..... Click the link for more information. ) or dwarf planet, in particular, the single natural satellite of the earth earth, in geology and astronomy, 3rd planet of the solar system and the 5th largest, the only planet definitely known to support life. Gravitational forces have molded the earth, like all celestial bodies, into a spherical shape.
..... Click the link for more information. .
The Earth-Moon System
The moon is the earth's nearest neighbor in space. In addition to its proximity, the moon is also exceptional in that it is quite massive compared to the earth itself, the ratio of their masses being far larger than the similar ratios of other natural satellites to the planets they orbit (though that of Charon Charon, in astronomy, the natural satellite, or moon, of Pluto.
..... Click the link for more information. and the dwarf planet Pluto Pluto, in astronomy, a dwarf planet and the first Kuiper belt, or transneptunian, object (see comet) to be discovered (1930) by astronomers. Pluto has an elliptical orbit usually lying beyond that of Neptune.
..... Click the link for more information. exceeds that of the moon and earth). For this reason, the earth-moon system is sometimes considered a double planet. It is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun in accordance with Kepler's laws Kepler's laws, three mathematical statements formulated by the German astronomer Johannes Kepler that accurately describe the revolutions of the planets around the sun. Kepler's laws opened the way for the development of celestial mechanics, i.e.
..... Click the link for more information. . It is also more accurate to say that the earth and moon together revolve about their common center of mass, rather than saying that the moon revolves about the earth. This common center of mass lies beneath the earth's surface, about 3,000 mi (4800 km) from the earth's center.
The Lunar Month
The moon was studied, and its apparent motions through the sky recorded, beginning in ancient times. The Babylonians and the Maya, for example, had remarkably precise calendars calendar [Lat., from Kalends], system of reckoning time for the practical purpose of recording past events and calculating dates for future plans. The calendar is based on noting ordinary and easily observable natural events, the cycle of the sun through the seasons
..... Click the link for more information. for eclipses and other astronomical events. Astronomers now recognize different kinds of months, such as the synodic month of 29 days, 12 hr, 44 min, the period of the lunar phases phase, in astronomy, the measure of how much of the illuminated surface of a planet or satellite can be seen from a point at a distance from that body; the term is most often used to describe the moon as seen from the earth.
..... Click the link for more information. , and the sidereal month of 27 days, 7 hr, 43 min, the period of lunar revolution around the earth.
The Lunar Orbit
As seen from above the earth's north pole, the moon moves in a counterclockwise direction with an average orbital speed of about 0.6 mi/sec (1 km/sec). Because the lunar orbit is elliptical, the distance between the earth and the moon varies periodically as the moon revolves in its orbit. At perigee, when the moon is nearest the earth, the distance is about 227,000 mi (365,000 km); at apogee, when the moon is farthest from the earth, the distance is about 254,000 mi (409,000 km). The average distance is about 240,000 mi (385,000 km), or about 60 times the radius of the earth itself. The plane of the moon's orbit is tilted, or inclined, at an angle of about 5° with respect to the ecliptic ecliptic , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic). Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the
..... Click the link for more information. . The line dividing the bright and dark portions of the moon is called the terminator.
Retarded Lunar Motion
Due to the earth's rotation, the moon appears to rise in the east and set in the west, like all other heavenly bodies; however, the moon's own orbital motion carries it eastward against the stars. This apparent motion is much more rapid than the similar motion of the sun. Hence the moon appears to overtake the sun and rises on an average of 50 minutes later each night. There are many variations in this retardation according to latitude and time of year. In much of the Northern Hemisphere, at the autumnal equinox equinox , either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north.
..... Click the link for more information. , the harvest moon harvest moon, full moon occurring nearest to the autumnal equinox, about Sept. 23. During harvest moon the retardation (later rising each night) of the moon is at a minimum because of the relation of the moon's path to the horizon.
..... Click the link for more information. occurs; moonrise and sunset nearly coincide for several days around full moon. The next succeeding full moon, called the hunter's moon, also shows this coincidence.
Solar and Lunar Eclipses
Although an optical illusion causes the moon to appear larger when it is near the horizon than when it is near the zenith, the true angular size of the moon's diameter is about 1-2°, which also happens to be the sun's apparent diameter. This coincidence makes possible total eclipses eclipse [Gr.,=failing], in astronomy, partial or total obscuring of one celestial body by the shadow of another. Best known are the lunar eclipses, which occur when the earth blocks the sun's light from the moon, and solar eclipses, occurring when the moon blocks the
..... Click the link for more information. of the sun in which the solar disk is exactly covered by the disk of the moon. An eclipse of the moon occurs when the earth's shadow falls onto the moon, temporarily blocking the sunlight that causes the moon to shine. Eclipses can occur only when the moon, sun, and earth are arranged along a straight line—lunar eclipses at full moon and solar eclipses at new moon.
