Scientists have revealed crucial new details about the South Pole–Aitken (SPA) basin, the moon’s largest and oldest confirmed impact crater. This discovery may significantly shape NASA’s Artemis mission, scheduled for 2028, which is set to land near the basin’s south pole. The research opens up the possibility of finding priceless samples from the moon’s mantle, marking a milestone in our exploration of the lunar surface.
South Pole–Aitken Basin: An Ancient Lunar Puzzle
The SPA basin, a colossal depression stretching over 1,200 miles (2,000 kilometers) across the moon’s far side, has puzzled scientists for decades. The basin’s elliptical shape has sparked debates about the trajectory of the impactor that created it. For years, researchers were divided between two competing theories: a northward impact or a southward one.
The new study, published in Science Advances, using advanced 3D simulations, has provided clarity on this long-standing mystery. It suggests that the impactor likely traveled from north to south, creating the unique tapering structure of the basin. The authors of the study state,
“Large basins on the moon and other solid bodies (e.g., Mars and Pluto) are ellipses that taper in the downrange direction. SPA’s tapering toward the south, a steeper crustal thickness gradient toward the north, and the presence of a thorium- and iron-rich deposit toward the southwest of SPA beyond the basin rim support a southward impact trajectory.”
This refined understanding of the impact’s direction will play a crucial role in guiding future lunar missions, especially the Artemis program, which aims to land in the region surrounding the SPA basin’s rim.
Crustal thickness distribution after the oblique impact.
Credit: Science Advances (2026). DOI: 10.1126/sciadv.aea1984
The Impactor: A Key to Understanding the Moon’s History
By running high-resolution simulations, the team was able to recreate various impact scenarios, adjusting parameters such as the impactor’s size, speed, and angle. Their analysis pointed to a 260-km-wide impactor, likely a differentiated object with a dense core. This object struck the moon at a shallow angle, creating the basin’s distinctive shape. The study’s lead authors suggest,
“Our best-fit favoring a 13 km/s impact velocity implies that the SPA impactor was on a low inclination Earth-like orbit before it struck the moon.”
This velocity also offers clues about the impactor’s origin. According to the team, the most likely source of the SPA impactor was the Mars zone, rather than closer to Earth or Venus. “Given the collisional and dynamical evolution of leftover planetesimals, the most likely source of the SPA impactor probably originated within the Mars zone,” they explain.
Time series of our best-fitting SPA forming impact.
Simulation is of a 260-km-diameter differentiated impactor striking at 30° and 13 km/s. Green color represents the impactor core, cream color represents the crust, and gray color represents the mantle, respectively. Black dashed lines indicate the original surface of the moon. (A) Initial state (i.e., before the impact), (B) decapitation (see also Fig. 2), (C) excavation, (D) formation of the uplift, (E) collapse of the uplift, and (F) final state of basin formation (see the main text for details). The white arrow in (A) indicates the impact direction. Movie S1 is an animation of this figure, and movie S2 is an animation in 3D.
Credit: Science Advances
A Golden Opportunity for Artemis to Sample the Moon’s Mantle
One of the most exciting aspects of the study lies in its implications for the upcoming Artemis mission. The simulations suggest that the SPA impactor ejected mantle material, which landed in a distinctive pattern. The ejecta from the moon’s mantle was spread across the surrounding region in a “butterfly-like” shape, with material extending 550 kilometers beyond the basin’s rim in the downrange direction and 650 kilometers in the cross-range direction.
This pattern presents a unique opportunity for Artemis to collect samples from the lunar mantle, something previous missions have not been able to do. The team writes,
“For the previously assumed south-to-north impact, the Artemis landing region just beyond the postcollapse topographic rim would be devoid of ejecta from the lunar mantle. In contrast, for a north-to-south trajectory, our models predict that the Artemis III mission will land downrange of the impact point assuming our interpretation of a southward trajectory is correct.”
These findings suggest that Artemis, by landing near the south rim of the SPA basin, may collect invaluable samples of lunar mantle material that were ejected during the ancient impact. This discovery would provide critical insights into the moon’s composition and its formation, which could, in turn, help us understand the early solar system’s evolution.