An ancient impact that formed the South Pole-Aitken basin, a vast impact structure on the far side of the Moon, would have created a massive plume of heat that propagated through the lunar interior, according to new research; that plume would have carried certain rare-Earth and heat-producing elements to the Moon’s nearside. That concentration of elements would have contributed to the volcanism that created the nearside volcanic plains.
Global map of the albedo from the 750 nm filter of the UV-VIS camera onboard NASA’s Clementine spacecraft. The image shows the near side and far side of the Moon in Lambert, equal-area projection. Image credit: NASA.
“We know that big impacts like the one that formed the South Pole-Aitken basin would create a lot of heat,” said Matt Jones, a Ph.D. candidate at Brown University.
“The question is how that heat affects the Moon’s interior dynamics.”
“What we show is that under any plausible conditions at the time that the South Pole-Aitken basin formed, it ends up concentrating these heat-producing elements on the nearside.”
“We expect that this contributed to the mantle melting that produced the lava flows we see on the surface.”
The differences between the near and far sides of the Moon were first revealed in the 1960s by the 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 planetary scientists have suspected a connection between the Procellarum KREEP terrane and the nearside lava flows, but the question of why that suite of elements was concentrated on the nearside remained.
The 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, Jones and colleagues 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 South Pole-Aitken impact.
According to the model, the KREEP material would have ridden the wave of heat emanating from the South Pole-Aitken 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 researchers 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 Procellarum KREEP terrane.
“Our work provides a credible explanation for one of the Moon’s most enduring mysteries,” Jones said.
“How the Procellarum KREEP terrane formed is arguably the most significant open question in lunar science.”
“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.”
The study was published in the journal Science Advances.
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Matt J. Jones et al. 2022. A South Pole-Aitken impact origin of the lunar compositional asymmetry. Science Advances 8 (14); doi: 10.1126/sciadv.abm8475
