Understanding the Moon’s Hidden History
The Moon wobbles through space in a quiet duet with Earth: it spins on its axis in roughly the same time it takes to orbit our planet, a phenomenon known as synchronous rotation. This lock means we always see the same face, while the far side remains largely mysterious to us. Among its most striking features is the colossal South Pole-Aitken (SPA) basin, a scar that stretches across the lunar landscape for roughly 1,930 kilometers in length and 1,600 kilometers in width.
Formed about 4.3 billion years ago by a gigantic asteroid impact, the SPA basin is one of the oldest and deepest basins in the Solar System. Its sheer size and preserved interior make it a natural archive of the Moon’s early evolution, offering a window into a time when the planets were still differentiating into crust, mantle, and core.
A New Twist on an Old Event
A recent study from the University of Arizona, led by Jeffrey Andrews-Hanna, reexamined the shape of the SPA basin. Basins created by massive impacts often show a teardrop form, narrowing in the direction the impact debris was sent. Previously, scientists assumed the asteroid struck from the south, but the new analysis shows the basin actually narrows toward the south. This implies the impact came from the north, a subtle correction with potentially major consequences for how and where we study the Moon’s interior.
The directionality of the impact matters because the ejecta—the material blasted from the Moon’s depths—doesn’t spread evenly. The downrange side typically buries itself under a thick layer of debris, while the uprange side receives much less. The Artemis missions, which are targeted at the southern rim of the SPA basin, will land in a position that now makes it possible to sample material from the Moon’s deep interior almost as a core sample, without drilling.
Why These Samples Matter: KREEP, Crust, and Interior Structure
What makes these samples especially exciting is the information they could reveal about KREEP—potassium, rare earth elements, and phosphorus. In the Moon’s early magma ocean stage, these elements did not fully crystallize and became concentrated in the last residual liquid. This material heated the crust and fueled volcanic activity that created the familiar dark basaltic plains on the near side, the side we can observe from Earth.
In contrast, the far side remained heavily cratered and less volcanic. The new study suggests the Moon’s crust was thicker on the far side, driving the last remaining magma toward the thinner near side. The SPA basin’s western flank, with elevated thorium—a hallmark of KREEP-rich material—offers a rare glimpse into the transitional zone between the near side’s KREEP-enriched crust and the far side’s more typical crust.
What Artemis Could Find
If astronauts sample rocks from the radioactive regions around SPA, they could test these geological models with unprecedented precision. The mission’s goal is to understand how the Moon evolved from a molten world to the geologically diverse body we study today. By analyzing rocks that record the end stages of the lunar magma ocean, scientists hope to unlock how heat, composition, and crustal thickness sculpted the Moon’s hemispheric differences.
Such discoveries would shed light on the broader story of how terrestrial planets differentiate early in the Solar System’s history and why the Moon presents two very different faces. The SPA basin, with its deep interior exposure and asymmetric crustal structure, stands as a natural laboratory for testing ideas about planetary formation and evolution.
Looking Ahead
Nature publishing this work underscores how refined measurements of basin geometry can alter our interpretation of planetary formation. Artemis planners are now encouraged to consider these new insights when selecting landing sites and collecting samples, ensuring that the mission maximizes scientific return. As samples return to Earth, researchers will be able to compare the near-side and far-side histories in fresh, high-resolution detail.
