The Moon – Our Closest Neighbor

1. The Moon – Our Closest Neighbor Chapter 16 Earth/Space Science Spring 2007

2. 6.1 Description of the Moon • General Features • Radius: 1,738 km • About ¼ of Earth’s radius • Orbital distance: 384,400 km • Orbital period: 27.32 days • Mass: 734.9 x1020 kg • About of Earth’s mass

3. 6.1 Description of the Moon • Surface Features • Craters • Circular pits, usually with a raised rim and sometimes with a central peak • Most craters are formed by the impact of solid bodies, such as asterioids. • On the Moon, the craters range in size from a few centimeters in diameter to around 240 km across. • Most lunar craters are named after famous scientists

4. 6.1 Description of the Moon • Surface features • Maria (MAR-ee-a) • Vast, smooth, dark, and congealed lava flows filling a basin on the Moon. • Mare (MAR-ay) is the Latin word for “sea.” • Early observers thought they looked like water deposits

5. 6.1 Description of the Moon • Surface features • Highlands • Bright areas surrounding the maria • They are different color due to their different composition than the maria • The highlands have a much more rugged topography than the maria. • Most of the highlands are pitted with craters.

6. 6.1 Description of the Moon • Surface features • Lunar rays • Long, light streaks of pulverized rock radiating from craters • Rilles • Long, narrow canyons • Believed to be the remnants of ancient lava flows. • May also be from crustal cracks

7. 6.1 Description of the Moon • Origin of lunar surface features • Nearly all of the surface features on the moon are a result of solid body impacts

8. 6.2 Structure of the Moon • Crust and Interior • The moon’s surface layer is shattered rock that forms a regolith around ten or twenty meters deep. • Regolith – “blanket of rock” • The regolith consists of both rock chunks and fine powder, the result of successive impacts breaking rock into smaller and smaller pieces

9. 6.2 Structure of the Moon • Crust and Interior • The Moon’s crust begins about ten or twenty meters down. • The average thickness of the crust is about 100 km. • The crust is thinner on the near side of the moon allowing more maria to form there

10. 6.2 Structure of the Moon • Crust and Interior • Extending about 1000 km below the crust is the Moon’s mantle. • The Moon’s mantle is made up of much the same stuff as Earth’s. • However, the Moon’s mantle is much cooler than Earth’s.

11. 6.2 Structure of the Moon • Crust and Interior • The Moon has a small core, roughly 200 km across. • Low in iron and nickel compared to Earth. • This may explain why there is no magnetic field on the Moon.

12. 6.2 Structure of the Moon • Absence of a lunar atmosphere • The Moon lacks an atmosphere for two reasons: • Its interior is too cool for any volcanic activity. • Its small mass does not create a strong enough gravitational field to hold an atmosphere in place.

13. 6.3 Orbit and Motions of the Moon • The Moon’s Rotation • The Moon’s rotation is synchronous. • This means that it rotates once for every revolution around the Earth • Rotation period = orbital period • Because of this, the same side of the Moon is always facing the Earth.

14. 6.3 Orbit and Motions of the Moon • Oddities of the Moon’s Orbit • The Moon’s orbit is tilted 5° with respect to the Earth’s orbit around the Sun and Earth’s equator. • While most moons have a mass less than 1/1000 of their planets, the Moon has a mass 1/81 of Earth’s.

15. 6.4 Origin and History of the Moon • The leading theory, based on the study of lunar rocks brought back from the Moon: • Early in Earth’s history, it collided with a Mars-sized body • The Moon formed from the debris blasted away by the collision. • This also explains the tilt of the Earth.

16. 6.6 Tides • Cause of tides • The Moon exerts a differential gravitational force on the Earth. • The gravitational force is stronger on Earth’s side that is closer to the Moon. • This stronger gravitational force pulls the oceans into a tidal bulge on the side of the Earth facing the Moon.

17. 6.6 Tides • Cause of Tides • The Earth’s rotation carries the land through the tidal bulges. • Because the oceans cannot flow over th continents, the water either bunches up at the edge of the continent (high tide) or recedes from the continent (low tide).

18. 6.6 Tides • Tidal Braking • This phenomenon due to tidal bulges works to slow the Earth’s rotation and speed up the Moon’s orbit. • This is one cause of the Moon always facing the Earth.