Origin of the Moon

Origin of the Moon 7 September 2012

Why study the origin of the moon?

• How do terrestrial planets form? • Effects of Moon on Earth? • Pluto’s moons formed the same way?

Why study the origin of the moon?

• How do terrestrial planets form? • Effects of Moon on Earth:

• Tides • Obliquity stabilized

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

Why 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 (I=0.39)

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

• heat removes volatiles How hot is the Impact? from debris disk

Evolution of the Protolunar disk • Centrally condensed hot disk = 2.53REarth • 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)

•

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

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

ReAccretion & the postimpact moon

• Earth spin and Moon orbit locked • Moon orbit expands a few cm/yr • Earth rotation slows

ReAccretion & the postimpact 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 postimpact 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