- The far side of the moon remains invisible from the surface of the Earth and has fuelled great theories and debates among scientists for ages.
- Scientists have conducted a joint research study into the geological features of the Moon to uncover some key facts which can help in explaining the differences between the near and far sides of the moon.
- This research can have huge potential in establishing areas for placing radio telescopes for use by astronomers.
The far side of the Moon is the lunar hemisphere that always faces away from Earth. Also known in common literature as the “dark side of the moon”, this is the hemisphere that is present diametrically opposite to the near side. The near side is the hemisphere that can be observed in the night sky from the Earth surface.
The mystery of the dark side
Why is it so that the inhabitants of Earth can’t see the dark side of the moon? The answer to this question lies in the tidal forces generated on Earth’s water bodies. The moon’s gravitational pull has a massive impact on the water bodies of the blue planet as it causes tide formation. These tidal forces cause the water on Earth to bulge out on the side of the hemisphere which lies farthest from the moon. This phenomenon itself has slowed the Moon’s rotation to the point where the same side that is the near side is always facing the Earth. This phenomenon is called tidal locking.
Over the course of time, in accordance with the various phases of the lunar cycle, some crescent-shaped glimpses of the far side of the moon can be seen due to libration. Libration is defined as an apparent or real oscillation of the moon by which edges of the lunar disc which aren’t visible under normal circumstances from Earth come into view. Approximately a total of 59 per cent of the Moon’s surface is visible from Earth at one time or another due to the low viewing angle.
Hence, here darkness is a term used to refer to the apparent lack of knowledge or unknown nature of this aspect of the moon. In primitive eras, it was considered that the far side appears darker due to the lack of exposure to the rays of the sun, or in other words, the moon doesn’t rotate on its axis. But it has now been effectively established that there is no such phenomenon to demonstrate the same. It was observed that both sides of the Moon experience two weeks of sunlight while the opposite side experiences two weeks of night, meaning both sides receive equal sunlight. However, that being said, the near side also receives sunlight reflected from the Earth, known as earthshine. Earthshine does not reach the area of the far side.
Nowadays, the use of the word “dark” to describe the moon has expanded to refer to the fact that communication with spacecraft can be blocked while the spacecraft is on the far side of the Moon. Hence it is a zone with the absence of proper radio communicability and acts as a blind spot which was noticed for the first time during the Apollo space missions for example.
The differences between the near and the far side
The two hemispheres of the Moon have significantly varied appearances. The near side is covered in multiple, large Marias. Maria is a Latin term for seas; the early astronomers falsely identified the barren plains on the near side as seas filled with lunar water. The far side, on the other hand, has numerous craters which give it a rocky and battered appearance, along with a few maria. The far side is home to the South Pole–Aitken basin which is the lowest elevation of the Moon and possesses a thin crust. There also exists a higher concentration of heat-producing elements on the near-side hemisphere as compared to the far side. This hypothesis has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer.
The differences between the two hemispheres of the moon have been debated over for ages and most scientists attribute the same to the Theia collision. Theia was considered to be a Mars-sized ancient planet in the early solar system which collided with a primitive form of the planet Earth called proto-earth according to the Giant impact hypothesis.
According to this theory, around 4.5 billion years ago, when Theia collided with Earth, the moon was created. Being smaller than Earth, it cooled down faster and geologically froze. During this course, scientists believe there might have been a second collision. One between the moon and a smaller companion moon that also originated from the Theia collision. This phenomenon has been widely regarded as the proposed theory that explains the differences between the two hemispheres of the moon.
Explorations into the dark side
Only around 18 per cent of the far side of the moon was known to mankind due to libration. The remaining 82 per cent remained unobserved until October 7, 1959, when it was photographed by the Soviet Luna 3 space probe which covered around one-third of the surface invisible from the Earth. The Soviet Academy of Sciences published the first atlas of the far side on November 6, 1960. It included a catalogue of 500 distinguished features of the landscape. In 1961, the first globe containing lunar features invisible from the Earth was released in the USSR, based on images from Luna 3.
On April 26, 1962, NASA’s Ranger 4 space probe became the first spacecraft to impact the far side of the Moon, although it failed to return any scientific data before impact. The Apollo 8 astronauts were the first humans to see the far side in person when they orbited the Moon in 1968. All manned and unmanned soft landings had taken place on the near side of the Moon, until 3 January 2019 when China’s National Space Administration launched the Change 4 spacecraft which made the first landing on the far side of the moon.
New research on the dark side
In the new study, which happens to be a combined effort on behalf of researchers 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 Centre and the University of New Mexico, a joint investigation into the moon’s geologic history was rigorously carried out.
And so the researchers might have found a new explanation for the asymmetry between its far and near sides. Through a number of experiments, computer modelling technology and the use of existing aids and observations of the lunar surface, these researchers found that concentrations of radioactive elements on the moon could help to explain the mystery of the differences between the near and far sides of the moon.
In particular, regions on the moon’s near side concentrations of certain radioactive elements like Uranium and Thorium are very high and unlike anywhere else on the moon. The joint study showed that radioactively unstable elements including Potassium (K), thorium (Th) and uranium (U) create heat through the process of radioactive decay. This heat can melt the rocks where these elements are located. So, 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.
Additionally, researchers also found, the asymmetry between the sides of the moon is also somewhat linked to a property of KREEP, a rocking signature which is short for potassium (chemical symbol K) enriched rock, rare‐earth elements (REE — which includes cerium, dysprosium, erbium, europium, among others) and phosphorus (chemical symbol P), which is associated with lunar maria and therefore volcanic and other geologic activity… These materials are abundant on the near side and are found in concentrations of around 3.2 per cent as compared to the 1 per cent on the far side.
According to this study, in addition to heating caused by radioactive decay from unstable elements, KREEP-enriched material on the lunar surface has lower melting points. This only added to expected geologic changes. The combined results of this study suggest that KREEP-enriched maria have been changing the lunar landscape since the rocky satellite was first formed nearly 4.5 billion years ago with the birth of the Theia collision.