GEOL 3045: Planetary Geology
Lysa Chizmadia
Formation of the Moon
Lunar Eclipses
Images from: http://en.wikipedia.org/wiki/Lunar_eclipse
20 Feb 2008
http://sunearth.gsfc.nasa.gov/eclipse/LEmono/TLE2008Feb21/TLE2008Feb21.html
Formation Theories
Fission: Moon was originally part of the Earth
& somehow separated early
Problem: compositions not ~ enough
Capture: Moon was created somewhere else
and captured by Earth’s gravity
Problem: O-isotopes identical
Mutual Accretion: They both formed together
out of the same materials
Problem: Moon has little or no core
Big Splash: ~Mars-sized bolide hit Earth &
Moon formed from ejecta ring
Fits O-isotopes, trace elements & low density of
Moon
Big Splash (1/3)
Impactor ~ size of Mars
Called Tethys
Collided ~4.4 Ga
Sprayed crust & some of
mantle into space
Both proto-Earth & Tethys
molten
Core of impactor
incorporated into Earth’s
core
Images from: http://www.psi.edu/hartmann/pic-cat/earth.html
Big Splash (2/3)
Ejecta from impact forms ring
around molten Earth
Both have magma oceans
Ring coalesces into Moon
Crust of both bodies cool down
No longer magma oceans
Moon sweeps up rest of ring
material
Images from: http://www.psi.edu/hartmann/pic-cat/earth.html
Big Splash (3/3)
Volcanism on Moon
Late Heavy Bombardment
Volcanism subsides on
Moon
Images from: http://www.psi.edu/hartmann/pic-cat/earth.html
Late Heavy Bombardment
3.8 - 4.1 Ma
Basin-forming impacts on
Moon, Earth, Mercury,
Venus, Mars
E.g. Caloris Basin, Mare
Basins
Materials sent in to inner
solar system by
perturbations from Giant
Planets
Image from: http://www.daviddarling.info/encyclopedia/L/late_heavy_bombardment.html
Lunar Volcanism
Mare Volcanism
Most 3.5 - 3.0 Ga
Ages range from 4.2 to 1.2 Ga
Occurs mostly on near side
Classification:
High Ti basalt
Low Ti basalt
Very low Ti basalt
Ti-rich Orange glass
3.7-3.8 Ga
Images from: http://www.psrd.hawaii.edu/Dec00/highTi.html
KREEP
K = Potassium
REE = Rare Earth
Elements
P = Phosphorus
Formed as a result of
magma ocean
Heavy minerals (e.g. olivine,
pyroxene) sank
Light minerals (e.g.
Plagioclase) rose
Fractional Crystallization
made magma KREEP-rich
KREEP = incompatible
Images from: http://www.lunarpedia.org/index.php?title=KREEP
Summary
Formed from Big Splash
Problems with other models
Moon ~ Earth composition, but not
identical
Moon has low density
Moon & Earth have same O-isotopes
Heavy Late Bombardment
Perturbations from Giant Planets
Lunar Volcanism
Ti important element
KREEP
Leftovers from fractional crystallization
of magma ocean