Astronomy & Astrology

Moon

I. Introduction

Moon, name given to the natural satellite of Earth, and sometimes applied to the satellites of the other planets in the solar system. The diameter of Earth's Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth, and the Moon's volume is about one-fiftieth that of Earth. The mass of Earth is 81 times greater than the mass of the Moon. Thus the average density of the Moon is only three-fifths, and the gravitational pull at the lunar surface only one-sixth, that of Earth. The Moon has no liquid water and essentially no atmosphere, so no weather exists to change its surface; yet it is not totally inert.

The Moon moves about Earth at an average distance of 384,403 km (238,857 mi), and at an average speed of 3,700 km/h (2,300 mph). It completes one revolution in an elliptical orbit about Earth in 27 days 7 hours 43 minutes 11.5 seconds with reference to the stars. For the Moon to go from one phase to the next similar phase, or one lunar month, requires 29 days 12 hours 44 minutes 2.8 seconds. The Moon rotates once on its axis in about the same period of time that elapses for its sidereal period of revolution, accounting for the fact that virtually the same portion of the Moon is always turned toward the Earth. Although the Moon appears bright to the eye, it reflects into space only 7 percent of the light that falls on it. The reflectivity, or albedo, of 0.07 is similar to that of coal dust.

II. The Moon Seen from Earth

At any one time, an observer can see only 50 percent of the Moon's entire surface. However, an additional 9 percent can be seen from time to time around the apparent edge because of the relative motion called libration. This is because of the slightly different angles of view from Earth, due to different relative positions of the Moon along its inclined elliptical orbit.

The Moon shows progressively different phases as it moves along its orbit around Earth. Half the Moon is always in sunlight, just as half Earth has day while the other half has night. The phases of the Moon depend on how much of the sunlit half can be seen at any one time. In the phase called the new moon, the face is completely in shadow. About a week later, the Moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted surface; a week afterward, in its last quarter, the Moon appears as a half-circle again. The entire cycle is repeated each lunar month. The Moon is full when it is farther away from the Sun than Earth; it is new when it is closer. When it is more than half illuminated, it is said to be in gibbous phase. The Moon is said to be waning when it progresses from full to new, and to be waxing as it proceeds again to full. Temperatures on its surface are extreme, ranging from a maximum of 127°C (261°F) at lunar noon to a minimum of -173°C (-279°F) just before lunar dawn.

Phases of the Moon (© Microsoft Corporation. All Rights Reserved.)

The appearance of the Moon from Earth depends on the relative positions of the Earth, Moon, and Sun. This illustration shows what the Moon looks like from Earth at different stages of the Moon's orbit.

III. Surface of the Moon

Ancient observers of the Moon believed that the dark regions on its face were oceans, giving rise to the name mare (Latin for “sea”), which is still used today; the brighter regions were likewise held to be continents.

Modern observation and exploration of the Moon has yielded far more comprehensive and specific knowledge. Since the Renaissance, telescopes have revealed a wealth of lunar detail, and lunar spacecraft have contributed further to this knowledge. Features discernible on the surface of the Moon include craters, mountain ranges, plains or maria, faults, domes, rilles, and rays. The largest distinct crater, called Bailly, is 295 km (183 mi) wide and 3,960 m (13,000 ft) deep. The largest mare or sea is Mare Imbrium (Sea of Rains), about 1,200 km (about 750 mi) wide. The highest mountains, in the Leibnitz and Doerfel ranges near the south pole of the Moon, have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. Craters as small as 1.6 km (1 mi) across have been defined in telescopic observations. The origin of lunar craters has been long debated. The latest evidence indicates that nearly all craters were formed by explosive impacts of high-velocity meteorites or small asteroids, mostly during the early part of lunar history, when the solar system still contained many such fragments. Some craters, rilles, and domes, however, display characteristics of indisputable volcanic origin.

In 1996 a team working with data gathered by the United States Department of Defense Clementine spacecraft announced that frozen water may exist in a large basin near the Moon's south pole. Clementine's radar showed what may be an 8,000 sq km (3,000 sq mi) area covered with a mixture of dirt and ice crystals. Clementine was launched in 1994 and gathered data for four months.

