The Earth's natural satellite. The Moon is a solid body orbiting our planet and is by far the brightest object in the night sky (Fig. 1). Many nations including the United States, China, members of the European Space Agency, India, Japan and Russia have all sent spacecraft that have orbited or landed on the Moon. The Apollo missions undertaken by the American government in the late 1960s and early 1970s represent the only times humans have visited the Moon. The Apollo crews landed, walked and roved upon the lunar surface, bringing back 382 kg (842 lb) of Moon rocks for study on Earth. After a long hiatus, significant interest has resumed in humans returning to and perhaps eventually residing upon the Moon. See also: Astronautics; Lunar and planetary mining technology; Space probe
Although many of the Moon's properties are now well-understood, significant mysteries about its inner structure, history and ultimate origin remain (Table 1). Theories regarding the Moon's origin include independent condensation and then capture by the Earth; formation in the same cloud of preplanetary matter with the Earth; fission from the Earth; and formation after the impact of a Mars-sized body on the proto-Earth. Because many of the Moon's geologic processes stopped long ago, its surface preserves a record of very ancient events. However, because the Moon's rocks and soils have been affected by geochemical and impact processes, their origins are partly obscured. Accordingly, working out the Moon's early history continues to pose a fascinating puzzle for scientists.
Values and remarks
|Diameter (approximate)||3476 km (2160 mi)|
|Mass||1/81.301 Earth's mass, or 7.348 × 1022 kg (1.62 × 1022 lb)|
|Mean density||0.604 Earth's, or 3.34 g/cm3 (209 lb/ft3)|
|Mean surface gravity||0.165 Earth's, or 162 cm/s2 (5.3 ft/s2)|
|Surface escape velocity||0.213 Earth's, or 2.38 km/s (1.48 mi/s)|
|Atmosphere||Surface pressure 10-12 torr (1.3 × 10-10 Pa); hints of some charged dust particles and occasional venting of volatiles; tenuous sodium and potassium clouds observed.|
|Magnetic field||Dipole field less than –0.5 × 10-5 Earth's; remanent magnetism in rocks shows past field was much stronger.|
|Dielectric properties||Surface material has apparent dielectric constant of 2.8 or less; bulk apparent conductivity is 10-5 mho/m or less.|
|Natural radioactivity||Mainly due to solar- and cosmic ray-induced background (about 1 milliroentgen per hour for quiet Sun).|
|Seismic activity||Much lower than Earth's; deep moonquakes occur more frequently when the Moon is near perigee; subsurface layer evident.|
|Heat flow||3 × 10-2 W/m2 (Apollo 15 site)|
|Surface composition and properties||
Basic silicates, three sites (Table 4); some magnetic material present; soil grain size is 2–60 μm and 50% is less than 10 μm; soil-bearing strength 15 lb/in.2 (1 kg/cm2) at depth of 1–2 in. (a few centimeters).
|Rocks||All sizes up to tens of meters present, concentrated in strewn fields; rock samples from Mare Tranquillitatis include fine- and medium-grained igneous and breccia.|
|Surface temperature range||At equator 127°C (260°F) at noon; –193°C to –173°C (–315°F to –280°F) night minimum; 1 m (3 ft) below surface, 7°C (–45°F); at poles –173°C (–280°F)|
The apparent motions of the Moon, its waxing and waning, and the visible markings on its face are reflected in stories and legends from every early civilization. At the beginning of recorded history, it was already known that time could be reckoned by observing the position and phases of the Moon. Attempts to reconcile the repetitive but incommensurate motions of the Moon and Sun led to the construction of calendars in ancient Chinese and Mesopotamian societies as well as, a thousand years later, by the Maya. By about 300 BCE, the Babylonian astronomer-priests had accumulated long spans of observational data and so were able to predict eclipses (Table 2). See also: Calendar
|Prehistory||Markings and phases observed; legends created connecting Moon with silver, dark markings with rabbit (shape of maria) or with mud.||1960||Eastern far side photographed by Luna 3. Slower cooling of Tycho detected during lunar eclipse.|
|∼ 300 BCE||Apparent lunar motions recorded and forecast by Babylonians and Chaldeans.||1961||United States commitment to human lunar flight.|
|∼ 150 BCE||Phases and eclipses correctly explained, distance to Moon and Sun measured by Hipparchus.||1962||Earth-Moon mass ratio measured by Mariner 2.|
|∼ 150 CE||Ancient observations compiled and extended by C. Ptolemy.||1964||High-resolution pictures sent by Ranger 7. Surface temperatures during eclipse measured by Earth-based infrared scan.|
|∼ 700||Ephemeris refined by Arabs.||1965||Western far side photographed by Zond 3.|
