Mysterious Radioactive Rock Discovered on the Far Side of the Moon

In the enigmatic depths of the lunar far side, a radioactive rock reveals the Moon’s hidden secrets.

The far side of the Moon, which always faces away from Earth, has long been a source of intrigue and wonder for astronomers and space enthusiasts. Unlike the familiar near side, which is marked by dark patches of volcanic plains called maria, the far side is mostly covered by rugged highlands and craters. However, there is one region on the far side that stands out from the rest: the Compton-Belkovich anomaly.

The Compton-Belkovich anomaly is a large, circular area that appears brighter than its surroundings in radar images. It is located near the equator, about 500 kilometers (310 miles) from the center of the South Pole-Aitken basin, the oldest and largest impact crater on the Moon. The anomaly was first detected by the Lunar Prospector orbiter in 1998, and later confirmed by the Lunar Reconnaissance Orbiter (LRO) in 2011.

What makes this region so unusual is its chemical composition, which is rich in elements such as potassium, thorium and uranium. These elements are collectively known as KREEP, an acronym for potassium (K), rare earth elements (REE) and phosphorus (P). KREEP is associated with volcanic activity on the Moon, as it is thought to have formed when molten magma from the lunar interior differentiated and concentrated these elements near the surface.

However, volcanic activity on the far side of the Moon was much less frequent and intense than on the near side, where most of the maria are found. So how did such a large amount of KREEP end up in this remote and isolated location?

A new study published in the journal Nature may have an answer. A team of researchers from the United States used microwave data from the obtained from public data released from two Chinese lunar orbiters, Chang’E-1 in 2010 and Chang’E-2 in 2012, to measure the geothermal activity of the Compton-Belkovich anomaly. They found that this region has a higher heat flow than its surroundings, indicating that there is something hot and radioactive beneath the surface.

The researchers then used numerical models to simulate how such a heat source could have formed and evolved over time. They concluded that the most likely explanation is that there is a huge mass of granite buried under the lunar crust, which is emitting natural radiation from its decay. Granite is a type of igneous rock that forms when magma cools slowly and crystallizes. It is commonly found on Earth, but very rare on the Moon, where most of the crust is made of basalt.

The research team estimates that this granite mass is over 30 miles in diameter and about 6 miles thick. They suggest that it was created about 3.5 billion years ago, when a massive volcanic eruption occurred on the far side of the Moon. This eruption may have been triggered by a large impact that hit near the South Pole-Aitken basin, creating a crack in the crust that allowed magma to rise to the surface.

The magma then cooled and solidified into granite, forming a large subsurface structure called a batholith. A batholith is a massive body of igneous rock that feeds volcanoes on the surface. For example, on Earth, the Sierra Nevada mountains are a batholith left from a volcanic chain that existed long ago.

The granite batholith on the Moon remained hidden for billions of years, until it was exposed by erosion and impacts that removed some of the overlying crust. The radiation from its KREEP elements then made it visible to radar and microwave sensors.

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“This is the first time that there’s been direct evidence for this kind of stratified upper mantle,” said Matthew Siegler, a professor at Southern Methodist University and co-author of the study, in a statement. “That has implications for things like the origin of life. Since the Moon is so inextricably linked to Earth through its giant impact formation, it tells us a lot about Earth as well.”

The discovery of this mysterious radioactive rock on the far side of the Moon also opens up new possibilities for future exploration and research. The Compton-Belkovich anomaly is one of several potential landing sites for NASA’s Artemis program, which aims to send humans back to the lunar surface by 2024. The granite batholith could provide valuable insights into the history and evolution of both Earth and Moon, as well as offer clues about how life emerged from primordial chemistry.

“Any big body of granite that we find on Earth used to feed a big bunch of volcanoes,” Siegler said. “So here’s this system with no water and no plate tectonics — but you have granite. Was there water on Moon — at least in this one spot? Or was it just especially hot?”

Sources: 1, 2, 3, 4, 5