Other studies of the Moon's poles could not confirm Clementine's findings, but the U.S. National Aeronautics and Space Administration (NASA) launched the Lunar Prospector spacecraft toward the Moon in 1998. Prospector returned data suggesting that a significant amount of water exists on the Moon in the form of ice crystals mixed with the soil at the lunar poles. Estimates of the amount of water on the Moon varied widely, from 10 million to 6 billion metric tons.

In 1999, at the end of the Lunar Prospector's mission, scientists programmed the spacecraft to crash at a specific spot likely to contain water, hoping that the debris that rose with the impact would contain detectable water vapor. Although no water was detected after the crash, scientists could not conclude that no water existed on the Moon. They acknowledged several other possible explanations for the result, such as that the spacecraft might have missed its target area or that the telescopes used to observe the crash might have been aimed incorrectly.

IV. Origin of the Moon

Before the modern age of space exploration, scientists had three major theories for the origin of the Moon: fission from Earth; formation in Earth orbit; and formation far from Earth. Then, in 1975, having studied moon rocks and close-up pictures of the Moon, scientists proposed what has come to be regarded as the most probable of the theories of formation, planetesimal impact.

A. Formation by Fission from the Earth

The modern version of this theory proposes that the Moon was spun off from Earth when Earth was young and rotating rapidly on its axis. This idea gained support partly because the density of the Moon is the same as that of the rocks just below the crust, or upper mantle, of Earth. A major difficulty with this theory is that the angular momentum of Earth, in order to achieve rotational instability, would have to have been much greater than the angular momentum of the present Earth-Moon system.

B. Formation in Orbit Near Earth

This theory proposes that Earth and the Moon, and all other bodies of the solar system, condensed independently out of the huge cloud of cold gases and solid particles that constituted the primordial solar nebula. Much of this material finally collected at the center to form the Sun (see Nebula).

C. Formation Far from Earth

According to this theory independent formation of Earth and the Moon, as in the above theory, is assumed; but the Moon is supposed to have formed at a different place in the solar system, far from Earth. The orbits of Earth and the Moon then, it is surmised, carried them near each other so that the Moon was pulled into permanent orbit about Earth.

D. Planetesimal Impact

First published in 1975, this theory proposes that early in Earth's history, well over 4 billion years ago, Earth was struck by a large body called a planetesimal. Early estimates for the size of the planetesimal were comparable to the size of Mars, but a computer simulation by American scientists in 1997 showed that the body would have had to have been at least two-and-a-half to three times the size of Mars. The catastrophic impact blasted portions of Earth and the planetesimal into Earth orbit, where debris from the impact eventually coalesced to form the Moon. This theory, after years of research on moon rocks in the 1970s and 1980s, has become the most widely accepted one for the Moon's origin. The major problem with the theory is that it would seem to require that Earth melted throughout after the impact. This seems to be the only way that the huge crater caused by the crash could have been erased, but Earth's geochemistry does not indicate such a radical melting.

V. Lunar Exploration

Throughout the 19th and 20th centuries, visual exploration through powerful telescopes yielded a fairly comprehensive picture of the visible side of the Moon. The hitherto unseen far side of the Moon was first revealed to the world in October 1959 through photographs made by the Soviet Luna 3 spacecraft. These photographs showed that the far side of the Moon is similar to the near side except that large lunar maria are absent. Craters are now known to cover the entire Moon, ranging in size from huge, ringed maria to those of microscopic size. In 1964 and 1966 photographs from U.S. spacecraft—Ranger 7, 8, and 9 and Lunar Orbiter 1 and 2—further supported these conclusions. The entire Moon has about 3 trillion craters larger than 1 m (3 ft) in diameter.

The successful landings of the unpiloted Surveyor series spacecraft by the United States and the Luna series by the USSR in the 1960s, and then the manned landings on the lunar surface as part of the U.S. Apollo program, made direct measurement of the physical and chemical properties of the Moon a reality. The Apollo astronauts collected rocks, took thousands of photographs, and set up instruments on the Moon that sent information back to Earth by radio telemetry. These instruments measured temperature and gas pressure at the lunar surface; the heat flow from the Moon's interior; molecules and ions of hot gases streaming out from the atmosphere of the Sun, called the solar wind; the magnetic field and gravity of the Moon; seismic vibrations of the lunar surface caused by landslides, meteoroid impacts, and so-called moonquakes; and through laser beams, the precise distance between Earth and the Moon.

