Moon

Moon

I

INTRODUCTION

Moon, name given to the only natural satellite of Earth. The Moon is the second brightest object in Earth’s sky, after the Sun, and has accordingly been an object of wonder and speculation for people since earliest times. The natural satellites of the other planets in the solar system are also sometimes referred to as moons.

Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 12 percent of the light that falls on it. This reflectivity, called albedo, of 0.12 is similar to that of coal dust.

The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth. The Moon’s mass is only 1.2 percent of Earth’s mass. The average density of the Moon is only three-fifths that of Earth, and gravity at the lunar surface is only one-sixth as strong as gravity at sea level on Earth. The Moon moves in an elliptical (oval-shaped) orbit around Earth at an average distance of 384,403 km (238,857 mi) and at an average speed of 3,700 km/h (2,300 mph). It completes one revolution in 27 days 7 hours 43 minutes. For the Moon to go from one phase to the next similar phase—as seen from Earth—requires 29 days 12 hours 44 minutes. This period is called a lunar month. The Moon rotates once on its axis in the same period of time that it circles Earth, accounting for the fact that virtually the same portion of the Moon (the “near side”) is always turned toward Earth.

II

THE MOON SEEN FROM EARTH

As the Moon orbits Earth in a counterclockwise direction, Earth itself rotates counterclockwise (from west to east) on its axis and revolves around the Sun in a counterclockwise orbit. All of these motions combined determine when and how the Moon appears in the sky to an observer on Earth. Seen from a single spot on Earth, the Moon rises about 50 minutes later every day. Since the Moon has moved 13.8 degrees further in its orbit in 24 hours, the Earth has to turn an extra 13.8 degrees on its axis for the Moon to rise above the horizon again.

The Moon shows progressively different phases as it moves along its orbit around Earth. Half the Moon is always in sunlight, just as half of Earth has day while the other half has night. Thus, there is no permanent “dark side of the Moon,” which is sometimes confused with the Moon’s far side—the side that always faces away from Earth. The phases of the Moon depend on how much of the sunlit half can be seen at any one time. In the phase called the new moon, the near side is completely in shadow. About a week after a new moon, the Moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted near side; a week afterward, in its last quarter, the Moon appears as a half-circle again. The entire cycle is repeated each lunar month. The Moon is full when it is farther away from the Sun than Earth; it is new when it is closer. When it is more than half illuminated, it is said to be in gibbous phase. When it is less than half illuminated, it is said to be in crescent phase. The Moon is said to be waning as it progresses from full to new, and to be waxing as it proceeds from new to full.

The Moon is in the sky about 12 hours a day. At new moon it is in the sky during daylight hours, rising just after dawn. At full moon it is visible throughout the night, rising at sunset. The phases of the Moon match its position in the sky. New moon is noticeable when the Moon is close to the western horizon at sunset. The full moon occurs when the Moon is above the eastern horizon at sunset about 14 days later. The dark phase of the Moon occurs when the Moon is in the daytime sky with its shaded night side facing Earth. Its unseen presence can be revealed in a spectacular way if the dark Moon passes directly in front of the Sun. When this happens, the view of the Sun is blocked and the Moon’s shadow falls on a small region of the surface of the Earth, an event called a solar eclipse.

By a cosmic coincidence, the apparent sizes of the disk of the Moon and the disk of the Sun are approximately the same (within about 0.5 of a degree) when seen from Earth. If the Moon’s orbit lay exactly in the plane of Earth’s orbit around the Sun, a solar eclipse would occur somewhere on Earth every month at new moon. However, the Moon’s orbit is tilted 5.1 degrees with respect to the plane of Earth’s orbit around the Sun. As a result, solar eclipses occur only about 2 to 5 times a year. Partial eclipses, when the Moon only partially covers the disk of the Sun, happen more often than total eclipses.

Another type of eclipse results when Earth comes directly between the Sun and the Moon. Lunar eclipses happen at full moon about twice a year and are visible over large areas of Earth. The round shadow of Earth passes over the Moon, giving it a red or copper hue from sunlight filtered through Earth’s atmosphere.

At any one time, an observer on Earth can see only 50 percent of the Moon’s entire surface. However, an additional 9 percent can be seen from time to time around the edges because the viewing angle from Earth changes slightly as the Moon moves through its elliptical orbit. This slight relative motion is called libration.

III

SURFACE OF THE MOON

Early observers of the Moon believed that the dark regions on its face were oceans, giving rise to their name maria (Latin for “seas”). This term is still used today although these regions are now known to be completely dry. The brighter regions were held to be continents. Modern observation and exploration of the Moon has yielded far more comprehensive and specific knowledge.

