5. The Moon: stepping stone to the planets

Basic data

Physical properties of the Moon*

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Mass 7.348 x 1022 kilograms = 0,0012 ME
Mean radius 1.738 x 106 meters = 0.2725 RE
Mean mass density 3,344 kilograms per cubic meter
Rotation period 27.322 days = sidereal month
Orbital period, the sidereal month 27.322 days = fixed star to fixed star
Synodic Month 29.53 days = new Moon to new Moon
Mean distance from Earth 3.844 x 108 meters
Increase in mean distance 0.0382 0.0007 meters per year
Mean orbital speed 1,023 meters per second
Angular radius at mean distance (geocentric) 15 minutes 32.6 seconds of arc
Angular radius at mean distance (topocentric) 15 minutes 48.3 seconds of arc
Age 4.55 x 109 years
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* Here ME and RE respectively denote the mass and radius of the Earth.

Eclipses

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Once or twice in a typical year, the Moon's orbital motion carries it through the Earth's shadow. This is an eclipse of the Moon, and it can be seen from half of the Earth. There are two regions in the Earth's shadow at the time of a lunar eclipse: the umbral region where there is no direct sunlight, and the penumbral region where the Sun's light is partially shadowed. The umbral shadow is darker, and it is in the shape of a narrow cone pointing away from the Earth. The full Moon turns a deep red when in the umbral shadow of the Earth.

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The outer atmosphere of the Sun, known as the corona, becomes momentarily visible to the unaided eye when the Sun's visible disk is bloked out by the Moon and it becomes dark during the day. The corona is then seen at the limb, or apparent edge, of the Sun, against the blackened sky as a faint, shimmering halo of pearl-white light. But be careful if you go watch an eclipse, for the light of the corona is still very hazardous to human eyes and should not be viewed directly. The million-degree corona can be seen all across the Sun's disk, and at any time, when viewing the Sun in X-rays with telescopes aboard satellites such as Yohkoh.

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Since the Moon and the Earth move along different orbits whose planes are inclined to each other, a total eclipse of the Sun does not happen very often. The Moon only passes between the Earth and the Sun about three times every decade on average. Even then, a total eclipse occurs along a relatively narrow region of the Earth's surface, where the tip of the Moon's shadow touches the Earth. At other nearby places on the Earth, the Sun will be partially eclipsed, and at more remote locations you cannot see any eclipse of the Sun.

The Moon's face

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When a full Moon rises or sets, it is a captivating sight. It looks huge, dwarfing everything in the foreground. But appearances can be deceiving. The Moon is no bigger when it is close to the horizon than when it is high in the sky. Its changing size is an illusion caused by comparing the Moon to other objects when it is viewed along the ground.

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This so-called Moon illusion arises from the way that the brain deals with apparent distance, not size. When people view the Moon near the horizon, there are large foreground objects, such as trees, buildings and hills, for comparison, so the Moon looks very far away and huge. When the Moon is overhead, alone in an otherwise empty sky, there are no other objects to gauge its distance; the Moon then appears to be closer and we think it is smaller than at the horizon.

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The Moon is the only planetary body that can be distinguished with the unaided eye as a globe, and even without a telescope you can tell that its surface is not uniform. Its face contains large, irregular features of light and dark material, familiarly known as "The Man in the Moon".

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The Moon's rough terrain is mostly confined to the brighter regions that Galileo called terrrae, Latin for "lands"; they are now known as the highlands because they are higher than the dark regions. Galileo also discovered that the dark patches are smooth and level, resembling seas seen from a distance. He called them maria, the Latin word for "seas"; mare, pronounced "MAHrey", is the singular for "sea". We now know there is no water on the Moon there. The dark maria cover about 17 percent of the lunar surface. When spacecraft were sent past the Moon to look at its averted face, they found that the far side contains very few maria.

