(ORDO NEWS) — No matter where you are on Earth, you can only see one face of the moon. Its other half is constantly turned away from our planet, and this far side is much more cratered than the one that faces us.
The near side of the Moon is covered with lunar maria – vast plains of volcanic basalt that look like dark spots when we look at our satellite.
The reason for this duplicity has remained a mystery that has persisted since the first spacecraft entered orbit around the Moon in the 1960s. But new modeling may have now solved the mystery of the Apollo era.
Computer models combining various features support the idea that lava flows were once formed on the near side of the Moon as a result of a powerful lunar impact. The differences are not only in the depth of the skin, but also in the geological composition of each side of the moon.
Astronomers have long suspected that the near side was once covered in a sea of magma that, as it cooled, flattened the rocky landscape, creating the dark patches we see today. However, the question of what triggered this volcanic activity is debatable.
A massive crater at the Moon’s south pole, known as the South Pole-Aitken Basin (SPA), may explain the differences.
This pool is the remnant of one of the largest and oldest collisions on the Moon. Simulations show that the SPA event, which happened about 4.3 billion years ago, happened at the right time and place to initiate changes to just one side of the lunar mantle.
The enormous heat released by the impact would have heated the upper mantle on the near side to such an extent that, according to experts, this would lead to a concentration of potassium, rare earth elements, phosphorus and heat-producing elements such as thorium.
To date, this is precisely the composition that scientists have found in samples of lunar rocks from the near side, especially in the Procellarum KREEP (PKT) terrane, a large area known for this compositional anomaly.
“We have shown that under any plausible conditions at the time the SPA formed, it eventually concentrates these heat-producing elements on the near side,” explains planetary scientist Matt Jones of Brown University.
“We expect that this contributed to the melting of the mantle, which led to the formation of the lava flows that we see on the surface.”
Fallout after the SPA event probably lasted hundreds of millions of years.
In the simulation, the oldest near-surface volcanic plain erupted 200 million years after the 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 that this cheek of the moon reacted more strongly to the impact lies both in the fact that the impact site facilitated the transfer of heat-producing materials, and in minor changes in gravity.
In every scenario the researchers considered, the upper mantle in the southern hemisphere heated up and began to flow towards the northern hemisphere, moving through the near side.
At the same time, the upper mantle on the far side remained too cold to distribute the same material in this way.
This difference could well have given rise to the asymmetry observed on the two sides of the Moon.
“How the PKT formed is perhaps the most important open question in lunar science,” says Jones.
“And the South Pole-Aitken collision is one of the most significant events in lunar history. This work brings these two together, and I think our results are really interesting.”
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