The face the moon shows to earth looks very different than what it hides on its other side. The Aside is dominated by the Lunar Mare – the vast, dark-hued remnants of ancient lava flows. The cratered other side, on the other hand, is practically devoid of extensive mare features. Why the two sides are so different is one of the moon’s most enduring mysteries.
Now researchers have a new explanation for the two-faced moon — one that relates to a giant impact billions of years ago near the moon’s south pole.
A new study published in the journal scientific advances shows that the impact, which formed the moon’s vast South Pole-Aitken (SPA) basin, would have created a massive heat plume that spread through the moon’s interior. That cloud would have carried certain materials — a range of rare earths and heat-generating elements — to the moon’s near side. This concentration of elements would have contributed to the volcanism that created the nearby volcanic plains.
“We know that large impacts like the one that formed SPA would generate a lot of heat,” said Matt Jones, a Ph.D. Brown University candidate and lead author of the study. “The question is how this heat affects the internal dynamics of the moon. What we show is that under all plausible conditions at the time SPA is formed, it concentrates these heat-generating elements on the near side. We expect this to have contributed to the mantle melting that produced the lava flows we see on 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, Arizona’s Lunar and Planetary Science Laboratory, Stanford University and NASA’s Jet Propulsion Laboratory.
The differences between the moon’s near and far sides were first revealed in the 1960s by the Soviet Luna missions and the US Apollo program. While the differences in volcanic deposits can be clearly seen, future missions would also reveal differences in geochemical composition. The aside hosts a compositional anomaly known as Procellarum KREEP terrane (PKT) – a concentration of potassium (K), rare earth elements (REE), phosphorus (P) along with heat-generating elements such as thorium. KREEP appears to be concentrated in and around Oceanus Procellarum, the largest of the nearby volcanic plains, but is sparse elsewhere on the moon.
Some scientists have suspected a connection between the PKT and the aside lava flows, but the question of why this group of elements was concentrated on the aside lingered. This new study provides an explanation linked to the South Pole-Aitken Basin, the second largest known impact crater in the Solar System.
For the study, researchers ran computer simulations of how the heat generated by a giant impact would alter convection patterns in the lunar interior and how this might redistribute KREEP material in the lunar mantle. KREEP is believed to represent the final portion of the mantle that solidifies after the moon’s formation. As such, it likely formed the outermost mantle layer just beneath the lunar crust. Models of the moon’s interior suggest it should be more or less evenly distributed beneath the surface. However, this new model shows that the even distribution would be disturbed by the thermal plume of the SPA impact.
According to the model, the KREEP material would have ridden the heat wave emanating from the SPA impact zone like a surfer. Eventually, as the thermal plume spread beneath the lunar crust, this material was carried en masse to the near side. The team ran simulations for a range of different impact scenarios, from a direct hit to a graze. While each produced different heat patterns and mobilized KREEP to different degrees, all produced near-side concentrations of KREEP, 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 important unanswered 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.”
Materials provided by University of Brown. Note: Content can be edited for style and length.