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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