Tidal Influence of the Moon
The gravitational influence of the moon is chiefly responsible for the tides tide, alternate and regular rise and fall of sea level in oceans and other large bodies of water. These changes are caused by the gravitational attraction of the moon and, to a lesser extent, of the sun on the earth.
..... Click the link for more information. of the earth's oceans, the twice-daily rise and fall of sea level. The ocean tides are caused by the flow of water toward the two points on the earth's surface that are instantaneously directly beneath the moon and directly opposite the moon. Because of frictional drag, the earth's rotation carries the two tidal bulges slightly forward of the line connecting earth and moon. The resulting torque slows the earth's rotation while increasing the moon's orbital velocity. As a result, the day is getting longer and the moon is moving farther away from the earth. The moon also raises much smaller tides in the solid crust of the earth, deforming its shape. The tidal influence of the earth on the moon was responsible for making the moon's periods of rotation and revolution equal, so that the same side of the moon always faces earth.
Physical Characteristics
The study of the moon's surface increased with the invention of the telescope by Galileo in 1610 and culminated in 1969 when the first human actually set foot on the moon's surface. The physical characteristics and surface of the moon thus have been studied telescopically, photographically, and more recently by instruments carried by manned and unmanned spacecraft (see space exploration space exploration, the investigation of physical conditions in space and on stars, planets, and other celestial bodies through the use of artificial satellites (spacecraft that orbit the earth), space probes (spacecraft that pass through the solar system and that may
..... Click the link for more information. ). The moon's diameter is about 2,160 mi (3,476 km) at the moon's equator, somewhat more than 1-4 the earth's diameter. The moon has about 1-81 the mass of the earth and is 3-5 as dense. On the moon's surface the force of gravitation is about 1-6 that on earth. It has been established that the moon completely lacks an atmosphere and, despite some tantalizing hints that there might be ice under the surface dust in shaded portions of Shackleton Crater (near the moon's south pole), there is no definite evidence of water. The surface temperature rises above 100°C; (212°F;) at lunar noon and sinks below −155°C; (−247°F;) at night. The gross surface features of the moon are visible to the unaided eye and were first studied telescopically in 1610 by Galileo.
Surface Features
The lunar surface is divided into the mountainous highlands and the large, roughly circular plains called maria (sing. mare; from Lat.,=sea) by early astronomers, who erroneously believed them to be bodies of water. The smooth floors of the maria, varying from flat to gently undulating, are covered by a thin layer of powdered rock that darkens them and accounts for the moon's low albedo albedo , reflectivity of the surface of a planet, moon, asteroid, or other celestial body that does not shine by its own light. Albedo is measured as the fraction of incident light that the surface reflects back in all directions.
..... Click the link for more information. (only 7% of the incident sunlight is reflected back, the rest being absorbed). The brighter regions on the moon are the mountainous highlands, where the terrain is rough and strewn with rocky rubble. The lunar mountain ranges, with heights up to 25,000 ft (7800 m), are comparable to the highest mountains on earth but in general are not very steep. The highlands are densely scarred by thousands of craters—shallow circular depressions, usually ringed by well-defined walls and often possessing a central peak. Craters range in diameter from a few feet to many miles, and in some regions there are so many that they overlap or several smaller craters lie within a large crater. Craters are also found on the maria, although there are nowhere near as many as in the lunar highlands. Other prominent surface features include the rilles and rays. Rilles are sinuous, canyonlike clefts found near the edges of mountain ranges. Rays are bright streaks radiating outward from certain craters, such as Tycho.
Mare and highland rocks differ in both appearance and chemical content. For example, mare rocks are richer in iron and poorer in aluminum than highland rocks. The maria consist largely of basalt, i.e., igneous rock formed from magma. In the highlands the majority of the rocks are breccias—conglomerates conglomerate, in geology, sedimentary rock composed largely of pebbles or other rounded particles whose diameter is larger than 2 mm (.08 in.). Essentially a cemented gravel, conglomerates are formed along beaches, as glacial drift, and in river deposits.
..... Click the link for more information. formed from basaltic rock and often studded with small, green, glassy spheres. These spheres probably were formed as the spray of molten rock, originally melted by the heat of meteorite impact, recongealed in midflight. The exposure ages of some rocks (the time their surfaces have been exposed to the action of cosmic rays that produce radioactive isotopes) are as short as 50 million years, much shorter than their crystallization ages. These rocks may have been shifted in position by meteorite impact or seismic activity (moonquakes). However, present lunar seismic activity is very low, corroborating the image of the moon as an essentially static, nonevolving world.
