Formation of our Moon:
The Giant Impact Hypothesis
Michelle Kirchoff
Southwest Research Institute
Center for Lunar Origin and Evolution
Southwest Research Institute in Boulder
http://www.boulder.swri.edu/
Center for Lunar Origin and Evolution (CLOE)
Goal: Learn more about how our Moon formed and
changed in its early history
History of Impacts on Our Moon
 Late heavy bombardment (many
large basins forming in a short time)
-> “Nice” model or end of
accretion?
Center for Lunar Origin and Evolution (CLOE)
Goal: Learn more about how our Moon formed and
changed in its early history
History of Impacts on Our Moon
 Late heavy bombardment (many
large basins forming in a short time)
-> “Nice” model or end of
accretion?
Center for Lunar Origin and Evolution (CLOE)
Goal: Learn more about how our Moon formed and
changed in its early history
1) History of Impacts on Our Moon
 explore different rates of impacts
 Computer models constrained by new information about evolution of comet
and asteroid populations
 Analyze chemistries and ages of early Earth and Moon rocks
 Analyze new images of Moon's surface to create a timeline of impact craters
2) Formation of Our Moon: Giant Impact Theory
 How the disk evolved into the Moon we see today
 Study physics of the disk -> motion, temperature
 Powerful computer models constrained by information about chemistry of
early Earth and Moon rocks
http://cloe.boulder.swri.edu/
Properties of the Moon
 Mass ratio of Earth to Moon -> large moon
 Moon formed near a rapidly-rotating Earth -> 5 vs. 24 hours
 Moon is depleted in iron
Giant Impact Model of Moon Formation
Mars-sized
body
hits Earth obliquely
& Moon forms from
debris disk
Early Earth
This model explains:
 Mass ratio Earth-Moon
 Earth fast rotation speed
Iron core / stony mantle
Animation from Robin Canup
 Lack of iron in Moon
Temperature
> 11,000°
104  11,000°
9100  10,000°
8200  9100°
7100  8200°
6200  7100°
5100  6200°
4200  5100°
3100  4200°
Animation from Robin Canup
Large impacts are common!
Very
Elongated
Elongation of
Orbit
Sea of bodies:
• Moon to Mars-sized
• smaller planetesimals
Location of
Asteroid Belt
⇒ MANY
COLLISIONS
Jupiter
Very
Circular
(Closer)
Distance From Sun
Animation from Alessandro Morbidelli
(Further)
Lunar Accretion Simulations
• Models allow us to track disk
particles forming into Moon
• The Moon could form in as
short as a few years or as long
as 10,000 years
Issues
 Moon forms too fast and hot => completely molten
– oxide, siderophile, and volatile ratios different than
expected (e.g., water!)
– diversity of basalts
– crust too thin
– global cracks from cooling
 New computer model
 Late veneer
 ~ 80% of material from impactor, but Earth-Moon
oxygen isotope ratios identical