The massive ancient influence associated with the differences between the near and far sides of the moon

New research shows how the impact from the Moon’s South Pole-Aitken Basin is related to the stark contrast in composition and appearance between the two sides of the Moon.

The face the Moon shows to Earth looks very different from the one it hides on its far side. The near side is dominated by lunar Persians – the vast, dark-colored remains of ancient lava flows. On the other hand, the crater-filled far side is virtually devoid of extensive mare features. The reason for the great difference between the two sides is one of the moon’s most enduring mysteries.

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-Aitken Basin (SPA) would have created a massive plume of heat spreading across the moon’s interior. This plume would have held certain materials – a combination of rare earth and heat-producing elements – as well as the nearby moon. This concentration of elements could 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 fewer. The reason for the big difference between the two sides is the mystery 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. Candidate at Brown University 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 the SPA forms, it ends up concentrating these heat-producing elements 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 assistant professor at Brown University, along with researchers from Purdue University, the Lunar and Planetary Science Laboratory in Arizona, Stanford University and[{” attribute=””>NASA’s Jet Propulsion Laboratory.

A new study reveals that an ancient collision on the Moon’s south pole changed patterns of convection in the lunar mantle, concentrating a suite of heat-producing elements on the nearside. Those elements played a role in creating the vast lunar mare 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 U.S. Apollo program. While the differences in volcanic deposits are plain to see, future missions would reveal differences in the geochemical composition as well. The nearside is home to a compositional anomaly known as the Procellarum KREEP terrane (PKT) — a concentration of potassium (K), rare earth elements (REE), phosphorus (P), along with heat-producing elements like thorium. KREEP seems to be concentrated in and around Oceanus Procellarum, the largest of the nearside volcanic plains, but is sparse elsewhere on the Moon.

Some scientists have suspected a connection between the PKT and the nearside lava flows, but the question of why that suite of elements was concentrated on the nearside remained. This new study provides an explanation that is connected to the South Pole–Aitken basin, the second largest known impact crater in the solar system.

For the study, the researchers conducted computer simulations of how heat generated by a giant impact would alter patterns of convection in the Moon’s interior, and how that might redistribute KREEP material in the lunar mantle. KREEP is thought to represent the last part of the mantle to solidify after the Moon’s formation. As such, it likely formed the outermost layer of mantle, just beneath the lunar crust. Models of the lunar interior suggest that it should have been more or less evenly distributed beneath the surface. But this new model shows that the uniform distribution would be disrupted by the heat plume from the SPA impact.

According to the model, the KREEP material would have ridden the wave of heat emanating from the SPA impact zone like a surfer. As the heat plume spread beneath the Moon’s crust, that material was eventually delivered en masse to the nearside. The team ran simulations for a number of different impact scenarios, from dead-on hit to a glancing blow. While each produced differing heat patterns and mobilized KREEP to varying degrees, all created KREEP concentrations on the nearside, consistent with the PKT anomaly.

The researchers say the work provides a credible explanation for one of the Moon’s most enduring mysteries.

“How the PKT formed is arguably the most significant open question in lunar science,” Jones said. “And the South Pole–Aitken impact is one of the most significant events in lunar history. This work brings those two things together, and I think our results are really exciting.”

Refernece: “A South Pole–Aitken impact origin of the lunar compositional asymmetry” by Matt J. Jones, Alexander J. Evans, Brandon C. Johnson, Matthew B. Weller, Jeffrey C. Andrews-Hanna, Sonia M. Tikoo and James T. Kean, 8 April 2022, Science Advances.
DOI: 10.1126/sciadv.abm8475