THE EARTH AND THE MOON

PHYSICAL FEATURES OF THE EARTH

• mass known from Newton's law of gravitation, and acceleration of gravity at earth's surface
• radius determined by Eratosthenes (modern measurements from satellite data)
• density = 5500 kg/m³; center must be very dense (cf. water, 1000 kg/m³; surface rocks: 2000-3000 kg/m³)
• distance to the moon is well known from round trip for radar: 2.56 s: distance = 384,000 km (250,000 mi) = 10 x circumference of earth
• period of rotation: 1 day
• period of revolution: 1 year

PHYSICAL FEATURES OF THE MOON

Lunar probes: Pioneer, Ranger, Surveyor, Lunar Orbiter, Apollo (U.S., 1968-1972); Luna, Zond (Soviet, 1959-1975); Clementine (U.S., 1994)

• mass known from gravitational effect on motion of earth (1/80 of earth's mass)
• radius of moon determined from apparent size and known distance (1/4 earth's)
• density: 3300 kg/m³
• gravity: 1/6 earth's
• period of revolution: 1 month
• period of rotation: also 1 month. Synchronous rotation, due to tidal forces
  • Note well: the moon has a near side and a far side, but the far side is not necessarily dark

Both earth and moon have an equatorial bulge due to rotation

STRUCTURE OF THE EARTH

properties of interior can be measured by studying the propagation of seismic waves (originating in earthquakes)

S (shear) waves can propagate in solids only
P (pressure) waves propagate in liquids and solids

• Core: dense, metallic; mostly iron/nickel; liquid outer core, possibly solid inner core. Density up to 12,000 kg/m³
• Mantle: rocky, mostly basalt (a heavy mineral containing iron and magnesium); density 5000 kg/m³. samples of mantle come to us from volcanoes
• crust: 15 km thick; lighter rocks such as granite (containing silicon and aluminum); density 3000 kg/m³ (floats on mantle)

Earth is differentiated; must once have been liquid. Heated in early times by impacts from space, differentiation, radioactive heating.

SURFACE FEATURES OF THE EARTH

continents and oceans.

• The crust floats on the mantle: plate tectonics.
• Driven by convection in the mantle
• Earthquakes and volcanoes are evidence of geological activity within the earth

ATMOSPHERE

lies mostly within 10-30 km of the surface

78% nitrogen, 21% oxygen; composition determined by biological activity.

Layers of the atmosphere:

• Troposphere: region of convection (weather); 15 km thick
• stratosphere: air stable and calm; 20-50 km (includes ozone layer)
• ionosphere: gases charged by interaction with radiation from space; reflects AM radio

HYDROSPHERE (Oceans)

tides, 2 per day, produced by the gravity of the moon (and to a lesser extent, the sun)

The tides lag behind the moon producing friction and slowing the earth's rotation: that's what happened to the moon

MAGNETOSPHERE

• magnetosphere; traps charged particles in Van Allen belts, protects the earth; aurora near the poles
• probably due to stirring of conducting liquid core by rotation of Earth

STRUCTURE OF THE MOON

determined by seismographs left by the Apollo astronauts. Moonquakes are very weak: there is little geological activity left in the moon.

Density fairly uniform, but chemically differentiated.

• iron rich core, not as dense as earth's; possibly molten
• crust is thicker than earth's (thicker on far side)
• mantle is more rigid than earth's

SURFACE FEATURES OF THE MOON

• highlands: oldest parts of the moon
• maria: seas of lava left from partial melting of the moon in its early days (melted by bombardment)
• craters: caused by impact of meteorites (maria are less cratered)

ATMOSPHERE OF THE MOON--none; gravity too weak

Water -- water ice detected near poles by Clementine in 1996 (confirmed by Lunar Prospector in 1998)

AGE OF THE EARTH AND THE MOON

determined by radioactive dating

compare amount of radioactive material with amount of decay product

useful isotopes:

• uranium-238 (half-life 4.5 billion years)
• uranium-235 (half-life 0.7 billion years)
• for shorter time scales, carbon-14

oldest surface rocks on Earth (Greenland, Labrador): 3.9 billion years old (tells when rocks solidified)

lunar highlands: 4.1 billion years old (rocks from lunar maria slightly younger: more recently melted)

meteorites: 4.5 billion years old: date to origin of solar system

ORIGIN OF THE MOON

theories:

1. Sister theory: Earth and Moon formed at same time in the same part of the solar system (problem: they have different compositions)
2. Capture theory: Earth captured the Moon as it passed by; need not have the same composition (problem: gravitational capture is improbable)
3. Daughter or fission: spinning Earth threw off the Moon (problem: how did it get to be spinning that fast?)
4. Impact: large body hits the Earth and is absorbed; part of Earth's mantle is knocked out. (Plausible: supported by computer simulations; but there's no direct evidence)

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