No matter where we are on Earth, we can only see one side of the Moon's face, while the other side is always away from our planet. The far side of the Moon is more pockmarked because it has more craters. Scientists try to explain why.
The near side of the Moon is covered in Lunar Maria, a vast plain of volcanic basalt that appears as a dark patch when we view Earth's natural satellite.
The reason for the appearance of these two faces has remained a mystery, even since the first spacecraft orbited the Moon in the 1960s. But today's latest simulations may have solved the puzzle of the Apollo mission era.
Combining these disparate features, the computer model supports the idea that a massive Moon impact once reappeared on the near side of the Moon in lava flows. The differences go beyond just the surface of the skin, as they are also described by the different geological compositions on each side of the Moon.
Astronomers have long suspected that the near side of the Moon was once covered in a sea of magma that when cooled, smoothed the rocky landscape and created the dark smudge we see today. However, the trigger for this volcanic activity is still being debated.
A large crater at the Moon's south pole, known as the South Pole-Aitken (SPA), may explain the difference. This basin is the remnant of one of the largest and oldest collisions on the Moon.
Simulations show the SPA event, which occurred about 4.3 billion years ago, occurred at the right time and place to initiate changes on only one side of the Moon's mantle.
The tremendous heat generated by the collision would have warmed the upper mantle on the near side of the Moon to such an extent. Experts think it will lead to concentrations of potassium, rare earth elements, phosphorus, and heat-generating elements such as thorium.
To date, that's the composition scientists have found in near-side lunar rock samples, particularly in the Procellarum KREEP Terrane (PKT), a large area known for this compositional anomaly.
"What we show is that under reasonable conditions at the time the SPA formed, it eventually concentrated this heat-producing element in the nearer part of the Moon," explained planetary scientist Matt Jones of Brown University. 2022).
"We hope this contributes to the melting of the mantle that produces the lava flows we see at the surface," he said.
The impact of the SPA event may last for hundreds of millions of years. In the simulation, the most ancient nearby volcanic plateau erupted 200 million years after the impact event. In fact, intense episodes of volcanic activity continued on the near side of the Moon for up to 700 million years post-impact.
According to experts, the reason why this side of the Moon reacts more to the collision is because this is the location of the collision centered on the transport of heat-generating material, as well as a slight change in gravity.
In each scenario the researchers studied, the upper mantle in the southern hemisphere warmed up and began to flow toward the northern hemisphere, moving through the near side.
Meanwhile, the upper coat on the far side remains too cold to distribute the same material in a similar way.
This difference has resulted in the asymmetry observed in the two faces of the Moon.
"How the PKT formed is arguably the most significant open question in lunar science. And the SPA collision was one of the most significant events in the history of the moon. This work brings those two things together, and I think our results are very interesting."