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Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30
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Lecture 30:
The Moon
Key Ideas:
Surface of the Moon:
- Old, heavily cratered highlands
- Younger, dark Maria
- Thick regolith of pulverized rock
Interior of the Moon
- Crust and Mantle, but no Core(?)
- No magnetic field today
The Origin of the Moon
The Neighbor Moon
After the Earth, the Moon is the best known body in the Solar
System:
- Closest companion in space
- We've visited it and returned samples of rock for
study.
- Planted instruments and seismographs on the surface to
probe the interior
- Study it up-close with robotic probes, Clementine
& Lunar Prospector most recently.
Lunar Terrain
Maria (seas)
- 16% of surface
- fewer craters
- (younger surface)
- older craters filled in by lava flows
- higher iron content in the rocks (basalts)
Highlands
- Light Colored
- Heavily Cratered
- Overlapping craters
- High Mountains
- Deep Valleys
Moon Rocks
Our most detailed knowledge of the Moon comes from ~382 kg of
rocks and regolith samples returned by 9 space missions:
United States:
- 6 Apollo Landings (Apollo 11-17) 1969-1972
- 12 astronauts visited maria and highlands
Soviet Union:
- 3 Luna robotic missions (1970, 1972, 1976)
- Returned rock samples by capsule
The results from these studies are summarized below.
Regolith
Regolith (literally "blanket rock") is the layer of dust and
fragmented rock produced by meteor impacts that covers the uppermost
surfaces of most moons, planets and asteroids in the inner Solar System.
Lunar Regolith:
- Single-mineral grains & rock fragments
- Impact breccias: heat-fused grains & rocks
- 4-5 meters thick in the Maria
- 10-15 meters thick in the older Highlands
Lunar History
The surface of the Moon was entirely shaped by meteor impacts and
their effects:
Maria:
- Younger (3.1 - 3.8Gyr) and lightly cratered
- Formed after the end of the epoch of heavy meteor bombardment
Highlands:
- Older (3.8 - 4.0 Gyr) and heavily cratered.
- Solidified ~4.3 Gyr ago, before which time the Moon was mostly molten.
Maria
Youngest terrain on the Moon.
- Composed mostly of dark "volcanic" basalts rich in
iron and magnesium.
- No water or hydrated minerals, unlike Earth basaltic lavas.
- Titanium content is 10x higher than Earth.
Maria are flows of magma from deep fractures in the crust:
- Caused by the impacts of asteroid-sized bodies (objects many km across)
- Ages of 3.1-3.8 Gyr: time of last major impacts
Highlands
Oldest terrain on the Moon.
- Highlands rocks are thoroughly pulverized by impacts.
- Older than the Maria as expected (3.8-4.0 Gyr)
- Age marks the end of an intense period of bombardment
that started 4.6 Gyr ago
Unusual mineral content of highland breccias:
- Suggests that the Moon was almost completely molten 4.35
Gyr ago!
The Lunar Interior
Crust:
- ~70 km thick on average, >100 in highlands
- ~10 km thick in the maria
- Thicker on far-side than near-side
Mantle:
- Solid, makes up 90% of lunar volume
- Moonquakes, triggered by tidal stresses from the Earth,
occur in the lower mantle
Core: None or very small
Is there a Lunar Core?
Moon has no global magnetic field now, so no molten core like the Earth.
Did the Moon have a core in the past?
- Very old Moon rocks show signs of residual "fossil" magnetism.
- This means that they cooled in the presence of a strong magnetic field.
Conclusion:
The Moon may have had a molten core and magnetic field 3.6-3.8 Gyr
ago, but it no longer has one.
Origin of the Moon
Any theory for the origin of the Moon must explain these facts:
- Moon has much less iron than the Earth
- Moon lacks water and other "volatiles"
- Moon rocks most resemble Earth's mantle
- Identical proportions of Oxygen isotopes in Earth and
Moon rocks (very different proportions are found in meteorites).
At least 4 basic formation models have been proposed. All of
these are testable with observations.
Theories of Moon Formation
Co-Formation:
- Earth & Moon formed together in place.
- Cannot explain the lack iron and volatiles.
Capture:
- Earth gravitationally captured the Moon
- Cannot explain the lack of iron and volatiles, or the
identical Oxygen isotope ratios.
- Also is hard to do (but not a theory-killer)
Fission:
- Moon split off from a fast-spinning proto-Earth
- Composition issues OK except the volatiles.
- Also hard to do.
Giant Impact: (favored theory)
- Proto-Earth struck by a Mars-sized body.
- Only iron-poor mantle stuff knocked off
- Moon formed from this debris.
The impact theory is currently favored.
The impact theory has many strong points in its favor:
- Impactor's iron would have been in the core, and
that core would have sunk into the Earth's mantle.
- A molten post-impact Moon would have boiled off all its
volatiles.
- Moon formed of mostly Earth's mantle debris, which
explains the compositional similarities, especially the
similar Oxygen isotope ratios.
There are still lingering questions, but it does the best so far at
explaining the observed properties of the Moon.
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Updated: 2007 November 1
Copyright © Richard W. Pogge,
All Rights Reserved.