|∼ 1600||Empirical laws of planetary motion derived by J. Kepler.||1966||Surface pictures produced by Luna 9 and Surveyor 1. Radiation dose at surface measured by Luna 9. Gamma radioactivity measured by Luna 10. High-resolution, broad-area photographs taken by Lunar Orbiter 1. Surface strength and density measurements made by Luna 13.|
|1620||Kepler's dream story, Somnium, uses correct description of lunar temperatures.||1967||Mare soil properties and chemistry measured by Surveyor 3, 5, and 6. Whole front face mapped by Lunar Orbiter 4; sites of special scientific interest examined by Lunar Orbiter 5. Particle-and-field environment in lunar orbit measured by Explorer 35.|
|1609||Lunar craters observed with telescopes by T. Harriot and Galileo.||1968||Highland soil and rock properties and chemistry measured by Surveyor 7. Mass concentrations at circular maria discovered.|
|1650||Moon mapped by J. Hevelius and G. Riccioli; features named by them in system still in use.||1968||Astronauts orbit Moon, return with photographs.|
|1667||Experiments by R. Hooke simulating cratering through impact and volcanism.||1969||Astronauts land and emplace instruments on Moon, return with lunar samples and photographs.|
|1687||Moon's motion ascribed to gravity by I. Newton.||1969–1972||Lunar seismic and laser retroreflector networks established. Heat flow measured at two sites. Remanent magnetism discovered in lunar rocks. Geologic traverses accomplished. Orbital surveys of natural gamma radioactivity, x-ray fluorescence, gravity, magnetic field, surface elevation, and subsurface electromagnetic properties made at low latitudes. Metric mapping photos obtained. Samples returned by both piloted (United States) and automated (Soviet) missions; sample analyses confirmed early heating and chemical differentiation of Moon, with surface rocks enriched in refractory elements and depleted in volatiles. Age dating of lunar rocks and soils showed that most of the Moon's activity (meteoritic, tectonic, volcanic) occurred more than 3 billion years ago.|
|1692||Empirical laws of lunar motion stated by J. D. Cassini.||1975||Giant-impact hypothesis for lunar origin advanced by W. K. Hartmann and D. R. Davis.|
|1700–1800||Lunar librations measured, lunar ephemeris computed using perturbation theory by T. Mayer. Secular changes computed by J. L. Lagrange and P. S. de Laplace. Theory of planetary evolution propounded by I. Kant and Laplace. Many lunar surface features described by J. H. Schroeter and other observers.||1982||Earth-based spectrometry reveals mineral variations over Moon's near side; central peaks of crater Copernicus found to be rich in olivine.|
|1800–1920||Lunar motion theory and observations further refined, leading to understanding of tidal interaction and irregularities in Earth's rotation rate. Photography, photometry and bolometry applied to description of lunar surface and environment. Lunar atmosphere proved absent. New disciplines of geology and evolution applied to Moon, providing impetus to theories of its origin.||1983||Antarctic meteorite, ALHA 81005, proved to have come from the Moon.|
|1924||Polarization measured by B. F. Lyot, showing surface to be composed of small particles.||1988–1992||Thin, extended atmosphere of sodium and potassium discovered. Earth-based multispectral observations yield compositional mapping of near side. Galileo images Moon during Earth flybys.|
|1927–1930||Lunar day, night, and eclipse temperatures measured by E. Pettit and S. B. Nicholson.||1994||Clementine maps entire Moon in 11 visible and near-infrared bands. Topography measured by laser altimetry.|
|1946||First radar return from Moon.||1998||Orbital geochemical and gravity mapping by Lunar Prospector spacecraft begins. Increased concentration of near-surface light elements observed at lunar poles, possibly indicating presence of hydrogen in cold-trapped water ice.|
|1950–1957||New photographic lunar atlases and geologic reasoning; renewed interest in theories of lunar origin by G. P. Kuiper, H. C. Urey, and E. M. Shoemaker. New methods (for example, isotope dating) applied to meteorites; concepts extended to planetology of Moon. Low subsurface temperatures confirmed by Earth-based microwave radiometry.||2004||Orbital imaging, mineral, chemical, and environmental particles and fields observation by SMART 1 spacecraft begins.|
|1959||Absence of lunar magnetic field (on sunlit side) shown by Luna 2.|
