Massive impact crater may rewrite the Moon’s origin story

When NASA’s Artemis astronauts touch down near the Moon’s south pole, they may step into a natural archive of the Moon’s deepest secrets.

A new study, led by Jeffrey Andrews-Hanna at the University of Arizona, argues that this region lies on the “downrange” rim of the Moon’s largest crater, the South Pole-Aitken (SPA) basin. The area is probably covered with rocks ejected from deep beneath the lunar surface.

The Moon’s greatest impact

SPA is the Moon’s largest impact structure, an oblong depression more than 1,200 miles long and 1,000 miles wide, gouged into the far side roughly 4.3 billion years ago.

Its odd shape has always hinted at a glancing blow rather than a head-on collision, but from which direction the impactor came had remained an open question.

Comparing SPA’s outline with other giant basins across the solar system, the research team found a consistent pattern: these enormous craters narrow in the downrange direction, giving them a teardrop – or avocado – silhouette.

Applying that logic to SPA flips the script. Rather than arriving from the south, the impactor appears to have struck from the north and plowed southward.

That is significant because the downrange end of such a basin tends to be draped in thick ejecta – material thrown up from deep inside the Moon.

By this new interpretation, Artemis landing zones at the south pole lie precisely where that ancient interior rubble should be piled highest.

A fresh look at the Moon impact

The study doesn’t rest on shape alone. Analyses of topography, crustal thickness, and surface chemistry all line up with a southward-moving impact.

The team notes a striking asymmetry in radioactivity around SPA: thorium – an easy-to-trace element associated with a suite of “leftover” magma-ocean materials – is abundant on the basin’s western flank but not its eastern side.

That pattern looks like a geological window torn open at a critical boundary: where the crust transitions from the last, lingering puddles of the Moon’s global magma ocean to more typical crust elsewhere.

This helps solve a mystery that has bothered lunar scientists for decades. The Moon’s near side, which always faces Earth, is thin-crusted and volcanic, packed with heat-producing elements.

The far side, by contrast, is thicker and pocked with craters. The answer, the new work suggests, may be written in how the Moon’s primordial “magma ocean” froze.

The Moon’s soda can secret

Shortly after the Moon formed, it likely melted globally. As that magma ocean cooled, heavy minerals sank to build the mantle while lighter ones floated up to form the crust. But not everything fit neatly into those two layers.

A small fraction of elements – including potassium, rare earth elements, and phosphorus, collectively nicknamed KREEP – refused to crystallize until the very end.

Like syrup in a freezing soda can, they concentrated in the last dregs of liquid trapped between the mantle and the crust.

Today, KREEP – and the heat it generated – shows up strongly on the near side, which helps explain the vast dark volcanic plains that define the Moon’s “face.”

But why did those leftovers end up so lopsided? Andrews-Hanna’s team points to the crustal asymmetry itself. As the far-side crust thickened, it squeezed the dwindling magma ocean beneath it sideways – like toothpaste from a tube – driving the last KREEP-rich liquids toward the near side.

The SPA impact then punched through a patchy remnant of that layer on the far side, scattering radioactive ejecta across one side of the basin while leaving the other comparatively clean. The observed thorium pattern fits the model.

Artemis samples may change everything

Remote sensing can map thorium and infer crustal properties from orbit, but only rock in hand tells the full story.

If Artemis crews bring home samples from the downrange rim of SPA, scientists will finally be able to test whether these materials truly come from deep inside the Moon. They can then determine whether the samples capture the late-stage chemistry of the magma ocean.

That would put hard constraints on long-debated problems, from the timing of the Moon’s early crust formation to the origins of its near-side/far-side dichotomy.

It would also sharpen our picture of how giant impacts sculpt rocky worlds. The teardrop geometry documented in this study appears in large basins across the solar system, hinting at universal rules for oblique impacts.

Confirming those rules on the Moon – where we can combine orbital data, sample analysis, and eventually field geology – would ripple into how we read cratered landscapes on Mars, Mercury, and beyond.

Secrets in the south-pole region

The practical upshot is simple and exciting: Artemis may land in the perfect place to study the Moon’s oldest, biggest impact with the richest haul of interior material stacked at astronauts’ feet.

The samples could reveal how the magma ocean froze and why heat-producing elements migrated. They could also show how that process set the stage for billions of years of volcanic activity on one hemisphere but not the other.

“SPA is the largest and oldest basin on the Moon, and the downrange rim is where the deepest interior rocks should be,” noted Andrews-Hanna.

With the new impact direction pinned down, the south-pole region becomes even more scientifically compelling.

Missing pages of the Moon’s story

Thorium maps and gravity models have brought us to this threshold. The next leap will happen in laboratories on Earth.

State-of-the-art instruments can tease out mineral textures, isotope ratios, and trace-element fingerprints that orbiters can’t. Those data won’t just refine models – they’ll anchor them.

The Moon still guards plenty of secrets from its earliest days. But if this study is right, Artemis astronauts won’t just be planting flags.

They’ll be collecting the missing pages of a story that began when the Moon was an ocean of fire – and when a glancing blow on its far side helped decide the face it shows us today.

The study is published in the journal Nature.

Image Credit: Jeff Andrews-Hanna/University of Arizona/NASA/NAOJ

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