Internal Structure
Diffraction of seismic waves provided the first clear-cut evidence for a lunar crust, mantle, and core analogous to those of the earth. The lunar crust is about 45 mi (70 km) thick, making the moon a rigid solid to a greater depth than the earth. The inner core has a radius of about 600 mi (1,000 km), about 2-3 of the radius of the moon itself. The internal temperature decreases from 830°C; (1,530°F;) at the center to 170°C; (340°F;) near the surface. The heat traveling outward near the lunar surface is about half that of the earth but still twice that predicted by current theory. This heat flow is directly related to the rate of internal energy production, so that the internal temperature profile provides information about long-lived radio isotopes and the moon's thermal evolution. The heat-flow measurements indicate that the moon's radioactive content is higher than that of the earth. The moon's magnetic field is a million times weaker than that of the earth, but it varies by a factor of 20 from point to point on the surface. Certain rocks retain a high magnetization, indicating that they crystallized in the presence of magnetic fields much higher than those presently existing on the moon. Mascons are large concentrations of unusually high density that are located below certain of the circular maria. The mascons may have been created by the implantation of very dense, iron-rich meteorites, whose impact formed the mare basins themselves.
Formation and Evolution
The moon probably formed by the cold accretion of small particles about 4.6 billion years ago at the same time that the rest of the solar system solar system, the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass.
..... Click the link for more information. formed; thus, it is now believed that the moon was never in an entirely molten state. The crust, showing pronounced chemical differentiation, formed early. Subsequent impact of very large meteorites depressed the mare basins, at the same time thrusting up the surrounding crust to form the highlands. The mare basins later filled with lava flow, which in turn was covered by a thin layer of lunar "soil"—fine rock dust pulverized by the very slow mechanisms of lunar erosion (thermal cycling, solar wind, and micrometeorites). The craters were probably also formed by meteorite bombardment rather than by internal volcanic action as once believed. The rays surrounding the craters are material ejected during the impacts that formed the craters. The moon's rock types are correlated with its major geological periods.
Bibliography
See P. Moore and P. J. Cattermole, The Craters of the Moon (1967); D. Thomas, ed., Moon (1970); G. Gamow, The Moon (rev. ed. 1971); S. R. Taylor, Lunar Science (1975); B. M. French, The Moon Book (1977); W. K. Hartmann, ed., The Origins of the Moon (1986).
Moon
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moon
Moon
moon [mün]
Moon
Moon
the earth’s only natural satellite and the celestial body closest to us. Its astronomical symbol is (.
Motion. The moon moves around the earth at a mean velocity of 1.02 km/sec in an approximately elliptical orbit in the same direction as most other bodies of the solar system, that is, counterclockwise if the moon’s orbit is viewed from the earth’s north pole. The semimajor axis of the moon’s orbit, which is equal to the mean distance between the centers of the earth and the moon, is 384,400 km (approximately 60 earth radii), which corresponds to a horizontal equatorial parallax of 57’2”.6. Because of the orbit’s ellipticity (the orbit’s eccentricity is equal to 0.0549) and perturbations, the distance to the moon varies from 356,400 to 406,800 km. As a result, the visible angular diameter of the moon, which is equal to 31’5” at the mean distance, varies from 33’32” to 29’20” (that is, is greater or less than the solar diameter). The moon’s period of revolution around the earth, known as the sidereal month, is equal to 27.32166 days but is subject to small fluctuations and a very small secular contraction.
The moon’s motion around the earth is very complex, and its study is one of the most difficult tasks of celestial mechanics. Elliptical motion is only a crude approximation; it is subject to many perturbations caused by the attraction of the sun and planets and by the oblateness of the earth. The most important perturbations, or nonuniformities (the equation of the center, evection, variation, annual equation) were discovered from observations long before they were theoretically derived from the law of universal gravitation. The moon’s attraction by the sun is 2.2 times stronger than that by the earth. Thus, strictly speaking, the moon’s motion around the sun and the perturbations of this motion caused by the earth should be examined. However, since the researcher is interested in the motion of the moon as viewed from the earth, the theory of gravitation, which has been developed by many prominent scientists beginning with I. Newton, considers the moon’s motion around the earth.
In the 20th century the American mathematician G. W. Hill developed a theory on the basis of which the American astronomer E. Brown in 1919 calculated mathematical series and compiled tables giving the latitude, longitude, and parallax of the moon as functions of time. The series contain about 1,500 terms derived on the basis of purely gravitational action. A significant empirical term that cannot be explained from the standpoint of the theory of gravitation was added to achieve better agreement with the results of lunar observations. In the 1930’s it was established that the introduction of this term was connected not with the deviation of the moon’s motion from the theory of gravitation, but with the inaccuracy of the time measurement system, which was based on the earth’s rotation about its axis, which proved to be nonuniform. In the current theory of lunar motion this empirical term is not taken into account but an appropriate correction is made in universal time; thus the transition to a uniformly flowing ephemeris time, which is used as the independent variable in Brown’s tables, is accomplished.
