Is the Moon concealing the icy core of an ancient asteroid within its interior?
Under the Moon’s greatest intact impact crater, its largest South Pole-Aitken (SPA) basin, researchers have found a colossal, dense never-before-seen body on our closest astronomical companion. The anomaly extends over 300 kilometers into the Moon’s mantle and is 2,000 kilometers long and likely weighs at least 2.18 × 10¹⁸ kilograms sufficient to cover five times the volume of the Big Island of Hawaii.
The discovery emerged from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, which used twin spacecraft to measure minute variations in the Moon’s gravitational field. By combining these gravity maps with high-resolution topography from the Lunar Orbiter Laser Altimeter (LOLA), researchers found the basin floor depressed by over half a mile under the weight of the hidden mass. “When we combined lunar gravity data with topography, we found a huge amount of extra mass hundreds of miles under the basin,” said Peter B. James, a planetary geophysics professor at Baylor University. “One explanation is that metal from the asteroid that created the crater is still embedded in the Moon’s mantle.”
Computer simulations of planetary impacts provide this as a possibility. Under certain circumstances, the iron–nickel core of a colossal asteroid can be scattered out into the upper mantle instead of falling to the planetary core. It might be sealed where it is for more than four billion years, James’ estimations indicate. This recalls research from Earth’s own deep interior, where seismic prospecting has imaged continent-sized blobs that could be remnants of Theia, the proto-planetary body believed to have created the Moon following an impact with accreting Earth. But there is more than one pony in the asteroid-core show.
The mass could also be a load of dense oxides that formed at the termination of the cooling of the Moon’s global magma ocean. As minerals cooled to form crystals, heavier substances could have settled in small areas in the mantle, producing gravitational anomalies that can be seen today. The make-up of the Moon’s crust and mantle, set by this early differentiation, is still a current subject of research, and the SPA basin provides one of the best windows into these events. SPA itself is an old scar, one that was likely created around 4.25 billion years ago in an energy-releasing impact equivalent to over a trillion atomic bombs.
Its sheer size 2,500 kilometers wide and as deep as 8.2 kilometers is the second-largest confirmed impact crater in our solar system. Volcanism, erosion, and plate tectonics wear features like this on Earth away. On the Moon, though, geologic quiescence has preserved this record of solar system violence early in the game. Latest analysis of Chang’e-6 mission samples, which brought back almost two kilograms of material from SPA’s farside surface, has confirmed volatile-poor mantle, extensive volcanic activity over 1.4 billion years, and even a surprise reawakening of the Moon’s magnetic field about 2.8 billion years ago.
These observations place the basin’s role in reorganizing the Moon’s chemical and thermal history center stage, and suggest that the concealed mass is associated with larger hemispherical asymmetries in crustal thickness, volcanic record, and elemental content. Relative geological coherence elevates science value of far-side.
In contrast to the nearside, where extensive plains of lava have blanketed the ancient surface, the farside still contains crust and mantle material in a state generally unchanged since formation. It is, therefore, an ideal target for missions with the aim of entering the Moon’s interior and bringing back samples of rock from the mantle or even asteroid impact material. Impactor material modeling predicts that in large complex craters such as SPA, unaltered impactor material can be transported to central peaks or deposited beneath the basin floor possibly intact with platinum group elements and other high-priority siderophile elements. GRAIL’s high-precision gravity mapping, which can recognize seven-kilometer features, has revolutionized planetary interior research. Scientists can now infer the existence of buried structures from ancient impact cores to mantle upwellings from minimal gravitational anomalies. The long-standing, deep, and wide anomaly in SPA confirms that it is no passing fancy; it is a remnant of the Moon’s earliest, most violent days.
Upcoming missions could be the answer.
A rover or lander carrying a deep-penetrating sensor would access the composition of the anomaly directly, and orbital remote sensing of sub-kilometer resolution would be able to sense metal concentrations. Combined with sample return capacity, these missions might definitively confirm that the Moon’s secret giant is a fossilized asteroid core a finding that would tie lunar history to the same explosive power that formed Earth and the other rock planets of the solar system.
