The Moon was volcanically active for a long time, but if we look at its mysterious far side, we now know exactly how long that was the case. Thanks to samples collected by the Chinese Chang’e-6 probe, researchers have been able to estimate that this volcanic period lasted for at least 1.4 billion years.

Chang’e-6 is the only spacecraft that has brought back material from the far side of the Moon. Just shy of two kilograms (4.4 pounds) of material was collected and yet it is enough to provide a wealth of new insights as to what goes on beyond the part of the Moon we can see. One fragment is estimated to be 4.2 billion years old. This is the oldest basalt sample from the Moon whose age has been precisely measured – but that’s not all they found.

They also found 107 other fragments of basalt that instead are "just" 2.8 billion years old. These were local basalts from an eruption that must have taken place at the Chang’e-6 landing site. There is no evidence of this eruption from near-side samples, making this a very important finding; the lander had previously found evidence of magmatic structures there.

The far side of the Moon is very different from the near side. The near side has the beautiful dark plains of basalts, the maria, or seas. The far side, however, is covered in craters. The crust thickness among the two hemispheres is also different, as is the concentration of certain elements, such as thorium. 

Why there is such a dichotomy is not clear, but the two papers describing these results are certain that the answer goes through understanding the volcanism on the far side.

“The samples returned by Chang’e-6 provide a best opportunity to investigate the lunar global dichotomy,” senior author Professor Yi-Gang Xu, from the Guangzhou Institute of Geochemistry, said in a statement.

It is a sentiment shared by Professor Qiu-Li Li's lab at the Institute of Geology and Geophysics of the Chinese Academy of Sciences. “Unraveling the volcanic history of the lunar farside is crucial for understanding the hemispheric dichotomy of the Moon,” said Li in a different statement.

The volcanic event of 2.8 billion years ago happened locally and it is consistent with the estimation of the age of the landing site of Chang’e-6, determined by statistically counting the craters in that region. The crate-counting chronology was established on the lunar near side; the fact that it also works on the far side is a very exciting result.

The Moon was likely already stuck with one face always towards Earth by the time the volcanism on the far side stopped.

The papers are published in the journal Science and Nature.