Origin of the Moon 2 September 2015

Why study the origin of the moon? How terrestrial planets form: they build up from impacts between smaller objects Moon effects on Earth: tides, change Earth spin Pluto’s moons likely formed the same way, from a giant impact on Pluto

Why study the origin of the moon? Effects of Moon on Earth: Tides Obliquity stabilized Day and month changes

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 Should produce ~20 terrestraial planetary “embroys” on circular orbits

Evidence for giant impacts Planets spin faster than they orbit Planets are tilted to orbital revolution Should produce ~20 terrestrial planetary “embroys” on circular orbits

Moon Properties Name some of the distinguishing properties of the Moon… Lunar density vs. earth density Lunar similarity to earth’s mantle Angular momentum Expansion of lunar orbit Circular orbit

Moon Properties Earth has only 1 Moon Depleted in Fe and volatiles; Oxygen isotopes similar to Earth Moon’s orbit: is not in Earth’s equatorial plane Circular Expanding due to tidal interaction Moon has very small core Lunar density vs. earth density Lunar similarity to earth’s mantle Angular momentum Expansion of lunar orbit VOLATILE: evaporates at lower temps (water, N, etc) Circular orbit

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 Flip back to “moon properties” slide to help them fill in the table

Giant Impact Stages Earth close to final size both differentiated 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 Where does Iron go?

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

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

the post-impact moon Impact: Mars-sized body collides with Earth Debris ejected into Earth orbit A. heated B. comes from mantle (no Fe) C. ~1 lunar mass = ~1% Earth mass = ~10% impactor mass Debris accumulates to form one large Moon, not multiple small moons… but maybe a second, smaller moon hits it later

ReAccretion & the post-impact moon Earth spin and Moon orbit locked Moon orbit expands a few cm/yr Earth rotation slows: conservation of angular momentum

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 A

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