How the Moon’s Far Side Got Its Radioactive Spots

The mystery of why the near and far sides of the Moon are so different may be written in thorium, a weakly radioactive element.
December 22, 2020, 2:00pm
​The South Pole Aitken basin. Image: JAXA
The South Pole Aitken basin. Image: JAXA

For decades, a mystery on the Moon has puzzled scientists: anomalous “hotspots” of radioactive material strewn across its surface. 

Now, scientists finally have an answer, and it could have far-reaching implications for our understanding of planetary formation and even the origins of life. 

The Moon had a rocky infancy. First, it was blasted into existence when Earth was hit by a Mars-sized object some 4.5 billion years ago, so the theory goes, causing it to spend its early years as a molten ball of magma. About a half-billion years later, it was cataclysmically struck by another space rock that left behind a gigantic impact crater called the South Pole–Aitken basin (SPA), which stretches some 1,600 miles across the far side of the Moon. 

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As the oldest, deepest, and largest impact basin on the Moon—and among the largest impact craters in the whole solar system—SPA has long fascinated scientists. In particular, the weird chemical composition of the giant ancient depression, which is unlike anything else on the far side, has defied clear explanation.

Now, a team of researchers has presented evidence that the ancient impact ejected radioactive material, in addition to other anomalous elements, from a long-lost layer that once existed between the molten mantle of the infant Moon and its crystallizing crust, and subsequently seems to have vanished from the lunar far side. 

The results “have important implications for understanding the formation and evolution of the Moon”—especially why its near and far sides are so drastically different—and suggest that samples from SPA “must be considered amongst the highest-priority targets for the advancement of planetary science,” according to a study published on Friday in the Journal of Geophysical Research: Planets.

“This is the first time that there's been direct evidence for this kind of stratified upper mantle,” said Daniel Moriarty, a lunar scientist at NASA Goddard Space Flight Center who led the study, in a call.

“That has implications for things like the origin of life,” he said. “Since the Moon is so inextricably linked to the Earth, through its giant impact formation, it tells us a lot about the Earth as well.”

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For years, scientists have been puzzled by the asymmetric distribution of the so-called KREEP signature on the Moon, which is an acronym for the chemicals potassium (atomic symbol K), rare Earth-elements (R-E-E), and phosphorus (atomic symbol P). 

These elements are linked to lunar volcanism, which can partially explain why they are so heavily concentrated on the near side of the Moon, as this face was far more volcanically active in the past. But this raises the question: how did anomalous hotspots of KREEP end up in SPA, on the far side, where volcanism was rare?

One hypothesis suggests that early lunar evolution led to a sequestration of KREEP on the near side, before the Moon had begun to cool and crystallize into its current form. In this model, the weird SPA deposits are explained by some random process, such as an impact on the near side that flung some of the KREEP over to the far side.

“One of the things that people were really looking into was whether these hotspots were the result of relocated materials from the near side,”  Moriarty said. “I think people were duped into this a little bit, because these materials are so prevalent on the near side and they're so closely tied to volcanic processes.” 

“We show that that might not be the case,” he continued. “From the data that we look at and integrate, it looks like that [KREEP] material was excavated by a basin on the lunar far side, so it couldn't have just been sequestered in the near side. It had to be globally distributed.”

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Moriarty and his colleagues were able to detect pristine remnants of this ancient radioactive ejecta, which they think is endemic to the far side, by combining data from two missions: NASA’s Lunar Prospector orbiter and the Moon Mineralogy Mapper (M3), an instrument that NASA contributed to India's Chandrayaan-1 lunar probe. 

Lunar Prospector, which spent the late 1990s in a polar orbit around the Moon, was equipped with gamma ray and neutron spectrometers that enabled it to pick up radioactive signals on the surface. The instruments detected abundant deposits of thorium, an element that is weakly radioactive and a key tracer for KREEP, in SPA.

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The impact that formed the vast South Pole – Aitken Basin on the lunar farside excavated thorium-bearing materials from the lunar mantle. This map shows the thorium concentration across the basin as measured by Lunar Prospector, illustrating how this mantle ejecta is currently distributed across the lunar surface. Image: Moriarty et al

The M3 instrument, which was carried into lunar orbit by Chandrayaan-1 in 2008, was a near-infrared spectrometer that focused on mapping out the broader mineralogical properties of the lunar surface. By combining these two datasets, both from missions that died more than a decade ago, Moriarty and his colleagues were able to reveal new insights about SPA’s mysterious hotspots.

“It's the gift that keeps on giving,” Moriarty said of past Moon exploration. “Stuff from the 90s and 2000s, we're still finding out random things from.”

“The power of this study, and the power of this approach, is integrating different questions,” he added. “You couldn’t have done this with just one dataset. You need to integrate the thorium abundance from the Lunar Prospector with the distribution of mineralogy from the Moon Mineralogy Mapper because otherwise, you'd only have an incomplete piece of the picture.”  

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The study suggests that the KREEP layer, with its radioactive elements, existed all around the infant Moon, sandwiched between the lunar mantle and the crust, when the impact that created SPA occurred. The sheer force of that crash may have actually been the catalyst that bumped the KREEP over to the near side, but it will take more observations, research, modeling, and ideally, sample returns, to understand if, and how, that may have happened.  

“Some people think that this asymmetric distribution of radioactive elements is kind of baked into the beginning,” Moriarty said. “Our paper’s showing that that's not the case. They were globally distributed, so you need something else to explain why the near side and far side are so different.”

The dimensions of this mystery extend far beyond the Moon. As humans explore the many worlds of our solar system—as well as exoplanets we have detected beyond it—our closest celestial companion, hewn from our own planet eons ago, continues to be a vital source of insights about planetary evolution writ large.

“The reason we care so much about the Moon is because it really does serve as a fundamental proxy for understanding other rocky bodies,” Moriarty said. “The variety of rocky bodies that we've observed and detected is growing by the day, so having a better fundamental understanding of how those bodies operate will help us understand the kind of the solar systems they're in and the conditions, through time, on those planets that may have been experienced.”

Fortunately, there is a pretty good chance that scientists will be able to analyze some of these highly sought-after samples soon. 

“Scientifically, some of this material is available at the South Pole for the Artemis missions,” he said, referencing NASA’s plan to return humans to the Moon this decade. “That becomes a sampling priority in terms of returning this material back to Earth, because it would tell us a ton about exactly how the Moon's mantle formed and evolved.”