The Earth and Moon now make one revolution about their barycenter, or common center of mass (a point about 4670 km or 2900 mi from the Earth's center), in 27d 7h 43m 12s (Table 3). This so-called sidereal period is slowly lengthening, and the distance (now about 60.27 Earth radii) between the centers of mass is increasing because of tidal friction in the oceans of the Earth. The tidal bulges raised by the Moon are dragged eastward by the Earth's daily rotation. The displaced water masses exert a gravitational force on the Moon, with a component along its direction of motion, causing the Moon to spiral slowly outward. The Moon, through this same tidal friction, acts to slow the Earth's rotation, lengthening the day. Tidal effects on the Moon itself have caused its rotation to become synchronous with its orbital period, so that it always turns the same face toward the Earth. See also: Earth; Earth rotation and orbital motion; Earth tides; Orbital motion; Tide
|Sidereal period (true period of rotation and revolution)||(27.32166140 + 0.00000016T) ephemeris days, where T is in centuries from 1900.|
|Synodic period (new Moon to new Moon)||(29.5305882 + 0.00000016T) ephemeris days|
|Apogee||406,700 km or 252,700 mi (largest); 405,508 km or 251,971 mi (mean)|
|Perigee||356,400 km or 221,500 mi (smallest); 363,300 km or 225,744 mi (mean)|
|Period of rotation of perigee||8.8503 years direct ("direct" meaning that the motion of perigee is in the direction of Moon's motion about the Earth).|
|Period of regression of nodes||18.5995 years|
|Eccentricity of orbit||0.054900489 (mean)|
|Inclination of orbit to ecliptic||5°8'43" (oscillating ±9' with period of 173 days)|
|Inclination of orbit to Earth's equator||Maximum 28°35', minimum 18°21'|
|Inclination of lunar equator|
*In conventional coordinates with origin at the center of the Earth, rather than the Earth-Moon barycenter.
Tracing lunar motions backward in time is difficult, because small errors in the recent data propagate through the lengthy calculations, and because the Earth's own moment of inertia may not have been constant over geologic time. Nevertheless, the attempt is being made by using diverse data sources, such as the old Babylonian eclipse records and the growth rings of fossil shellfish. At its present rate of departure, the Moon would have been quite close to the Earth about 4.6 billion years ago, a time which other evidence suggests as the approximate epoch of the Earth's formation.
The Moon's present orbit (Fig. 2) is inclined about 5° to the plane of the ecliptic (Table 3). As a result of differential attraction by the Sun on the Earth-Moon system, the Moon's orbital plane rotates slowly relative to the ecliptic (the line of nodes regresses in an average period of 18.60 years) and the Moon's apogee and perigee rotate slowly in the plane of the orbit (the line of apsides advances in a period of 8.850 years). Looking down on the system from the north, the Moon moves counterclockwise. It travels along its orbit at an average speed of nearly 1 km/s (0.6 mi/s) or about 1 lunar diameter per hour; as seen from the Earth, its mean motion eastward among the stars is 13°11′ per day.
Due to the Earth's annual motion around the Sun, the direction of solar illumination changes about 1° per day, so the lunar phases do not repeat in the sidereal period given above but in the synodic period, which averages 29d 12h 44m and varies some 13 h because of the eccentricity of the Moon's orbit. See also: Earth rotation and orbital motion; Phase (astronomy)
When the lunar line of nodes (Fig. 2) coincides with the direction to the Sun, and the Moon happens to be near a node, eclipses can occur. Because of the 18.6-year regression of the nodes, groups of eclipses recur with this period. When it passes through the Earth's shadow in a lunar eclipse, the Moon remains dimly visible because of the reddish light scattered through the atmosphere around the limbs of the Earth. When the Moon passes between the Earth and Sun, the solar eclipse may be total or annular. As seen from Earth, the angular diameter of the Moon (31′) is almost the same as that of the Sun, but both apparent diameters vary because of the eccentricities of the orbits of Moon and Earth. Eclipses are annular when the Moon is near apogee and the Earth is near perihelion at the time of eclipse. A partial solar eclipse is seen from places on Earth that are not directly along the track of the Moon's shadow. See also: Eclipse
The Moon's polar axis is inclined slightly to the pole of the lunar orbit (Fig. 2) and rotates with the same 18.6-year period about the ecliptic pole. The rotation of the Moon about its polar axis is nearly uniform, but its orbital motion is not, owing to the finite eccentricity and Kepler's law of equal areas, so that the face of the Moon appears to swing east and west about 8° from its central position every month. This is the apparent libration in longitude. The Moon does rock to and fro in a very small oscillation about its mean rotation rate; this is called the physical libration. There is also a libration in latitude because of the inclination of the Moon's polar axis. The librations make it possible to see about 59% of the Moon's surface from the Earth.
