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?