Moon

Our solitary and prominent Moon orbits Earth at a mean distance of only 382,000 kilometers (236,840 miles). The nearest planet, Venus, is never closer than 40 million kilometers (25 million miles). The Moon's mass is just under one-eightieth that of Earth, its volume just over one-fiftieth; the difference mainly stems from the Moon lacking a large metallic iron core and therefore having a much lower overall density than Earth. Its low mass is responsible for the low surface gravity (one-sixth that at Earth's surface), popularly recognized in the jumping, bouncing gait of Apollo astronauts. The mass is much too low for the Moon to hold any significant atmosphere—it is essentially in a vacuum —or for its surface to have liquid water.

The surface area of the Moon is only about four times that of the land area of the United States. The Moon is not as large as any planet other than distant little Pluto but is of the same scale as the Galilean satellites of Jupiter. These moons are much smaller in comparison with the planet they orbit. Earth's Moon is very different in chemical composition and structure—and probably origin—from any other body in the solar system.

Orbit and Rotation

The 29.53-day orbit provides us with the lunar phases, as well as the occasional eclipses of the Sun and the more frequent eclipses of the Moon. The orbit is tilted only slightly (5.1°) from the plane of the ecliptic , but because Earth itself has a tilted axis of rotation (23.5°), the Moon's orbit is tilted substantially with respect to Earth's equator. The Moon's own axial rotation period is exactly the same as its orbital period, and so it shows almost the same face to Earth continuously. It is not exactly the same face because of the tilt of the Moon's rotational axis (1.5°) to its orbital plane around Earth, and the slight ellipticity of that orbit (the position of the observer on Earth also has a slight effect). Altogether, only 41 percent of the Moon's surface is permanently invisible to observers on Earth.

The gravitational pull of the Moon provides the twice-daily tides on Earth as Earth spins under the Moon. The Moon is gradually receding because of the tidal effects. As the Moon recedes, its angular momentum increases, compensated by a decrease in the spin rate of Earth. Thus, Earth's day is increasing in length; 600 million years ago it was only about eighteen hours long. The Moon stabilizes the tilt of Earth's own axis of rotation over long periods of time, and this has been important for stabilizing climate and thus life habitats.

The Exploration of the Moon

Even to the naked eye the Moon's face has darker and lighter patches. Italian mathematician and astronomer Galileo Galilei used a telescope in 1610 to discover its rugged, varied, and essentially unchanging features. He distinguished the brighter areas as higher and more rugged, the darker as lower, flatter, and smoother. He called the former "terra" (meaning "land"; pl. "terrae") and the latter "mare" (meaning "sea"; pl. "maria"), although that is not what they are.

For three centuries the Moon remained an object of astronomical study, with the collection of data about its shape, size, movements, and surface physical properties, as well as mapping. Not until the middle of the twentieth century were observations and a combination of natural and terrestrial analogs advanced enough that the volcanic origin of its dark plains and the impact origin of its craters and basins could be considered as settled. In the 1960s, a program of geological mapping, using techniques such as crater counting and overlapping relationships, confirmed and elucidated the nature of geological units and the order in which they were produced.

The study of the Moon reached peak activity in the space age, when spacecraft sent back detailed information from orbiters, hard-landers , and soft-landers (mainly from 1959 to 1970), and Apollo astronauts conducted experiments and made observations from equatorial orbit and at the surface (from 1968 to 1972). Six Apollo missions and three robotic samplereturn

vehicles collected samples of the Moon (from 1970 to 1976). Samples are particularly useful for understanding the processes that created the rocks and for the dating of events using radiogenic isotope techniques . Two flybys by the Galileo mission^* to Jupiter (in 1990 and 1992), the Clementine lunar polar orbiter (in 1994) and the Lunar Prospector polar orbiter (in 1998) have provided substantially more global imaging, topographic, chemical, and mineralogical data.

Global and Interior Characteristics

The Moon is nearly homogeneous, as shown by its motions in space, and by the fact that rocks near the surface are not much different in density from the Moon as a whole. Nonetheless, samples show that the Moon was thoroughly heated at its birth about 4.5 billion years ago, possibly to the point of total melting, and then quickly solidified to produce a comparatively thin (60 to 100 kilometers [37 to 62 miles]) crust of slightly lighter material. This structure was confirmed by seismic experiments performed on the early Apollo missions. There may be an iron core, but if so it is very tiny, and there is no significant magnetic field.

Samples show that the Moon is very depleted in volatile elements (those that form gases and low-temperature boiling-point liquids), to the extent that it lacks any water of its own at all, even bonded into rocks. Water delivered to the Moon by cometary impact might exist, frozen in crater floors near the poles. The Moon is very reduced chemically, such that iron metal exists, but rust (oxidized, ferric iron) does not. The Moon is very depleted in the siderophile elements ("iron-loving") that go with metallic iron into a core, except for the surface rubble to which such elements have been delivered by eons of meteorite impact.

