LECTURE 16: THE MOON
- How do we learn about the Moon?
- What are the basic geological features of the Moon?
- How do craters form?
- How can we use craters to infer the relative ages of different
parts of the Moon's surface?
- What was the era of heavy bombardment?
- What are the fission, joint formation, and capture theories of
the Moon's origin, and what are their problems?
- Why is giant impact the leading theory for the origin of the Moon?
THE MOON: BASICS
- Average: 384,000 km (0.0026 AU, 60.3 Earth radii).
- Varies by 10% in elliptical orbit.
- Growing by few cm per year due to tides on Earth.
- Radius: 1700 km (0.272 Earth radii).
- Mass: 7.3 x 1022 kg (1/80 Earth mass).
- Average Density: Mass / Volume = 3340 kg / m3
(60% of Earth density).
- More than water, less than iron, like compressed rock.
- Surface gravity: 1/6 Earth gravity.
- No atmosphere.
- No magnetic field.
- Geologically inactive (no plate tectonics, minor activity caused
by Earth tides).
INVESTIGATIONS OF THE MOON
- Naked eye. Mottled surface, see one side, radius, distance.
- Telescopes: Craters, mountains, maria.
- Unmanned missions: flybys, orbiters, crashers, landers,
sample return missions.
- Manned missions: allow detailed mapping of surface and gravity,
rock analyses, installation of seismometers and reflectors.
- Highlands: 83% of total surface
- Lightest rock on Moon
- Oldest rock on Moon, 4-4.5 Gyr
- Maria: 17% of surface, mostly on near side
- MAR-eeh-ah, singular Mare (MAR-ay), Latin for "sea."
- Not really water -- darker, heavier rock.
- Several km lower than highlands.
- Younger rock, 3.5 - 3.8 Gyr.
- Craters: everywhere, much heavier on highlands.
- Mountains: mostly near edges of craters.
- Formed by impacts of asteroids, big and small.
- Explosive impact, vaporizing rock, leads to circular shape.
- Rim of mountains, with some ejecta beyond.
- "Bounce" of crust produces central peak in big craters.
- Why are craters obvious on Moon, not on Earth?
- Earth is geologically active, surface constantly "repaved."
- Moon is inactive, preserves record.
- Also no erosion by water, wind.
A key idea, useful for investigating other planets: degree of cratering
can be used to infer age of surface.
- Why are highlands more heavily cratered than maria?
- Maria formed later, covered over cratered surface.
- Formed by lava flows, probably through cracks formed by largest impacts.
- Not much later, 3.5 vs. 4 Gyr. But dramatic difference in cratering.
- Conclusion: bombardment much heavier in early solar system.
- "Era of heavy bombardment" ended about 3.8 Gyr ago, 0.7 Gyr after
formation of sun and planets.
- Cratering rate roughly constant since then.
ORIGIN OF THE MOON: FACTS TO BE EXPLAINED
- Unusually large moon relative to size of planet.
- Orbit near ecliptic (plane of sun and planet motions), not Earth equator.
- Orbit nearly circular, not elongated.
- Composition similar to Earth's mantle. Not much iron.
- Proportions of oxygen isotopes identical to Earth, different from
- Much less volatile (easily vaporized) material than Earth.
ORIGIN OF THE MOON: THEORIES AND THEIR PROBLEMS
- Fission: Rapidly spinning, young Earth ejects
- Problems: Moon's orbit should be close to Earth equator, not ecliptic.
Why no volatiles?
- Joint Formation: Earth and Moon condense out of
common, rotating cloud.
- Problems: Same problems as fission, plus Moon should have iron like Earth.
- Capture: Moon formed elsewhere in solar system,
captured by Earth's gravity.
- Problems: Composition, especially oxygen isotopes, are too similar for
independent formation. Gravitational capture is improbable, wouldn't
produce circular orbit.
ORIGIN OF THE MOON: GIANT IMPACT
Suggested in 1970s, now leading theory for the Moon's origin.
- Proto-Earth smacked off-center by Mars-sized body.
- Knocks off mantle material, vaporizes volatiles.
- Debris settles into circular orbit, pulls together by gravity,
- Accounts reasonably well for all the main facts. Leading theory
by process of elimination.
- Implies violent early history of the solar system.
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Updated: 2005 May 14 [dhw]