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A Decades-Old Moon Mystery May Be Tied to an Ancient Impact

When you gaze up at the Moon from Earth, you are always looking at its near side, with those familiar dark patches, known as maria, that have inspired countless stories over the centuries. So when Soviet and American missions glimpsed the far side of the Moon for the first time in the 1960s, it came as a surprise that this foreign landscape was so different in appearance, with barely any traces of maria.

The origin of this remarkable distinction, known as the nearside-farside lunar asymmetry, has puzzled scientists for decades. Now, a team led by Matt Jones, a PhD candidate at Brown University, proposes that a cataclysmic impact that created one of the largest craters in the solar system, the South Pole–Aitken (SPA) basin, may have forged the stark asymmetry.  

“The formation of the largest and most ancient lunar impact basin, South Pole–Aitken (SPA), was a defining event in the Moon’s evolution,” Jones and his colleagues write in a paper published on Friday in Science Advances

“Using numerical simulations, we show that widespread mantle heating from the SPA impact can catalyze the formation of the long-lived nearside-farside lunar asymmetry in incompatible elements and surface volcanic deposits, which has remained unexplained since its discovery in the Apollo era.” 

The lunar near side is decorated with iconic features like Mare Tranquillitatis, where humans first stepped foot on the Moon, and Mare Imbrium, which forms the right eye of the traditional image of a “Man in the Moon.” These maria were created by lava that flowed some four billion years ago, producing patches of smooth and dark terrain that contrast with the cratered landscapes surrounding them. 

Much of the lunar nearside is also rich in the so-called KREEP signature, which is an acronym for the chemicals potassium (atomic symbol K), rare Earth-elements (R-E-E), and phosphorus (atomic symbol P). In the new study, Jones and his colleagues suggest that a massive impact in the Moon’s infancy may have asymmetrically distributed these volatile materials, along with the heat-producing radioactive element thorium, to the lunar nearside, where they eventually helped to drive the volcanism that created the maria.

The team points to the SPA basin, a crater that stretches for an astonishing 1,600 miles across the south lunar region, as a key piece of evidence for their hypothesis. This immense formation was blasted into existence by a projectile about 4.2 billion years ago. The crash was powerful enough to excavate material buried more than 100 miles under the Moon’s surface, spraying ejecta around the lunar south pole.

In simulations of the impact and its aftermath, Jones and his colleagues demonstrated that the SPA impact could have also spurred convection within the Moon’s mantle. This process erupted into volcanism, fueled by KREEP materials, hundreds of millions of years later. The location of the impact, among other factors, ended up affecting the nearside far more than the farside, according to the team’s models.

“A warm initial upper mantle facilitates generation of a pronounced compositional asymmetry consistent with the observed lunar asymmetry,” Brown and his colleagues said in the study.

In addition to shedding light on the weird idiosyncrasies of the Moon, the hypothesis could “provide insight to the giant impact origin hypothesis” on Mars, which has been proposed to explain the difference between the north and south hemispheres of the red planet.  

“Our results presented here indicate that SPA-induced convection is a fundamental consideration for lunar history, and the associated hemisphere-scale processes could have many implications beyond what is discussed here,” the team concluded.

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