Volcanism on the Moon Ended About 2 Billion Years ago

The most popular theories state that the Moon formed around 4.5 billion years ago when a Mars-sized object (Theia), collided with Earth. The Moon experienced a long cooling process after the debris formed the Earth-Moon system. This resulted in lakes of lava flowing across the Moon's surface a few billion year ago. These lava layers eventually hardened to create the large dark patches (lunar maria) that still exist today.
Scientists are now learning more about the Moon's formation and evolution thanks to lunar rock samples brought back by China's Change 5 mission. A recent study by the Chinese Academy of Geological Sciences, (CGAS) revealed that an international team looked at these samples to determine when lunar volcanism ended. These results not only fill in the gaps in Moon's geological history, but also provide information about other Solar System bodies.

The study was published in Science by Xiaochao Che, of the Beijing Sensitive High Resolution Ion Micro Probe Center. It is located at the CGAS Institute of Geography. He was joined by scientists from the Planetary Science Institute, McDonnell Center for the Space Sciences and the Swedish Museum of Natural History.

This image was taken by the Apollo 15 mission to lunar orbit and shows Mons Rmker in Oceanus Procellarum. Credit: NASA

The Change-5 rover has returned the first samples to Earth since the Apollo era (45 Years ago). They were taken from Oceanus Procellarum, a volcanic plain. This lunar region is unusual among lunar terrae because it is thought to have been home to the latest basalt lava flows. Jim Head, a Browns Department of Earth, Environmental and Planetary Sciences research professor, was co-author of the new study.

The Change-5 spacecraft arrived in the region on December 1st 2020 and collected approximately 1,730g (61.1 oz.) lunar rock. This included a core sample taken from a depth of 1m (3.3 feet) below the surface. In a News from Brown press release, he explained:

These samples are from an area of the Moon that has been mostly unexplored by landed spacecraft. All previous samples taken by the Apollo and Soviet Luna missions came from the Moon's central and eastern side. As we collected more remote sensing data, it became obvious that most recent lunar volcanism was in the western region. Therefore, that area became the prime target for sample collection. The samples were taken from Mons Rmker near Oceanus Procellarum, the largest lunar maria.

Mons Rmker is a volcanic structure in Oceanus Procellarum, on the Moon. A mosaic of photos taken by Lunar Reconnaissance Orbiter using Wide Angle Camera. Credit to NASA

High levels of radioactive elements are found in the Oceanus Procellarum area, including uranium and potassium. They are thought to have contributed to the prolongation of magmatic activity near the Moon by generating heat through long-lived radioactive decomposition. The team determined that the samples were on average 2 billion years old after analyzing them using radiometric dating.

Head said that we did not see any elevated radioactive elements in the samples. We expect enhanced radioactivity to be seen in these samples if radioactive elements are driving volcanism in this area. But we didnt. The composition was more like mare basalts from older deposits. This casts doubt on the hypothesis of long-lasting volcanism.

Their investigation revealed that there are alternative explanations for why Oceanus Procellarum experienced prolonged periods of lunar magmatism. The most important takeaway from this study was how it managed to limit the age of some recent basalticlava samples taken from the Moon. This provides an endpoint for Moon's most active volcanic period and is crucial to modeling its thermal evolution, geological history, and other aspects.

It also, as Head pointed out, serves to calibrate the timing of events in the Moon's geological history as well as on other bodies in our Solar System.

We use radiometric dating to determine the age of a Moon feature or surface. This is done by analyzing the frequency distribution of impact craters and their size. As time passes, more frequent impacts are occurring. We can calculate the relative age of a surface by counting craters with different sizes.

Understanding the Moon's geological history can help us reconstruct the history of the Solar System. Credit: Hernn Caellas/Benjamin Weiss

These examinations also allow scientists to fill in critical gaps in our knowledge of the Moon's history. Head said that we don't have any good data points between one billion and three trillion years ago to determine what the impact flux looked like. This radiometric date helps us calibrate the flux curve and date other surfaces. This is not only true for the Moon. This allows us to calibrate the ages for Mars, Venus and other planets.

The Change-5 rover also collected samples that are the first to be returned from Earth since the Apollo era (45 year ago). These results could be interpreted as a preview of what our renewed lunar exploration efforts can yield. These could help us understand how we created and sustained habitable conditions on Earth, but not on other planets in the Solar System.

Additional Reading: Brown University Science