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Chemical examination of rock samples returned from the Moon has shown that the maria are ancient volcanic outflows composed of dark lava. This material flowed out from inside the Moon to fill large impact basins that were formed at about the same time as the lunar highlands. One of them, the Imbrium Basin that contains Mare Imbrium, now forms the "eyesocket" in the face of the "Man on the Moon"; it has a diameter of 1.5 million meters.

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Craters form one of the most striking features of the Moon's landscape. The word crater is derived from the Greek word for "cup or bowl", and it is a good description of the bowl-shaped depressions. They are just beyond the limit of visibility with the unaided eye, but a pair of binoculars will reveal a few of the larger ones. When seen through a telescope, the bright highlands are resolved into an enormous number of overlapping craters that have been visible to generations of telescopic observers.

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At around the time of full Moon, a pair of binoculars will also show bright streaks that radiate from several craters like the spokes of a wheel. These are the lunar rays, and they were produced by the debris of crater-formation. Some of the rays go more than one-quarter of the way around the Moon.

Expeditions to the Moon

Race to the Moon

The Space Age began on 4 October 1957, when the Soviet Union launched the first artificial Earth satellite, Prosteyshiy Sputnik, the simplest satellite. Two years later the Soviets launched their Luna 3 probe that was sent around the Moon, taking the first pictures of the normally invisible far side. And on 12 April 1961, cosmonaut Yuri A. Gagarin became the first human in space, orbiting the Earth in the Vostok 1 capsule. Soviet officials used these accomplishments in a Cold War with the United States, citing them as evidence that communism is a superior form of social and economic organization.

Stimulated by the worldwide excitement generated by the first human fight in space, the brave, visionary young President, John F. Kennedy, decided that the United States had to surpass the Soviet Union in some dramatic way in space. After expert advice, he concluded that there was a good chance of beating the Soviets to the first manned landing on the Moon. On 25 May 1961, just six weeks after the Gagarin flight, Kennedy delivered his now-famous address to a joint session of Congress, including the declaration: "I believe that this nation should commit itself to achieving the goal, before the decade is out, of landing a man on the Moon and returning him safely to Earth". The president's call to action struck a responsive chord in the American public and galvanized their space program.

The Apollo program

The Apollo spacecraft was designed to carry three men into orbit around the Moon. A small, Spartan landing craft, the Lunar Excursion Module or LEM for short, would ferry two of the crewmen from lunar obit to the Moon's surface and then back to the mother ship, while the third astronaut remained orbiting the Moon in the larger Command Service Module.

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On 21 December 1968 three Apollo 8 astronauts became the first humans to break free of the Earth's gravity. Although the crew would only orbit the Moon and not land on it, the unprecedented voyage provided the first sight of the Earth seen from afar - a radiant blue-and-white sphere rising beyond the battered face of the Moon in the dark void of space. We then saw our home world in a new perspective, beautiful and vulnerable, a tiny, fragile oasis shimmering all alone in the vast, deep chill of outer space. The sheer isolation of the Earth became plain to every person on the planet. It stimulated a world-wide awareness of the Earth as a unique and vulnerable place, fostering the ecology movement and helping us to get a better feeling for planet's place in our lives and the Universe.

On 20 July 1969, the spindly-legged, Lunar Module Eagle carried two Apollo 11 astronauts to the lunar surface. While an estimated half-billion people watched, Neil A. Armstrong took the controls to avoid a hazardous crater, and radioed the first words from another world "Houston, Tranquillity Base here. The Eagle has landed". It was enough to take your breath away.

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In all, twelve humans have walked on the lunar surface, to gather samples, take photographs and make other scientific measurements. All of the landing sites were on the near side and close to the lunar equator because these were the only places the astronauts could go safely. Direct radio contact from Earth would be lost if they landed on the far side of the Moon. Sites near the equator were chosen to always be able to get astronauts back from the lunar surface quickly in case something bad happened down there. A landing near the edge or limb of the Moon, as viewed from Earth, was ruled out if the spacecraft was to return to Earth in daylight. Within these constraints, the landing sites were chosen to provide samples of wide variety of terrain, from the smooth maria to the heavily cratered highlands.

