1. Origin of the Moon
13 February 2018

7. 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

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

10. 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

11. Big Muley

12. 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

13. 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

14. 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

15. Lunar Far Side

16. 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

18. 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

19. Giant Impact Stages Earth close to final size both differentiated
Mars-sized impactor
both differentiated
both formed near 1 AU

22. Where does Iron go?

23. 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?

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

25. Evolution of the Protolunar disk
Centrally condensed hot disk <a> = REarth
Cooling: condensation/solidification
Collisional spreading of disk
Accretional growth of moonlets
Tidal evolution of moonlets
Collisions between moonlets yield moon

26. Moon Video

27. 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

28. 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

29. The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago; about 20 to 100 million years after the solar system coalesced. The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.[Analysis of lunar rocks, published in a 2016 report, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies

30. The newly formed Moon orbited at about one-tenth the distance that it does today, and spiraled outward because of tidal friction transferring 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 elements  not being as depleted as expected from such an energetic impact.

31. 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 re-accretion 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.

32. 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

33. 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

34. The giant-impact hypothesis is currently the favored scientific hypothesis for the formation of the Moon. Supporting evidence includes:
Earth's spin and the Moon's orbit have similar orientations.
Moon samples indicate that the Moon’s surface was once molten. 
The Moon has a relatively small iron core.
The Moon has a lower density than Earth.
Evidence exists of similar collisions in other star systems.
Giant collisions are consistent with the leading theories of the formation of Solar Systems.
The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin

35. Summary
Apollo mission had the scientific objective to discover the origin of the Moon
It did not, but Apollo results were key in developing later models
The Moon is not the sister, daughter or wife of Earth
Instead, it was created form a giant collision: the Big Splash, when Earth was struck by a Mars-sized object

36. Discussion
Was Apollo worth it?
What could have improved its return?