The Moon has no movement of wind or water to alter its surface, yet it was geologically active in the past and is still not totally unchanging. Craters cover the surface, and meteors continue to create new craters. Micrometeorites also slowly erode surface features and alter the lunar soil. Billions of years ago volcanic eruptions sculpted large areas of the surface. Volcanic features such as maria, domes (low, rounded, circular hills), and rilles (channels or grooves) are still discernable. Small amounts of gas from deep in the Moon may still reach the surface. Scientists have also recently discovered possible evidence of ice in permanently shadowed areas of the surface. Such ice could have come from comet impacts.

A

Craters

The Moon’s surface is covered with craters overlain by a layer of soil called regolith. Nearly all the craters were formed by explosive impacts of high-velocity meteorites. Billions of years of this meteorite bombardment ground up the Moon’s surface rocks to produce the finely divided rock fragments that compose the regolith. Craters range in size from microscopic to the South Pole-Aitken Basin, which measures over 2,500 km (1560 mi) in diameter and would nearly span the continental United States. The highest mountains on the Moon, in the Leibnitz and Doerfel ranges near the south pole, make up the rim crest of the South Pole-Aitken Basin and have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. At full Moon long bright streaks that radiate from certain craters can be seen. These streaks are called ray systems. Ray systems are created when bright material ejected from the craters by meteorites splashes out onto the darker surrounding surface.

The biggest of the Moon’s craters were created by the impacts of large remnants from the formation of the planets billions of years ago when the young solar system still contained many such remnants. Astronomers, however, have directly observed meteorites forming small craters on the Moon’s surface. Seismometers operating on the lunar surface have also recorded signals indicating between 70 and 150 meteorite impacts per year, with projectile masses from 100 g to 1,000 kg (4 oz to 2,200 lb). Hence the Moon is still being bombarded by meteorites, although neither as often nor as violently as in the distant past.

B

Volcanic Features

Maria, domes, rilles, and a few craters display indisputable characteristics of volcanic origin. Maria are plains of dark-colored rock that cover approximately 40 percent of the Moon's visible hemisphere. The maria formed when molten rock erupted onto the surface and solidified between 3.16 billion and 3.96 billion years ago. This rock resembles terrestrial basalt, a volcanic rock type widely distributed on Earth, but the rock that formed the maria has a higher iron content and contains unusually large amounts of titanium. The largest of the maria is Oceanus Procellarum, an oval-shaped plain on the near side of the Moon 2,500 km by 1,500 km wide.

Photographs of the side of the Moon not visible from Earth have revealed a startling fact: The far side generally lacks the maria that are so conspicuous a feature of the visible side. This probably reflects the fact that the Moon’s crust is thicker on the far side than on the near side, and therefore the lavas that form the maria were more easily erupted through the thinner crust of the near side.

Rilles are of two basic types: sinuous and straight. Sinuous rilles are meandering channels that are probably lava drainage channels or collapsed lava tubes formed by large lava flows. Straight rilles are large shallow troughs caused by movement of the Moon’s crust; they may be up to a thousand kilometers long and several kilometers wide.

Domes are small rounded features that range from 8 to 16 km (5 to 10 mi) in diameter and from 60 to 90 m (200 to 300 ft) in height. Domes, thought to be small inactive volcanoes, often contain a small rimless pit on their tops.

Magnetic and other measurements indicate a current temperature at the Moon’s core as high as 1600°C (2900°F), above the melting point of most lunar rocks. Evidence from seismic recordings suggests that some regions near the lunar center may be liquid. However, no recent eruptions of liquid rock have been observed and the Moon evidently has had no volcanic activity on its surface over the last 1 billion years. At most, trapped gas from deep in the Moon may still reach the surface in some places.

Astronomers reported possible evidence of “out-gassing” on the surface of the Moon in the last 1 to 10 million years in a paper published in 2006. The unusually bright soil around a feature 3 km (2 mi) wide on the Moon’s equator indicates some process has turned over fresh regolith that has not had enough time to be “weathered” by solar wind and micrometeorites. Called Ina, the feature was first photographed from Apollo spacecraft orbiting the Moon in the 1970s, and was later examined by the Clementine probe. Gases from inside the Moon may have erupted on the surface, lifting and exposing fresh lunar soil. Scientists do not know the exact source and nature of the gases. At least three other lunar features that look similar to Ina have been identified. They may have been formed by bursts of gas, as well.

