Why the far side of the Moon so different from near side

The composition of the Moon’s near side that is perpetually Earth-facing is oddly different from its far side which always faces away from Earth and scientists think they finally understand why.

These are linked to an important property of rock signature KREEP — 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), according to a study published in the journal Nature Geoscience.

On the Moon’s perpetually Earth-facing near side, on any given night, or day, one can observe dark and light patches with the naked eye.

Early astronomers named these dark regions “maria”, Latin for “seas”, thinking they were bodies of water by analogy with the Earth.

Using telescopes, scientists were able to figure out over a century ago that these were not in fact seas, but more likely craters or volcanic features.

Back then, most scientists assumed the far side of the Moon, which they would never have been able to see, was more or less like the near side.

In the late 1950s and early 1960s, non-crewed space probes launched by the then USSR returned the first images of the far side of the Moon, and scientists were surprised to find that the two sides were very different.

The far side had almost no maria. Only one per cent of the far side was covered with maria compared with approximately 31 per cent for the near side.

Scientists were puzzled, but they suspected this asymmetry was offering clues as to how the Moon formed.

In the late 1960s and early 1970s, NASA’s Apollo missions landed six spacecraft on the Moon, and astronauts brought back 382 kg of Moon rocks to try to understand the origin of the Moon using chemical analysis.

Having samples in hand, scientists quickly figured out the relative darkness of these patches was due to their geological composition and they were, in fact, attributable to volcanism.

They also identified a new type of rock signature they named KREEP which was associated with the maria.

But why volcanism and this KREEP signature should be distributed so unevenly between the near and far sides of the Moon again presented a puzzle.

Now, using a combination of observation, laboratory experiments and computer modelling, scientists from the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology, the University of Florida, the Carnegie Institution for Science, Towson University, NASA Johnson Space Center and the University of New Mexico have brought some new clues as to how the Moon gained its near- and far-side asymmetry.

These clues are linked to an important property of KREEP.

Potassium (K), thorium (Th) and uranium (U) are radioactively unstable elements. This means that they occur in a variety of atomic configurations that have variable numbers of neutrons.

These variable composition atoms are known as “isotopes”, some of which are unstable and fall apart to yield other elements, producing heat.

The heat from the radioactive decay of these elements can help melt the rocks they are contained in, which may partly explain their co-localisation.

This study showed that, in addition to enhanced heating, the inclusion of a KREEP component to rocks also lowers their melting temperature, compounding the expected volcanic activity from simply radiogenic decay models.

After conducting high temperature melting experiments of rocks with various KREEP components, the team analysed the implications this would have on the timing and volume of volcanic activity at the lunar surface, providing important insight about the early stages of evolution of the Earth-Moon system.

“Because of the relative lack of erosion processes, the Moon’s surface records geological events from the Solar System’s early history. In particular, regions on the Moon’s near side have concentrations of radioactive elements like U and Th unlike anywhere else on the Moon,” said study co-author Matthieu Laneuville from ELSI.

“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.