The plane of the moon’s orbit is inclined to the plane of the ecliptic at an angle of 5°8’43”, which is subject to small variations. The points of intersection of the orbit and the ecliptic, which are called the ascending and descending nodes, have a nonuniform, retrograde motion—called retrogression of the nodes—and complete a full revolution about the ecliptic in 6,794 days, or about 18.6 years. As a result, the moon returns to a given node after a period of time known as the draconic period, or nodical month, which is shorter than the sidereal month and on the average equals 27.21222 days. The periodicity of solar and lunar eclipses is related to the nodical month. The moon rotates about an axis inclined to the plane of the ecliptic at an angle of 88°28’ with a period precisely equal to the sidereal month, as a result of which it always presents the same side to the earth. The coincidence of the periods of the moon’s rotation and revolution is not accidental and is the result of the tidal friction which the earth produced in the solid or formerly liquid mantle of the moon. However, the combination of uniform axial rotation with nonuniform orbital motion causes small periodic deviations from constant orientation toward the earth, which reach 7°54’ in longitude, and the inclination of the moon’s axis of rotation to the plane of the orbit is responsible for deviations of up to 6°50’ in latitude. As a result of these periodic deviations, known as librations, as much as 59 percent of the lunar surface is visible from the earth at different times (although the areas near the edges of the lunar disk can be seen only in strong foreshortening). The planes of the moon’s equator, the ecliptic, and the lunar orbit always intersect in a single line (Cassini’s law).
Figure. The shape of the moon is very close to a sphere with a radius of 1,737 km, which is equal to 0.2724 times the earth’s equatorial radius. The surface area of the moon is 3.8 X 107 km2 (that is, 0.0743 ≈ 3/40 of the earth’s surface), and its volume is 2.2 X 1025 cm3 (that is, 0.0203 ≈ 1/49 of the earth’s volume). An exact determination of the moon’s shape poses problems because, as a result of the absence of oceans, the moon has no clearly defined fiducial surface with respect to which heights and depths can be determined. Moreover, since the moon keeps the same side toward the earth, it is possible to measure from the earth the radii of points on the surface of the moon’s visible hemisphere (except for points on the very edge of the lunar disk) only on the basis of the weak stereoscopic effects caused by the libration. The study of the libration has made it possible to estimate the difference between the principal semiaxes of the moon’s ellipsoid. The polar axis is 700 m shorter than the equatorial axis facing the earth and 400 m shorter than the equatorial axis that is perpendicular to the direction of the earth. Thus, the moon is slightly elongated in the direction of the earth because of tidal forces.
The moon’s mass can be determined most accurately from observations of artificial lunar satellites. It is 1/81.3 that of the earth and equals 7.35 X 1025 g. The moon’s mean density is 3.34g/cm3 (0.61 that of the earth). The acceleration of gravity at the moon’s surface is one-sixth that for the earth and is equal to 162.3 cm/sec2; 1 km above the surface it decreases to 0.187 cm/sec2. The orbital velocity is 1,680 m/sec, and the surface escape velocity is 2,375 m/sec. Because of its small gravitational field, the moon cannot retain a gaseous envelope or water in the free state.
Phases. The moon is not a self-luminous body, and only that part of the moon that is illuminated by sunlight either directly or after reflection from the earth is visible. This explains the phases of the moon. Each month the moon, moving in orbit, passes approximately between the sun and the earth and faces us with its dark side. This is the new moon. One or two days after this the narrow bright crescent of the new moon appears in the western part of the sky. At this time the rest of the lunar disk is weakly illuminated from the earth, which faces the moon with its sunlit hemisphere. This weak luminosity is the moon’s earth-shine, or earthlight. After seven days the moon has moved away from the sun by 90° and the first quarter sets in. This is when exactly half of the moon’s disk is illuminated and the terminator—the line that separates the bright and dark sides—becomes a straight line and thus a diameter of the lunar disk. In the days that follow, the terminator becomes convex, the shape of the moon approaches a bright circle, and the full moon occurs after 14 or 15 days. Then the western limb of the moon begins to wane. On the 22nd day the last quarter is observed. This is when the moon appears once again as a semicircle, but this time with its convex side to the east. The moon’s angular distance from the sun decreases, the moon again becomes a crescent, and the new moon occurs once again after 29.5 days.
The interval between two successive new moons is called the synodic month and has an average length of 29.53059 days. The synodic month is longer than the sidereal month, since in this time the earth passes through approximately 1/13 of its orbit and the moon, in order to pass again between the earth and the sun, must traverse still another 1/13 of its orbit, which requires a little more than two days. If the new moon occurs near one of the nodes of the lunar orbit, a solar eclipse occurs. When the full moon is near a node, a lunar eclipse occurs. The easily observable succession of lunar phases has served as the basis of many calendar systems.
