New research shows how the Moon’s South Pole-Aitken Basin effect relates to the stark contrast in composition and appearance between the two sides of the Moon.
The face of the moon shows that the Earth looks very different from the one it hides on its far side. The near side is dominated by the Lunar Persians – the vast, dark-colored remains of ancient lava flows. On the other hand, the cratered far side is virtually devoid of broad marine features. One of the moon’s most enduring mysteries is the cause of the vast difference between the two sides.
Now, researchers have a new explanation for the two-faced moon – an explanation related to a giant impact billions of years ago near the moon’s south pole.
A new study published in Science Advances shows that the impact that formed the Moon’s Giant South Pole Basin (SPA) would have created a massive cloud of heat spreading across the interior of the Moon. This column will contain certain materials – a mixture of rare earth and heat-producing elements – as well as the nearby moon. This concentration of elements may have contributed to the volcanic activity that led to the creation of the nearby volcanic plains.
Extensive volcanic deposits dominate the near side of the Moon (left), while the far side (right) contains much less. The reason for the great difference between the two sides is the secret of the perpetual moon. Credit: Brown University
“We know that big impacts like the one that shaped the spa will create a lot of heat,” said Matt Jones, Ph.D. Brown University candidate and lead author of the study. The question is how this temperature affects the internal dynamics of the Moon. What we show is that under any reasonable conditions at the time of SPA formation, these heat-producing elements end up being concentrated on the near side. We speculate that this contributed to the mantle melting that led to the lava flows we see at the surface. “
The study was a collaboration between Jones and his advisor, Alexander Evans, an associate professor at Brown University, along with researchers from Purdue University, the Lunar and Planetary Science Laboratory in Arizona, Stanford University, and NASA’s Jet Propulsion Laboratory[{“attribute=””>NASA’sJetPropulsionLaboratory[{”attribute=””>NASAمعملالدفعالنفاث[{”attribute=””>NASA’sJetPropulsionLaboratory
A new study reveals that an ancient collision on the moon’s south pole has altered convection patterns in the lunar mantle, focusing on a group of heat-producing elements on the near side. These elements played a role in the formation of the vast lunar horseshoe visible from Earth. Credit: Matt Jones
The differences between the near and far sides of the Moon were first revealed in the 1960s by the Soviet Luna missions and the American Apollo program. While differences in volcanic sediments are clearly visible, future missions will reveal differences in geochemical composition as well. The proximal part is home to a compositional anomaly known as the Procellarum KREEP Topography (PKT) – a concentration of potassium (K), rare earth elements (REE), and phosphorous (P) along with heat-producing elements such as thorium. KREEP appears to be concentrated in and around Oceanus Procellarum, the largest of the nearby volcanic plains, but is few elsewhere on the Moon.
Some scientists suspect a connection between PKT and nearby lava flows, but the question of why the group of elements is focused on the near side has remained. This new study provides an explanation associated with the Antarctic-Aitken Basin, the second largest known impact crater in the Solar System.
For the study, the researchers ran computer simulations of how the heat from a giant collision changes convection patterns in the moon’s interior, and how this might redistribute KREEP in the lunar mantle. KREEP is thought to represent the last portion of the mantle that freezes after the moon’s formation. As such, it likely formed the outermost layer of the mantle, just below the lunar crust. Models of the moon’s interior suggest that it should have been distributed fairly evenly under the surface. But this new model shows that the uniform distribution will be disrupted by the heat column from the SPA effect.
According to the model, the KREEP material would have traversed the heat wave emitted by the SPA’s area of impact like a surfer. As the heat plume spread under the lunar crust, this material was eventually delivered collectively to the near side. The team ran simulations of a number of different impact scenarios, from the knockout to the lightning strike. While each produced different heat patterns and induced KREEP to varying degrees, all created KREEP concentrations on the proximal side, consistent with a PKT anomaly.
The researchers say the work provides a reliable explanation for one of the moon’s most enduring mysteries.
“How the PKT formed is the most important open question in lunar science,” Jones said. The South Pole-Aitken effect is one of the most important events in the history of the Moon. This work brings those two things together, and I think our results are really exciting.”
References: “The Origin of the South Pole-Aitken Effect of Lunar Synthetic Variation” by Matt J. Jones, Alexander J. Evans, Brandon C. Johnson, Matthew B. Wheeler, Jeffrey C. Andrews, Hannah, Sonya M. Techo, and James T. Keen, April 8, 2022, science progress.
DOI: 10.1126 / sciadv.abm8475
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