The lunar ephemeris, derived from precise astronomical observations and refined through lengthy computations of the effects perturbing the movements of the Moon, has now reached a high degree of accuracy in forecasting lunar motions and events such as eclipses. Laser ranging to retroreflectors landed on the Moon, aided by radio ranging to spacecraft, provides measurements of Earth-Moon distances to a precision of the order of meters (Fig. 3). See also: Perturbation (astronomy)
The Moon's small size and low mean density (Table 1) result in surface gravity too low to hold a permanent atmosphere, and therefore it was to be expected that lunar surface characteristics would be very different from those of Earth. However, the bulk properties of the Moon are also quite different—the density alone is evidence of that—and the unraveling of the Moon's internal history and constitution is a great challenge to planetologists. See also: Earth
The Earth, with its dense metallic fluid core, convective mantle, strong and variable magnetic field with trapped radiation belts, widespread seismic tremors, volcanoes and folded mountain ranges, moving lithospheric plates, and highly differentiated radioactive rocks, is plainly a planet seething with inner activity. Is the Moon also an active, evolving world or is it something very different? The answer lies in a group of related experiments: seismic investigations, heat-flow measurements, surface magnetic and gravity profiles, determination of abundances and ages of the radioactive isotopes in lunar material, and comparison of the latter with those found in the Earth and meteorites. Present theories and experimental data yield the following clues to the problem. See also: Earth's core; Geology
1. The Moon is too small to have compressed its silicates into a metallic phase by gravity; therefore, if it has a dense core at all, the core should be of nickel-iron. But the low mass of the whole Moon does not permit a large core unless the outer layers are of very light material; available data suggest that the Moon's iron core may have a diameter of approximately 354 km (220 mi).
2. The Moon has no radiation belts, and behaves as a nonconductor in the presence of the interplanetary field. Moon rocks are magnetized, though there is now little or no general lunar magnetic field.Re-analyses of rocks collected during the Apollo missions and ongoing theoretical work suggest, however, that Earth’s gravity separated the Moon’s mantle from its liquid core early in lunar history. This situation could have created convection, powering a field-generating dynamo lasting up until 3.56 billion years ago.
3. The Moon's natural radioactivity from long-lived isotopes of potassium, thorium, and uranium, expected to provide internal heat sufficient for partial melting, was roughly measured from orbit by Luna 10. The component above the cosmic-ray-induced background radiation was found to be at most that of basic or ultrabasic earthly rock, rather than that of more highly radioactive, differentiated rocks such as granites. Apollo 11 and 12 rock samples confirmed this result; Apollo 15, 16, and 17 mapped lunar composition and radioactivity from orbit. X-ray experiments showed higher aluminum-silicon concentration ratios over highland areas and lower values over maria, while magnesium-silicon ratios showed a converse relationship—higher values over maria and lower values over highlands. See also: Cosmic rays; X-rays
4. The Moon is seismically much quieter than the Earth. Moonquakes are small, many of them originate deep in the interior, and activity is correlated with tidal stress: more quakes occur when the Moon is near perigee. See also: Earthquake; Seismology
When all of the Apollo observations are taken together, it is evident that the Moon was melted to an unknown depth and chemically differentiated about 4.5 billion years ago, leaving the highlands relatively rich in aluminum and an underlying mantle relatively rich in iron and magnesium, with all known lunar materials depleted in volatiles. The subsequent history of impacts and lava flooding includes further episodes of partial melting until about 3.9 billion years ago, with the final result being a thick, rigid crust with only minor evidence of recent basaltic extrusions. The temperature profile and physical properties of the Moon's deep interior are, despite the Apollo seismic and heat-flow data, under active debate.
