The Moon by Oliver Morton

Author:Oliver Morton [Morton, Oliver]
Language: eng
Format: epub
Publisher: Profile
Published: 2019-05-02T23:00:00+00:00

IMPACTS HAVE CONTINUED FOR THE REST OF LUNAR HISTORY, the only real exception to Robert Heinlein’s tongue-in-cheek dictum that “nothing ever happens on the moon”. And they brought the Moon at least two things that humans might treasure.

The first is water. Many asteroids are made of minerals that contain a bit of water; those known as “carbonaceous chondrites” can be over 20% water by mass. Comets are a good bit wetter still. When a body of either sort hits the Moon, the water it contains is vaporised and much of that vapour is immediately lost back to space. But some sticks around. On the hot, sunlit side of the Moon’s night-edge, it forms a tenuous atmosphere; on the dark, cold side, an all but undetectable frost. As the night-edge sweeps round the planet, the volatiles move from ice below to vapour above and back again on a monthly basis.

In time, most of this asymmetric atmosphere is lost—the Moon is too small to keep such a wrapping around it. The Sun’s ultraviolet light ionises the volatile molecules, after which the charged particles of the solar wind strip them away. But some of them remain as frost in perpetuity—because some of the Moon never sees the light of day.

The Moon has a low obliquity; it sits almost straight up with respect to the ecliptic. This means that the Moon’s poles are lit tangentially, with the Sun never rising far above the horizon. The shadows are long—so long that some of them never end. In craters at the poles there are places where the horizon-hugging Sun cannot shine. It may rise high enough to light the inner rim of a crater, creating the morning-lit side which Galileo, when first convincing people that the craters were craters, compared to the western side of an Alpine valley. And as the Moon slowly turns, the part of the inner rim that is lit changes, too, as if being broiled on a sluggish rotisserie. But though most of the rim is illuminated at some time or other, the floor never is. The only light it sees is the secondary light reflected from the rim.

And some of the crater’s interior does not even see that—because there are craters within craters, and from those inner craters the rim of the outer one often is invisible. The depths of such craters see the Sun neither directly nor indirectly.

Most of the craters which contain this perpetual darkness are around the South Pole: the crater named after Gene Shoemaker is one. Being in the depths of the South Pole-Aitken basin gives the region a head start when it comes to avoiding sunlight. But there are pools of perpetual darkness in the north, too. And at both poles the darkness is phenomenally cold—colder, remarkably, than the surface of Pluto, which is 30 times farther from the Sun. Pluto may get sunlight a thousand times weaker than that which bathes the Moon, but every square metre of it gets some of that light some of the time.

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