No matter where you stand on Earth, you can only ever see one face of the Moon. Its other cheek is always farther away from our planet, and this far side is filled with a lot more craters than the one in front of us.
The near side of the Moon is covered in lunar maria, vast plains of volcanic basalt that appear as dark spots when viewed by our satellite. The reason for this two-faced appearance remains a mystery—one that has persisted since the first spacecraft orbited the Moon in the 1960s. But a new simulation may have now solved the Apollo-era puzzle.
Combining the various features together, computer models support the idea that the once-massive lunar impact re-emerged on the near side of the Moon in lava flows. The differences are more than skin deep, as they are also reflected by the different geological compositions on each side of the Moon.
Astronomers have long suspected that the near side was once covered in 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 giant crater at the Moon’s south pole, known as the South Pole-Aitken Basin (SPA), may explain the differences.
This basin is the remnant of one of the largest and oldest collisions on the Moon. Simulations show that the SPA event, which occurred about 4.3 billion years ago, happened at the right time and was the right place to start the change on just one side of the lunar system.
The extreme heat generated by the impact would have heated the upper mantle to such an extent that experts believe it would have increased the concentration of heat-generating elements such as potassium, rare earth elements, phosphorus and thorium.
To date, this is precisely what scientists have found in samples of lunar rock from the Near East, notably in the Procellarum Creep Terrain (PKT), a large region known for this compositional anomaly.
“What we show is that under any appreciable conditions at the time of SPA’s formation, it concentrated these heat-generating elements nearby,” explains Brown University planetary scientist Matt Jones.
“We expect that this contributed to the mantle melting producing the lava flows seen at the surface.”
The result of the SPA program may have been around for hundreds of millions of years.
In simulations, the most ancient nearby volcanic plain erupted 200 million years after impact events. In fact, intense episodes of volcanic activity continued on the near side of the Moon for 700 million years after the impact.
According to experts, the reason why this cheek of the Moon reacted more to the hit is because of where the site of impact concentrates the transport of heat-generating material, and also because of slight changes in gravity.
In every scenario the researchers examined, the upper mantle in the Southern Hemisphere warmed up and began flowing toward the Northern Hemisphere, traveling closer.
Meanwhile, the upper mantle on the far side remained too cold for the same material to be distributed more evenly.
This difference could very well have generated the asymmetry seen in the two faces of the Moon.
“How the PKT formed is arguably the most important open question in lunar science,” Jones says.
“And the South Pole-Aitken effect is one of the most important events in lunar history. This work brings those two things together, and I think our results are really exciting.”
The study was published in science progress,