C

Ice

Temperatures on most of the Moon’s surface are too extreme for water or ice to exist, ranging from a maximum of 127°C (261°F) at lunar noon to a minimum of -173°C (-279°F) just before lunar dawn. Temperatures in permanently shadowed areas near the lunar poles, however, may consistently be as low as -220°C (-364°F). Comets and micrometeoroids that strike the Moon release gases that contain water. The gases would form an extremely thin atmosphere that would then migrate to the coldest regions of the poles and condense out, forming ice that combines with the lunar soil. In 1996 a team working with data gathered by the Clementine spacecraft announced that frozen water may exist in one of these shadowed areas near the Moon’s south pole. Clementine was a joint venture by the Department of Defense and the National Aeronautics and Space Administration (NASA). The spacecraft’s radar showed what may be an 8,000 sq km (3,000 sq mi) area covered with a mixture of dirt and ice crystals. Clementine was launched in 1994 and gathered data for four months.

NASA launched the Lunar Prospector spacecraft toward the Moon in 1998. Prospector returned data that appeared to confirm the Clementine discovery and suggested that a significant amount of water exists in the dark areas near the lunar poles in the form of ice crystals mixed with soil. The evidence was indirect, however, and consisted of finding elevated levels of hydrogen, a component of water, around the poles. Estimates of the possible amount of water on the Moon varied widely, from 10 million to 6 billion metric tons.

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

In 2003 researchers used the giant Arecibo Observatory radio telescope to bounce radar signals off the surface of craters at the Moon’s poles. The returned radar signal indicated that large, thick layers of ice were not present. The findings failed to rule out the existence of smaller amounts of ice at the lunar poles preserved in thin layers or as scattered ice crystals mixed with dust. The Arecibo Observatory conducted a higher resolution radar study of the lunar south pole in 2006 and found that similar radar signals came from both sunlit and shaded areas. The issue of ice at the lunar poles was not resolved, however. NASA’s Lunar Reconnaissance Orbiter (LRO), scheduled for launch in 2008, will carry a special satellite called LCROSS (Lunar CRater Observation and Sensing Satellite) that will look for evidence of water in the debris plume when the LRO’s booster stage crashes into Shackleton Crater at the south pole.

IV

ORIGIN OF THE MOON

Measuring the ages of lunar rocks has revealed that the Moon is about 4.6 billion years old, or about the same age as Earth and probably the rest of the solar system. Before the modern age of space exploration, scientists had three major models for the origin of the Moon. The fission from Earth model proposed that the young, molten Earth rotated so fast that it flung off some material that became the Moon. The formation in Earth orbit model claimed that the Moon formed independently, but close enough to Earth to orbit the planet. The formation far from Earth model proposed that the Moon formed independently in orbit around the Sun but was subsequently captured by Earth’s gravity when it passed close to the planet. None of these three models, however, is entirely consistent with current knowledge of the Moon. In 1975, having studied Moon rocks and close-up pictures of the Moon, scientists proposed what has come to be regarded as the most probable of the theories of formation: a giant, planetary impact.

The giant impact model proposes that early in Earth’s history, well over 4 billion years ago, Earth was struck by a large planet-sized body sometimes referred to as Theia. Early estimates for the size of this object were comparable to the size of Mars, but other research suggests that the object may have been more massive and that it struck Earth at a glancing angle. The catastrophic impact blasted portions of Earth and the impacting body into Earth’s orbit, where debris from the impact eventually coalesced to form the Moon. After years of research on lunar rocks during the 1970s and 1980s, this model became the most widely accepted one for the Moon’s origin.

The giant impact model seems to account for most of the available evidence: the similarity in composition between Earth and Moon indicated by analysis of lunar samples, the near-complete global melting of the Moon (and possibly Earth) in the distant past, and the simple fact that the other models are all inadequate to one degree or another. Research continues on the ramifications of such a violent lunar origin to the early history of Earth and the other planets.

Similar giant impacts may have affected the planets Mercury and Venus, but without forming moons–at least none that have survived. Mercury may have had most of its outer crust blasted away, leaving a dense iron core. Venus’s slow backward (retrograde) rotation may have been caused by one or more collisions with planet-sized bodies.