Surface. The surface of the moon is quite dark and its albedo is equal to 0.073; that is, on the average the moon reflects only 7.3 percent of the sun’s rays. The visual stellar magnitude of the full moon at the mean distance is — 12.7. When the moon is full, it sends 465,000 times less light to the earth than the sun. In its variation with phase, this amount of light decreases much more rapidly than the area of the illuminated part of the moon, so that when the moon is at a quarter and we see half of its disk illuminated, it transmits to us not 50 percent but only 8 percent of the light of the full moon. The color index of moonlight is + 1.2, appreciably redder than sunlight.
The moon rotates with respect to the sun with a period equal to the synodic month, and therefore both the day and the night on the moon each last nearly 15 earth days. Being unprotected by an atmosphere, the surface of the moon heats up to +100° C in the day and cools to — 120°C at night. However, as radio observations have demonstrated, these enormous fluctuations in temperature penetrate only a few decimeters because of the extremely low thermal conductivity of the surface layers. For the same reason, the heated surface cools rapidly during total lunar eclipses, although some places retain their heat longer, probably because of their high specific heat (the “hot spots”).
Irregular extended darkish markings that have been taken as seas can be seen on the moon even by the unaided eye. The term “maria” (seas) has been retained even though it has been established that these formations have nothing in common with seas on the earth. Telescopic observations begun in 1610 by Galileo made it possible to detect the mountainous structure of the moon’s surface. The maria are plains of darker color than other areas, which are sometimes called continental regions, and abound in mountains, most of which are circular (craters).
Detailed maps of the moon have been compiled on the basis of many years of observations. The first such maps were published in Danzig (Gdansk) in 1647 by J. Hevelius. Retaining the term “mare,” Hevelius also named the principal lunar mountain ranges after similar features on the earth: the Apennines, Caucasus, and Alps. In 1651, G. Riccioli of Ferrara gave imaginative names to the extensive dark lowlands, such as the Oceanus Procellarum (Ocean of Storms), Mare Crisium (Sea of Crises), Mare Tranquillitatis (Sea of Tranquillity), and Mare Imbrium (Sea of Showers). He called small dark regions bordering on these maria, bays—for example, Sinus Iridum (Bay of Rainbows)— and small irregular spots, marshes—for example, Palus Putredinis. He named individual mountains, chiefly circular ones, after outstanding scientists, such as Copernicus, Kepler, and Tycho Brahe. These names are still retained on lunar maps, and many new names of outstanding individuals and scientists of later date have been added. The names of K. E. Tsiolkovskii, S. P. Korolev, Iu. A. Gagarin, and others appear on maps of the far side of the moon that have been compiled as a result of observations made from space probes and artificial lunar satellites.
In the 19th century the German astronomers J. von Mädler and J. Schmidt compiled detailed and accurate maps of the moon from telescopic observations. The maps were prepared in orthographic projection for the middle phase of the libration, that is, approximately as the moon is seen from the earth. Photographic observations of the moon were begun at the end of the 19th century. Between 1896 and 1910 a large atlas of the moon was published by the French astronomers M. Loewy and P. Puiseux from photographs taken at the Paris Observatory. Later, a photographic album of the moon was published by the Lick Observatory in the United States, and in the mid-20th century G. Kuiper (United States) compiled several detailed atlases of photographs of the moon that were taken through large telescopes at different astronomical observatories. Modern telescopes make it possible to detect (but not to examine in detail) craters about 0.7 km in size and crevasses a few hundred meters wide.
Surface relief. The relief of the lunar surface has been clarified chiefly as a result of many years of telescopic observations. The maria, which occupy about 40 percent of the visible surface of the moon, are flat plains traversed by crevasses and low winding ridges. There are comparatively few large craters in the maria. Many of the maria are surrounded by concentric circular rims (walls). The remaining, lighter, surface is covered with numerous craters, circular ridges, rills, and other features. Craters smaller than 15-20 km are bowl-shaped. Larger craters (up to 200 km) consist of circular rim with steep interior slopes and have a comparatively flat depressed bottom, often with a central knoll. The heights of mountains above the surrounding terrain are determined photometrically or by the length of the shadows cast on the lunar surface. Hypsometric maps with a scale of 1:1,000,000 have been compiled in this manner for most of the visible side. However, absolute heights and the distances of points on the lunar surface from the moon’s mass, or figure, center can be only very inaccurately determined, and the hypsometric maps based on them provide only a general idea of the lunar relief.
