The moon’s largest impact crater scattered something priceless—and Artemis may be heading straight into it

A new study, published in Science Advances, has refined some important details about the moon’s largest and oldest impact crater, which stretches more than 1,200 miles (2,000 km) on the far side of the moon. The new details can help guide some of the planning for NASA’s upcoming Artemis mission to the moon, which is planned for 2028.

The moon’s South Pole–Aitken (SPA) basin

The South Pole–Aitken (SPA) basin is the moon’s largest and oldest confirmed impact basin. The basin has a unique, tapered elliptical shape that has puzzled scientists and sparked some debate over the direction and nature of the impact that formed it. Some asymmetries in the crust suggest a northward trajectory of the impactor, while the shape and the structure of the basin suggest a southward trajectory.

The authors of the new study write, “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.”

The direction further determines where ejecta, including material from the moon’s mantle, may have landed during and after the impact. Previous models have suggested a size range of 200 to 400 km for the width of the impactor with an impact angle of 30° to 45°. However, past models have not analyzed the relationship between the impact direction and the observed SPA crustal thickness distribution, or its tapered shape and ejecta distribution. Determining such details can help to elucidate the moon’s history and plan for future lunar missions—especially Artemis.

Refining knowledge about a critical event in the moon’s history

To understand more about the SPA basin and the object that formed it, the team used high-resolution 3D simulations to model the impact with varying parameters on a moon-like target. They used both differentiated and undifferentiated bodies in their simulations, where the materials in the differentiated bodies had been separated into a dense core and outer layers through a heating process early in their formation. The impactor size, angle and velocity were also varied in the simulations. The team compared the outcome of each simulation to observed basin features.

Their best match showed that the SPA basin’s shape is best explained by a 260-km-wide, differentiated impactor striking from north to south at a shallow angle, not fully penetrating into the lunar surface. They say that the impactor’s dense core is responsible for the basin’s distinctive tapering. The models show that material is ejected, then near the end, the transient crater collapses under gravity and creates a large asymmetric central uplift. Most of the moon’s mantle material ejected by the impact fell back into the basin.

Variations in the velocity of the objects resulted in differing shapes of the crater. At 10 km/s, the models showed a similar shape to the team’s best-fit case, but the tapering was overexaggerated. At 16 km/s, the crater was too circular, compared to the real SPA, meaning the object’s velocity was likely somewhere in between. The velocity also provides clues about its origin.

“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. Given the collisional and dynamical evolution of leftover planetesimals, the most likely source of the SPA impactor probably originated within the Mars zone rather than within the Venus-Earth zone. The bodies in the Mars zone were transported there and/or formed in situ during the early solar system.”

An opportunity for Artemis

A major goal of the study was to determine the distribution of mantle ejecta around the SPA and find out whether samples can be obtained during upcoming missions. The simulations showed that the ejecta landed in a “butterfly-like” pattern with the mantle material distributed approximately 550 km beyond the rim in the downrange direction and 650 km in the cross-range direction. They found that no mantle ejecta in the uprange direction, however.

Since the upcoming Artemis lunar mission is planning to land in the south polar region of the moon, near the south rim of SPA, the team says sampling the lunar mantle is a likely outcome—that is, if their simulations are correct.

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. If a north-to-south impact produced SPA, the Artemis III mission may land within the ejecta deposit that contained excavated mantle material by the SPA-forming impact.”

The team notes that the study’s resolution, while high for 3D models, may still miss finer details of crustal deformation and ejecta distribution. However, Artemis mission samples will likely directly test these predictions in the next few years. The team says that these samples can also help reveal the age of SPA and the composition of the lunar mantle.

Who’s behind this story?

Krystal Kasal

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Lisa Lock

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Robert Egan

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