According to the most widely accepted theories, the Moon formed about 4.5 billion years ago after a Mars-sized object (Theia) collided with Earth. After the resulting debris accreted to create the Earth-Moon system, the Moon spent many eons cooling down. This meant that a few billion years ago, lakes of lava were flowing across the surface of the Moon, which eventually hardened to form the vast dark patches (lunar maria) that are still there today.

Thanks to the samples of lunar rock brought back to Earth by China’s Chang’e 5 mission, scientists are learning more about how the Moon formed and evolved. According to a recent study led by the Chinese Academy of Geological Sciences (CGAS), an international team examined these samples to investigate when volcanism on the Moon ended. Their results are not only filling in the gaps of the Moon’s geological history but also of other bodies in the Solar System.

The study, which recently appeared in the journal Science, was led by Xiaochao Che of the Beijing Sensitive High-Resolution Ion Micro Probe Center, located at the CGAS Institute of Geography. He was joined by researchers from the Planetary Science Institute (PSI), McDonnell Center for the Space Sciences, the Swedish Museum of Natural History, Shandong Institute of Geological Sciences, and several universities from the US, UK, and Australia.

Mons Rümker is visible in Oceanus Procellarum in this image taken from the Apollo 15 mission in lunar orbit. Credit: NASA

The samples obtained by the Chang’e-5 rover are the first to be returned to Earth since the Apollo era (45 years ago) and were obtained from the volcanic plain known as Oceanus Procellarum (Latin for “Ocean of Storms”). This lunar region is unique among lunar terrae, as it is believed to have hosted the most recent basalt lava flows on the Moon. Jim Head, a research professor in Brown’s Department of Earth, Environmental and Planetary Sciences, was a co-author on the new study.

The Chang’e-5 spacecraft landed in this region on Dec. 1st, 2020, and managed to collect about 1,730 g (61.1 oz) of lunar rock from this region, including a core sample obtained from a depth of ~1 m (3.3 ft) beneath the surface. As he explained in a recent News from Brown press release:

“These samples come from a region of the Moon that’s been largely unexplored by landed spacecraft. Previous samples from the Apollo missions and the Soviet Luna missions all come from the central and eastern part of the Moon’s near side.

“But it became clear as we collected more remote sensing data that the most recent volcanism on the Moon was absolutely in that western portion, so that region became a prime target for sample collection. Specifically, the samples came from near Mons Rümker, a volcanic mound in the largest of the lunar maria, Oceanus Procellarum.”

Mons Rümker, a volcanic construct in Oceanus Procellarum on the Moon. Mosaic of photos by Lunar Reconnaissance Orbiter, made with Wide Angle Camera. Credit: NASA

The Oceanus Procellarum region is characterized by high concentrations of radioactive elements such as potassium, uranium, and particularly thorium. These generate heat through long-lived radioactive decay and are believed to have played a role in prolonging magmatic activity on the near side of the Moon. After examining the samples through radiometric dating, the team concluded that they were (on average) 2 billion years old.

“However, in these samples, we didn’t actually see an elevated radioactive element composition,” said Head. “If these radioactive
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