The relief of the limb zone, which limits the lunar disk as a function of the phase of the libration, has been studied in much greater detail and with much better accuracy. The German scientist F. Heyn, the Soviet scientist A. A. Nefed’ev, and the American scientist C. Watts compiled hypsometric maps that are used to calculate irregularities of the lunar limb for the purpose of determining the coordinates of the moon (such observations are made with transit circles and from photographs of the moon taken against the background of surrounding stars and from occultations of the stars by the moon). The selenographic coordinates of several principal reference points, which are used to interlink a large number of other points on the lunar surface, are determined through micrometric measurements with respect to the lunar equator and central meridian. The starting point is the Mosting A crater, a small regular formation that is easily visible in the center of the lunar disk. The structure of the moon has been studied chiefly through photometric and polarimetric observations augmented by radio-astronomical investigations.
The craters on the lunar surface are of different relative ages, ranging from ancient, scarcely discernible, heavily modified formations to young clearly defined craters that are sometimes surrounded by bright streaks called rays. Also, the young craters overlap older ones. In some cases the craters cut into the surface of the maria, and in others the rocks of the maria overlap the craters. Tectonic fractures sometimes traverse the craters and maria and sometimes they themselves are overlapped by younger formations. These and other features make it possible to establish the sequence of the appearance of the various structures on the lunar surface.
In 1949 the Soviet scientist A. V. Khabakov divided lunar formations into several successive age complexes. Further development of this approach toward the end of the 1960’s made it possible to compile intermediate-scale geological maps for much of the lunar surface. The absolute age of lunar formations is known only at a few sites, but by using certain indirect methods it has been determined that the youngest large craters are tens or hundreds of millions of years old and that most of the large craters appeared in the “premaria” era, 3 or 4 billion years ago.
Both internal forces and external influences have had a part in the formation of the lunar relief. Thermal calculations show that soon after the moon’s formation the interior was heated by radioactive sources and was melted to a considerable extent, leading to intensive volcanism on the surface. As a result, huge lava fields, a number of volcanic craters, and numerous clefts, scarps, and other features were formed. At the same time an enormous number of meteorites and asteroids (remnants of the protoplanetary cloud) struck the surface of the moon at different stages, resulting in craters ranging in size from pinholes to ringed structures with a cross section of several tens and perhaps several hundred km. Because of the absence of an atmosphere and hydrosphere, a large number of these craters have been preserved to the present day. Meteorites now strike the moon much less often; volcanism has basically halted, since the moon has expended much of its thermal energy and the radioactive elements have been raised to the outer strata of the moon. Escaping carbon-bearing gases in lunar craters, spectrograms of which were obtained for the first time by the Soviet astronomer A. N. Kozyrev, attest to residual volcanism.
Origin. The origin of the moon has not yet been fully established. Three different hypotheses have been developed in detail. At the end of the 19th century G. Darwin posited that the moon and earth originally constituted a single molten mass whose rate of rotation increased as the mass cooled and contracted. As a result, the mass broke up into two parts: the larger one was the earth, and the smaller, the moon. Darwin’s hypothesis explains the low density of the moon, which was formed from the outer layers of the original mass. However, it encounters serious objections from the standpoint of the mechanics of the process. Moreover, there are significant geochemical differences between the rocks of the earth’s mantle and lunar rocks.
The capture hypothesis proposed by the German scientist C. von Weizsacker, the Swedish scientist H. Alfven, and the American scientist H. Urey assumes that the moon was originally an asteroid that, on passing near the earth, became a satellite as a result of the effects of gravity. The probability of such an event is extremely small, and if it happened, there would be a greater difference between earth rocks and lunar rocks.
According to the third hypothesis, worked out by the Soviet scientist O. Iu. Schmidt and his followers in the mid-20th century, the moon and the earth were formed at the same time through the consolidation and compaction of a large cluster of small particles. However, since the moon as a whole has a lower density than the earth, the matter of the interplanetary cloud must have divided with a concentration of the heavy elements in the earth. This led to the assumption that the earth, surrounded by a thick atmosphere enriched with relatively volatile silicates, began to form first. On subsequent cooling, the matter of this atmosphere condensed into a ring of planetesimals, from which the moon formed. This hypothesis seems to be the most plausible at the current level of knowledge (1970’s).
Recent advances. A new stage of lunar research began with the launching of the first unmanned interplanetary probes to the moon. Studies have been conducted in the USSR with the Luna (21 spacecraft had been launched by September 1973) and the Zond probes, and in the United States the Ranger, Lunar Orbiter, Surveyor, and Apollo programs have been carried out. In early 1959, the Soviet Luna 1 was accelerated to escape velocity for the first time, and thus the first artificial planet was created. On Sept. 14, 1959, Luna 2 spacecraft delivered on the moon a banner bearing the USSR State Seal, and on Oct. 7, 1959, Luna 3 took the first photographs of about two-thirds of the moon’s far side at a distance of about 65,000 km. The transmitted television pictures made it possible to compile the first atlas of the far side. On July 20, 1965, the Zond 3 probe transmitted much clearer pictures of almost all the rest of the far side, which differs from the visible side in the absence of maria, with a few exceptions (for example, the Mare Moscoviense [Sea of Moscow]). Virtually the entire surface is mountainous and covered with craters of different sizes. Chains of craters up to several hundred kilometers long were also observed on the far side. As a result of studies of photographs of the far side taken by Luna 3 and Zond 3, the Atlas of the Far Side of the Moon cataloging about 4,000 newly observed formations was published in the USSR. In 1966-67 the first complete map of the moon and a complete globe of the moon were made in the USSR from the data of this atlas and photographs of the visible side of the moon. An atlas consisting of seven maps of the equatorial zone of the visible lunar hemisphere was published in 1968.