Large-scale surface features
As can be seen from the Earth with the unaided eye, the Moon has two major types of surface: the dark, smooth maria and the lighter, rougher highlands (Fig. 4). Photography by spacecraft shows that, for some unknown reason, the Moon's far side consists mainly of highlands. Both maria and highlands are covered with craters of all sizes. Craters are more numerous in the highlands than in the maria, except on the steeper slopes, where downhill movement of material apparently tends to obliterate them. Numerous different types of craters can be recognized. Some of them appear very similar to the craters made by explosions on the Earth; they have raised rims, sometimes have central peaks, and are surrounded by fields of hummocky, blocky ejecta. Others are rimless and tend to occur in lines along cracks in the lunar surface. Some of the rimless craters, particularly those with dark halos, may be gas vents bringing dark pyroclastic material to the surface. Most prominent at full moon are the bright ray craters whose grayish ejecta appear to have traveled for hundreds of kilometers across the lunar surface. Observers have long recognized that some erosive process has been and is probably still active on the Moon. For example, when craters overlap so that their relative ages are evident, the younger ones are seen to have sharper outlines than the older ones. Bombardment of the airless Moon by meteoritic matter and solar particles, and extreme temperature cycling, are now considered the most likely erosive agents, but local internal activity is also a possibility. Rocks returned by the Apollo astronauts are covered with tiny glass-lined pits, confirming erosion by small high-speed particles.
The lunar mountains, though very high [8000 m (26,000 ft) or more], are not extremely steep, and lunar explorers see rolling rather than jagged scenery (Fig. 5). There are steep slopes (30–40°) on the inside walls and central peaks of recent craters, where the lunar material appears to be resting at its maximum angle of repose, and rocks can be seen to have rolled down to the crater bottoms.
Though widespread networks of cracks are visible, there is no evidence on the Moon of the great mountain-building processes seen on the Earth. There are some low domes suggestive of volcanic activity, but the higher mountains are all part of the gently rolling highlands or the vast circular structures surrounding major basins. One of these is the Mare Orientale (Fig. 6), a large concentric structure which is almost invisible from Earth because it lies just past the Moon's western limb; at favorable librations, parts of its basin and mountain ramparts can be seen. A great region of radial sculpture surrounds the Orientale basin, strongly suggesting a catastrophic origin, with huge masses of matter thrown outward from the center. However, the lowest parts of the concentric rings are flooded by dark mare material which displays a gentle appearance. Other basins, namely, Imbrium, Serenitatis, and Crisium, appear more fully flooded. These maria were created by giant impacts, followed by subsidence of the ejecta and (probably much later) upwelling of lava from inside the Moon. Examination of small variations in Lunar Orbiter motions has revealed that each of the great circular maria is the site of a positive gravity anomaly (excess mass). The old argument about impact versus volcanism as the primary agent in forming the lunar relief, reflected in lunar literature over the past 100 years, appears to be entering a new, more complicated phase with the confirmation of extensive flooding of impact craters by lava on the Moon's near side, while on the far side, where the crust is thicker, the great basins remain mostly empty.
In some of the Moon's mountainous regions bordering on the maria are found sinuous rilles (Fig. 7). These winding valleys, some of them known since the eighteenth century, have been shown from orbit to have an exquisite fineness of detail. Some of them originate in small circular pits and then wriggle delicately across the Moon's gentle slopes for hundreds of kilometers, detouring around even slight obstacles, before vanishing on the plains. Though their resemblance to meandering rivers is strong, the sinuous rilles have no tributaries or deltas. Though to form from lava spilling onto the lunar surface, they remain an active area of research.
Other strange large-scale features, observed by telescope and then revealed in more detail by spacecraft cameras, are the ghost craters, circular structures protruding slightly from the maria, and the low, ropy wrinkle ridges that stretch for hundreds of kilometers around some mare borders. Large, light-toned swirly features, with no topography but sometimes with magnetic anomalies, are unexplained (Fig. 8).
Small-scale surface features
Careful observations, some of them made decades before the beginning of space flight, revealed much about the fine-scale nature of the lunar surface. Since the smallest lunar feature telescopically observable from the Earth is some hundreds of meters in extent, methods other than direct visual observation had to be used. Photometry, polarimetry, and later radiometry and radar probing gave the early fine-scale data. Some results of these investigations suggested bizarre characteristics for the Moon. Nevertheless, many of their findings have now been confirmed by spacecraft. The Moon seems to be totally covered, to a depth of at least tens of meters, by a layer of rubble and soil with very peculiar optical and thermal properties. This layer is called the regolith. The observed optical and radio properties are as follows. See also: Photometry; Polarimetry; Radar; Radiometry
1. The Moon reflects only a small portion of the light incident on it (the average albedo of the maria is only 7%). See also: Albedo
2. The full moon is more than 10 times as bright as the half moon.
3. At full moon, the disk is almost equally bright all the way to the edge; that is, there is no “limb darkening” such as is observed for ordinary spheres, whether they be specular or diffuse reflectors.
