1. Section 1: Earth’s Moon Preview Key Ideas Exploring the Moon
The Lunar Surface
The Interior of the Moon
The Formation of the Moon
Formation of the Moon

2. Key Ideas List four kinds of lunar surface features.
Describe the three layers of the moon.
Summarize the three stages by which the moon formed.

3. Exploring the Moon
satellite a natural or artificial body that revolves around a celestial body that is greater in mass
moon a celestial body that revolves around a body that is greater in mass; a natural satellite
Six of the planets in our solar system have natural satellites, or moons. Our moon is Earth’s satellite.
The Apollo space program sent six spacecraft to study the moon. Scientists were able to gather data about the moon’s weak gravity and its effect on astronauts, as well as data about the moon’s surface.

4. The Lunar Surface Any feature of the moon is referred to as lunar.
Light and dark patches on the moon can be seen with the unaided eye.
The lighter areas are rough highlands composed of rocks called anorthosites. The darker areas are smooth, reflect less light, and are called maria.
mare a large, dark area of basalt on the moon
Maria are plains of dark, solidified lava which formed more than 3 billion years ago when lava slowly filled basins that were created by impacts of massive asteroids.

5. The Lunar Surface, continued
Craters, Rilles, and Ridges
crater a bowl-shaped depression that forms on the surface of an object when a falling body strikes the object’s surface or when an explosion occurs
The surface of the moon is covered with craters, rilles, and ridges. Most of the craters formed when debris struck the moon about 4 billion years ago.
Rilles are long, deep channels that run through the maria. Rilles are thought to be leftover lava channels from the formation of the maria.
The moon’s surface also has several ridges, which are long, narrow elevations of rock that rise out of the surface and criss-cross the maria.

6. Reading check
Name two features of the moon. The moon’s features include maria, craters, ridges, and rilles.

7. The Lunar Surface, continued
Regolith
More meteorites have reached the surface of the moon than have reached Earth’s surface because the moon has no atmosphere for protection.
Over billions of years, these meteorites crushed much of the rock on the lunar surface into a layer of dust and small fragments called regolith.
The depth of regolith layer varies from 1 m to 6 m.

8. The Lunar Surface, continued
Lunar Rocks
Lunar rocks are igneous, and most rocks near the surface are composed mainly of oxygen and silicon.
Rocks from the lunar highlands are light-colored, coarse-grained anorthosites rich in calcium and aluminum.
Rocks from the maria are fine-grained basalts and contain titanium, magnesium, and iron.
Breccia is found in both maria and the highlands. Lunar breccia formed when meteorites struck the moon.

9. The Interior of the Moon
The interior of the moon is less dense than the interior of Earth.
Most of the information about the interior of the moon comes from seismographs that were placed on the moon by the Apollo astronauts.
More than 10,000 moonquakes have been detected. From these moonquakes, scientists learned that the moon’s interior is layered.

10. The Interior of the Moon, continued
The Moon’s Crust
The side of the moon that faces Earth is called the near side. The other side of the moon that faces away from Earth is called the far side.
The pull of Earth’s gravity during the moon’s formation caused the crust on the far side of the moon to become thicker than the crust on the near side.
The crust on the near side is about 60 km thick. The crust on the far side is up to 100 km thick.

11. Reading check, continued
Name two features of the far side of the moon. The crust of the far side of the moon is thicker than the crust of the near side is. The crust of the far side also consists mainly of mountainous terrain and has only a few small maria.

12. The Interior of the Moon, continued
The Moon’s Mantle and Core
Beneath the crust is the moon’s mantle. The mantle is thought to be made of rock that is rich in silica, magnesium, and iron.
Scientists think that the moon has a small iron core that has a radius of less than 700 km.
Studies of the core have shown that the moon’s rotation is not uniform and that the core is neither completely solid nor completely liquid.

13. The Formation of the Moon
Most scientists agree that the moon formed in three stages.
The Giant Impact Hypothesis
The giant impact hypothesis states that the formation of the moon began when a large object collided with Earth more than 4 billion years ago.
The collision ejected chunks of Earth’s mantle into orbit around Earth. The debris eventually clumped together to form the moon.
This hypothesis explains when moon rocks share many of the chemical characteristics of Earth’s mantle.

14. The Formation of the Moon, continued
Differentiation of the Lunar Interior
Early in its history, the lunar surface was covered by an ocean of molten rock.
Over time, the densest materials moved toward the center of the moon and formed a small core.
The least dense materials formed an outer crust.
The other materials settled between the core and the outer layer to form the moon’s mantle.

15. The Formation of the Moon, continued
Meteorite Bombardment
As the outer surface of the moon cooled to form the crust, debris struck the solid surface and produced craters and regolith.
About 3 billion years ago, less material struck the lunar surface, and few new craters formed.
Craters that have rays formed during the most recent meteor impacts. During this stage, virtually all geologic activity stopped on the moon.

16. The Formation of the Moon, continued

17. The Formation of the Moon, continued
Lava Flows on the Moon
After impacts on the moon’s surface formed deep basins, lava flowed out of cracks, or fissures, in the lunar crust.
This lava flooded the crater basins to form maria.
The presence of maria suggest that fissure eruptions once characterized the moon.
Because of the uneven thickness of the moon’s crust, more maria formed on the near side of the moon than on the far side.

18. The Formation of the Moon, continued
Lava Flows on the Moon
Scientist do not yet know how magma formed in the lunar interior or how magma reached the surface.
There is no evidence of plate tectonics or convection currents in the moon’s mantle.
Some scientists think the energy needed to produce magma may have come from a long period of intense meteorite bombardment. Other scientists think radioactive decay of materials may have heated the moon’s interior enough to cause magma to form.

19. Formation of the Moon
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