It is now known, from measuring the ages of lunar rocks, that the Moon is about 4.60 billion years old, or about the same age as Earth and probably the rest of the solar system. Rocks from the lunar maria were formed when molten rock solidified between 3.16 billion and 3.96 billion years ago. These rocks most nearly resemble terrestrial basalts, a volcanic rock type widely distributed on Earth, but with certain important differences. Evidence indicates that the lunar highlands, or continents, may be formed of a less dense plutonic igneous rock called anorthosite, which consists almost entirely of the mineral plagioclase. Other important lunar sample types include glasses, breccias (complex assemblages of rock fragments cemented together by heat or pressure, or both), and the soils or regolith (finely divided rock fragments produced by many millions of years of meteoritic bombardment).

The Moon's magnetic field is not as strong or widespread as that of Earth. Some lunar rocks are weakly magnetic, indicating that they solidified in a somewhat stronger magnetic field. The Moon's magnetic field seems to be strongest in areas that are opposite from large craters. Astronomers theorize that impacts with asteroids created these craters when the Moon's magnetic field was stronger than it is today. When an asteroid crashed into the Moon, material on the other side of the Moon flew up and crashed back down. When rock undergoes a strong shock, such as being hit with a hammer or falling from a great height, it sometimes takes on the magnetic field that surrounds it. Astronomers theorize that the moon rocks took on the relatively strong magnetic field and have kept it, even after the Moon's magnetic field has faded. Regions of strong magnetic fields repel the charged particles that stream from the Sun in the solar wind. Astronomers believe that that interaction with the solar wind darkens the Moon, and that lighter regions of the Moon are protected by magnetic fields.

Magnetic and other measurements indicate an internal temperature of the Moon as high as 1600°C (2912°F), which is above the melting point of most lunar rocks. Evidence from seismic recordings suggests that some regions near the lunar center may be liquid. Seismometers operating on the lunar surface have also recorded signals of between 70 and 150 meteorite impacts per year, with masses from 100 g to 1,000 kg (4 oz to 2,200 lb). Hence the Moon is still being bombarded by meteorites, although not as often as in has been the past. This could pose a problem to engineers who design permanent bases for the lunar surface. The surface is covered by a layer of rubble, which may be several kilometers deep in the maria and of as yet unknown depth in the highlands. The rubble zone is believed to have been formed by the impacts of meteorites (see Meteor). The atmosphere of the Moon is so thin that it cannot be duplicated even in the best vacuum chambers on Earth.

All six manned landings on the Moon—Apollo 11, 12, and 14 to 17—returned samples of rock and soil to Earth, weighing 384 kg (847 lb) in all. It was not until the final mission, Apollo 17, that the astronaut crew included a geologist, Harrison Schmitt. He spent 22 hours exploring the Taurus-Littrow Valley region, covering 35 km (22 mi) in a lunar roving vehicle. Intensive analysis of the data and rocks obtained by the lunar missions continues.

NASA sent its first spacecraft in 25 years to the Moon in 1998, when the agency launched the orbiter Lunar Prospector. Lunar Prospector orbited around the Moon's north and south poles and returned data until July 1999. Its mission was to search for natural resources on the Moon such as water and minerals. Scientists used the spacecraft until its final hours. They ended its mission by programming the orbiter to crash into the Moon's surface and then observing the cloud of debris that rose afterward.

Many active research programs are observing and measuring many other regions of the solar system, including the craters of Mercury, the volcanoes and canyons of Mars, the gigantic magnetic and radiation fields of Jupiter, and the enormous energies of the Sun. Combined with the scientific information gathered from the Moon, scientists are beginning to greatly improve our understanding of the complex origin, structure, and evolution of the solar system, and of its relationship to life and the human race.

Contributed By: Gary V. Latham, Ph.D. Professor of Geophysics, University of Texas. Associate Director, Marine Science Institute Geophysical Laboratory.