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Unraveling the History of
the Moon
The Lunar Surface
• Highlands
• low density rocky slag formed on top
of molten Moon
• heavily cratered
17% of Lunar surface
Huge lava flows from volcanoes
Basalts - similar to volcanic rock on Earth
• but no water
• fewer volatile (= easily melted)
• Much less iron.
Crater Bullialdus
Craters as Chronometers
The number of craters on a surface
can be used to estimate its age:
older surfaces have been exposed to
impacts for longer and show more
Any changes in the impactor population over time
Secondary impacts formed by ejecta from a single large
Saturation: when a surface is so heavily cratered that a
new crater can form only at the expense of an older one
There is a need for measured surface ages in order to
calibrate the crater counting
Solid line: trend for southern highlands
Dotted points: average for Maria
Calibrating the rate of lunar
Moon rocks
(mostly recovered
during the Apollo
missions but a few
by the Soviet Luna
Dating Rocks
Radioactive elements
• U-235 half-life = 710 million years
• U-238 half-life = 4.5 billion years
• Th-232 half-life = 13.9 billion years
Ages of lunar features
Heavily cratered highlands: 4.1-4.5 billion years
Large basins: 3.9 – 4.1 billion years old
Maria flooding: 3.0 – 3.9 billion years old
Compared to the Earth, the typical lunar surface
is very ancient
High early cratering rate
Late heavy bombardment?
A proposed interval about 3.8 to 4
billion years ago in which the moon
and the inner solar system were
subjected to heavy asteroid
Time of the formation of impact
The crust facing the earth may be
thinner than on the farside
The Interior of the Moon
Center of mass
offset 2 km
towards Earth.
Most maria are
on near side.
Crust is 107
km thick on
far side.
Crust ~ 0 km
thick on near
From Modern Astrophysics, by Carroll & Ostlie.
3476 km
• Geologically inactive.
• No magnetic field  no molten core.