Samples from far side of the moon show history of ancient volcanoes

The first surface samples retrieved from the dark side of the moon show that there used to be erupting volcanoes there, just as there were on the near side billions of years ago, according to new research.

China’s Chang’e-6 robotic spacecraft made history in June by retrieving the samples on the far side of the moon, which perpetually faces away from Earth. Now, two new studies examining those samples — which include the oldest evidence to date of lunar volcanism — is painting a deeper picture of the moon’s geological history. 

Researchers said on Friday the soil brought back from the Chang’e-6 landing site contained fragments of volcanic rock — basalt — dating to 4.2 billion years ago and to 2.8 billion years ago. This points to a period of volcanic activity at least 1.4 billion years long on the far side during the first half of the moon’s history, when it was a more dynamic world than it is today.

Volcanism on the moon, Earth and other planetary bodies involves the eruption of molten rock from the mantle — the layer just under the outer crust — onto the surface. The landing site in the South Pole-Aitken Basin, an impact crater, is an area with the thinnest crust on the moon, helpful for finding evidence of volcanism.

The Chang’e-6 probe used a scoop and drill to obtain about 1,935 grams of soil, containing more than 100 basalt fragments. It then returned the material to Earth, landing in the Chinese region of Inner Mongolia.

“The Chang’e-6 samples provide a unique opportunity to study far side volcanism,” said lunar scientist Qiu-Li Li of the Chinese Academy of Sciences, who helped lead the study in the journal Nature, one of two published on Friday detailing the findings, the other in the journal Science.

Researchers used radioisotope dating to determine the age of the various volcanic rock fragments found in the samples. 

A more explosive place years ago

We’ve known for a long time that the moon used to be a far more explosive place — lunar basalt samples previously were obtained from the moon’s near side, which perpetually faces Earth, during U.S. Apollo, Soviet Luna and Chinese Chang’e-5 missions. 

These showed that volcanism on the near side had occurred as long ago as four billion years ago and continued for at least two billion years, Li said.

“The exact timing and duration of lunar volcanism is elusive and maybe varied across different regions. Some small-scale volcanism may have also occurred on the near side as late as about 120 million years ago as recorded by volcanic glass beads from Chang’e-5 samples” collected in 2020, Li said.

While previous studies, including data from NASA’s Lunar Reconnaissance Orbiter, suggested the far side might also have a volcanic past, this is the first time that samples have been able to confirm that. The evidence of volcanic activity dating to as recently as 2.8 billion years ago on the far side is also something that has never been observed on the near side. 

Different volcanic sources

Researchers also found that the basalt dating to 4.2 billion years ago differed in composition from the basalt dating to 2.8 billion years ago, meaning they originated from different sources of molten rock — magma — in the mantle, Li said.

The Chang’e-6 samples, Li said, also differ in composition compared with previously collected lunar samples from the near side.

There is no evidence of active volcanism on the moon, unlike other places in the solar system such as Venus and Earth.

“The cessation of volcanism on the moon is due to internal heat sources dwindling over time. Initially, the moon’s volcanic activity was driven by the decay of radioactive elements and the residual heat from its formation, which created enough internal energy to sustain mantle melting and, consequently, surface volcanic eruptions,” Li said.

As the heat sources diminished, the moon’s mantle cooled, he added — a process hastened by the moon’s smaller size. 

The moon’s diameter of about 3,475 kilometres is a bit more than a quarter of Earth’s diameter.