Origin of the Moon
August 31, 2011 Addie Dove
Why study the origin of the moon?
• How do terrestrial planets form?
• Effects of Moon on Earth?
Why study the origin of the moon?
• How do terrestrial planets form? • Effects of Moon on Earth?
• Tides • Obliquity changes
Moon Properties
• Can you name some of the distinguishing properties of the Moon?
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)
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
Clues to giant impacts
• Planets that spin fast
• Planets are tilted Image: Lynette Cook for Gemini Observatory/AURA.
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
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
Evaluate the Hypotheses
• Pros vs. Cons
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
How big of an impactor is needed to produce angular momentum
of current system?
• To the board!
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
• 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
Evolution of the Protolunar disk
Evolution of the Protolunar disk
• Accretion in the disk • collisional growth • Vesc • aRoche= closest to a planet a body can hold
together via self-gravity • aRoche= 3REarth
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
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
New discussions about the Moon’s origins
• Near-side vs. far-side dichotomy
Companion Moon?
Late Volcanism?