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The astronauts' cameras recorded an eerie wasteland below a blackened sky, battered and scarred with craters of all sizes and covered with dust. It clung to the astronauts' clothing and equipment and showed the sharp outline of their footprints; but there were no clouds of dust above the airless surface. Walking on the lunar surface was like walking on plowed soil or wet sand, and most of the finer dust had evidently been plowed down into the Moon by the churning of the meteorites.

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Armstrong and Aldrin never strayed more than a hundred meters from their lander, like a timid child testing the water when entering a lake or sea for the first time. The astronauts of the next two missions (Apollo 12 and 14), had greater confidence and took longer moonwalks. During the last three missions (Apollo 15, 16 and 17) astronauts roamed as far as 7 thousand meters from the landing site, visiting some of the most spectacular places on the Moon in a battery-powered car called the Lunar Rover.

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The astronauts left behind the Apollo Lunar Surface Experiments Package, abbreviated ALSEP. This nuclear-powered array of instruments included seismometers to monitor vibrations of moonquakes and meteorite impact, magnetometers to measure possible magnetic fields, and other instruments to analyze gases and charged particles streaming from the Sun to the Moon. The astronauts also brought lunar soil and rocks back home with them, altogether 382 kilograms and not an ounce of cheese.

Return to the Moon

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After the Apollo missions, no one had even a single glimpse of the Moon's far side for nearly two decades, and then it was obtained by the Galileo spacecraft on its way to explore Jupiter's realm. In order to reach the giant planet, Galileo pumped up its orbit and gained speed by swinging past the Earth, once on 8 December 1990, just 14 months after launch, and again on 8 December 1992, passing by the Moon in the process. It obtained images of the lunar limb and far side from vantage points not previously obtained. For instance, the Sun illuminated the western limb of the Moon during the 1990 Galileo flyby, while sunlight brightened the opposite eastern limb during all of the Apollo missions.

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Composites of Galileo images taken in three colors, violet, red and near infrared, have been used to depict compositional variations of the lunar surface. They have been calibrated by Apollo sample returns that specify the chemistry at specific sites on the near side of the Moon. Some mare basalts are rich in titanium, while many others are relatively low in titanium but rich in iron and magnesium. The heavily cratered highland are typically poor in titanium, iron and magnesium.

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In early 1994, the United States Department of Defense placed the small Clementine spacecraft in orbit about the Moon. Like its namesake, the spacecraft was "lost and gone forever" after orbiting the Moon for two months, but not without first chalking up an impressive list of accomplishments. Unlike the Apollo Command Modules, that circled the Moon in low, near equatorial orbits, Clementine orbited across the lunar poles, permitting a global perspective as different regions rotated into view. The surface composition and topography of the entire satellite were mapped in unprecedented detail, the South Pole - Aitken basin was completely mapped with high resolution for the first time, and possible deposits of water ice deposits were found in the cold permanently dark places within the poles.

Inside the Moon

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Moonquakes have been recorded by sensitive seismometers left on the lunar surface, and used to infer the internal structure of the Moon. The Moon is slightly asymmetrical in bulk form, with a thicker crust on the far side. Most of the volcanic maria occur on the near side where the crust is thinner. On average, the lunar crust is about 70 thousand meters thick. It is only a few tens of thousands of meters thick beneath the mare basins, but over 100 thousand meters thick in the highlands and the far side of the Moon. As a result, the Moon's center of mass is offset from its geometric center by about 2 thousand meters in the direction of the Earth. Data from Lunar Prospector obtained in 1998-1999, indicate that the Moon's iron-rich core has a radius of 350 thousands meters, or 20 percent of the satellite's radius, and only about 2 percent of the body's mass.

The lunar surface

Rocks from the Apollo missions

During the Apollo landings from 1969 to 1972 a dozen people roamed the Moon taking hundreds of rock samples, placing them in labeled bags, and returning 382 kilograms of Moon rocks to Earth in sealed containers. These specimens from another world have permitted scientists to decipher the composition of the lunar crust, and to reconstruct our satellite's history.

