Formation of our Moon: The Giant Impact

HypothesisMichelle Kirchoff

Southwest Research InstituteCenter for Lunar Origin and Evolution

Southwest Research Institute in Boulder

http://www.boulder.swri.edu/

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

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

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

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

Iron core / stony mantleAnimation from Robin Canup

Impactor Trajectory

Early Earth

This model explains: Mass ratio Earth-Moon Earth fast rotation speed

Lack of iron in Moon

> 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

Sea of bodies:• Moon to Mars-sized• smaller planetesimals

⇒ MANY COLLISIONS

Distance From Sun (Further)(Closer)

Elon

gatio

n of

O

rbit

Very Circular

Very Elongated

Location ofAsteroid Belt

Animation from Alessandro Morbidelli

Large impacts are common!

Jupiter

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