by By Will Dunham
(Reuters) – China’s robotic spacecraft Chang’e-6 made history in June by collecting the first surface samples from the far side of the moon, which is constantly facing away from Earth. This material now provides new insights into the Moon’s geological history, including the oldest evidence of lunar volcanism to date.
Researchers said Friday that soil brought back from the Chang’e-6 landing site contained fragments of volcanic rock – basalt – dating back to 4.2 billion years ago and 2.8 billion years ago. This suggests a long period of volcanic activity – at least 1.4 billion years – on the other hand, during the first half of the moon’s history, when the world was more dynamic than it is today.
The Moon, like the Earth, was formed about 4.5 billion years ago. Volcanism on the Moon, Earth, and other planetary bodies involves molten rock erupting from the mantle—the layer directly beneath the outer crust—to the surface. The South Pole-Aitken Basin landing site, an impact crater, is an area of the thinnest crust on the Moon and helps search for evidence of volcanism.
The Chang’e-6 probe used a shovel and drill to extract about 4-1/4 pounds (1,935 grams) of soil. It then returned the material to Earth and landed in China’s Inner Mongolia region.
“The Chang’e-6 samples provide a unique opportunity to study volcanism on the other side,” lunar scientist Qiu-Li Li of the Chinese Academy of Sciences, who led the study, said in the journal Nature of two published details of the results on Friday, the other in the journal Science.
The samples contained various volcanic rock fragments, and researchers used a method called radioisotope dating to determine their age.
During the US Apollo missions, the Soviet Luna missions and the Chinese Chang’e-5 missions, lunar basalt samples were previously collected from the Earth-facing side of the Moon. These show that volcanism on the opposite side occurred as early as 4.0 billion years ago and lasted at least two billion years, Li said.
“The exact timing and duration of lunar volcanism is difficult to determine and may vary across regions. Even on the near side, small-scale volcanism may have occurred as recently as about 120 million years ago, as shown by volcanic glass beads from Chang’e. “5 samples” were collected in 2020, Li said.
The new study also found that the basalt from 4.2 billion years ago was different in composition from the basalt from 2.8 billion years ago, meaning it came from different sources of molten rock – magma – in the Earth’s mantle, Li said.
The Chang’e-6 samples, Li said, also differ in composition compared to previously collected lunar samples from the near side.
Unlike other places in the solar system such as Venus and Earth, there is no evidence of active volcanism on the Moon.
“The cessation of volcanism on the Moon is due to internal heat sources diminishing over time. “Initially, the Moon’s volcanic activity was driven by the decay of radioactive elements and the residual heat of its formation, generating enough internal energy to sustain the melting of the Earth’s mantle, hence leading to surface volcanic eruptions,” Li said.
“As these heat sources diminished, the lunar mantle increasingly cooled and solidified, reducing the ability of magma to reach the surface. The Moon’s smaller size compared to Earth also contributed to this faster cooling rate, as smaller planetary bodies lose heat more quickly,” Li said.
The Moon’s diameter is about 2,160 miles (3,475 km), just over a quarter of Earth’s diameter.
“Eventually, the temperature inside the moon fell below the threshold required to sustain volcanic activity, marking the end of significant volcanism,” Li said.
(Reporting by Will Dunham in Washington; Editing by Daniel Wallis)
