Origin of the Moon

Origin of the Moon • 22 September 2017

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

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

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

Big Muley

Evidence for 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 • 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 Far Side

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 • both differentiated • both formed near 1 AU • Earth close to final size • Mars-sized impactor

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

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

The newly formed Moon orbited at about one-tenth the distance that it does today, and spiraled outward because of tidal frictiontransferring angular momentum from the rotations of both bodies to the Moon's orbital motion. Along the way, the Moon's rotation became tidally locked to Earth, so that one side of the Moon continually faces toward Earth. Also, the Moon would have collided with and incorporated any small pre-existing satellites of Earth, which would have shared the Earth's composition, including isotopic abundances. The geology of the Moon has since been more independent of the Earth. Although this hypothesis explains many aspects of the Earth–Moon system, there are still a few unresolved problems, such as the Moon's volatile elementsnot being as depleted as expected from such an energetic impact.

Computer simulations show a need for a glancing blow, which causes a portion of the collider to form a long arm of material that then shears off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. The energy involved in this collision is impressive: possibly trillions of tons of material would have been vaporized and melted. In parts of the Earth, the temperature would have risen to 10,000 °C (18,000 °F). The Moon's relatively small iron core is explained by Theia's core accreting into that of Earth. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccreation of material in orbit around Earth would have been sufficient to melt a large portion of the Moon, leading to the generation of a magma ocean.

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

Origin of the Moon