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

• Distance:
  • 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.

SURFACE FEATURES

• 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.

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.

HEAVY BOMBARDMENT

• 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.

• 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 meteorites.
• Much less volatile (easily vaporized) material than Earth.

ORIGIN OF THE MOON: THEORIES AND THEIR PROBLEMS

• Fission: Rapidly spinning, young Earth ejects Moon.
• 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, forms Moon.
• Accounts reasonably well for all the main facts. Leading theory by process of elimination.
• Implies violent early history of the solar system.