most of the volcanic material on the far side of the Moon.

“Imagine having rock samples from maybe 10 places in North America, and that’s what you know about Earth,” says planetary scientist Stephen Elardo of the University of Florida in Gainesville, who was not involved in the new work. “Then all of a sudden you get your first stones from South Africa or Australia. Now you can have it as another data point to learn about the entire planet. That’s basically what this is about the moon.”

Two groups examined the rocks using radiometric dating, a technique for estimating an object’s age from the relative amounts of certain radioactive elements it contains. (SN: 10/5/21).

Geochemist Le Zhang of the Chinese Academy of Sciences in Guangzhou and colleagues studied 35 fragments of volcanic rocks called basalt and found their cumulative ages to be about 2.830 billion years, the researchers report in Science. Li and colleagues examined 108 basalt fragments and found similar ages: 2.807 billion years, the team reports in Nature.

“That’s a little younger than I would have predicted for that region of the moon,” says Elardo.

This is due to another feature that both groups found in the samples. The rocks lack heat-producing elements such as potassium, rare earth elements (such as uranium and thorium) and phosphorus, collectively known as KREEP. On the near side, broken KREEP elements may have kept the lunar mantle warm enough to support volcanism up to 2 billion years ago. But without these elements, it’s not clear how the last sphere stayed molten for so long, Elardo says.

The results also suggest long-term volcanism. Li’s team found one rock in particular that stood out: an aluminum-rich fragment dating back 4.2 billion years. The only known lunar rock that is older is a meteorite whose point of origin on the Moon is unknown. Together with younger samples, the rocks imply that distant volcanism stretches back at least 1.4 billion years.

Given the known differences between the moon’s hemispheres, it’s not surprising that the first distant samples look so different from the nearby ones, says Ryan Zeigler, curator of the Apollo sample at the Johnson Space Center in Houston. But the data is still exciting.

“I think this is just the first step,” says Zeigler. “I think they will come up with more techniques to deal with these particles with more time. And I think there may be more surprising things to come.”