Decapitated Asteroid Created Moon's Largest Crater and Left Treasure for Future Astronauts

A Violent Collision That Changed the Moon Forever

More than 4 billion years ago, a massive asteroid traveling at eight miles per second slammed into the moon's surface at a shallow angle, creating what would become the largest impact crater in our solar system. A new study suggests the South Pole–Aitken (SPA) basin, an impact crater more than 1,200 miles (2,000 kilometers) wide on the moon's far side, was likely created by a differentiated asteroid. But this wasn't just any ordinary space rock—it was what scientists now call a "decapitated" asteroid.

Using high-resolution 3D simulations, a team of researchers led by Shigeru Wakita of Purdue University found that SPA's distinctive tapered-ellipse shape is best explained by a 160-mile-wide (260-kilometer-wide) differentiated impactor — a large asteroid that had already separated into a dense iron core and a rocky outer layer, much like a tiny planet. At that trajectory, the impactor essentially gets "decapitated." Its upper layers shear off while the dense iron core continues plowing forward.

This unique collision mechanism explains why the SPA basin has its characteristic elongated shape rather than the circular craters typically formed by asteroid impacts. "The impactor's core is responsible for the tapered shape of SPA," the authors wrote in the study.

Ancient Treasures Scattered Across the Lunar South Pole

The violent impact did more than just carve out a massive crater—it may have scattered precious scientific samples across the moon's surface. Crucially, the team found the impact would have flung ejecta from the mantle toward the lunar south pole. This means material from deep within the moon's interior, normally inaccessible to scientists, could now be lying on the surface waiting to be collected.

The SPA basin is one of the moon's most scientifically valuable impact structures because it may expose material excavated from deep within the moon's mantle. These samples could provide unprecedented insights into the moon's formation and early history, answering fundamental questions about how our celestial neighbor evolved over billions of years.

The timing couldn't be better for lunar exploration. According to the paper, Artemis astronauts landing near the pole could encounter deposits containing material excavated from depths greater than 56 miles (90 km). This represents a unique opportunity to study material that formed during the moon's earliest days, when it was still a molten world slowly cooling and differentiating into distinct layers.

Perfect Timing for NASA's Return to the Moon

NASA's Artemis program plans to land astronauts at the lunar south pole, precisely where this ancient impact may have deposited its scientific treasures. "Our work suggests that NASA's Artemis III mission , which will send astronauts to the moon, is likely to sample SPA ejecta, if it lands as planned in the south polar region of the moon," the researchers wrote in the study. While NASA has since revised its timeline, with the first crewed landing now planned for the Artemis 4 mission no earlier than 2028, the scientific opportunity remains intact.

The research also helps resolve long-standing debates about how the SPA basin formed. Scientists have long debated exactly how the basin formed, including the size, speed and direction of the impactor. The new findings suggest that differentiated asteroids—essentially failed planets that never grew large enough to become full worlds—played a crucial role in shaping the early solar system.

As future Artemis missions prepare to explore the lunar south pole, they may be walking on ground that contains some of the oldest and most pristine samples of the moon's interior. These materials could unlock secrets about planetary formation, the early solar system, and the violent processes that shaped both Earth and its companion in the cosmos. The "decapitated" asteroid that created this opportunity billions of years ago may ultimately help us understand our own origins.