The Moon’s south pole hides a 4-billion-year-old secret

The study, published Oct. 8 in Nature, paints a vivid picture of the moon’s violent early history. It could also shed light on one of lunar science’s enduring mysteries: why the far side of the moon is heavily cratered while the near side, which hosted the Apollo landings of the 1960s and 1970s, is comparatively smooth.

Around 4.3 billion years ago, when the solar system was still young, a massive asteroid struck the far side of the moon. The colossal impact carved out the South Pole-Aitken basin (SPA), an immense crater measuring roughly 1,200 miles from north to south and 1,000 miles from east to west. Its elongated, oval shape suggests the asteroid hit at an angle rather than head-on.

By comparing SPA with other giant impact sites across the solar system, Andrews-Hanna’s team found a consistent pattern: these enormous craters narrow in the direction the impactor was traveling, forming a shape similar to a teardrop or avocado. Contrary to earlier assumptions that the asteroid came from the south, their analysis shows the SPA basin tapers toward the south, meaning the asteroid likely arrived from the north. The researchers determined that the southern, or down-range, rim should be buried under thick layers of debris blasted from deep within the moon, while the northern, up-range end should contain less of this material.

“This means that the Artemis missions will be landing on the down-range rim of the basin — the best place to study the largest and oldest impact basin on the moon, where most of the ejecta, material from deep within the moon’s interior, should be piled up,” he said.

Further evidence for a north-to-south impact came from studying the moon’s topography, crustal thickness, and surface chemistry. Together, these clues not only strengthen the case for the asteroid’s northern origin but also reveal new details about the moon’s inner structure and how it evolved over time.

Scientists have long believed that the early moon was once completely molten, forming a global “magma ocean.” As it cooled, denser minerals sank to create the mantle, while lighter ones floated upward to form the crust. Some elements, however, failed to fit neatly into these solid layers and accumulated in the last remnants of molten material. Those residual ingredients included potassium, rare earth elements, and phosphorus — collectively known as “KREEP,” with the “K” representing potassium’s chemical symbol. Andrews-Hanna noted that these elements are unusually concentrated on the moon’s near side.

“If you’ve ever left a can of soda in the freezer, you may have noticed that as the water becomes solid, the high fructose corn syrup resists freezing until the very end and instead becomes concentrated in the last bits of liquid,” he said. “We think something similar happened on the moon with KREEP.”

As it cooled over many millions of years, the magma ocean gradually solidified into crust and mantle. “And eventually you get to this point where you just have that tiny bit of liquid left sandwiched between the mantle and the crust, and that’s this KREEP-rich material,” he said.

“All of the KREEP-rich material and heat-producing elements somehow became concentrated on the moon’s near side, causing it to heat up and leading to intense volcanism that formed the dark volcanic plains that make for the familiar sight of the ‘face’ of the Moon from Earth, according to Andrews-Hanna. However, the reason why the KREEP-rich material ended up on the nearside, and how that material evolved over time, has been a mystery.

“The moon’s crust is much thicker on its far side than on its near side facing the Earth, an asymmetry that has scientists puzzled to this day. This asymmetry has affected all aspects of the moon’s evolution, including the latest stages of the magma ocean,” Andrews-Hanna said.

“Our theory is that as the crust thickened on the far side, the magma ocean below was squeezed out to the sides, like toothpaste being squeezed out of a tube, until most of it ended up on the near side,” he said.

The new study of the SPA impact crater revealed a striking and unexpected asymmetry around the basin that supports exactly that scenario: The ejecta blanket on its western side is rich in radioactive thorium, but not in its eastern flank. This suggests that the gash left by the impact created a window through the moon’s skin right at the boundary separating the crust underlain by the last remnants of the KREEP-enriched magma ocean from the “regular” crust.

“Our study shows that the distribution and composition of these materials match the predictions that we get by modeling the latest stages of the evolution of the magma ocean,” Andrews-Hanna said. “The last dregs of the lunar magma ocean ended up on the near side, where we see the highest concentrations of radioactive elements. But at some earlier time, a thin and patchy layer of magma ocean would have existed below parts of the far side, explaining the radioactive ejecta on one side of the SPA impact basin.”

Many mysteries surrounding the moon’s earliest history still remain, and once astronauts bring samples back to Earth, researchers hope to find more pieces to the puzzle. Remote sensing data collected by orbiting spacecraft like those used for this study provide researchers with a basic idea of the composition of the moon’s surface, according to Andrews-Hanna. Thorium, an important element in KREEP-rich material, is easy to spot, but getting a more detailed analysis of the composition is a heavier lift.

“Those samples will be analyzed by scientists around the world, including here at the University of Arizona, where we have state -of-the-art facilities that are specially designed for those types of analyses,” he said.

“With Artemis, we’ll have samples to study here on Earth, and we will know exactly what they are,” he said. “Our study shows that these samples may reveal even more about the early evolution of the moon than had been thought.”