The Moon's South Pole-Aitken basin, a colossal scar from the early solar system, may hold a secret even more enigmatic than the impact that created it. Beneath the basin's floor lies a massive, dense mass, preserved deep within the lunar mantle. This discovery, made through a combination of gravity data and topography measurements, suggests a violent chapter in the Moon's history and a stable interior that has surprised many researchers.
The basin, spanning 2,000 kilometers, is the largest and oldest impact basin on the Moon. It formed during a period when impacts were common, and its preservation of ancient scars is unique compared to Earth, which has been reshaped by plate tectonics, erosion, and volcanism. The new finding points to a dense mass hundreds of miles under the basin, possibly metal from the asteroid that created the crater, or dense lunar material left over from the Moon's early cooling and magma ocean.
The excess mass, estimated at 2.18 × 10¹⁸ kilograms, is significant, accounting for 0.003% of the Moon's total mass. It extends to depths of at least 300 kilometers and explains the basin floor's depression. This discovery challenges previous interpretations of the depression as a result of impact melt sheet contraction, favoring a simpler explanation: the floor is sagging under the weight of the dense material below.
Two theories emerge for the origin of this dense mass. One suggests that if the impactor had a differentiated iron-nickel core, metal could have been scattered into the upper mantle. The other theory points to the Moon's early cooling and the formation of a global magma ocean. As the molten body cooled, dense oxide-rich material could have sunk unevenly, potentially becoming stranded beneath the South Pole-Aitken basin.
However, the study also presents a complication: the mass anomaly is offset from the basin center. This offset raises questions about the mechanism that concentrated the material and adds complexity to future models of the impact.
The basin itself is a fascinating subject. Improved data from LOLA and GRAIL reveal a more detailed picture, including an exterior scarp beyond the outer rim. The crust inside the basin is also a puzzle, with models suggesting a thickness of at least 16 kilometers, complicating the understanding of crust formation after the massive impact.
The dense mass beneath the basin has important implications for the Moon's interior. It suggests that the lower mantle remained rigid enough to preserve ancient structures for billions of years. This stability implies that the Moon lost a significant amount of its original heat, and parts of the mantle stayed cool and stiff, providing a clear record of the early solar system's impacts.
For planetary scientists, the South Pole-Aitken basin is a treasure trove. Unlike the near side, which is resurfaced by volcanic activity, the far side offers a clearer view of the Moon's earliest history. The basin is a natural laboratory for studying giant impacts, crustal excavation, and the deep aftermath of basin formation.
Future lunar exploration missions to the basin could provide valuable insights. They could help determine whether the anomaly is impactor metal or dense lunar material from the magma ocean, shaping our understanding of the Moon's formation and cooling. The basin, in this sense, is not just a crater but a historical archive, preserving evidence of impacts that shaped rocky worlds across the early solar system, including Earth.
In conclusion, the discovery of the dense mass beneath the South Pole-Aitken basin adds a layer of complexity and intrigue to our understanding of the Moon's history. It highlights the Moon's unique preservation of ancient structures and provides a window into the early solar system's violent past, offering valuable insights for planetary science and exploration.
