Active volcanoes were erupting on the far side of the moon 2.8 billion years ago, the first lunar samples returned from the far side reveal.
A gravity map of the moon’s surface with the moon’s far side in the middle. (Photo courtesy of MIT/NASA/ARC)
Volcanoes were erupting on the moon’s far side 2.8 billion years ago, according to the first examination of lava samples taken from that region.
Since the moon and Earth are tidally locked, our planet is constantly facing the same side of the moon. Compared to the close side, the far side is less explored. The moon’s far side has only been reached by two landers, both of whom are from China.
In a study that was published on Nov. 15 in the journal Science, scientists examined rock samples that the Chang’e 6 lander brought back to Earth. A little more than 4 pounds (1.9 kilograms) of rock from the South Pole-Aitken basin were returned by the 2024 mission, marking the first time samples from the moon’s far side have ever been transported to Earth.
To determine the age and origin of these samples, Zexian Cui of the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences and Associates examined the isotopes present in the samples together with their chemical composition. Atoms of an element that differ in the number of neutrons in their nuclei but have the same number of protons are called isotopes. The ratio of various isotopes in a sample is a useful approach to determine how old that sample is because the quantity of neutrons fluctuates with time during radioactive decay.
According to the study, the rocks were 2.8 billion years old and were made of solidified lava known as basalt. Up to at least 2 billion years ago, previous studies had discovered volcanism on the moon’s near side. The new dates show that the moon’s far side was also experiencing volcanic activity. Volcanoes may have been erupting on the moon as recently as 120 million years ago, according to a new analysis of materials from the Chang’e 5 rover, which touched down on the moon’s near side in 2020.
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Additionally, the scientists discovered that the lava that formed the basalt originated from a region of the moon’s mantle that was deficient in phosphorus, potassium, and rare earth elements. The lava on the moon’s near side contains large amounts of these elements. Cui and his colleagues stated that the South Pole-Aitken basin may have formed as a result of an impact crater, which might explain the perplexing imbalance. The impact may have melted the mantle directly under the impact site, depriving it of certain elements, and redistributed rocks containing those elements since it was massive enough to resonate all the way across the moon.
Another peculiar distinction between the moon’s two sides might be explained by the elemental imbalance: Only 2% of the far side is covered with massive lava flows known as mare basalts, which make up about 30% of the close side. According to the authors, some of the far side’s missing elements, such as uranium and potassium, are radioactive and emit heat during their disintegration. The lack of these melty basalts may be explained by their absence in the mantle beneath the far side of the moon.