V

MAGNETIC PROPERTIES OF THE MOON

The Moon has no global magnetic field as does Earth. Some lunar rocks are weakly magnetic, indicating that they solidified in the presence of a magnetic field. The Moon has small, local magnetic fields that seem to be strongest in areas that are on opposite hemispheres from large basins. The origin of these local magnetic fields is unknown. Some scientists speculate that the magnetic fields were induced by the extreme shock pressures associated with the large asteroid collisions that created the basins. Others believe that the Moon originally had a global magnetic field generated by the movement of liquid metal in the core as on Earth. This global field shut down for some reason and only remnants of it exist in certain places on the lunar surface, preserved in material ejected by the asteroid collisions. The “fossil” magnetism found in some lunar rocks supports the former global field model, whereas the regional distribution of the magnetic surface anomalies tends to support the local field model. Regions of strong magnetic fields repel the charged particles that stream from the Sun in the solar wind. Scientists believe that interaction with the solar wind darkens the Moon, and that some lighter swirl-shaped regions of the Moon are protected by local magnetic fields.

VI

GRAVITATIONAL INFLUENCE OF THE MOON

The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. The Moon, being much nearer to the Earth than the Sun, is the principal cause of tides. Because the force of gravity decreases with distance, the Moon exerts a stronger gravitational pull on the side of the Earth that is closer to it and a weaker pull on the side farther from it. The Earth does not respond to this variation in strength because the planet is rigid—instead, it moves in response to the average of the Moon’s gravitational attraction. The world’s oceans, however, are liquid and can flow in response to the variation in the Moon’s pull. On the side of the Earth facing the Moon, the Moon’s stronger pull makes water flow toward it, causing a dome of water to rise on the Earth’s surface directly below the Moon. On the side of the Earth facing away from the Moon, the Moon’s pull on the oceans is weakest. The water’s inertia, or its tendency to keep traveling in the same direction, makes it want to fly off the Earth instead of rotate with the planet. The Moon’s weaker pull does not compensate as much for the water’s inertia on the far side, so another dome of water rises on this side of the Earth. The dome of water directly beneath the Moon is called direct tide, and the dome of water on the opposite side of the Earth is called opposite tide.

As the Earth rotates throughout the day, the domes of water remain aligned with the Moon and travel around the globe. When a dome of water passes a place on the Earth, that place experiences a rise in the level of the ocean water, known as high tide or high water. Between successive high tides the water level drops. The lowest water level reached between successive high tides is known as low tide or low water. Low and high tides alternate in a continuous cycle. The variations that naturally occur in the level between successive high tide and low tide are referred to as the range of tide. At most shores throughout the world, two high tides and two low tides occur every lunar day, the average length of a lunar day being 24 hours, 50 minutes, and 28 seconds. One of these high tides is caused by the direct-tide dome and the other by the opposite-tide dome. Two successive high tides or low tides are generally of about the same height.

VII

LUNAR EXPLORATION

Throughout the 19th and 20th centuries, visual exploration with powerful telescopes yielded fairly comprehensive knowledge of the geography of the visible side of the Moon. The hitherto unseen far side of the Moon was first revealed to the world in October 1959 through photographs made by the Soviet Luna 3 spacecraft. These photographs showed that the far side of the Moon is similar to the near side except for the absence of large maria. Craters are now known to cover the entire Moon. In 1964 and 1966 photographs from U.S. spacecraft—Ranger 7 through 9 and Lunar Orbiter 1 through 5—further supported these conclusions. The entire Moon has about 3 trillion craters larger than 1 m (3 ft) in diameter.

The successful landings of the robotic U.S. Surveyor series spacecraft and the USSR Luna series in the 1960s, and then the manned landings on the lunar surface as part of the U.S. Apollo program, made direct measurement of the physical and chemical properties of the lunar surface a reality (see Space Exploration). The Apollo astronauts collected rocks, took thousands of photographs, and set up instruments on the Moon that radioed information back to Earth even after the astronauts departed. These instruments measured temperature and gas pressure at the lunar surface; heat flow from the Moon’s interior; molecules and ions of hot gases, called the solar wind, that stream out from the atmosphere of the Sun; the Moon’s magnetic field and gravity; seismic vibrations of the lunar surface caused by landslides, meteorite impacts, and so-called moonquakes; and the precise distance between Earth and the Moon.

All six manned landings on the Moon—Apollo 11, 12, 14, 15, 16, and 17—returned samples of rock and soil to Earth. These samples weighed a total of 384 kg (847 lb). The astronauts explored increasingly wider areas on the Moon with each successive flight, culminating with the 35 km (22 mi) explored using a lunar roving vehicle by the Apollo 17 crew. This final mission included the only geologist ever to walk on the Moon, Harrison (Jack) Schmitt. Analysis of the data and rocks obtained by the lunar missions continues.

