Chang'e-6 sample studies cast light on the moon's far side

The latest discoveries achieved via studies of lunar rock samples retrieved by China's Chang'e-6 spacecraft were published in the world's leading scientific journals on Friday, revealing the volcanic history of the moon's lesser-known far side, which is distinct from its near side.

Rock samples returned by Chang'e-6, which had touched down on the lunar surface a few months earlier, have revealed two volcanic events that occurred on the moon's far side 2.8 billion years ago and 4.2 billion years ago, respectively, according to a new study published in the journal Nature.

In the study, researchers at the Institute of Geology and Geophysics of the Chinese Academy of Sciences analyzed 108 basalt fragments from lunar samples collected at the Chang'e-6 landing site. They determined that 107 pieces were formed approximately 2.8 million years ago, revealing a unique volcanic event dating back 2.8 billion years that had not been observed in previous samples obtained from the near side of the moon.

The remaining fragment, dating back to 4.2 billion years ago, is believed to have originated from a region south of the landing site. It is the oldest lunar rock sample with a precisely determined age to be returned to Earth, the study noted.

Another study published in the journal Science, emphasizes the distinctions between the moon's two sides.

Researchers from the Guangzhou Institute of Geochemistry under the CAS used various techniques to confirm that the low-titanium rock samples collected by Chang'e-6 are approximately 2.83 billion years old – which suggests recent volcanic activity on the far side of the moon.

This discovery also adds an important notch to the moon's crater timeline, suggesting that after an earlier significant decrease, the frequency of crater impacts on the moon has remained relatively stable from at least 2.83 billion years ago, according to the research team.

The study also challenged the traditional belief that the thicker lunar crust on the far side of the moon prevents volcanic eruptions. However, the researchers discovered a scarcity of large volcanic rocks even in areas with a thin crust near the Chang'e-6 landing site. They suggested that other factors, such as the composition of the mantle source, may also have an impact on lunar volcanic activity.

"The samples returned by Chang'e-6 provide the best opportunity to investigate lunar hemispheric differences," said Professor Xu Yigang, leader of the study published in Science.

People on Earth can only ever see one lunar face because the moon takes about the same amount of time to rotate once as it does to complete a single orbit of Earth. As a result, scientists divide the moon into two hemispheres, namely the side that always faces us, known as the near side, and the one that is always turned away from us, which we refer to as the far side of the moon.

The moon's far side is very different to its near side. For example, volcanic eruptions flooded parts of the surface with lava, producing rocks known as mare basalt – which occupy over 30 percent of the near side of the moon, but less than 2 percent of its far side.

Previous missions, such as those involving Apollo of the United States and China's Chang'e-5, retrieved lunar samples from the moon's near side. These samples suggest that volcanic activity on the lunar near side can be traced back to 4 billion years ago and that such activity continued until at least 2 billion years ago. In addition, Chang'e-5 samples indicate that small-scale volcanic activity even occurred on the near side as recently as 120 million years ago. However, it remains uncertain whether similar long-term volcanic activity took place on the moon's far side, said Li Qiuli, lead author of the study appearing in Nature.

Apart from the prevalence of mare basalt, the moon's two hemispheres also differ in terms of geological structures, landscapes, chemical composition, and crustal thickness.

To gain a comprehensive understanding of the dual nature of the Earth's celestial neighbor, it is crucial to also examine samples from its less-studied far side. The significance of this need was emphasized by the two Chinese research teams.

On June 25 this year, the Chang'e-6 lunar probe returned to Earth with 1,935.3 grams of lunar rock and dust samples collected from the previously unexplored far side of the moon.

More discoveries about the enigmatic moon will likely be revealed in the near future, now that China's space agency has started its application process for other researchers keen to have a closer look at the lunar specimens returned by Chang'e-6 earlier this year.

The agency said that further study of these samples would enhance the understanding of lunar evolution, accelerate the pace of peaceful exploration and utilization of lunar resources, and serve as an important asset for all humanity.