The Moon's South Pole-Aitken basin, a colossal scar from the early solar system, may be hiding a secret stranger than the impact that created it. Beneath the basin's floor lies a massive, unusually 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 more stable interior than previously thought.
The basin, one of the Moon's largest and oldest, spans approximately 2,000 kilometers and dates back to a time when impacts were common. Unlike Earth, which has been reshaped by plate tectonics and other geological processes, the Moon's surface retains many of these ancient scars. Now, one of these scars appears to conceal a clue buried in the mantle.
Peter B. James, a planetary geophysics professor at Baylor University, led the research. By combining lunar gravity data from NASA's GRAIL mission and topography data from the Lunar Orbiter Laser Altimeter (LOLA) aboard the Lunar Reconnaissance Orbiter, the team uncovered a significant excess of mass beneath the basin floor. This excess mass, estimated to be at least 2.18 × 10¹⁸ kilograms, is about 0.003% of the Moon's total mass and extends to depths of at least 300 kilometers.
The researchers propose two plausible explanations for this dense mass. One theory suggests that metal from the asteroid that formed the crater is still embedded in the Moon's mantle. Simulations support this idea, indicating that if the impactor had a differentiated iron-nickel core, the metal could have been scattered into the upper mantle. The detected excess mass aligns with the dispersal of a roughly 95-kilometer-wide iron-nickel core.
The other possibility is linked to the Moon's early cooling history. After its formation, the Moon is believed to have had a global magma ocean. As this molten body cooled, dense oxide-rich material could have formed and sunk unevenly. If some of these dense leftovers became stranded beneath the South Pole-Aitken basin, they could explain the gravity signal.
However, the study also highlights a complication. The mass anomaly is offset by about 400 kilometers to the southeast of the basin center. This offset is crucial for future models of the collision, as it suggests that the anomaly may not be directly related to the impactor. If the anomaly came from the impactor, this offset would need to be considered.
The research also refines our understanding of the basin itself. Using improved LOLA and GRAIL data, the team updated the basin's inner rim and identified an exterior scarp beyond the previously mapped outer rim. These findings largely agree with earlier work, but the new data near the south pole enhances the reliability of the basin's boundaries.
The crust inside the basin remains a puzzle. Models suggest that the undisturbed crust in the central basin interior is at least 16 kilometers thick, which is thicker than some previous estimates. This thickness complicates efforts to explain how the crust formed after such a massive impact. The collision likely excavated preexisting crust and upper mantle material, and any crust left today is likely a result of later processes.
Despite these complexities, the South Pole-Aitken basin is a unique natural laboratory for studying giant impacts and the deep aftermath of basin formation. The basin's preservation of ancient structure suggests that the Moon's interior lost a substantial amount of its original heat and that parts of the mantle stayed cool and stiff enough to preserve this record.
The practical implications of this research are significant for future lunar exploration. A mission to the basin could help determine whether the buried anomaly reflects impactor metal or dense lunar material from the magma ocean. Either answer would provide valuable insights into the Moon's formation and cooling history.
In conclusion, the discovery of this massive, dense mass beneath the Moon's South Pole-Aitken basin offers a fascinating glimpse into the Moon's violent past and the stability of its interior. It also highlights the scientific value of the basin for future exploration, as it may hold clues to the Moon's formation and the impacts that shaped rocky worlds across the early solar system.
