The nearby side of the Moon is covered by Lunar Mary, vast plains of volcanic basalt that appear as dark patches when we look up at our satellite. The reason for this two-faced appearance has remained a mystery – one that has persisted since the first spacecraft orbited the Moon in the 1960s. But a new simulation could now have solved the puzzle of the Apollo era. Combining different features, computer models support the idea that a huge lunar impact once reappeared on the nearby side of the Moon in lava flows. The differences are more than profound, as they are also reflected by separate geological compositions on each side of the Moon. Astronomers have long suspected that the nearby side was once covered by a sea of magma that, when cooled, smoothed the rocky landscape, creating the dark spots we see today. But the trigger for this volcanic activity is controversial. A huge crater at the Moon’s south pole, known as the Aitken South Pole Basin (SPA), could explain the differences. This basin is a remnant of one of the largest and oldest collisions on the Moon. The simulations show that the SPA event, which occurred about 4.3 billion years ago, happened at just the right time and the right place to start changes on only one side of the lunar mantle. The enormous heat generated by the impact would have warmed the upper mantle nearby to such an extent that experts believe it would have led to a build-up of potassium, rare earth elements, phosphorus and heat-producing elements such as thorium. To date, this is exactly the composition that scientists have found in samples of nearby lunar rocks, especially in the Procellarum KREEP Terrane (PKT), a large area known for this complex anomaly. “What we are showing is that under any reasonable circumstances at the time the SPA was formed, it ends up collecting these heat-generating elements nearby,” explains Matt Jones, a planetary scientist at Brown University. “We expect this to have contributed to the melting of the mantle that produced the lava flows we see on the surface.” The consequences of the SPA event would probably have lasted for hundreds of millions of years. In simulations, the oldest volcanic plain erupted 200 million years after the collision events. In fact, intense volcanic activity continued on the nearby side of the Moon for up to 700 million years after the collision. According to experts, the reason that this moon’s cheek reacted more to the impact is both due to the position of the impact that focused the transfer of heat-producing materials, and due to slight changes in gravity. In each scenario examined by the researchers, the upper mantle in the southern hemisphere warmed up and began to flow toward the northern hemisphere, traveling through the nearby side. Meanwhile, the top mantle on the far side remained too cool to distribute the same material in a similar way. This difference could well have created the asymmetry observed on both sides of the Moon. “How the PKT was formed is arguably the most important open-ended question in lunar science,” says Jones. “And the South Pole-Aitken collision is one of the most important events in lunar history. This work brings these two things together and I think our results are really exciting.” The study was published in Science Advances.