None of the rocks brought back from the Moon contain any moisture or hydrated minerals, and they show no signs of having been exposed to water. The oxygen that is on the Earth in rocks and water forms only rocks on the Moon. So, there is no water in the Moon rocks, and there hasn't been any for billions of years. The Moon is lifeless, as we might expect from the lack of water. Extensive testing revealed no evidence for life, past or present, among the lunar samples. They contain no living organisms, fossils or native organic compounds. Thus, the Moon is a desolate place, barren of life.

Scanning the surface

The Apollo rock and soil samples came from only six sites on the near side, chosen mainly to be safe and easy to get to. A global view of the Moon's surface composition therefore had to wait until the Clementine spacecraft surveyed the unexplored regions on both the near and far sides.

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The Clementine global data was used to map the abundance and distribution of iron on the Moon, showing that the dark, near-side maria consist of iron-rich lava, containing up to 14 percent iron by weight. In contrast, iron is practically absent in the near-side highland crust and across vast tracts of the far side, at about 3 percent iron by weight. These regions of very low iron content are dominated by aluminum-rich anorthosite.

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Until Clementine, we also had no global map of the topography of the Moon. The laser altimeter on the spacecraft fired pulses of light at the Moon once every second and timed how long it took for the light beam to travel down to the lunar surface and back again. This enabled scientists to determine the distance to the surface, over and over again, with an accuracy of 50 meters. When these distances were combined with knowledge of the spacecraft orbit, maps of the elevation, or topography, of the entire lunar surface were obtained.

Possible water ice at the lunar poles

Bright radar echoes, returned to Clementine from the south pole of the Moon, suggested that this region may contain radar-reflective water ice. Lunar Prospector then strengthened the possibility of water ice at the south pole, and also discovered what appears to be additional ice near the north pole. During its passes over the poles, an instrument on Lunar Prospector detected substantial quantities of hydrogen, which mission scientists attributed to water ice found in permanently shaded areas near both poles. They estimate that there could be as much as 6 billion tons, or 6 x 1012 kilograms, of water ice located in the polar regions.

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Since the Moon's rotation axis is orientated nearly perpendicular to the ecliptic plane, the lunar poles are never tilted toward the Sun by more than a very small amount. This means that the bottoms of craters at the poles are in constant shadow and in a perpetual deep freeze of 50 to 70 degrees kelvin. Any ice deposited in these frozen reservoirs would be preserved indefinitely in the eternal dark and cold.

The magnetized Moon

The Moon has no overall dipolar magnetic field, at least none that is strong enough to be detected. Its magnetic moment is at least 10 million times weaker than the Earth's. Yet, some of the lunar rocks returned to Earth are magnetized. They have survived since the time that molten rocks covered the Moon and solidified 3 to 4 billion years ago, preserving fossilized remnants of ancient magnetic fields.

The Moon's history

The age of the oldest rocks - Moon, Earth and meteorites

The time at which different features on the Moon originated can be determined from rocks returned from them. These relics have remained unaffected by the erosion that removed the primordial record from most terrestrial rocks. The ages of the lunar rocks can be determined by examining unstable radioactive elements and their stable decay products.

The daughter isotopes must be trapped in the rock and not escape or the estimated age will be too short. In fact, the daughters can escape quite easily when the rock is molten; only when it cools and solidifies do the daughters start to accumulate. For this reason, the ages determined for the rocks are really the times since the rock became solid. And if the rock is re-melted, say by the impact of a meteorite, its radioactive clock is reset, and the age will measure the time since the last solidification.

Formation of the highlands and maria

The retrieved lunar rocks have taken us back into time, to the formative stages of the Moon. They record events from the earliest history of the solar system that have been erased on Earth by water, wind and geologic activity. Radioactive dating of Moon rocks and primitive meteorites indicates, for example, that the Moon was assembled a mere 50 million years after the solar system itself was born 4.6 billion years ago.

