Future astronauts could walk across rocks from deep inside the Moon

The findings come from two companion studies led in part by researchers from the Center for Lunar Origin and Evolution (CLOE), Southwest Research Institute's team within NASA's Solar System Exploration Research Virtual Institute. Together, the studies shed light on how the enormous South Pole-Aitken (SPA) basin formed and where valuable lunar material may be located today.

Exploring the Moon's Largest Impact Basin

The South Pole-Aitken (SPA) basin lies on the Moon's far side and ranks among the oldest preserved structures in the solar system. Because of its immense size and age, scientists view it as a unique record of the Moon's early history. Areas near the basin are also being considered for future NASA Artemis missions near the lunar south pole.

"The basin offers scientists a rare opportunity to study the Moon's earliest history," said Dr. William Bottke, director of CLOE and executive director of SwRI's Science Directorate in Boulder, Colorado. He is a co-author of the studies. "The collision struck the lunar surface with such force that it may have excavated material from deep inside the Moon, including portions of the lunar mantle."

To better understand what happened, researchers used advanced computer simulations to recreate the ancient impact that produced the SPA basin. The results point to an object that approached from the north and traveled south before striking the Moon at a shallow angle.

According to the simulations, this low-angle impact explains the basin's distinctive elongated and tapered shape.

"Our simulation produces the right shape and nature of the impact basin. It also tells us about the projectile that created it and the direction of the impact," said Dr. Shigeru Wakita of Purdue University, lead author of the SPA impact study.

Evidence of a Differentiated Impactor

The modeling indicates that the object responsible for the impact was not a simple chunk of rock. Instead, it was likely a differentiated body with an iron core surrounded by rocky material, similar to a small protoplanet or a differentiated asteroid.

When this object collided with the Moon, it carved out a deep and uneven cavity while generating intense heat that melted rock in the basin's center. The impact also blasted enormous quantities of material from both the lunar crust and mantle into space. Much of that material eventually fell back and accumulated within the basin.

Lunar Mantle Material Near Artemis Regions

In the second study, scientists focused on how this ejected material is distributed beneath and around the basin. Using high-resolution gravity measurements along with models that accounted for both crustal and mantle material, the team found evidence that the SPA basin likely contains large amounts of rock originating from the Moon's mantle.

The analysis suggests that mantle-derived material is mixed throughout the basin and within the ejecta blanket surrounding it. Later impacts inside the SPA basin may have dug into these buried deposits and exposed some of them at the surface, potentially making them accessible to robotic missions and future astronauts.

"The precise distribution of mantle material has been a big unknown," said Dr. Gabriel Gowman of the University of Arizona, lead author of the gravity-based study. "Our models indicate that the SPA impact ejected enough deep material to form a significant deposit that should still be accessible today. Most importantly, some of that material at a trace level may exist in regions being considered for the Artemis landings."

New Targets for Future Moon Exploration

Previous ideas suggested that the deepest material excavated by the impact might be concentrated in parts of the basin far from the lunar south polar regions targeted for exploration. However, the new simulations and gravity analysis paint a different picture.

The researchers found that deposits containing mantle material may be spread across portions of the Moon's south polar region, including areas that future Artemis astronauts could potentially visit and study in person.

"The combination of impact and gravity modeling gives us a powerful roadmap," Bottke said. "It tells us not just how SPA formed, but where to look for the rocks that can answer some of our biggest questions about the Moon's origin and evolution."

The study "A southward differentiated impactor forms the tapered shape of the South Pole-Aitken impact basin on the Moon" was published in Science Advances. A companion paper examining the gravity and ejecta structure of the basin, "Gravity Mapping of Lunar Mantle Material in South Pole-Aitken Basin Ejecta," was published in the Journal of Geophysical Research: Planets.