In 1994, the joint Defense Department/NASA spacecraft Clementine orbited the Moon for 71 days, mapping the color and precise altitude of the lunar surface. From Clementine data, astronomers obtained their first global look at the topography and mineralogy of the Moon, finding that the Moon’s crust is indeed made of a low-iron, low-density rock called anorthosite and mapping the large, ancient basins that make up the structural framework of the Moon. Clementine also discovered possible evidence of ice on the Moon in the permanently dark areas near the south pole.

NASA sent a spacecraft of its own, an orbiter called Lunar Prospector, to the Moon in 1998. Lunar Prospector orbited around the Moon’s north and south poles and returned data until July 1999. The spacecraft mapped the gravitational field of the Moon, determined the distribution of radioactive elements in its crust, and found additional evidence that could indicate the presence of ice at the lunar poles. Scientists used the spacecraft right up to its final moments. They ended Prospector’s mission by programming it to crash into the Moon’s surface and then observed the cloud of debris that rose from the impact.

In 2003 the European Space Agency (ESA) launched it first lunar probe, called SMART-1 (Small Missions for Advanced Research and Technology). The solar-powered orbiter used an innovative form of ion propulsion. It carried a number of instruments to study the chemical elements that make up the Moon’s surface. At the end of its mission in 2006, the probe was deliberately crashed into the lunar surface. Earth-based telescopes studied the composition of the debris thrown up by the impact.

An ambitious series of international Moon probes have been planned. China’s first Moon probe, the lunar orbiter Chang’e No. 1 (pronounced CHAHNG-UH), was scheduled to be launched in 2007. It is named for the fairy maiden Chang’e, who traveled to the Moon in Chinese mythology. Based on the design of the Dongfanghong-3 communications satellite, the Chinese probe will send back photos and data about the Moon. The Chinese are working with Russian experts to build a robot Moon lander for launch by 2010.

India’s first Moon mission, called Chandrayaan-1 (pronounced CHUN-dry-ahn, meaning “Moon craft” in Sanskrit), is scheduled for launch in 2008. The orbiting probe will carry European scientific instruments and two instruments provided by NASA, the M³ (Moon Mineralogy Mapper) and the Mini SAR (Mini Synthetic Aperture Radar). M³ (pronounced “em cube”) is a highly advanced imaging spectrometer designed to map the entire lunar surface to study its mineral composition. Mini SAR is an imaging radar device designed to map the lunar poles and look for water ice. It will see into permanently shadowed areas at the poles and can distinguish the radar signature of ice from surface roughness.

Japan planned a 2007 launch date for SELENE (SELenological and ENgineering Explorer). The orbiter will map the Moon and deploy a small satellite to study the gravitational field on the far side of the Moon. The orbiter’s propulsion unit will later separate and land on the lunar surface.

NASA has proposed a series of unmanned Moon probes over the next decade, leading to a manned landing by 2020. The Lunar Reconnaissance Orbiter is slated for launch in 2008, carrying six different instruments. Also on board will be a small, separate satellite called LCROSS (Lunar CRater Observation and Sensing Satellite). The main probe will map small areas of the Moon’s surface in high resolution. The upper stage of the probe’s booster rocket will impact Shackleton Crater at the south pole, sending up a plume of debris that could reveal water ice. The LCROSS satellite will study the plume for signs of ice then crash into the crater itself, creating a second impact for observers on Earth. Possible future missions may include sample returns from the lunar surface.

The first step toward a return to manned exploration of the Moon came in 2006 with NASA’s official announcement of the design and contractor for the Orion space capsule, part of the Constellation program of manned space flights. Plans call for a lunar landing in 2020, with an unmanned cargo mission in 2019. A crew of four will reach the Moon in an Orion capsule accompanied by a lunar lander. The four astronauts will land on the surface and spend about a week on the Moon. By 2024 a permanent Moon base may be established, probably near the lunar south pole. China has also announced plans for manned flights to the Moon for sometime after 2020.

Although much has been learned about the Moon in the past few decades, much still remains mysterious. Understanding the Moon and its history is important for two reasons. First, the Moon is a natural laboratory to study the geological processes—meteorite impacts, volcanism, and large-scale movements of the crust—that have shaped all of the rocky planets. Second, the Moon’s ancient surface retains a record of events in this part of the solar system that has been erased from the much more active, dynamic surface of Earth. The impact record, which has been almost entirely erased on Earth, is especially clear on the Moon, and may contain important clues to the history of life on Earth. Thus, the Moon serves as a touchstone, allowing us to better comprehend the complex stories of all the planets in our solar system.

Reviewed By:
Paul D. Spudis

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