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History of the Moon
It’s in the rocks
4/9/02
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Major features
• Maria - (Mare Imbrium) - basalt
– already known from Surveyor
– guessed at from the presence on Earth
• Highlands - anorthosite (Genesis rock)
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Basalt
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Dark color (black)
dense
fine-grained
lava
oceanic crust
most large volcanoes
– Hawaii, Iceland
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Basalt
• Forms (on Earth) by partial melting
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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
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Formation of Basalt
Surface of the Moon
cold and low pressure
hotter and higher pressure
But - there is no melting (pressure)
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After basin formation
path of lava
fractures - lower pressure
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Conclusions
• Basalts are younger than the Highlands
• They cannot tell us about the formation or
early history of the Moon
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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)
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Anorthosites (pl)
• Formed by fractional crystallization
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–
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4/9/02
from melted rock,
which cools and,
some crystals form - these are
anorthite (mineral): a Ca Al silicate and
olivine (mineral): a Mg Fe silicate
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1
powder
liquid
anorthite
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anorthite
olivine
4/9/02
olivine
2
crust
4
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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
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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
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History of the Moon
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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
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• Lava formation (old)
• lava formation (young)
• minor impacts
– Eratosthenes
– Copernicus
– Tycho
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3.8
3.2
1.0
44 million
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Frequency of impacts
5
4
3
2
1
0
Time (billions of years)
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Interior
Moon
toward
Earth
mantle
crust
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Far
side
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Asymmetry of the Moon
• Near side - (thinner crust)
– maria (impact basins)
– highlands (older crust)
• Far side - (thicker crust)
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–
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only highlands
no impact basins
a little basalt in the crater Tsiolkovsky
?
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