The American Ranger 7 unmanned spacecraft, which was launched on July 28, 1964, transmitted about 200 photographs taken at distances of 1,800 to 0.3 km from the moon. The photographs revealed the presence of craters ranging in size from those visible from the earth to others 1-2 m in diameter even on the apparently smooth surface of the maria. Luna 9, launched on Jan. 31, 1966, made the first soft landing on the moon on Feb. 3, 1966. A panoramic picture of the surrounding terrain was transmitted to the earth. Individual rocks or clods that were probably ejected on the impact of meteorites or during volcanic eruptions could be seen on the microgranular surface. Luna 10, launched on Mar. 31, 1966, became the first artificial satellite of the moon on Apr. 3, 1966. Between June and December 1966, American and Soviet spacecraft conducted experiments on the mechanical properties of the soil, determining the soil’s density and bearing strength. The outermost layer has a density of 1.1-1.2 g/cm3 and can withstand loads of up to 1 kg/cm2; at a depth of a few decimeters the density and bearing strength increase considerably. American artificial lunar satellites of the Lunar Orbiter series transmitted medium-scale photographs of virtually the entire lunar surface and large-scale photographs of a number of particular areas. Measurement of the velocity of motion of these satellites around the moon made it possible to compile gravitational maps of the moon. In the process, it was discovered that masses of high-density matter (mascons) occur in the vicinity of the circular maria.
On July 21, 1969, the American astronauts N. Armstrong and E. Aldrin, who traveled to the moon aboard the Apollo 11 spacecraft, were the first humans to set foot on the moon. Another ten men visited the moon on subsequent flights of Apollo spacecraft. The astronauts brought back to the earth several hundred kilograms of lunar rock samples; they also conducted a number of studies on the moon itself, including measurements of the heat flux, magnetic field, radiation level, and the intensity and composition of the solar wind (the stream of particles emanating from the sun). It was shown that the heat flux from the lunar interior is approximately one-half that from the earth’s interior. Residual magnetization was found in lunar rocks, attesting to the presence, at one time, of a magnetic field on the moon. Instruments that automatically transmit information to the earth, including seismometers that record oscillations in the body of the moon, were left on the moon. The seismometers have recorded shocks from the impact of meteorites and “moonquakes” of internal origin. On the basis of seismic data, it has been determined that to a depth of a few tens of kilometers the moon consists of a relatively light “crust”; below that lies a denser “mantle.” The duration of seismic oscillations on the moon, which is several times longer than on the earth, is apparently connected with the strongly fractured nature of the upper “crust.”
At the same time, studies of the moon were conducted by the Soviet Luna probes. In September 1970 Luna 16 drilled a 35-cm core sample and brought it back to the earth. In November 1970, Luna 17 delivered to the moon’s Mare Imbrium Lunokhod 1, a self-propelled lunar vehicle, which traveled a distance of 10,540 m in 11 lunar days (or 10.5 months) and transmitted a large number of panoramic pictures, individual photographs of the lunar surface, and various scientific data. A French reflector mounted on the Lunokhod made it possible, using a laser beam, to measure the distance to the moon with an accuracy of a fraction of a meter. In February 1972, Luna 20 probe brought back samples of lunar soil that were picked up in an almost inaccessible region of the moon. In January 1973 Luna 21 placed Lunokhod 2 in the Le Monnier crater (Mare Serenitatis) to conduct a comprehensive study of the transition zone between the mare and continental regions. Lunokhod 2 operated for five lunar days (four months) and traversed a distance of about 37 km.
Soil. Everywhere that spacecraft have made landings, the moon is covered with regolith, a layer of loose rock material, ranging in thickness from a few meters to several tens of meters. The regolith formed as a result of the crushing, intermixing, and sintering of lunar rocks under the impact of meteorites and micrometeorites. As a result of the solar wind, it is saturated with neutral gases. Particles of meteoritic matter are found in the regolith. It has been established from radioisotopes that some fragments on the surface of the regolith have been there tens or hundreds of millions of years.
