Mystery of why the dark side of the moon is so different could be solved

Rob Waugh
The moon is 'tidally locked' to Earth so there is a side we never see. (Getty)

A new study might have finally unravelled one of the mysteries of Earth’s moon – why the ‘dark side’ we never see is such a strange shape. 

Our moon is ‘tidally locked’ to Earth, which means that it rotates at the same rate it orbits our planet – so one side is never visible. 

The far side, which faces away from us, has a paler surface, with fewer black splotches, which astronomers used to believe were ‘seas’ on the surface. 

It also has a thicker crust.

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A study suggests that the differences on the ‘other’ side of the moon may be due to radioactivity in rocks on the near side of the moon. 

Scientists believe the Earth-moon system formed when a Mars-sized body dubbed Theia crashed into the young Earth, and the moon formed from debris. 

In the 50s and 60s, space probes returned images of the ‘far side’ showing it had no ‘maria’ (seas), with just 1% covered with maria compared with around 30% on the near side. 

Distribution of thorium on the lunar surface from the Lunar Prospector mission. (NASA)

Researchers believe the lack of maria is associated with a particular form of rock signature known as KREEP, which is short for rock enriched in potassium (chemical symbol K), rare-earth elements (REE, which include cerium, dysprosium, erbium, europium, and other elements which are rare on Earth) and phosphorus (chemical symbol P).

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Researchers from the Earth-Life Science Institute at Tokyo Institute of Technology and the University of Florida analysed KREEP in the lab. 

This study shows that, in addition to enhanced heating, the inclusion of a KREEP component to rocks also lowers their melting temperature.

The finding hints at ancient volcanic activity and could also offer important insights into the history of both the moon and the Earth. 

ELSI co-author Matthieu Laneuville said: “Because of the relative lack of erosion processes, the Moon’s surface records geological events from the solar system’s early history. 

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“In particular, regions on the moon’s near side have concentrations of radioactive elements like U and Th unlike anywhere else on the Moon.

“Understanding the origin of these local U and Th enrichments can help explain the early stages of the moon’s formation and, as a consequence, conditions on the early Earth.’

The results from this study suggest that the Moon’s KREEP-enriched maria have influenced lunar evolution since the moon formed. 

Laneuville thinks evidence for these kinds of non-symmetric, self-amplifying processes might be found in other moons in our solar system, and may be ubiquitous on rocky bodies throughout the universe.