Origin of the Moon

Origin of the Moon

2 September 20152 September 2015

Why study the origin of the moon?

• How terrestrial planets form: they build up from impacts between smaller objects

• Moon effects on Earth: tides, change Earth spin

• Pluto’s moons likely formed the same way, from a giant impact on Pluto

Why study the origin of the moon?

• Effects of Moon on Earth:

• Tides

• Obliquity stabilized

• Day and month changes

terrestrial planets formation

• Disk of gas and dust around Sun

• Interparticle collisions: if impact velocities are low enough, we get gravitationally bound aggregates

• 10,000 yrs: 10 km-sized bodies

• 100,000 yrs: Moon-Mars sized (~2000 km, ~20 “embryos”)

• 1 million-10 million yrs: planet-sized “giant impacts” will reduce number of embryos to 4 terrestrial planets

Evidence for giant impacts

• Planets spin faster than they orbit

• Planets are tilted to orbital revolution

Moon Properties

• Name some of the distinguishing properties of the Moon…

Moon Properties

1.Earth has only 1 Moon

2.Depleted in Fe and volatiles;

3.Oxygen isotopes similar to Earth

4.Moon’s orbit:

• is not in Earth’s equatorial plane

• Circular

• Expanding due to tidal interaction

5.Moon has very small core

Moon Origin Hypotheses

• Co-accretion: Earth and Moon formed together. Like sister

• Fission: Earth spun so fast that it split off a Moon-sized chunk. Like daughter

• Capture: Earth captured an independently-formed Moon as it passed by. Like wife. THESE WERE THE 3 HYPOTHESES BEFORE APOLLO!

• Giant Impact: Mars-sized body collided with proto-Earth and excavated material eventually coalesced to form Moon

Evaluate the Hypotheses

Co-accretion: Moon has little iron, volatiles.

Fission: Earth never spun fast enough

Capture: too unlikely

AFTER APOLLO WE STILL HAD THE SAME THREE POSSIBILITIES

Giant Impact Stages• Earth close to final

size

• Mars-sized impactor

• both differentiated

• both formed near 1 AU

Where does Iron go?

Where does Iron go?•Both Fe cores stay with Earth•1 lunar mass in orbit outside Roche radius•Moon is mostly impactor material

How hot is the Impact?

•heat removes volatiles from debris disk

Evolution of the Protolunar disk

• Centrally condensed hot disk <a> = 2.5-3REarth

• Cooling: condensation/solidification

• Collisional spreading of disk

• Accretional growth of moonlets

• Tidal evolution of moonlets

• Collisions between moonlets yield moon

the post-impact moon

• Impact: Mars-sized body collides with Earth

• Debris ejected into Earth orbit

• A. heated

• B. comes from mantle (no Fe)

• C. ~1 lunar mass = ~1% Earth mass = ~10% impactor mass

• Debris accumulates to form one large Moon, not multiple small moons… but maybe a second, smaller moon hits it later

ReAccretion & the post-impact moon

• Earth spin and Moon orbit locked

• Moon orbit expands a few cm/yr

• Earth rotation slows: conservation of angular momentum

ReAccretion & the post-impact moon

• In the past, which is a possible state of the Earth/Moon system?

•A. Moon orbits closer in, Earth’s day is 18 hours

•B. Moon orbits farther away, Earth’s day is 36 hours

•C. Moon orbits closer in, Earth day is same as now

•D. Same conditions as today

ReAccretion & the post-impact moon

• In the past, which is a possible state of the Earth/Moon system?

•A. Moon orbits closer in, Earth’s day is 18 hours

•B. Moon orbits farther away, Earth’s day is 36 hours

•C. Moon orbits closer in, Earth day is same as now

•D. Same conditions as today