The moon is the nearest celestial object to earth and the only one on which a manned spacecraft has landed. For these reasons, scientists today probably know more about the moon than any other heavenly body.
In the entire solar system, the moon is the second largest satellite in terms of mass relative to its parent planet: its mass is about of the earth’s. In comparison, the proportionally largest satellite is Charon, which is about | as massive as its governing planet, Pluto. (Ganymede, the most massive satellite in absolute terms, has a mass only about yji—that of Jupiter.) The moon’s equatorial diameter is 2,160 miles (3,476 kilometers), more than a quarter that of earth.
Because of the moon’s low absolute mass, its gravity (and, therefore, escape velocity) is also low—about | that of the earth. As a result, the moon has virtually no atmosphere—except for minute traces of hydrogen, helium, argon, and neon. The lack of an atmosphere—and of water—means that the lunar terrain has been (and still is) shaped by forces other than the forces of erosion (wind and rain, for example) that constantly change the earth’s surface features. Instead, the moon’s surface has been fashioned principally by volcanic activity and by the impact of meteorites. Because themoon lacks an appreciable atmosphere, these meteorites are not burnt up as they approach the planet—unlike the meteors that enter the earth’s atmosphere.
The origin of the moon
Despite the American Apollo and Soviet Lunokhod missions to the moon in the late 1960’s and early 1970’s, and the many centuries of detailed scientific investigations carried out from terrestrial observatories, the origin of the moon remains a mystery.
According to one theory, the fission hypothesis (which was first put forward in 1879 by the British astronomer Sir George Darwin), the moon was originally part of the earth. In the early phases of the development of the solar system, while the planets were still hot and fluid, the primordial earth was rotating so rapidly that it became distended and eventually threw off a spherical globule, which became the moon. Although not part of the original theory, it has even been suggested that the present-day Pacific Ocean represents the enormous hollow left when the moon broke away from the earth. There are, however, several problems with the fission hypothesis. Scientists have calculated that for fissioning to occur, the primordial earth would have had to spin about eight times faster than it does at present, which is generally thought to be impossible. Furthermore, the earth and moon have different densities and chemical compositions, factors that make a common origin extremely unlikely.
The binary accretion hypothesis—similar in some respects to the fission theory—maintains that particles spun off from the rapidly-rotating, primordial earth and later condensed to form the moon. The objections to this hypothesis are basically the same as those to the fission theory. The primordial earth is unlikely to have been spinning fast enough to eject particles, and the different compositions and densities of the earth and moon indicate that they originated separately (although analysis of lunar rocks suggests that the earth and moon were formed at about the same time).
A more probable related origin theory is the precipitation hypothesis. According to this theory, energy released by the condensation of the primordial earth heated the thick cloud of dust that then surrounded the planet, simultaneously transforming the dust into various metals and metal oxides—a process that explains the density and composition differences between the earth and moon. Shortly afterward, the transformed dust cooled and condensed to form the moon.
The fourth main lunar origin theory ignores the exact process by which the moon was formed and concentrates instead on explaining how the earth acquired its satellite. According to this last theory—the lunar capture hypothesis—the primordial earth and moon condensed into planets in different parts of the solar system (which explains their different densities and compositions). The theory also postulates that the moon’s orbit brought it close enough to the earth to be captured (possibly after having been slowed down by passing through the dust cloud then surrounding the earth), thereby becoming a satellite.
Of these four principal theories of the origin of the moon, the last two are the more credible, although none provides an entirely satisfactory explanation.
The orbit of the moon
The gravitational attraction between the sun and moon is about twice as strong as that between the earth and the moon. The overall effect of these two forces is that the moon has a primary orbit around the sun, superimposed on which is its elliptical orbit around the earth. The sun also influences the shape and orientation of the moon’s secondary orbit around the earth. One of the most marked of these effects is the periodic rotation of the entire lunar orbit, which moves around the earth (in the same direction as the earth’s rotation) with a period of 8.85 years.
The moon always presents the same “side” toward the earth. This is because the moon’s rotation is synchronous—that is, the time taken for it to complete one rotation about its axis is the same as that taken for it to orbit once around the earth. But because the moon’s axial rotation rate is constant, whereas its orbital speed is not (a phenomenon caused by the elliptical shape of its orbit), and because its orbit is inclined at 5° to the plane of the earth’s orbit, it is possible to see about 59 per cent of the moon’s total surface area from earth. The other 41 per cent remained unknown until 1959, when the Soviet spacecraft Luna 3 took the first photographs of the moon’s far side.
