History of the Moon

1
History of the Moon

• Its in the rocks

2
Major features

• Maria - (Mare Imbrium) - basalt
• already known from Surveyor
• guessed at from the presence on Earth
• Highlands - anorthosite (Genesis rock)

3
Basalt

• Dark color (black)
• dense
• fine-grained
• lava
• oceanic crust
• most large volcanoes
• Hawaii, Iceland

4
Basalt

• Forms (on Earth) by partial melting
• pre-existing rock
• heated until part of the rock melts
• liquid (molten rock) moves to the surface
• extruded on surface as lava flow
• this is a secondary rock

5
Formation of Basalt
But - there is no melting (pressure)
6
After basin formation
7
Conclusions

• Basalts are younger than the Highlands
• They cannot tell us about the formation or early
  history of the Moon

8
Anorthosite

• Rare on Earth
• Adirondack Mountains
• Wyoming
• a few other places
• Very old (several billion years) on Earth
• but not the oldest rock
• No young examples
• Origin on Earth is unknown (and controversial)

9
Anorthosites (pl)

• Formed by fractional crystallization
• from melted rock,
• which cools and,
• some crystals form - these are
• anorthite (mineral) a Ca Al silicate and
• olivine (mineral) a Mg Fe silicate

10
(No Transcript)
11
Conclusions

• The original crust of the Moon is the rock
  anorthosite
• therefore, the outer part (or the whole thing) of
  the Moon was molten, early on
• a magma ocean
• the olivine (similar to basalt) sinks and is then
  remelted when the large basins form

12
Age dating

• Look at radioactive elements
• with time, the parent (radioactive) element
  decays (decreases in quantity) to the daughter
  element(s) which increase in quantity
• rate is fixed
• need rocks from the Moon

13
History of the Moon

• Formation 4.6 billion years ago
• melting (short time)
• crust formation
• Crust cools 4.4
• impact basins (old) 4.4?
• impact basins (young) 3.9

14

• Lava formation (old) 3.8
• lava formation (young) 3.2
• minor impacts
• Eratosthenes
• Copernicus 1.0
• Tycho 44 million

15
(No Transcript)
16
Interior
17
Asymmetry of the Moon

• Near side - (thinner crust)
• maria (impact basins)
• highlands (older crust)
• Far side - (thicker crust)
• only highlands
• no impact basins
• a little basalt in the crater Tsiolkovsky
• ?