The nearby side is dominated by the lunar mare – the huge, dark-colored remnants of ancient lava flows. The far side of the crater, on the other hand, is virtually devoid of large-scale mare features. Why the two sides are so different is one of the most enduring mysteries of the Moon. Now researchers have a new explanation for the two-sided Moon – an explanation for a giant collision billions of years ago near the Moon’s south pole. A new study published in the journal Science Advances shows that the collision that formed the Moon’s giant South Pole-Aitken (SPA) basin would have created a huge cloud of heat that propagated inside the Moon. This plume would have carried some material – a series of rare Earth elements that produce heat – to the nearby side of the Moon. This collection of data would contribute to the volcanic eruption that created the neighboring volcanic plains. “We know that big impacts like the one the SPA created would generate a lot of heat,” said Matt Jones, Ph.D. candidate at Brown University and lead author of the study. “The question is how this heat affects the internal dynamics of the Moon. What we are showing is that under any reasonable circumstances at the time the SPA was formed, it ends up collecting these elements that produce heat nearby. We expect that this contributed to melt the melting that produced the lava flows we see on the surface “. The study was a collaboration between Jones and Alexander Evans, an assistant professor at Brown, along with researchers from Purdue University, the Lunar and Planetary Science Laboratory in Arizona, Stanford University, and NASA’s Jet Propulsion Laboratory. 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 deposits are obvious, future missions would also reveal differences in geochemical composition. The neighborhood is home to a compositional disorder known as Procellarum KREEP terrane (PKT) – a concentration of potassium (K), rare earth elements (REE), phosphorus (P), along with elements that produce heat 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 link between PKT and lava flows nearby, but the question of why this suite of data was concentrated on the nearby side remained. This new study provides an explanation for 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 the heat generated by a giant impact would change the transport patterns inside the Moon and how this could redistribute KREEP material into the lunar mantle. KREEP is believed to represent the last part of the mantle that solidified after the formation of the Moon. As a result, it probably formed the outermost layer of the mantle, just below the lunar crust. Lunar interior models suggest that it should be more or less evenly distributed below the surface. But this new model shows that the uniform distribution will be disturbed by the heat cloud from the SPA impact. According to the model, the KREEP material would have led the heat wave coming from the SPA impact zone like a surfer. As the heat cloud spread beneath the Moon’s crust, this material was eventually delivered en masse to the nearby side. The team conducted simulations for a number of different collision scenarios, from a dead hit to a one-shot hit. While each produced different heat patterns and mobilized KREEP to varying degrees, they all created KREEP concentrations on the near side, according to the PKT anomaly. Researchers say the work provides a reliable explanation for one of the Moon’s most enduring mysteries. “How the PKT was formed is arguably the most important open-ended question in lunar science,” Jones said. “And the South Pole-Aitken collision is one of the most important events in lunar history. This work brings these two things together and I think our results are really exciting.” Follow SpaceRef on Twitter and Like us on Facebook.