The Dark Side of the Moon
What does the ‘dark side' of the Moon really mean, and why are we so interested in exploring it?
Capturing the dark side
The 7 October 2019 marks the 60th anniversary of the first image of the ‘dark side’ of the Moon (left). This image was taken by the Soviet Luna 3 spacecraft, and while the picture is fuzzy and indistinct, this was the first time that this side of the Moon had ever been seen.
The ‘dark side’ of the Moon refers to the hemisphere of the Moon that is facing away from the Earth. In reality it is no darker than any other part of the Moon’s surface as sunlight does in fact fall equally on all sides of the Moon. It is only ‘dark’ to us, as that hemisphere can never be viewed from Earth due to a phenomenon known as ‘Tidal Locking’. A better term for the side we don’t see is the ‘far side’, rather than the ‘dark side’, which leads to all kinds of misconceptions. For consistency, we’ll refer to the ‘far side’ for the rest of the article.
Over the millions of years in which the Moon has orbited around the Earth, the gravitational interactions between the two bodies have subtly altered their orbits and the speed at which they rotate.
As the Earth is much larger than the Moon, the Moon’s rotation is slowed down until it reaches a balance point. This balance point is where the time for the Moon to have a full rotation around its axis, is the same as the time for the Moon to fully orbit around the Earth, becoming ‘tidally locked’.
As this NASA animation shows (right), this means that the same portion of the Moon always faces towards the Earth, and we can never see the far side.
But it gets more complicated still.
The unseen portion of the Moon does not make up exactly 50% of the Moon’s surface due to libration. Libration causes, over time, around 59% of the Moon’s surface to be visible from the Earth. Libration is caused by the Moon having an eccentric orbit around the Earth, the slight tilt of the Moon’s rotation, and the fact that the Earth rotates. These effects result in the Moon being viewed from slightly different angles, and more of the surface being viewed over time. The video below demonstrates this ‘wobbling’ view of the Moon.
Further lunar exploration
The next images of the Moon’s far side were not taken until six years later in 1965 by the Soviet probe Zond 3. This mission captured much higher resolution images, revealing large chains of craters and a hemisphere that looked very different than the near side of the Moon.
The US Lunar Orbiter programme then undertook the first detailed mapping of the far side of the Moon, but it was not until 1968 that the far side was first seen directly by human eyes, on the Apollo 8 mission.
Jim Lovell was one of the astronauts on board the Apollo 8, and recounts what it was like to experience this piece of history:
“We entered lunar orbit on the dark side, and the Moon, nowhere to be seen. As we continued to orbit, shards of sunlight started to illuminate the peaks of craters just 60 miles below. Finally the far side was bathed in sunlight and we stared in silence as the ancient far side craters slowly passed underneath. I was observing alive that part of the Moon that had been hidden from man for millions of years.”
The first successful soft landing on the far side of the Moon took place in early 2019 by the China National Space Administration’s Chang’e 4 mission. This mission has collected samples to determine the age and composition of the surface at the bottom of the South Pole-Aitken Basin.
This 13 km deep crater was created by an impact that is thought to have been large enough to expose the deep lunar crust and some of the mantle material, giving an incredible opportunity to learn more about the Moon’s internal structure and origins. The Chang’e 4 mission is part of a wider international goal to develop a human lunar colony near the South Pole. The South Pole has been chosen as the best location as water ice is present, which is a critical resource for any long term human exploration. NASA has also chosen the South Pole as a future landing site.
To find out more, check out our blog: Why go back to the Moon?
Near side vs far side
The first image of the far side of the Moon showed some surprising differences when compared with the face that we normally see. The far side has far fewer ‘maria’, which are large dark patches caused by ancient volcanic flows. Instead it is much more densely covered with craters compared to the near side.
But why should the near side of the Moon have more volcanic activity and lava flows than the far side? One of the commonly accepted theories is that in the early history of our Solar System, a young dwarf planet collided with the Moon. This impact would have thrown up huge amounts of material which would eventually fall back onto the Moon’s surface, burying the far side in five to ten kilometres of debris. This debris would go on to form a large part of the crust, and could theoretically still be detected today.
Another theory also involves the far side of the Moon having a thicker crust. While the Earth and Moon were forming, heat from the still-molten Earth slowed the cooling process of the near side of the Moon. The far side could solidify faster, forming a thicker crust. This resulted in meteoroid impacts on the near side sometimes punching through the thinner surface to the still molten mantle, releasing lava to the surface to create the maria.
Alternatively if the same meteoroid was to impact on the far side of the Moon, the thicker crust would not be punctured, so more valleys, craters and highlands are created, but no maria.
A radio telescope on the far side?
Another reason to explore the far side of the Moon is for radio astronomy. One of the largest limitations for our current radio telescopes is the background noise from our global radio communications signals. Another limitation is that the Earth’s atmosphere blocks the longest wavelengths from reaching the telescopes.
One way to combat both of these challenges is to build a radio telescope on the far side of the Moon. The Moon would shield the telescopes from Earth-related signals and there is no atmosphere to absorb radiation. By being able to detect these low-frequency radio waves, astronomers would be able to collect light from the very creation of the universe, from the first trillionth of a trillionth of a trillionth of a second after the Big Bang.
To find out more, read astrophysicist Joseph Silk’s case for building these lunar telescopes.
About the author: Scott Davis is a physicist and former Science Interpreter at the National Space Centre.