The surface of the moon
The lunar surface can be divided into two main regions: highlands, which cover about 85 per cent of the moon’s total surface area, and plains (called maria), which cover the remaining 15 per cent. The formation of these two types of terrain is intimately connected with the evolution of the moon itself.
About 4.6 billion years ago, the moon was a hot, fluid mass. As it cooled, relatively low density matter rose to the surface and solidified, forming a primordial crust. After the crust had been formed, it was heavily bombarded by meteorites and other celestial debris (of which there was a much larger amount in the solar system than there is now), which gave rise to the impact craters and other surface features characteristic of the rugged highland terrain.
The era of heavy bombardment ended about 3.9 billion years ago and was followed by a period of intense volcanic activity, during which the maria were formed. Some of the earlier bombardments created vast basins, which became flooded with lava from the lunar volcanoes. When the lava in the basins cooled, it left large, smooth plains—the maria.
After the volcanic activity, which ended about 3.2 billion years ago, the lunar surface appeared much as it does today, apart from craters formed by subsequent meteorite impacts.
The interior of the moon
Although the lunar surface shows no obvious signs of activity, the interior of the moon is still hot (a discovery made by means of heat-flow experiments carried out by the American Apollo 15 and Apollo 17 missions) because of the presence of subsurface radioactive material. The Apollo missions also found that about 3,000 tremors (“moonquakes”) occur every year. By investigating these moonquakes, selenologists (who specialize in studying the moon) have determined the moon’s inner structure.
The crust extends to a depth of about 31 miles (50 kilometers) on the near side of the moon and to about 40 miles (65 kilometers) on the far side. It consists of several layers. Down to a depth of a few miles is the regolith, comprising the shattered remnants of bedrock that
was broken up by meteorite bombardment The thickness and composition of this region varies from highlands to maria; the highlands are composed mainly of anorthesite, whereas the maria are chiefly basaltic.
At about 12 miles (20 kilometers) below the surface, the upper crust’s composition changes, becoming similar to that of the surface highlands. Below the crust is a layer of denser material that extends to a depth of about 90 miles (150 kilometers), where the lithosphere begins. Moonquakes originate within this region, at a depth of about 600 miles (1,000 kilometers). Under the lithosphere is the asthenosphere, the partly-molten outer core. Finally, at the center of the moon, is the molten inner core, which is about 900 miles (1,500 kilometers) across.
An interesting feature of the moon’s interior was discovered in 1968 as a result of unexpected variations in the orbital velocity of the American probe Orbiter 5. The cause was found to be localized subsurface areas of abnormally high density. Called mass concentrations, or mascons, these regions may have formed as a result of lava from the outer core rising through cracks in the lithosphere (these cracks possibly having resulted from a large meteorite impact early in the moon’s history) and then solidifying into localized high-density masses just below the surface.
The moon and the tides
t has been known since antiquity that the tides on earth and the positions of the moon are related in some way. However, scientists were not able to explain the phenomenon until Isaac Newton published his theory of gravitation in 1687. It was then found that both the moon and the sun play a part in producing the tides: the moon produces about 54 per cent of the tide-raising effect, and the sun, the remaining 46 per cent.
The tides are probably best explained by considering the earth to be a solid sphere covered by a layer of water. The moon’s gravitational attraction is greatest on the water on the part of the earth facing the moon. This attraction causes the water to bulge slightly toward the moon—a high tide. On the opposite side of the earth, the moon’s gravitational attraction is at a minimum. But the solid earth, being nearer the moon, is attracted more than the water, which is therefore “left behind”; the effect is to produce a second bulge (and thus another high tide) on the opposite side of the earth.
During the earth’s daily rotation, the tidal bulges move with the moon so that one bulge is always at the part of the earth nearest the moon and the second is directly opposite. The overall effect is to cause a high tide—and a low tide—every 12.42 hours. The tides do not recur at exactly 12-hour intervals because, in addition to the earth’s rotation, the moon is orbiting the earth and is not in the same position on consecutive days. The sun also produces two tidal bulges that move around the earth approximately every 24 hours; these tides are about half the amplitude of the lunar tides because of the sun’s lesser tide-raising effect. When sun, moon, and earth are directly in line—at every full and new moon—the solar and lunar tides reinforce each other, producing larger than average tides (called spring tides). The sun’s tidal influence is at its least during the first and third quarters of the moon, when the moon is at right angles to the line of the earth and sun. In this situation, the solar and lunar tides partly cancel each other, producing smaller than average tides (neap tides).