A series of recent analyses focusing on the Moon’s vast South Pole-Aitken basin is rewriting the history of this enormous scar, the oldest and largest confirmed impact crater on the lunar surface. For decades, scientists believed the feature was carved by a colossal object striking the Moon at a shallow angle. New evidence, however, points toward a much more direct, vertical impact, a finding that significantly alters theories about the Moon’s early evolution and has profound implications for future human exploration.
This revised understanding stems from multiple independent studies that leveraged high-resolution orbital data, advanced computer simulations, and the first-ever samples returned from the basin. Researchers have now established that the impact was not a glancing blow but a more perpendicular strike that scattered primordial materials from the Moon’s deep interior across the south polar region. This discovery enhances the scientific prospects of NASA’s upcoming Artemis missions, which are slated to land astronauts in the very area now believed to hold these once-inaccessible geological treasures. The new data also provides a more precise date for the cataclysmic event and helps explain one of the Moon’s most enduring mysteries: why its two hemispheres are so dramatically different.
Challenging the Glancing Blow Theory
For many years, the prevailing model for the formation of the South Pole-Aitken (SPA) basin suggested it was created by an impactor that hit the Moon at a low angle, akin to a stone skipping across water. This theory was based on what appeared to be an elongated or elliptical shape of the basin. An oblique impact would have created a predictable, somewhat contained debris field, ejecting lunar mantle material in a specific direction away from the immediate impact zone. However, recent work led by the University of Maryland challenges this long-held assumption.
Evidence from Orbital Data
Using high-resolution data from NASA’s Lunar Reconnaissance Orbiter, a research team developed a new method to study the basin’s complex and eroded structure. By identifying and mapping more than 200 ancient rock formations believed to be remnants of the original impact, they pieced together its true shape. Their analysis, published in Earth and Planetary Science Letters, concluded that the basin is far more circular than previously thought. A circular crater strongly implies a more vertical impact, one that would have excavated and thrown debris much more symmetrically around the site. This finding fundamentally changes predictions about where to find key materials for study. Further supporting this conclusion, India’s Chandrayaan-3 rover recently detected minerals near the South Pole consistent with mantle debris, a discovery that aligns with the widespread dispersal pattern of a direct hit.
Tracing the Impactor’s Trajectory
While one team redefined the crater’s shape, another focused on the direction of the object that created it. Research from the University of Arizona, detailed in the journal Nature, analyzed the basin’s topography and the distribution of radioactive materials to reconstruct the impact event. Their simulations concluded that the asteroid or comet likely struck from the north and traveled southward.
This trajectory means the bulk of the material blasted out from deep within the Moon—known as ejecta—would have been thrown “down-range” onto the basin’s southern rim. This finding is particularly exciting because the Artemis program’s designated landing zone sits squarely within this southern region. The research suggests that astronauts will not have to travel far to find what scientists have long sought: rocks from the lunar mantle. Accessing this material is a primary scientific goal, as it holds direct clues about the Moon’s formation and the processes that shaped its interior 4.3 billion years ago.
Solving the Mystery of the Two-Faced Moon
The new impact models also shed light on the stark dichotomy between the Moon’s near and far sides. The Earth-facing side is characterized by dark, smooth volcanic plains called maria, while the far side is more heavily cratered with a thicker crust. This difference is tied to a concentration of heat-producing elements—potassium, rare earth elements, and phosphorus, collectively known as KREEP—on the near side.
The Role of a Giant Impact
The SPA impact occurred when the young Moon was still largely molten, covered by a magma ocean. As this ocean cooled, heavier minerals sank to form the mantle while lighter ones created the crust, trapping the KREEP layer in between. The new analyses suggest the colossal SPA impact was powerful enough to excavate and redistribute this KREEP-rich material. One simulation indicates the impact occurred before this layer had fully settled and migrated to the near side, splashing a radioactive KREEP marker, thorium, globally. Another study suggests the impact exposed a boundary between crust rich in KREEP and crust that was not. In either case, the event played a critical role in the thermal evolution of the Moon, helping to establish the geological division that defines the lunar landscape today.
Pinpointing the Age of the Basin
While scientists have long known the SPA basin was ancient, its exact age remained a subject of debate, with estimates ranging from 4.26 to 4.35 billion years. Resolving this timeline is crucial for understanding the chaotic early history of the solar system. China’s Chang’e-6 mission provided the final piece of the puzzle. The robotic lander touched down within the massive crater, collected the first-ever physical samples from the lunar far side, and returned them to Earth.
By analyzing about 1,600 fragments from the returned samples using a technique called lead-lead dating, scientists were able to precisely determine the age of the rocks that were melted and recrystallized by the impact. The results provided a definitive age for the SPA basin’s formation: 4.25 billion years ago. This precise date serves as a vital anchor point for the chronology of lunar history, confirming the basin was formed just 320 million years after the Moon itself.
Implications for Lunar Exploration
Together, these new findings are reshaping the scientific strategy for humanity’s return to the Moon. The confirmation of a more direct impact, combined with a clear trajectory model, transforms the south polar region from a hopeful site to a near-certain repository of deep lunar material. Artemis astronauts will arrive with a much clearer picture of the region’s geology and a precise map of where to look for samples that could unlock the secrets of the Moon’s origin. These rocks, blasted from the mantle billions of years ago and now lying on the surface, offer a direct window into the formation of rocky worlds and the violent, creative history of our own solar system.