The Uppermost Surface of the Moon

The Moon has been bombarded by meteorites ranging in size from numerous tiny dust particles to rare objects hundreds of kilometers in diameter. The surface is covered everywhere with a thin fragmental layer (known as soil, or "regolith") that consists mainly of ground-up and remelted lunar rocks, with an average grain size of less than 0.1 millimeters (0.004 inch). This soil contains pebbles, cobbles, and even boulders of lunar rocks. A small percentage of the regolith consists of the meteoritic material that did the bombarding. The regolith is about 5 meters (16.5 feet) thick on basalts that were poured out about 3 billion years ago, while older surfaces have even thicker regoliths. This regolith layer, exposed to cosmic radiation and the solar wind , contains materials, such as hydrogen, that do not reach the surface of Earth because of its protection by both a magnetic field and an atmosphere.

The Older Crust of the Moon

Much of the crust consists of material that formed within a few tens of millions of years of the Moon's origin, partly by the floating of light (in both density and color) feldspar minerals , which crystallized from a vast ocean of silicate magma. The magma formed because of the Moon's rapid formation, and because of the generation of radioactive heat, which was greater then than now. Continued melting and remelting added to the crust, and the final dregs of the crystallizing magma ocean, richer in those elements that do not easily fit into common crystallizing minerals (feldspar, pyroxene, and olivine), also ended up in the crust. The rocks from the dregs are commonly called "KREEP"-rich because they are richer in potassium (K), rare Earth elements (REE) such as lanthanum, and phosphorus (P) than are typical rocks. Most, though not all, of this crust was in place by 4.3 billion years ago.

At its birth and at about 3.9 billion years ago (what happened in the time between remains somewhat unknown) the Moon was subjected to enormous bombardments that created deep basins as well as numerous small craters, partly disrupting the crust. This crust is somewhat thinner on the front side (about 60 kilometers [37 miles]) than on the farside (about 100 kilometers [62 miles]).

The Younger Crust of the Moon

Impacts decreased substantially after 3.8 billion years ago, to a level close to that of today by about 3.2 billion years ago. The Moon's deep basins, partly filled with overlapping thin flows of mare basalt, formed from the melting of small amounts of the lunar interior. These basins (150 kilometers [93 miles] to perhaps 500 kilometers [310 miles] deep) are prominent as the dark plains—the maria—of the Moon and show many signs of volcanic flow. Some of the volcanic lava erupted as fiery fountains, forming heaps of glass spherules . These lavas comprise only about 1 percent of the crust, but as the latest, topmost rocks, least affected by impacts, they remain clearly visible. They are much less abundant on the lunar farside, and everywhere their formation had ceased by 2 billion years ago. The Moon is now magmatically dead, and its uppermost crust is being continually gardened and converted into regolith.

The Origin of the Moon

Earth and the Moon show an identical relationship of oxygen isotope ratios (oxygen being the most common element in both planets), a relationship that is different from all other measured solar system objects (including Mars) except yEH chondrites. This indicates that Earth and the Moon formed in the same part of the solar system and gives credence to ideas that the Moon formed from Earth materials.

The pre-Apollo ideas of either capture, fission from Earth (by rapid spinning), or formation together as a double planet are not consistent with what scientists now know from geological or sample studies, nor with the orbital and angular momentum constraints. Thus a new concept was developed in the 1980s: Earth collided during its growth with an approximately Mars-sized object, producing an Earth-orbiting disk of material that accumulated to form the Moon. This idea can account for many features, including the chemistry of the Moon, its magma ocean, and even the tilt of Earth's axis. It is compatible with concepts of how planets develop by accumulation of solid objects. One of the implications of this theory is that the Moon actually must have accumulated very rapidly, on the order of days to years, rather than older ideas of tens of millions of years, and this explains the early melting of the Moon.

see also Apollo (volume 3); Apollo Lunar Landing Sites (volume 3); Exploration Programs (volume 2); Galilei, Galileo (volume 2); Lunar Bases (volume 4); Lunar Outposts (volume 3); Lunar Rovers (volume 3); Nasa (volume 3); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2); Shoemaker, Eugene (volume 2).

Graham Ryder

Bibliography

Heiken, Grant H., David Vaniman, Bevan M. French, and Jack Schmidt, eds. The Lunar Sourcebook: A User's Guide to the Moon. Cambridge, UK: Cambridge University Press, 1991.

Ryder, Graham. "Apollo's Gift: The Moon." Astronomy 22, no. 7 (1994):40-45.

Spudis, Paul D. "An Argument for Human Exploration of the Moon and Mars."American Scientist 80, no. 3 (1992):269-277.

——. The Once and Future Moon. Washington, DC, and London: Smithsonian Institution Press, 1996.

——. "The Moon." In The New Solar System, eds. J. Kelly Beatty, Carolyn C. Peterson, and Andrew Chaikin. Cambridge, UK: Cambridge University Press, 1999.

Taylor, G. Jeffrey. "The Scientific Legacy of Apollo." Scientific American 271, no. 1(1994):26-33.

Wilhelms, Donald E. To a Rocky Moon: A Geologist's History of Lunar Exploration. Tucson: University of Arizona Press, 1993.

National Aeronautics and Space Administration See NASA (Volume 3).

^*The Galileo mission successfully used robots to explore the outer solar system. This mission used gravity assists from Venus and Earth to reach Jupiter, where it dropped a probe into the atmosphere and studied the planet for nearly seven years.