origin of the moon september 1, 2010 bonnie meinke september 1, 2010 bonnie meinke
Post on 22-Dec-2015
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TRANSCRIPT
Why study the origin of the moon?
• How do terrestrial planets form?
• Effects of Moon on Earth?
• Tides
• Obliquity changes
How terrestrial planets form
• 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
Moon Properties
1.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
• Fission: Earth spun so fast that it split off a Moon-sized chunk
• Capture: Earth captured an independently-formed Moon as it passed by
• Giant Impact: Mars-sized body collided with proto-Earth and excavated material eventually coalesced to form Moon
Giant Impact Stages• Earth close to final
size
• Mars-sized impactor
• both differentiated
• both formed near 1 AU
Where does Iron go?•Both Fe cores stay with Earth•1 lunar mass in orbit outside Roche radius•Moon is mostly impactor material
QuickTime™ and aBMP decompressor
are needed to see this picture.
ReAccretion & the post-impact moon
• Mars-sized body collides with Earth
• Debris ejected into Earth orbit
A. heated
B. comes from mantle of both bodies (no Fe)
C. ~1 lunar mass = ~1% Earth mass = ~10% impactor mass
• Debris accumulates to form one large Moon, not multiple small moons
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
ReAccretion & the post-impact moon
• Earth spin and Moon orbit locked
• Moon orbit expands a few cm/yr
• Earth rotation slows
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