Moon

1. Formation of our Moon:
The Giant Impact Hypothesis
Michelle Kirchoff
Southwest Research Institute
Center for Lunar Origin and Evolution

2. Southwest Research Institute in Boulder
http://www.boulder.swri.edu/

3. Center for Lunar Origin and Evolution (CLOE)
History of Impacts on Our Moon
 Late heavy bombardment (many
large basins forming in a short time)
-> “Nice” model or end of
accretion?
Goal: Learn more about how our Moon formed and
changed in its early history

4. Center for Lunar Origin and Evolution (CLOE)
History of Impacts on Our Moon
 Late heavy bombardment (many
large basins forming in a short time)
-> “Nice” model or end of
accretion?
Goal: Learn more about how our Moon formed and
changed in its early history

5. Center for Lunar Origin and Evolution (CLOE)
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
Goal: Learn more about how our Moon formed and
changed in its early history

6. http://cloe.boulder.swri.edu/

7. 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

8. Giant Impact Model of Moon Formation
Mars-sized body
hits Earth obliquely
& Moon forms from
debris disk
Iron core / stony mantle
Animation from Robin Canup
Early Earth
This model explains:
 Mass ratio Earth-Moon
 Earth fast rotation speed
 Lack of iron in Moon

9. > 11,000°
9100  10,000°
8200  9100°
7100  8200°
6200  7100°
5100  6200°
4200  5100°
3100  4200°
104  11,000°
Temperature
Animation from Robin Canup

10. Sea of bodies:
• Moon to Mars-sized
• smaller planetesimals
⇒ MANY
COLLISIONS
Distance From Sun (Further)
(Closer)
Elongation
of
Orbit
Very
Circular
Very
Elongated
Location of
Asteroid Belt
Animation from Alessandro Morbidelli
Large impacts are common!
Jupiter

11. 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

12. 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