How radioactive spots appeared on the far side of the moon

The near and far sides of the moon are very different. The reason for this may be the chemical element thorium – a heavy, weakly radioactive metal

Crater South Pole – Aitken basin.  Source: JAXA
Crater South Pole – Aitken basin. Source: JAXA

For decades, scientists have been puzzled by a mystery: abnormal “hot spots” with radioactive traces on the lunar surface. Scientists now have an answer that could influence our understanding of planetary formation and even the origin of life.

Luna had a difficult childhood. First, it appeared as a result of the collision of the Earth with a space object the size of Mars. This happened about 4.5 billion years ago. According to the theory, the Moon spent its early years as a molten ball of magma. About half a billion years later, the Moon collided with a huge cosmic rocky object that left a giant impact crater called the South Pole-Aitken Basin (South Pole – Aitken basin, SPA). It is on the far side of the moon and stretches for about 1,600 miles. Further in the article, when we mention this crater, we will briefly say “SPA”.

As the oldest, deepest, and largest impact structure on the Moon and one of the largest in the solar system, SPA has long fascinated scientists. In particular, its strange chemical composition, unlike anything else on the far side of the Moon, defies clear explanation.

Now the team of researchers has presented evidence that the impact emitted radioactive material from a deep layer that once existed between the molten mantle of the young moon and its crystallizing crust, which subsequently disappeared from the far side of the moon.

“The results are important for understanding how the moon formed and evolved, especially why its near and far sides are so different. It is assumed that the SPA samples “should be considered among the highest priority targets for the development of planetary research” – says in study published in JGR Planets (Journal of Geophysical Research: Planets, Rus. Journal of Geophysical Research: Planets).

“This is the first direct evidence of this kind of upper mantle stratification,” said study lead Daniel Moriarty, scientist at the Goddard Space Flight Center.NASA Goddard Space Flight Center).

“This is important for many essential issues, such as the origin of life,” he said. “Since the Moon is inextricably linked to the Earth due to its giant impact formation, this also says a lot about the Earth.”

For many years, scientists have been puzzled by the asymmetric distribution on the Moon of the so-called KREEP – short for the acronym for lunar rocks, consisting of: potassium – K, rare earth elements – REE and phosphorus – P. The presence of these elements is associated with volcanic activity. This may partially explain the reason for their concentration on the near side of the Moon, since this side was much more volcanically active in the past. But the question arises: how did the anomalous hot spots KREEP end up in the SPA, on the far side of the moon, where volcanism was rare?

One hypothesis suggests that early lunar evolution led to sequestration of KREEP on the near side before the Moon began to cool and crystallize into its current form. In this model, strange deposits in the SPA are explained by the influence of chance, for example, the impact of a space body on the near side of the Moon, as a result of which part of the KREEP was thrown to its far side.

“Some were very interested in whether these hot spots were due to the movement of particles from the near side,” said Moriarty. “I think people were a little deceived because these particles are common on the near side and they are closely related to volcanic processes.”

“From the data we’re looking at and integrating, it looks like the KREEP particles were released from the SPA on the far side of the moon, meaning they couldn’t be sequestered on the near side only. And they spread throughout the entire territory. “

Moriarty and his colleagues were able to discover the intact remnants of this ancient radioactive release, which they believe is characteristic of the far side of the moon. In this they were helped by data from two projects: NASA’s Lunar Prospector (American automatic interplanetary station for exploration of the Moon, created as part of the NASA “Discovery” program) and Moon Mineralogy Mapper (M3) (“Lunar Mineralogy Cartographer“) – a tool that NASA has contributed to the development of the Indian lunar probe Chandrayan-1 (Chandrayaan-1).

The Lunar Prospector, which spent the late 1990s in polar orbit around the Moon, was equipped with gamma and neutron spectrometers that allowed it to record radioactive signals from the surface. The instruments detected abundant deposits of thorium, a weakly radioactive element that is also a key indicator of KREEP in SPA.

Image: moriarty et al
Image: moriarty et al

Commentary on the illustration: The impact that created the SPA, the largest known crater of the Moon (South Pole – Aitken basin, South Pole Basin – Aitken) on the far side of the moon, caused the eruption of thorium-containing materials from the depths of the lunar mantle. The map shows the concentration of thorium in the crater, measured by the automatic interplanetary station Lunar Prospector, and illustrates how the traces of the ejection of the lunar mantle are distributed over the lunar surface.

Instrument M3, which was launched into lunar orbit by the Chandrayan-1 lunar probe in 2008, was a near-infrared spectrometer designed to map the broader mineralogical properties of the lunar surface. By combining these two datasets from projects that ended more than a decade ago, Moriarty and his colleagues were able to uncover new insights into the mysterious hot spots in SPA.

“This is a gift that continues to bestow on us,” Moriarty said of past exploration of the moon. “We still learn random things from the 90s and 2000s.” “The breadth of this study and approach brings together different issues,” he added. “It couldn’t have been done with just one dataset. It is necessary to combine the fact of the thorium content from the Lunar Prospector with the mineralogical distribution from the Moon Mineralogy Mapper, because otherwise we get only an incomplete part of the picture. “

The study suggests that the KREEP layer, with its radioactive elements, existed throughout the young Moon, between the lunar mantle and crust, when the collision occurred, causing the SPA. The sheer strength of this collision may have actually been the catalyst that pushed the KREEP particles toward the near side of the Moon, but more observation, research, modeling, and ideally, samples will be needed to understand if and how this could happen.

“Some people think that this asymmetric distribution of radioactive elements was laid in the beginning,” said Moriarty. “Our newspaper shows that this is not the case. These elements are found throughout the territory, so arguments are needed to explain why the near and far sides of the moon are so different. “

The dimensions of this puzzle extend far beyond the Moon. As humans explore the many worlds in our solar system, as well as the exoplanets we have discovered beyond, our closest celestial satellite continues to be an important source of understanding planetary evolution in general.

“The reason we are so interested in the Moon is because it serves as a fundamental indicator for understanding other rocky bodies,” Moriarty said. “The variety of rocky bodies that we have observed and discovered is growing every day, so a deeper understanding of how these bodies work will help us understand what solar systems they are in and what conditions may be found on these planets over time.”

Fortunately, it is possible that scientists will soon be able to analyze some of these scarce samples.

“Scientifically, some of this material is available on SPA for missions of NASA’s Artemis Manned Space Program (Artemis), ”He said, referring to NASA’s plan to return humans to the moon this decade. “This is an important point, because from the results of studying these samples, we can learn a lot about how exactly the mantle of the moon was formed and developed.”

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