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As the external cratering rate was declining rapidly, internal processes set to work. The radioactive decay of long-lived unstable elements, such as uranium and thorium, produced heat that gradually warmed up the interior. There followed an era of volcanism, lasting for 700 million years, from 3.9 to 3.2 billion years ago. The outer zone of solid rock gradually cooled from the outside in, becoming thicker, and lava worked its way from deeper and deeper in the Moon. The magma flow may have stopped 3.2 billion years ago, since the youngest lunar samples are that old, but some mare basalts could be as young as 1 billion years old.

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As molten basaltic rock welled up from the interior, it penetrated the thin crust beneath the great impact basins on near side of the Moon, flooding them with lava and producing the dark circular maria that can be seen today. Successive lava flows set their marks in some maria, showing that they were not formed in a single quick pulse of volcanism, but by repeated outpourings that gradually filled the near-side basins. The lava inundated all craters in its path, wiping the slate clean of previous impacts. preparing a fresh surface to record new impacts which, by this time, had greatly diminished in intensity. Thus the maria are relatively unscarred and most of their craters are small and relatively young.

Tides and the once and future Moon

The pattern of the tides

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Walking along the ocean beach some morning, we might notice that the waves seem to be reaching farther and farther up the sand. The tide is flooding the beach. A few hours later, it hesitates and then begins to ebb, retreating onto the flats where the clams may often be found. The high tides occur simultaneously and symmetrically on opposite sides of the Earth; they return every 12 hours 25 minutes in each location, although not precisely to the same height. The time between consecutive high tides is slightly more than half a day because the Moon's revolution around Earth is in the same direction as the Earth's rotation on its axis, so Earth needs an extra 25 minutes of rotation to out-race the Moon and get into position. On a slowly rotating planet without continents, the tide would be highest along the line joining the centers of the Earth and Moon, that is, when the Moon is overhead. This is not the case for the Earth. The friction of the continents and the rapid rotation of the Earth carry the ocean's tidal bulge forward so it precedes the Earth-Moon line by about 3 degrees. This means that in the open ocean the high tide actually occurs about 12 minutes after the Moon is overhead.

The Moon creates two high tides because the gravitational force of the Moon draws the ocean out into an ellipsoid, or the shape of an egg. We can understand this by remembering that the gravitational force decreases with distance, so the Moon pulls hardest on the ocean facing it, and least on the opposite ocean; the Earth between is pulled with an intermediate force. As a result, the water directly beneath the Moon is pulled up away from the Earth's center, and the Earth's center is pulled away from the water on the opposite side, causing another high tide. Thus the differences of the gravitational attraction of the Moon on opposite sides of the Earth produce two tidal bulges - one facing the Moon and one facing away.

On a slowly rotating planet without continents, the tide would be highest along the line joining the centers of the Earth and Moon, that is, when the Moon is overhead. This is not the case for the Earth. The friction of the continents and the rapid rotation of the Earth carry the ocean's tidal bulge forward so it precedes the Earth-Moon line by about 3 degrees. This means that in the open ocean the high tide actually occurs about 12 minutes after the Moon is overhead.

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The Sun and the Moon both contribute to the formation of the tides, but the major portion of this rhythmic ebb and flood is driven by the Moon, whose tide is 2.2 times as high as the Sun's. In the course of a month, the changing alignment of the Sun and Moon causes the tides produced by these two bodies to alternately reinforce and interfere, leading to the cycle of spring tides and neap tides. The spring tide occurs near new and full moon, when the Sun and Moon reinforce each other's tides, and the neap tide occurs near first and third quarter, when they interfere with each other. The spring tides can be 2 or 3 times as high as the neap tides. The Sun's tides also vary by a small amount over the year as the Earth travels around its eccentric orbit and the Sun-Earth distance changes, with the greatest solar tides when the Earth is nearest the Sun.

