The 29.53-day synodic period* provides people on Earth 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 axial rotation period is exactly the same as its sidereal period* , so the Moon 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% 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 since Earth rotates on its axis much faster than the Moon orbits Earth. The Moon is gradually receding because of the tidal effects. As the Moon recedes, its angular momentum* increases, which is compensated for by a decrease in the spin rate of Earth. Thus, Earth's day is increasing in length; where six hundred 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.

Even to the naked eye the Moon's face has darker and lighter patches. Italian mathematician and astronomer Galileo Galilei (1564–1642) 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”; plural, “terrae”) and the latter “mare” (meaning “sea”; plural, “maria”), although that is not what they are in reality.

For three hundred more years 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. Observations, along with a combination of natural and terrestrial analogs, were not advanced enough technologically until the middle part of the twentieth century 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 at the beginning of 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 lunar equatorial orbit* and at the surface (from 1968 to 1972). Six U.S.

Apollo missions and three Soviet robotic sample-return 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* .

Then, in the last decade of the twentieth century and the first decade of the twenty-first, a variety of missions to the Moon added substantially to humanity's knowledge base in regards to lunar imaging, topography, the lunar gravitational field, and the chemical and mineralogical makeup of the Moon. Those missions included two flybys* by the U.S. Galileo mission to Jupiter in 1990 and 1992; along with several orbital missions to the Moon: the U.S. Clementine lunar polar orbiter in 1994, the U.S. Lunar Prospector polar orbiter in 1998, the European Space Agency (ESA) Small Missions for Advanced Research in Technology (SMART 1) lunar orbiter in 2004, the Japanese SELenological and ENgineering Explorer (SELENE) and Chinese Chang'e 1 lunar orbiters in 2007, the Indian Chandrayaan-1 lunar orbiter (Chandrayaan translates as “moon vehicle” in Sanskrit) in 2008, and the U.S. Lunar Reconnaissance Orbiter (LRO) polar orbiter in 2008. This last spacecraft—the LRO—was launched from Earth in tandem with the Lunar CRater Observation and Sensing Satellite (LCROSS). The LCROSS spacecraft was designed to gather data about the presence of water ice on the Moon by crashing a 2000 kilogram upper stage of the rocket into the Moon's surface. Detection devices on the rocket's lower stage were used to analyze the plume of materials ejected as a result of the impact. The results of this experiment radically changed scientists' understanding of the presence of water on the Moon (see below). Two later Moon projects have been a second Chang'e orbiter, Chang'e 2, launched on October 1, 2010, and the U.S. Gravity Recovery And Interior Laboratory-A (GRAIL), launched on September 10, 2011. The purpose of the GRAIL mission is to use gravitational technology to map the Moon's interior. A number of additional lunar explorations are either under development or in the planning stages. These include the Chinese Chang'e 3 orbiter, scheduled for launch in 2013; the U.S. Lunar Atmosphere and Dust Environment Explorer (LADEE), scheduled for January 15, 2013; two private projects, the GLXP (Google Lunar X Prize) Astrobotic Technology spacecraft (2013), and the GLXP Moon Express (2013); a second Chandrayaan orbiter (2014); the British MoonLITE and MoonRaker spacecrafts (2014); and the Russian Luna-Glob-1 and Luna-Glob-2 spacecrafts (2014 and 2015). The only manned trips to the Moon under development are those in the privately-operated Deep Space Expedition Alpha (DSE-Alpha) program. This program intends to use Russian Soyuz capsules to provide private trips into space and around the Moon beginning in 2015.

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

BASIC DATA ABOUT THE MOON

thoroughly heated at its birth about 4.6 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 around the Moon.

Samples show that the Moon is very depleted in volatile elements (those that form gases and low-temperature boiling-point liquids). To a great extent the Moon is 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.

One of the most intriguing questions in lunar studies has long concerned the possible existence of water on the Moon. Schemes to one day establish a human colony on Earth's satellite depend to a large extent on the possibility that this essential compound is present to at least some extent on the Moon. Until the early twenty first century, however, the conventional view among scientists was that water was absent from the Moon or, at best, present in vanishingly small amounts. Recent research, however, has produced some astonishing contradictions to that belief. The first of these breakthroughs occurred in 2009 when three spacecrafts, India's Chandrayaan-1 lunar orbiter and NASA's

This is a false-color composite image of the Moon, photographed through three color filters by the Galileo spacecraft. The colors aid in the interpretation of the satellite's surface soil composition: red areas typically correspond to lunar highlands; orange to blue shades suggest ancient volcanic lava flow or lunar sea; and purple sections indicate pyroclastic deposits. Courtesy of NASA/JPL/Caltech.

Cassini spacecraft and Deep Impact probe all detected spectra for water in light reflected from the Moon's surface. The discovery was made not only at the poles, where scientists had long suspected that water might be present, but across all other portions of the satellite as well. These initial studies were unable to detect the precise physical and chemical form in which water exists, but there was no longer any question that it was present on the Moon.

Confirmation of this discovery became available only a month later when the upper portion of the U.S. LCROSS spacecraft was intentionally crashed onto the Moon's surface. The plume produced by this impact provided further incontrovertible evidence of the presence of water on the Moon's surface. Perhaps the most remarkable point about this discovery was the quantity of water detected, more than 340 pounds (about 40 gallons) from a hole less than 100 feet across. According to this result, about 5.6 percent of the Moon's surface at the point of impact consists of liquid or solid water. LCROSS showed that, not only is water present on the Moon, but it is present in quantities far greater than had ever been appreciated. According to one estimate, more than 600 million tons of water may be hidden just below the Moon's surface. A somewhat different, but perhaps even more significant, discovery about lunar water

was announced in May 2011. Researchers at the Carnegie Institution of Washington reported that they had found water in samples of volcanic rock returned during the Apollo 17 flight of 1972. The water was apparently trapped in rocks formed during the Moon's formation about 3.7 million years ago. The interesting feature of the discovery was that the amount of water contained in the rocks was more than a hundred times as great as that previously estimated, meaning that the Moon's surface may actually hold a treasure trove of that (for humans) most precious of resources.

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 millimeter (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 averages about five meters (16 feet) thick and lies atop basalts* that were poured out about three billion years ago; 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.

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 near side (about 60 kilometers [40 miles]) than on the far side (about 100 kilometers [60 miles]).

Astronaut and geologist Harrison H. Schmitt working at the Taurus-Littrow landing site, where he first spotted orange soil. The lunar surface is covered everywhere with a thin fragmental layer (“regolith”) that consists mainly of ground-up and remelted rocks. NASA.

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 [90 miles] to perhaps 500 kilometers [300 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% 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 far side, and everywhere their formation had ceased by two billion years ago. The Moon is now magmatically dead, and its uppermost crust is being continually pulverized and converted into regolith.

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 for a special class of meteorites. 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 called Theia, 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. The discovery of water on the Moon requires a modification of that theory, however, which was originally based on the assumption that the Moon contains no water. One variation on the theory has been proposed by the same researchers who found water encased in volcanic rock in 2011. These researchers suggest that the formation of the Moon was accompanied by a massive shower of comets striking the new satellite. The comets provided a “snowstorm” of frozen water which became incorporated into the soil of the newly-forming body. More detailed study of additional Moon rocks will help determine the validity of this hypothesis.