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 <a> = 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