Moon is asymmetric: Scientists provide explanation why far and near side are different

The composition of the Moon’s near side is oddly different from that of its far side, and scientists think they finally understand why. Credits: NASA/NOAA
The Moon is our planet’s natural satellite, the closest neighbour, and the only extraterrestrial body where humans have ever set foot. Still, there is a lot about the Moon that we do not know— one of which is its asymmetry. However, a recent paper published in the journal Nature Geoscience proposes an explanation for this asymmetry backed by evidence.

Since the Moon is tidally locked with Earth, we only see one side of the Moon, which is also known as the ‘near side’. This Earth-facing side is perpetually different from the ‘far side’ that always faces away from Earth.

The near side features dark regions called ‘lunar maria’, which are believed to be craters or volcanic features. While scientists earlier believed the far side to be like the near side, the first images of the other half taken by non-crewed space probes launched by the USSR in the late 1950s and early 1960s revealed that it had almost no ‘maria’.

The far side is covered with 1 per cent maria compared with around 31 per cent for the near side. Scientists suspected that this asymmetry could offer clues as to how the Moon was formed.

Now, using a combination of observation, laboratory experiments, and computer modelling, scientists from the Earth-Life Science Institute 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.
Thorium concentrations corresponding with KREEP. (Image: NASA)
When NASA’s Apollo missions brought back 382 kg of Moon rocks, scientists figured out the relative darkness of these patches was due to their geological composition and they were, in fact, attributable to volcanism. They identified a new type of rock signature associated with the ‘maria’, called KREEP.

It is short for rock enriched in potassium (chemical symbol K), rare-earth elements (REE), and phosphorus (chemical symbol P). It also contains elements such as Uranium and Thorium, the radioactive decay of which generates heat. The thermal modelling of the lunar interior suggests that the radioactive decay of these elements would have provided a near-side heat source for billions of year and could have something to do with this side’s prominent volcanism and the imminent asymmetry of the Moon.

However, scientists are yet to find why volcanism and this KREEP signature distributed so unevenly between the near and far sides of the Moon. It is believed that it is probably a consequence of how the Moon formed when a body the size of Mars called Theia slammed into Earth around 4.5 billion years ago, sending debris flying into space that recombined into the Moon, but not homogeneously.
Distribution of thorium on the lunar surface from the Lunar Prospector mission. Thorium is highly correlated with other radioactive elements (heat producing), with most of it being present on the Earth-facing side (near side). The relationship between this region and many observed features of lunar history is a key question in lunar sciences. Credits: Laneuville, M. et al (2013) Journal of Geophysical Research: Planets.
Since most of these lava flows were emplaced early in lunar history, the study adds constraints about the timing of the Moon’s evolution and the order in which various processes occurred on the Moon.

“Because of the relative lack of erosion processes, the Moon’s surface records geological events from the Solar System’s early history,” explained planetary scientist and co-author Matthieu Laneuville of the Earth Life Science Institute in Japan.

“In particular, regions on the Moon’s near side have concentrations of radioactive elements like uranium and thorium unlike anywhere else on the Moon. Understanding the origin of these local uranium and thorium enrichments can help explain the early stages of the Moon’s formation and, as a consequence, conditions on the early Earth,” Laneuville added.

The study suggests that the Moon’s KREEP-enriched maria influenced lunar evolution since the Moon formed. Laneuville thinks the evidence for these kinds of non-symmetric, self-amplifying processes might be found in other moons in our Solar System, which may be ubiquitous on rocky bodies throughout the Universe.

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