The Moons Interior

1
The Moons Interior

• The Core
• Unlike Earth, Moon is now a completely solid body
  and contains little or no pure metals (low
  overall density)
• At most, core contains very small iron-nickel
  center (0-200 km) and is mostly solid
  metal-oxides or just dense rock
• Moon has no magnetic field
• Core may be hot enough to be in semi-liquid
  plastic state

2
The Moons Interior

• The Mantle
• Unlike Earth, Moons mantle is solid
• Composed of basaltic rock (silicates)

3
The Moons Interior

• The Crust
• Composed of the lightest rock (except for lava
  flood planes)
• 60 km thick on the Earth-facing side and 100 km
  thick on the backside
• Because of the solid interior, the Moon is
  geologically inactive
• Very small moon-quakes (Mantle-Core)
• No volcanoes or plate tectonics

4
Craters

• As with all terrestrial planets, crust became
  solid during Heavy Bombardment Period
• Original crust formed of the least dense material
  and was heavily cratered? Highlands
• Craters and early lava flooding are most
  prominent features on lunar surface

5
Lava Plains (Maria)

• Dark features on lunar surface
• Formed when large impacts cracked the crust
• Younger features than original crust
• 3.1-3.8 Gyrs vs. 4.0-4.4 Gyrs for Highlands
• Ages determined from radioactive dating but also
  from cratering rates
• Highlands have many craters
• Maria have few

6
Lava Plains (Maria)

• Multi-ringed basin forms from largest impacts
• Basins flood with lava to become Maria
• Positions opposite multi-ringed basins are
  regions of jumbled terrain where the shock-waves
  from impact were concentrated and created
  extremely large earthquakes

7
Formation of Moon

• Moon has some peculiarities that are difficult to
  account for
• Significantly different composition than Earth
• ?3.4 g/cc vs. 5.5 g/cc for Earth
• No water or volatiles on surface or chemically
  bound in surface rocks
• Similar in composition to the mantle of Earth
• Large amount of angular momentum
• Until mid-1980s, no good model for formation of
  Moon

8
Origins of Moon

• Fission hypothesis
• Earth spun so fast it broke into 2 parts (after
  differentiation)
• Why had Earth spun so fast?
• What happened to the angular momentum?
• Why is Moon not in plane of Earths equator?

9
Origins of Moon

• Co-accretion hypothesis
• Double planet from same cloud of material
• Why are composition and densities different?
• Moon is iron/nickel poor but has some oxygen
  isotopes as Earths mantle

10
Origins of Moon

• Capture hypothesis
• Moon formed inside orbit of Mercury and was flung
  out and captured by Earth
• Unlikely chain of events
• Moons speed would not have allowed capture w/o
  being ripped apart

11
Origins of Moon

• Large-Impact hypothesis
• After differentiation but before 4.4 Gyrs ago
  (oldest Moon Rocks), Earth struck by large (Mars
  size) planetesimal
• Glancing blow impact
• Impact ejected large portions of the planetesimal
  and Earths surface and mantle up into orbit
• Any iron core in planetesimal was absorbed into
  the proto-Earth

12
Origins of Moon

• Large-Impact hypothesis
• Material blown into orbit was molten, so
  volatiles would evaporate and be blown away by
  solar light or solar wind
• Orbital material (mostly mantle and crust)
  coalesced gravitationally to form Moon

13
Origins of Moon

• Large-Impact hypothesis
• Results
• Moon is iron poor (lower density)
• Moon is volatile poor (no water)
• Moon has composition similar to Earths crust and
  mantle (Oxygen isotope ratios)
• Moon has lots of angular momentum (from original
  impactor)
• Data necessary to make this model comes largely
  from rocks returned during Apollo missions