The Moon
Steven Dutch, Natural and Applied Sciences,
University of Wisconsin - Green Bay
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Vital Statistics
- Averages 236,000 miles from the earth.
- Closest distance (perigee) is about 220,000 miles,
- Greatest distance (apogee) is about 255,000 miles.
- Diameter about 2160 miles, roughly 1/4 that of the earth
- Mass 1/81 that of earth. Surface gravity 1/6 that of earth
- Takes 29-1/2 days to circle earth.
Significant Features
- Craters
- Rays around more recent craters
- Highlands and Maria
- Crust of Moon is largely anorthosite and gabbro (or norite)
- Maria are mostly basalt
- Maria almost exclusively on earth-facing side
- Tidally locked to earth
- Same side always faces earth
- Moon slightly elongated
- Small moonquakes occur when moon nearest earth
- Moon causes tides on Earth
- Most marked in oceans but a small tide occurs in the solid earth, too.
- Tides are slowing Earth's rotation - Earth rotated about 400 times/year 500,000,000 years ago.
- As Earth slows, it transfers its rotation (angular momentum) to the Moon, causing it to get farther from Earth.
- Interior of Moon probably much like Earth's mantle
- Moonquakes occur about 800 mi. deep in Moon, just outside core
- Small molten core, probably magnesium-iron silicate or iron sulfide but may be nickel-iron like Earth's.
- Core boundary not sharp.
- Diameter of core about 500 miles.
- No magnetic field now but seems to have had one early in its history.
- Moon differs chemically from Earth
- Very poor in water
- Depleted in volatile elements
- Richer in some metals, such as titanium
- No surface water, life or atmosphere
- Appears to have formed in hotter part of Solar System
Theories for Origin of Moon
- Fission: Originally part of Earth but torn free.
- Problem: would have fallen back or been flung into space, not into orbit.
- Should orbit in Earth's equatorial plane
- Fails to explain why lunar chemistry differs from Earth's
- Co-Creation Formed in its present orbit.
- Should orbit in Earth's equatorial plane
- Fails to explain why lunar chemistry differs from Earth's
- Capture Formed as a separate planet but captured by Earth
- Explains why Moon orbits in same plane as other planets
- Conditions for successful capture very stringent
- Impact Formed from Mega-Impact of Mars-sized planet
- Computer modeling suggests solar system forms 100 or so small planets which then collide to make larger objects.
- Explains why Moon orbits in same plane as other planets
- Can explain why lunar chemistry differs from Earth's
- Avoids fatal problems of other theories
- Currently favored model
Geology of the Moon
Geologic Map of the Moon
Geologic Evolution of the Moon
- A. Initial Accretion of the Moon, probably from debris launched into Earth orbit by a mega-impact.
- B. In the last stages of accretion, so much heat accumulates that the outermost 100 km of the lunar crust melts to form a magma ocean. Light feldspars rise to accumulate an anorthosite crust.
- C. Late impacts excavate giant basins. One of the earliest is the South-Pole-Aitken Basin.
- D. Mare Nectaris and other bains form.
- E. Mare Imbrium forms.
- F. Mare Orientale forms
- G. Mare basalts erupt and flood many of the impact basins.
- H. Since 3000 Ma, only a few large rayed craters like Tycho and Copernicus have formed.
Histories of Earth and Moon Compared
| Million Years Ago | Events on Earth | Events on Moon |
|---|---|---|
| 4600 | Formation of Earth | Formation of Moon |
| 4500 | Intense cratering??? | Intense cratering |
| 4400 | ||
| 4300 | ||
| 4200 | ||
| 4100 | ||
| 4000 | ||
| 3900 | Oldest known rocks | |
| 3800 | ||
| 3700 | ||
| 3600 | Surface Water | Earliest Maria |
| 3500 | ||
| 3400 | ||
| 3300 | ||
| 3200 | ||
| 3100 | ||
| 3000 | Earliest Indications of Life | Latest Maria |
| 2900 | ||
| 2800 | ||
| 2700 | ||
| 2600 | ||
| 2500 | Shield areas of continents formed by now | |
| 2400 | ||
| 2300 | Gowganda Ice Age | |
| 2200 | ||
| 2100 | ||
| 2000 | ||
| 1900 | ||
| 1800 | Trans-Hudson Orogeny | |
| 1700 | ||
| 1600 | ||
| 1500 | ||
| 1400 | ||
| 1300 | ||
| 1200 | ||
| 1100 | ||
| 1000 | Grenville Orogeny | |
| 900 | ||
| 800 | Vendian Ice Ages | |
| 700 | Breakup of Rodinia | |
| 600 | Earliest abundant fossils | |
| 500 | ||
| 400 | Appalachians form. Land life | |
| 300 | Coal forests | |
| 200 | Dinosaurs appear. First mammals, birds. | Rayed craters forming (Copernicus, Tycho, etc.) |
| 100 | Dinosaurs Extinct | |
| 0 | Ice Ages, Human History |
Surfaces of Earth and Moon
On the Earth, weathering and erosion are constantly breaking down old rocks, moving the fragments to new locations and cementing them together again. Much of the Earth is covered with a few kilometers or less of Sedimentary Rocks
- Weathering
- Mechanical
- Chemical
- Transport
- water
- glaciers
- wind
- Deposition
On the Moon, the major process which breaks down and transports rock is meteor impact. Most of the impacts occurred early in the history of the Moon, but small impacts are still occurring at a much lower rate. The surface of the Moon is covered with perhaps a kilometer or so of mixed-up and broken rocks and fine fragments called Regolith or Breccia.
- Primary Craters
- Secondary Craters
- Regolith or Breccia
References
- John A. Wood, 1975, The Moon. Scientific American, vol. 233, no. 3, pp. 92-105.
- Brian Mason, 1971, The Lunar Rocks. Scientific American, vol. 225, no. 4, pp. 48-62
- G. Jeffrey Taylor, 1994, The Scientific Legacy of Apollo; Scientific American, v. 271, p. 40-47, no. 1 (July, 1994)
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Created 20 May 1997, Last Update 9 April 1999
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