Two types of rocks are found among the samples brought back to the earth: volcanic rocks (lava) and rocks formed by the crushing and melting of lunar formations under the impact of meteorites (glasses and breccias). Most of the volcanic rocks are similar to terrestrial basalts and contain plagioclases, pyroxenes, ilmenite, olivine, spinel, zircon, apatite, metallic iron, copper, and other minerals. All the lunar maria apparently consist of such rocks. Moreover, fragments of other rocks, which are similar to such terrestrial rocks as norites, anorthosites, and dacites, have been observed in the lunar soil, as well as KREEP—a rock that is rich in potassium, rare-earth elements, and phosphorus. Apparently these rocks represent fragments of the matter of the lunar continent. Luna 20 and Apollo 16, which soft-landed in continental regions, brought back anorthosite-type rocks from these sites. All types of rocks (see Table 1) were formed as a result of the prolonged evolution of melts in the lunar interior. Lunar rocks differ from terrestrial rocks in a number of respects: (1) they contain very little water and little potassium, sodium, and other volatile elements, and (2) there is a great deal of titanium and iron in some samples, but on the whole the moon is poor in siderophile elements. The age of these rocks, as determined from the relationships among the radioactive elements, is 3-4.5 billion years, which corresponds to the earliest periods of the earth’s development.
| Table 1. Principal varieties of lunar rocks (composition by percent) | |||||
|---|---|---|---|---|---|
| Mare basalt1 | Gabbro- anorthosite2 | Anorthosite3 | Norite, or nonmare basalt4 | Dacite5 | |
| SiO2 | 40.5 | 42.4 | 44.1 | 50 | 61 |
| Al2O3 | 9.7 | 20.2 | 35.5 | 20 | 12 |
| FeO | 19.0 | 6.4 | 0.2. | 7.7 | 10 |
| Ti02 | 11.4 | 0.4 | – | 1.3 | 1.2 |
| CaO | 9.6 | 18.6 | 19.7 | 11 | 6.3 |
| MgO | 8.0 | 12.2 | 0.1 | 8 | 6 |
| Na2O | 0.53 | 0.40 | 0.34 | 0.63 | 0.69 |
| K2O | 0.16 | 0.52 | – | 0.53 | 2.0 |
| 1 Apollo 11, average of four samples2 Luna 203 Apollo 15, no. 154154 Apollo 14, no. 143105 Apollo 12, no. 12013 | |||||
International legal problems. The principal legal problems of exploring the moon have been resolved by the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies (known as the Outer Space Treaty). But the significant achievements in the study of the moon raised the necessity of concluding a special international treaty that would regulate different aspects of the activities of various countries on the moon. The need for a treaty whose range of force would be limited exclusively to the moon is due to the special status of the moon, since the study of the moon is conducted directly by human beings. In June 1971 the USSR proposed a draft of an international treaty regarding the moon at the 26th session of the UN General Assembly; this draft was submitted for appropriate study to the UN Committee on the Use of Outer Space for Peaceful Purposes. The Soviet draft is intended to ensure that the moon is used exclusively for peaceful purposes.
In conducting scientific research on the moon, no state has the right to infringe on the interests of other states or interfere with similar research conducted by other states. By making specific the Outer Space Treaty, which prohibits the appropriation of celestial bodies, the Soviet draft treaty stipulates that the surface and interior of the moon cannot be owned by any state. The question of the responsibility of states for damages inflicted during exploration of the moon are also covered.
REFERENCES
Luna. Edited by A. V. Markov. Moscow, 1960.Atlas obratnoi storony Luny, parts 1-2. Moscow, 1960-67.
Novoe o Lune. Moscow-Leningrad, 1963.
Pervye panoramy lunnoi poverkhnosti, vols. 1-2. Moscow, 1967-69.
Vvedenie v fiziku Luny. Moscow, 1969.
Khabakov, A. V. Ob osnovnykh voprosakh istorii razvitiia poverkhnosti Luny. Moscow, 1949.
Problemy geologii Luny. Moscow, 1969.
Vinogradov, A., and S. Sokolov. “‘Lunokhod-2’: Programma vypolnena.” Pravda, Nov. 20, 1973.
Wilkins, H. P., and P. A. Moore. The Moon, 2nd ed. London, 1961.
Physics and Astronomy of the Moon. Edited by Z. Kopal. New York-London, 1962.
Callatay, V. de Atlas de la Lune. Paris, 1962.
Baldwin, R. B. The Measure of the Moon. Chicago, 1963.
Ranger VII Photographs of the Moon, parts 1-3. Washington, D.C., 1964-65.
Measure of the Moon. Edited by Z. Kopal and C. L. Goudas. Dodrecht—New York, 1967.
Alter, D. Lunar Atlas. New York, 1968.
A. A. MIKHAILOV and A. P. VINOGRADOV
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