The Moon

Steven Dutch, Professor Emeritus, 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 basins 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 craters have formed. Older craters like Eratosthenes lack rays, but younger craters like Tycho and Copernicus still have them.

Histories of Earth and Moon Compared

The diagram above compares the geologic histories of the Earth and Moon. The central bar is the terrestrial geologic time scale. The bottom three large divisions, each a billion or so years long, are called eons. The time since 540 million years ago is called the Phanerozoic eon, from Greek roots meaning "visible life." The divisions of the Phanerozoic are called eras. They are in turn subdivided into periods (not shown). On Earth, everything before the Phanerozoic is often called Precambrian (the Cambrian is the earliest period of the Phanerozoic).

Just as with human history, we subdivide planetary history at major break points. The Moon has its own history which doesn't necessarily correspond to ours. For example, the end of the Mesozoic era is defined by a large meteor impact, but that event had no major effect on the Moon at all.

The short Nectarian eon (red) is the time when most of the lunar mare basins formed. Everything before that is Pre-Nectarian. The Imbrian is the time when the last very large basins formed and were filled by lava flows. The Eratosthenian generally corresponds to the interval after the main mare formation when older craters formed. The Copernican loosely corresponds to the interval of young craters with bright rays.

The two outer bars show the preservation of information on the Earth and Moon. On Earth, rocks have a "half life" of a couple of hundred million years, that is about half of the rocks of any given age will have eroded away in a couple of hundred million years. Do not confuse this with radioactive decay. So the farther back in time we go, as in human history, the sketchier the record gets. We have only a very partial understanding of the Precambrian. On the Moon, though, almost everything since the end of the Imbrian is preserved. So the Moon fills a gap in geologic time where evidence on the Earth is almost completely gone.

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. Cosmic ray and micrometeorite bombardment degrade regolith material. These processes are called space weathering.

• 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