4. Color variations are slight; the Moon is a uniform dark gray with a small yellowish cast. Some of the maria are a little redder, some a little bluer, and these differences do correlate with large-scale surface morphology, but the visible color differences are so slight that they are detectable only with special filters. Infrared spectral differences are more pronounced and have provided a method for mapping variations in the Moon's surface composition.
5. The Moon's polarization properties are those of a surface covered completely by small, opaque grains in the size range of a few micrometers.
6. The material at the lunar surface is an extremely good thermal insulator, better than the most porous terrestrial rocks. The cooling rate as measured by infrared observations during a lunar eclipse is strongly variable; the bright ray craters cool more slowly than their surroundings.
7. The Moon emits thermal radiation in the radio wavelength range; interpretations of this and the infrared data yield estimates of surface and shallow subsurface temperatures (Fig. 9).
8. At wavelengths in the meter range, the Moon appears smooth to radar, with a dielectric constant lower than that of most dry terrestrial rocks. To centimeter waves, the Moon appears rather rough, and at visible light wavelengths it is extremely rough (a conclusion from observations 1–5 above).
These observations all point to a highly porous or underdense structure for at least the top few millimeters of the lunar surface material. The so-called backscatter peak in the photometric function, which describes the sudden brightening near full Moon, is characteristic of surfaces with deep holes or with other roughness elements that are shadowed when the lighting is oblique. In addition to shadowing, light reflections within soil particles may contribute to the brightening.
Orbiter missions have made it clear that these strange electromagnetic properties are generic characteristics of the dark-gray, fine soil that appears to mantle the entire Moon, softening most surface contours and covering everything except occasional fields of rocks (Figs. 10). This soil, with a slightly cohesive character like that of damp sand and an average chemical composition similar to that of some basic silicates on the Earth, is a product of the radiation, meteoroid, and thermal environment at the lunar surface. A surface texture called patterned ground is common on the moderate slopes of the Moon. This widespread phenomenon is unexplained, though there are some similar surfaces developed on the Earth when unconsolidated rock, lava, or glacial ice moves downhill beneath an overburden. At many places on the Moon, there is unmistakable evidence of downward sliding or slumping of material and rolling rocks. There are also a few instances of apparent upwelling, as well as numerous “lakes” where material has collected in depressions.
Magnets on landed lunar probes have collected magnetic particles from the soil, demonstrating the presence of either meteoritic or native iron minerals at the sites examined. Meteoroid experiments using orbiting craft have showed about the same flux of small particles as is observed at the Earth, so that the lunar soil would be expected to contain a representative sample of meteoritic and possibly also cometary matter. Apollo results confirmed also indicated that glassy particles are abundant in and on the soil. Evidence of micro-meteoroid bombardment is seen in the many glass-lined microcraters found on lunar rocks. See also: Comet
Chemical, mineral, and isotopic analyses of minerals from the Apollo 11 site showed that mare rocks there are indeed of the basic igneous class and are very ancient (3–4 billion years). The Apollo 12 samples are significantly younger, suggesting that Mare Tranquillitatis and Oceanus Procellarum were formed during a long and complex lunar history. The Apollo 12 astronauts visited Surveyor 3 and brought back parts of that spacecraft to permit analysis of the effects of its 2½-year exposure on the surface of the Moon. The lunar rock and soil samples returned by the Apollo and Luna missions have continued to yield new information on the composition and history of the Moon (Table 4). Among the dominant characteristics of these rocks are enrichment in refractories, depletion in volatiles, much evidence of repeated breaking up and rewelding into breccias, and ages since solidification extending back from the mare flows of 3–4 billion years ago into the period of highland formation more than 4 billion years ago, but not as yet including the time of the Moon's original accretion. Soil characteristics, through the analysis of embedded solar-wind atoms, have also yielded information on the ancient history of the Sun. See also: Solar wind
Main sample properties
