Chapter FourChapter FourThe Moon: The Earth’sThe Moon: The Earth’s

Closest NeighborClosest Neighbor

The Moon’s FormationThe Moon’s FormationThe Moon is approximately 4.5 billion years old The Moon is approximately 4.5 billion years old – – this is just as old as the Earth, which suggests that they this is just as old as the Earth, which suggests that they were formed at about the same timewere formed at about the same time..

The origin of the Moon is still being debated.The origin of the Moon is still being debated.

– – One theory proposes that the Moon formed elsewhere in One theory proposes that the Moon formed elsewhere in the solar system and was captured by the Earth’s gravity the solar system and was captured by the Earth’s gravity as it passed close to our planet.as it passed close to our planet.

– – Another theory suggests that debris orbiting the Earth Another theory suggests that debris orbiting the Earth that was left over from the formation of the planet that was left over from the formation of the planet eventually came together to form the Moon.eventually came together to form the Moon.

– – The latest theory of how the Moon formed is gaining The latest theory of how the Moon formed is gaining the most popularity the most popularity this theory states that a large this theory states that a large object, object, perhaps as large as the planet Mars, struck the perhaps as large as the planet Mars, struck the Earth Earth and sent a large amount of the Earth’s crust into and sent a large amount of the Earth’s crust into space.space.

Evidence to support this theory comes from rock Evidence to support this theory comes from rock samples recovered from the Moon.samples recovered from the Moon.– – They reveal that the Moon is mainly composed of They reveal that the Moon is mainly composed of silicatesilicate minerals and is minerals and is deficientdeficient in iron. in iron.

DefinitionDefinition: a chemical compound that is composed : a chemical compound that is composed of atoms of oxygen and siliconof atoms of oxygen and silicon

DefinitionDefinition: lacking something essential to life: lacking something essential to life

– – The lack of iron on the Moon, as compared with Earth, The lack of iron on the Moon, as compared with Earth, suggests that it is formed from the outer crust of the Earth, suggests that it is formed from the outer crust of the Earth, which contains much less iron than is located in its core.which contains much less iron than is located in its core.

The Composition of the MoonThe Composition of the MoonThe rocks that were brought back from the Moon are The rocks that were brought back from the Moon are very similar to very similar to volcanic rocks volcanic rocks produced on the Earth.produced on the Earth.

DefinitionDefinition: igneous rocks that form from cooled lava : igneous rocks that form from cooled lava produced by a volcanoproduced by a volcano

The volcanic eruptions that formed these rocks on the The volcanic eruptions that formed these rocks on the Moon occurred early in the Moon’s history, between 4 Moon occurred early in the Moon’s history, between 4 and 2.5 billion years ago.and 2.5 billion years ago.– These ancient eruptions created large plains of These ancient eruptions created large plains of basaltbasalt

lava that are called mares.lava that are called mares.

Definition: Definition: a common fine-grained volcanic rock that is a common fine-grained volcanic rock that is dark, mafic and densedark, mafic and dense

Definition: Definition: a Latin word for “seas” used to describe the a Latin word for “seas” used to describe the flat dark plains that cover the surface of the Moonflat dark plains that cover the surface of the Moon

– Since the formation of the mares, the Moon has had no Since the formation of the mares, the Moon has had no known volcanic eruptions.known volcanic eruptions.

– The The most recognizable feature most recognizable feature of the Moon’s surface are of the Moon’s surface are its its impact cratersimpact craters..

DefinitionDefinition: large bowl-like depressions that are left on the : large bowl-like depressions that are left on the surface of a celestial object as the result of an impact by surface of a celestial object as the result of an impact by another celestial object, usually an asteroid or cometanother celestial object, usually an asteroid or comet

The Moon’s SurfaceThe Moon’s SurfaceAstronauts who landed on the Moon discovered that Astronauts who landed on the Moon discovered that the surface is covered with a the surface is covered with a fine volcanic dustfine volcanic dust, , which is formed by the impact craters.which is formed by the impact craters.

The Moon’s surface The Moon’s surface has no atmospherehas no atmosphere, and its , and its gravity is gravity is 17 percent 17 percent of the Earth’s gravity.of the Earth’s gravity.– The average surface temperature of the moon varies The average surface temperature of the moon varies

greatly during the day.greatly during the day.– At noon the temperature on the Moon can reach almost At noon the temperature on the Moon can reach almost

200oF (93200oF (93ooC), and during the lunar night the C), and during the lunar night the temperature plummets to lower than -250temperature plummets to lower than -250ooF (-157F (-157ooC).C).

Until recently it was believed that the Moon had no Until recently it was believed that the Moon had no water, but explorations of the Moon’s polar regions water, but explorations of the Moon’s polar regions suggest that suggest that there might be some frozen waterthere might be some frozen water on on the Moon.the Moon.

The Moon’s diameter is approximately 2,160 miles, The Moon’s diameter is approximately 2,160 miles, which is roughly which is roughly 27 percent 27 percent of the Earth’s diameter.of the Earth’s diameter.

The Moon is approximately 238,866 miles from Earth The Moon is approximately 238,866 miles from Earth and and completes one orbit around the Earth in completes one orbit around the Earth in 27 27 daysdays and and 7 hours7 hours..– The period of the Moon’s revolution around the Earth and The period of the Moon’s revolution around the Earth and

the Moon’s rotation on its own axis are the same; therefore the Moon’s rotation on its own axis are the same; therefore the same side of the Moon always faces the Earth.the same side of the Moon always faces the Earth.

The Moon’s OrbitThe Moon’s OrbitAs the Moon orbits the Earth, one side of it is always As the Moon orbits the Earth, one side of it is always illuminated by the Sun.illuminated by the Sun.– This causes the different Moon phases as viewed from This causes the different Moon phases as viewed from

the Earth’s surface.the Earth’s surface.– Different phases of the Moon occur at different points in Different phases of the Moon occur at different points in

its revolution around the Earth.its revolution around the Earth.