• Learn about:
  • a typical large satellite
  • the Earth
  • the early history of the solar system – why?
• The Moon's surface:
  • unmodified by atmosphere or water erosion
  • Reflects history of impacts and early volcanism

The Moon: Basic Facts

• Reminder:
  • The Moon is in synchronous rotation because of tidal friction.
  • On account of librations, 59% (not 50%) of the Moon’s surface can be seen from Earth.
• Mass: 1/80 of Earth's; no gases or water (ice??) - no atmosphere
• Density is 3.3 gm/cm³ , Earth's is 5.5
• Two types of surfaces: "seas" or maria (the lowlands) and the heavily cratered highlands
• Most of our knowledge is from the Apollo program: 1968 - 72:
  • From rocks learn about age, composition, history of impacts, etc
• Little or no seismic activity: a dead world - no magnetic field.

The Moon: Highlands and Lowlands

• Moon’s surface has lighter (higher elevations) and darker (lower elevations).
• The dark areas were caused by extensive volcanism early in Moon’s history. Surface is younger than highlands.
• Galileo noted that the darker areas are relatively smooth. He called them "seas" or Maria (plural). (Mare is singular).
• Maria concentrated on side of moon that faces the Earth. Most of rest of Moon called "highlands".
• Maria concentrated on lunar near side (maria shown in blue/violet).

Clementine Mosaic of the Moon

Lunar Regolith

• The lunar surface covered with dust, most of it is lunar crust that has been pulverized by impacts. About 10m thick.

Lunar Highlands

• Lighter color.
• Richer in calcium (Ca) and aluminum (Al).
• Saturated with craters.
• An old surface, not altered since heavy cratering era of planetary formation.

Age of Lunar Surface

• Apollo samples (some 400 kg of rock returned by six lunar missions) show:
  • Age of highlands: ~ 4 billion years
  • Oldest highland rock shown is 4.4 billion years old!
  • Age of lowlands or maria: ~3.5 billion years

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Crater Formation

• Impact at speeds up to 73 km s^-1 but not more – WHY?
• Deep penetration and vaporization
  • A rock (R = 1 km) at 73 km s^-1 carries 3 x 10²² joules of kinetic energy that is converted into heat at impact.
  • This energy is equivalent to 10 million Megatons of TNT.
• Circular crater and ejecta
  • The 1 km rock produces a complex crater 100 km diameter with 5 km high walls.
  • This is the size of some of the more prominent lunar craters such as Copernicus 
• Ejecta form walls and a surrounding blanket.
  • "Rebound" produces a central peak.
  • Surface underneath is fractured.
• A "glancing blow" can produce a chain of secondary craters.

Cratering History of the Moon

• Short period for formation: lunar highlands formed a few 100 Myr after formation of moon itself, about 4.6 Byr ago.
  • Almost all measured rock samples have very narrow age range: 3.8-4.0 Byr ago. Older rocks have been completely pulverized
  • Unlike Earth, erosion is inefficient at obliterating craters.
• Impact craters are swept away by big lava flows and new ones form.

Lunar Maria

• Richer in magnesium (Mg) and iron (Fe)
• Younger surfaces.
  • Formed after the highlands by flooding of low-lying areas.
  • Note comparatively little cratering of maria.

Formation of Maria

• Round shape implies that giant impacts triggered their formation.
• It took several 100 Myr after impact before lava welled up from below.
• Ages are 3.8-3.1 Byr: younger than highlands, but formed over long period of time.
• Relative lack of craters implies younger age than heavily cratered highlands.
• Lunar samples proved that they were from lava flows.
• Only a few 100 meters thick
• Lava from 100s of kms below surface, probably due to radioactive heating
• No evidence for any volcanic activity after formation of maria: the moon froze – not enough radioactive elements so no heat
• Age of Maria probably 3.8 to 3.1 Byr
  • Oldest rocks from other areas are 4.6 Byr

Summary of Steps to Form Maria

• Ejection of large amount of material
• Formation of central peak
• Ringed structure
• Flow of lava into the crater through cracks in the floor
• Heat from radioactivity in interior melts the rock.

Lunar Surface

• No evidence for volcanoes (in contrast to Earth, Venus, and Mars).
• But: Rilles, domes, and wrinkled ridges are evidence of past lava flows.

Key Generalization for Entire Solar System

• Can get good estimate for age of planetary surface (or that of a satellite) from number density of craters and location within solar system:
  • high density of craters - old
  • low density - young, surface renewal
  • how far from sun?
  • how close to giant planets?

  

Differences Between Lunar and Terrestrial Rocks

• All lunar rocks are igneous.
• Lunar rocks do not have a trace of water.
  • Earth rocks contain up to 3% water.
• Iron in lunar rocks is not oxidized.
• Lunar rocks are depleted in elements with low boiling points.

Summary Comparison of Surfaces of Earth and Moon

• Mountains: No mountains like the earth; all lunar mountains due to impacts
• No evidence for plate tectonics or erosion Impact cratering - no volcanoes, but much lava.
• Lower gravity means impacts throw out much more material

Lunar Interior

• Moon is differentiated, yet geologically dead, i.e. inactive.
  • How do we know differentiated?
  • Know mass and surface density
    • pre-Apollo - good guess
    • post-Apollo - a rock in the hand is worth two on the moon.
  • Smaller, less massive than Earth, so shorter cooling time after formation; less radioactivity.

Lunar Atmosphere

• Moon has virtually no atmosphere –WHY?
• Retention of atmosphere depends on:
  • escape velocity from planet’s surface
  • typical speed of particles in atmosphere
  • depends on their mass and temperature
  • planets most able to retain atmosphere are thus massive (high escape velocity) and cold (slow moving molecules in atmosphere

Origin of Moon

• Probable origin: impact of Mars-sized protoplanet with differentiated Earth.
  • Accounts for composition differences.

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