Chapter 8

The Moon

Orbital PropertiesDistance between Earth and Moon has been measured to accuracy of a 3 cm using lasers

Distance of the moon from earth is 384,000 km or about 240,000 miles

Physical Properties

Moon Earth

Radius 1700 km 6380 km

Mass 7.3 × 1022 kg 6.0 × 1024 kg

Density 3300 kg/m3 5500 kg/m3

Escape Speed

2.4 km/s 11.2 km/s

Escape speed-the speed needed for any object to escape forever from its surface.

Rotation Rates

Moon’s rotation rate is the same as the time it takes to make one revolution, so the same side of the Moon always faces Earth.

Rotational Period: 27.3 days

Revolution: 29.5 days

Phases are due to different amounts of sunlit portion being visible from Earth

New Moon-no part of the moon is visible from earth

Waning Moon-part of the moon that is visible decreases

Waxing Moon-part of the moon that is visible increases

Crescent Phase-less than half of the moon is visible from earth

Gibbous Phase-more than half of the moon is visible from the earth

Moon Phases

Surface Features on the Moon Moon has 3 main

surface features:

1.Maria

2.Highlands

3.Carters

Maria-

Large dark flat areas, due to lava flow (early observers thought they were oceans)

                                 

                                                                                  

Highlands: Highlands on the moon are higher, lighter, older areas on the moon's surface.

Altitudes of the moon's highlands can reach 8000 meters! (That's about 3 miles!)

Craters, from meteorite impacts:

Crater impacts are caused by meteoroid strikes that eject material; after impact explosion ejects

more material, leaving crater

King crater on the Farside of the moon.

The surface of the moon is scarred with millions of impact craters. There is no atmosphere on the moon to help protect it from bombardment. Also, there is no erosion and little geologic activity to wear away these craters, so they remain unchanged

until another new impact changes it.

The size, mass, speed, and angle of the falling object determine the size, shape, and complexity

of the resulting crater.

Messier and Messier A

Crater 308 on Farside

Lunar Cratering and Surface Composition

• Craters are typically about 10 times as wide as

the meteoroid creating them, and twice as deep.

• Rock is pulverized to a much greater depth.

• Most lunar craters date to at least 3.9 billion

years ago; much less bombardment since then.

Regolith: thick layer of dust left by meteorite impacts. It covers the lunar landscape to an average depth of about 20m.

Moon is still being bombarded, especially by very small “micrometeoroids”

More than 3 billion years ago, the moon was volcanically active.

Rille is formed from an open lava channel

Evolutionary History of the Moon

Time before present:

Event:

4.6 billion yr Formation of Moon; heavy bombardment liquefies surface

3.9 billion yr Bombardment much less intense; lunar volcanism fills maria

3.2 billion yr Volcanic activity ceases

Meteorites also hit Earth; this crater is in Arizona:

Between 20,000 to 50,000 years ago, a small asteroid about 80 feet in diameter impacted the Earth and formed the crater. The crater is the best preserved crater on Earth and measures 1.2

km in diameter.

Barringer Meteorite

Crater