|Apollo 11||Mare basalts, differentiated from melt at depth 3.7 billion years ago. Some crystalline highland fragments in soils. Unexpected abundance of glass. Much evidence of impact shock and microcratering. No water or organic materials.||Study of seismic properties showed low background, much scattering, and low attenuation.|
|Apollo 12||Basalts 3.2 billion years old. One sample 4.0 billion years old includes granitic component. Some samples with high potassium, rare-earth elements, and phosphorus (KREEP) may be Copernicus crater ejecta.||Surveyor parts returned showed effects of solar and cosmic bombardment.|
|Apollo 13||Spacecraft failure—no samples.||Despite emergency, some lunar photos returned.|
|Luna 16||Basalt 3.4 billion years old, relatively high Al content.|
|Apollo 14||Shocked highland basalts, probably Imbrium ejecta, 3.95 billion years old, higher Al and lower Fe than mare materials.||Deep moonquakes.|
|Apollo 15||Highland anorthosites including one sample 4.1 billion years old, mare basalts similar to Apollo 11 samples.||Orbital remote sensing began mapping of surface compositions.|
|Luna 20||Possibly Crisium ejecta, 3.9 billion years old.|
|Apollo 16||Highland anorthosite breccias 3.9–4 billion years old, also possibly Imbrium ejecta.||Seismic network began recording locations of impacts and deep moonquakes; orbital compositional mapping extended.|
|Apollo 17||Variety of basalts and anorthosites 3.7–4 billion years old, possibly volcanic glass, few dunite fragments 4.48 billion years old, possibly surviving from before the great highland bombardment.||Orbital mapping, and study of sesimic particle-and-field, and subsurface electrical properties yielded comprehensive picture of Moon.|
|Luna 24||Very low titanium basalts from Mare Crisium. Sample includes rock 3.3 billion years old.|
In 1983 an Antarctic meteorite was found to resemble some of the lunar samples, and it was determined by analysis to have come from the Moon. Many more lunar and Martian meteorites are now known and have been added to the Apollo and Luna sample collections. See also: Meteorite
As the Apollo missions progressed, each new landing site was selected with the aim of elucidating more of the Moon's history. A main objective was to sample each of the geologic units mapped by remote observation, either by landing on it or by collecting materials naturally transported from it to the landing site. Although this process did result in collection of both mare and highland materials with a wide range of ages and chemical compositions, it did not result in a complete unraveling of the history of the Moon. Apparently, the great impacts of 3–4 billion years ago erased much of the previous record, resetting radioactive clocks and scrambling minerals of diverse origins into the complicated soils and breccias found today. See also: Space flight
Though the Moon may at one time have contained appreciable quantities of the volatile elements and compounds (for example, hydrogen, helium, argon, water, sulfur, and carbon compounds) found in meteorites and on the Earth, its high daytime surface temperature and low gravity would cause rapid escape of the lighter elements. Solar ultraviolet and x-ray irradiation would tend to break down volatile compounds at the surface, and solar charged-particle bombardment would ionize and sweep away even the heavier gas species.
Visual observations from the Earth, looking for a twilight glow of the lunar atmosphere just past the terminator on the Moon, and watching radio-star occultations have all been negative, setting an upper limit of 10−12 times the Earth's sea-level atmospheric density for any lunar gas envelope. However, subsequent ground-based spectroscopic observations have revealed a tenuous cloud of sodium and potassium atoms extending hundreds of kilometers above the lunar surface. Except for this cloud, either the lunar volatile compounds have vanished into space or they are trapped beneath the surface. A slight, transient atmosphere does exist on the night side of the Moon as a result of the trapping and release of gas molecules at the very low temperatures prevailing there. See also: Atmosphere
The samples returned by Apollo are enriched in refractory elements, depleted in volatiles, and impregnated with hydrogen, helium, and other gases from the solar wind. Very little, if any water appears to have ever been present at the Apollo sites, and carbonaceous materials were present, if at all, only in very small amounts.
Enough is known about the Moon to show that it is a huge storehouse of metals, oxygen (bound into silicates), and other materials potentially available for future human use in space. Because of the Moon's weak gravity, lunar materials could be placed into orbit at less than one-twentieth of the energy cost for delivering them from Earth. An objective of numerous ongoing and planned lunar missions is to augment lunar scientific knowledge and thus add to understanding the possible uses of the natural resources of the Moon. See also: Gravity