The days are getting longer

As the Earth rotates, the bulge raised on its surface by the Moon's gravity is always a little ahead of the Moon rather than directly under it. The Moon pulls back on the bulge, and in the process it slows the whole planet down. In other words, our planet meets resistance in its daily rotation caused by the tidal interaction of the Moon with the Earth.

As the tides flood and ebb, they create eddies in the water, producing friction and dissipating energy at the expense of the Earth's rotation. The ocean water is heated ever so slightly by the motion of the tides and the Earth's rotation is slowed. The tides therefore act as brakes on the spinning Earth, slowing it by friction in much the way that the brakes of a car slow its wheels and become warm. The friction of tides dissipates energy at the rate of 5 billion horse-power (4 x 1012 watt). Tidal friction is slowing the rotation of the Earth, and the day is becoming longer at a rate of 2 milliseconds, or 0.002 seconds, per century. In other words, the days are getting longer at the rate of one second every 50,000 years, and tomorrow will by 60 billionths of a second longer than today.

Earth's tidal influence on the Moon

The Moon pulls the Earth's oceans, and the oceans pull back, in accord with Newton's third law that every action has an equal and opposite reaction. The net effect is to swing the Moon outward into a more distant orbit. This is because the tidal bulge on the side facing the Moon is displaced ahead of the Moon, and this bulge pulls the Moon forward.

As the Earth slows down, the angular momentum it loses is transferred to the Moon, which speeds up in its orbit around us. It is not hard to see that this will swing the Moon away from the Earth if we look at the key equations. When we do the arithmetic, we find that the change of 0.002 seconds per century in the length of the day implies an outward motion of the Moon amounting to about 0.04 meters per year. Small as it is, this value is just measurable with the laser reflectors planted on the Moon by the Apollo astronauts. The lunar laser ranging data indicate that the Moon is moving away from the Earth at a rate of 0.0382 0.0007 meters per year.

Stabilizing the Earth

The orientation of the Earth's rotation axis causes the annual seasonal variations of our climate, and small variations in its orientation contribute to the advance and retreat of the ice ages. The obliquity of the Earth, the angle that its spin axis makes with the perpendicular to its orbital plane, is now a modest 23.5 degrees, but this is sufficient to bring summer and winter as the northern or southern hemisphere is tilted toward or away from the Sun. Variation in the Earth's obliquity as small as 1.3 degrees, around a mean value of 23.3 degrees, may contribute to, or trigger, the ice ages.

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The climate forecast for a Moon-less Earth would be a lot bleaker. The gravitational pull of our large Moon acts as an anchor, limiting excursions in the Earth's rotation axis and keeping the climate relatively stable. Without the Moon, the tilt of Earth's spin axis would vary chaotically between 0 and 85 degrees. Such large variations in the planet's obliquity would result in dramatic changes in climate. With an obliquity of 0 degrees, there would be no seasonal variation in the distribution of sunlight on Earth. At 85 degrees, the Earth's axis would be tipped completely over. The equatorial tropics could then be permanently in cold winter snows, and the poles would be alternately pointed almost directly at or away from the Sun over the course of a single year. Such wide climate changes might be hostile to many forms of life on Earth.

Origin of the Moon

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There are three classical hypothesis for the origin of the Moon that have been advocated for more than a century. They are the fission, capture and accretion models, nicknamed the daughter, pickup and sister theories. But as Sherlock Holmes said in The Adventure of Silver Blaze, "I am afraid that whatever theory we state has very grave objections to it". So, we will move on to the more successful giant impact hypothesis.

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The giant impact mechanism permits the Moon to form initially in the same part of the solar system as the Earth and to undergo a process that explains both the dearth of metals and volatile elements, as well as an enrichment of refractory elements, before solidification. It might have shattered the colliding object to smithereens and vaporized parts of the iron-poor upper layers of the Earth, blasting off a mix of terrestrial and impactor material into orbit where it coalesced to form the Moon that we know. If the impacting object had a heavy iron core, it might have tumbled into the still-forming Earth, merging with the planet's core.

Summary Diagram

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