Volcanism on the Moon ended about 2 billion years ago

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 accumulated to create the Earth-Moon system, the Moon spent many eons cooling down. This meant that a few billion years ago, lakes of lava flowed across the surface of the Moon, which eventually hardened to form the vast dark spots (lunar maria) that are still there today.

Thanks to lunar rock samples brought to Earth by China Chang’e 5 mission, scientists learn more about the formation and evolution of the Moon. According to a recent study by the Chinese Academy of Geological Sciences (CGAS), an international team examined these samples to investigate the end of volcanism on the Moon. Their results not only fill the gaps in the geological history of the Moon, but also other bodies in the solar system.

The study, recently published in the journal Science, was led by Xiaochao Che of the Beijing Sensitive High-Resolution Ion Microprobe 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, Swedish Museum of Natural History, Shandong Institute of Geological Sciences and several universities in the United States, United Kingdom and Australia.

Mons Rümker can be seen 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 return 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 the lunar lands, as it is believed to have hosted the most recent basaltic lava flows on the Moon. Jim Head, a research professor in Brown’s Department of Earth, Environmental and Planetary Sciences, was a co-author of the new study.

The Chang’e-5 spacecraft landed in this region on December 1, 2020 and managed to collect approximately 1730 g (61.1 oz) of moon rock from this region, including a core obtained at a depth of approximately 1 m (3.3 ft) below the surface. As he explained in a recent News from Brown Press release:

“These samples come from a region of the Moon that has 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 visible face of the Moon.

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

Mons Rümker, a volcanic construction in Oceanus Procellarum on the Moon. Lunar Reconnaissance Orbiter photo mosaic, made with a wide-angle camera. Credit: NASA

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

“However, in these samples, we didn’t actually see a high composition of radioactive elements,” Head said. “If these radioactive elements are the source of volcanism in this region, we would expect to see increased radioactivity in the samples. But we didn’t. Instead, the composition was similar to that of marine basalts from older deposits. This therefore casts doubt on this hypothesis of long-lasting volcanism. “

Essentially, their examination revealed that alternative explanations are needed as to why the region of Oceanus Procellarum experienced a prolonged period of lunar magmatism. However, the most important finding of this study is how it succeeded in constraining the age of some of the Moon’s most recent basaltic lava samples. This not only establishes an end point for the Moon’s most active volcanic period, but is also essential for modeling its thermal evolution and geological history.

And as Head has indicated, it also serves as a means to calibrate the timing of other events in the geological history of the Moon and on other bodies in the solar system:

“When we look at a surface or feature on the Moon that we don’t have samples for for radiometric dating, we’re trying to estimate its age through the size-frequency distribution of the impact craters. Basically, over time larger impacts become rarer. Thus by counting craters of different sizes, we can establish a relative age of a surface.

Understanding the geological history of the Moon will help piece together the history of our solar system. Credit: Hernán Cañellas / Benjamin Weiss

Finally, these reviews allow scientists to fill critical gaps in our understanding of the history of the Moon. “But between about a billion and three billion years ago, we don’t have a lot of good data points to tell us what the impact flow looks like,” Head added. “So having an absolute radiometric date for that surface helps us calibrate the flux curve, which helps us date other surfaces. And that’s not just true for the Moon. It helps us calibrate the ages. for Mars, Venus and elsewhere. “

The samples obtained by the Chang’e-5 rover are also the first to return to Earth since the Apollo era (45 years ago). You could say that the results of this research are a glimpse of how our renewed lunar exploration efforts will yield new and valuable information about the formation and evolution of the Earth-Moon system. These, in turn, could shed light on how habitable conditions emerged and lasted on Earth, but no other body in the solar system.

Further reading: Brunette college, Science

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Johnnie Gross

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