The Far Side of the Moon

Originally published in the Informanté newspaper on Thursday, 5 July, 2018.

While listening to song recently, the lyrics claimed that, to be a man, one had to be ‘mysterious as the dark side of the moon.’ Yet, strangely enough, the dark side of the moon, or as it’s correctly called, the far side of the moon, is not mysterious anymore. It, in fact, hasn’t been since 7 October 1959, when the Soviet Luna 3 spacecraft took the first images of that hard-to-see surface.

The reason we call it the ‘far’ side of the moon, and not the ‘dark’ side is because it is, in fact, not dark all the time. The reason we cannot see the far side of the moon from earth, is because it is tidally locked. As the moon normally creates tides on earth, so too does the earth create ‘tides’ on the moon. Without liquid water to show that force, however, it is not easily observed. If there were, we’d note that due to the massive size difference between the earth and the moon, tides would be MUCH stronger there. 

The gravitational tides therefore serves to induce torque on the moon’s rotation, and eventually caused its rotation to match exactly its orbital period. Once this happens, it’s ‘tidally locked’ and rotates so that once side faces the larger gravitational body constantly even as it orbits it. In other words, it rotates around its axis in such a way that we on Earth only ever see one side of the moon – the near side of the moon.

The moon, however, orbits the earth, not the sun, and as such, its whole surface at one time or another DOES in fact receive sunlight. Whenever a portion of the moon is not lit up on the near side of the moon, a corresponding fraction of the far side receives light. When we have a new moon, the far side of the moon is lit up, and the new moon on 7 October 1959 gave the Luna 3 orbiter the chance to photograph the far side for the first time in history. 

In 1968, the Apollo 8 mission orbited the moon, and humans for the first time saw that side directly. In this case, however, the dark side took on a different meaning, as it meant Apollo 8 went radio silent – the moon blocking all radio transmissions from Earth. This property of the far side of the moon means that it would be an ideal place to set up a radio telescope, as it would be shielded from all of Earth’s radio transmissions… at least, until we start colonising the rest of the solar system, and inter-planetary radio traffic becomes more pronounced. 

So what is the far side of the moon really like? For one, it’s actually quite a different place from the near side which we can easily see with our own eyes. The near side has widespread basaltic plains called ‘maria,’ created by volcanic activity long ago. They enable us to imagine we see a ‘man in the moon,’ or a tree, or hands. Yet on the far side of the moon, volcanic activity was much more limited, and there are only a few maria. As astronauts Williams Anders described it, “The backside looks like a sand pile my kids have played in for some time. It's all beat up, no definition, just a lot of bumps and holes.”

Since the maria covers only 1% of the far side of the moon, compared to the 31% of the near side, it’s much more riddled with craters. One might think that this could be due to the Earth shielding the moon from asteroid impacts on the near side, but this is not so – the Earth only obscures 4 square degrees out of 41 thousand square degrees of sky as seen from the moon. 

Newer research has shown that the real reason is that during the formation of the Earth-Moon system, the moon started to cool down first. Since it was already tidally locked at that time, the far side of the moon condensed first, forming thicker plagioclases out of aluminium and calcium combining with the silicate in the moon’s mantle. As a result, the far side of the moon has a much thicker crust than the near side, which remain liquid for longer. When meteors impacted on the young moon, those on the far side merely caused craters, while those on the near side penetrated the crust, and caused the release of basaltic lava, which formed the maria.

As a result, the far side of the moon boasts the largest crater in the solar system, the South-Pole-Aitken basin, that’s roughly 2500 kilometers in diameter, and the moon’s largest mountain ranges. It is also the region known to have water, or ice, hidden away on permanently shadowed crater walls, and in regions just below the surface. The next expedition to explore the far side of the moon will be launched in December of this year – the Chinese Chang’e 4 mission. It will land a robotic lander and rover, controlled via a relay satellite. The Chang’e missions are named after the Chinese moon goddess, and apt name for what it will be accomplishing. 

So the next time you hear lyrics, poems, or even prose referring to the dark side of the moon, and how ‘mysterious’ it is, you too can now be annoyed by the presumption of artists to assume that humanity has not set its sights on our closest neighbour. Perhaps they feel that the mystique they’ll create outweighs the cost of marginalizing humanity’s history of space exploration? In any case, they should perhaps set their sights on the mystique of space as yet unexplored, for if we are to take our place amongst the stars, we should set our sights higher than the moon.

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