The Moon — The Metallic Mass Beneath the South Pole-Aitken Basin

The Moon — The Metallic Mass Beneath the South Pole-Aitken Basin

The Discovery

In 2019, a team of researchers from Baylor University led by Dr. Peter James published a study in the journal Geophysical Research Letters reporting the detection of an enormous mass anomaly beneath the Moon's South Pole-Aitken Basin. The study combined data from NASA's GRAIL (Gravity Recovery and Interior Laboratory) mission — two spacecraft that had mapped the Moon's gravitational field in unprecedented detail — with topographic data from the Lunar Reconnaissance Orbiter.

The finding: there is a mass concentration buried beneath the South Pole-Aitken Basin that is:

• Estimated to be at least 2.4 quintillion tonnes (2.4 x 10^18 tonnes)
• Spread across an area approximately five times the size of the Big Island of Hawaii
• Located at depths varying from approximately 300 km beneath the surface

Dr. James stated: "Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That's roughly how much unexpected mass we detected."

Why This Is Significant

The South Pole-Aitken Basin is already one of the most remarkable features in the solar system:

• Approximately 2,500 km in diameter — wider than the continental United States
• Approximately 8 km deep — making it one of the deepest impact craters known
• Among the oldest features on the Moon's surface
• Located on the far side of the Moon (partially extending to the south pole)

A mass anomaly of this magnitude beneath an already anomalous basin compounds the mystery. The mass is enormous by any scale — it is not a small pocket of dense material but an object of extraordinary size buried in the deep lunar interior.

The Three Proposed Explanations

Explanation 1: Remnants of the impacting asteroid

The asteroid or comet that created the South Pole-Aitken Basin was itself an enormous object — it must have been to excavate a crater 2,500 km across and 8 km deep. One hypothesis is that the impacting body's metallic core (asteroids differentiate, with dense iron-nickel settling to the centre) was not vaporised in the impact but survived largely intact and sank into the Moon's mantle, where it remains buried.

Plausibility: This would require an unusually "gentle" impact — most modelling of impacts at the velocities involved suggests the impactor's core would be vaporised. However, at the low end of plausible impact velocities for the early solar system, preservation of some metallic material is possible.

Explanation 2: Concentration of dense oxides from magma ocean crystallisation

As the early Moon's magma ocean cooled and crystallised, dense minerals (including ilmenite, a titanium-iron oxide) sank while lighter minerals floated. In some models, a dense "ilmenite cumulate" layer formed at the base of the crust or upper mantle. The South Pole-Aitken Basin impact may have brought this layer closer to the surface or concentrated it.

Plausibility: The ilmenite cumulate is a real predicted consequence of magma ocean cooling, but whether it could account for a mass this large is debated.

Explanation 3: An unknown feature of lunar evolution

The feature is large enough and unusual enough that some planetary scientists have acknowledged it may represent a process not yet identified in models of lunar interior evolution.

What Remains Unknown

As of 2025, no sample return from the South Pole-Aitken Basin interior has been conducted. The mass anomaly is detected gravitationally but has not been directly sampled. Its composition, structure, and precise depth remain inferred from orbital measurements.

China's Chang'e missions have targeted the South Pole-Aitken Basin and have returned samples from its edge; direct sampling of the deep interior mass anomaly would require drilling to depths far beyond current technology.

The Extraordinary Interpretations

Within the artificial Moon framework, the metallic mass beneath the South Pole-Aitken Basin is among the most discussed recent findings. The convergence of factors — a massive metallic object buried deep in the most mysterious part of the Moon, in the largest impact basin in the solar system, on the permanently hidden far side — is cited as consistent with a metallic shell structure that the basin impact partially exposed or disrupted.

No scientific evidence supports this interpretation; the mainstream geological explanations are plausible if not yet fully confirmed.