1. Origins of the Earth/Moon System Ryan Huberman and Aaron Bloom

2. The basics: Earth’s only satellite Diameter 27% of the Earth’s 60% as dense as the Earth Orbits at roughly 30 Earth diameters away »View of our small world from afar

3. Formation The moon formed roughly 4.5 billion years ago, 30 to 50 million years after the formation of the Solar System The most prevalent theory suggests that a large planet, Theia, roughly the size of Mars, collided with the Earth to form the moon (Giant Impact Hypothesis)

4. Theia Name derived from mythical Greek titan who gave birth to Moon goddess Theia was one of several Mars-sized bodies that existed 4.5 billion years ago –Formation of Moon fits into context of formation of Earth (dozens of collisions)

5. Alternative theories The Fission Theory –The moon separated from the Earth The Capture Theory –The moon was simply an object passing by the Earth The Condensation Theory –The Earth and moon formed from a Solar Nebula

6. …and their issues Fission Theory –Where did the energy required for separation come from (angular momentum)? –Present Earth-Moon system should contain “fossil evidence” The Capture Theory –The Earth’s gravity isn’t strong enough –The chemistry between Earth and Moon too similar The Condensation Theory –Why does the moon lack iron?

7. The prevalent theory… Russian astrophysicist, V.S. Safronov, pioneered the concept of planet aggregation from multiple planetesimals William K. Hartmann and Donald R. Davis suggested that one of several orbiting planets may have collided with the Earth This collision would form large amounts of debris around the Earth »Moon Forming out of rings of debris

8. The mechanics… The planet that collided with Earth was suggested to be roughly Mars sized (1/3 – 1/2 times the diameter of the Earth) Current computer simulations suggest an impact angle of 45 degrees and an initial impact velocity around 4 km/s

9. Choosing the theory… In 1984, an international meeting was organized in Kona, Hawaii The origin of the moon was discussed The Giant Impact Hypothesis emerged as the dominant theory

10. History of Theory 1898 – George Darwin suggests Earth and Moon had once been one body –Molten moon had been spun from Earth because of centrifugal forces 1946 – Reginald Aldworth Daly challenged Darwin’s explanation –Argued that creation of moon was caused by impact rather than centrifugal forces

11. History of Theory 1974 – William K. Hartmann and Donald R. Davis reintroduce Daly’s idea –Their model suggests at end of planet formation period, several satellite-sized bodies had formed that could collide with planets or be captured Similar approaches taken by Alastair Cameron and William Ward –Moon formed by tangential impact upon Earth of a body the size of Mars –More volatile materials emitted during collision would escape the Solar System whereas the silicates would tend to coalesce

12. Problems with the theory As recently as 15 years ago, scientists struggled to show how the particles created by the collision could have coalesced to form the moon Later work, as early as 1997, substantiated the theory more Despite controversy and alternative theories, the hypothesis has withstood 25 years of scrutiny

13. Support of the theory Explains the presence of a relatively small metallic core on the moon, relative to other planets in the solar system The idea of a moon-sized object forming from smaller objects around the earth reflects the principles behind the proto- planetary disk theory Moon craters over 150 kilometers across support the idea of an even larger object striking the earth

14. Evidence Rocks collected during Apollo Moon landings show oxygen isotope ratios identical to Earth Several physical factors such as mean density suggest radius of core is less than 25% the radius of the Moon (50% for most terrestrial bodies) Comparisons between zinc isotopic composition for the Moon with that of Earth and Mars

15. Details of the theory The Earth was probably not in a fully constructed phase, when the amount of angular momentum required to break free of Earth’s gravity and enter orbit was roughly half what it is now As the projectile hit the Earth, recent computer simulations suggest that a large portion rebounded and impacted once more. The entire process of rebounding is suggested to have only taken ~30 minutes

16. The moon’s birth Simulated snapshots of lunar formation Blue areas are metallic iron Red/Orange are rocky mantles

17. More details After rebounding, the second collision integrated the core of the impactor with the core of the Earth The debris formed by this collision eventually coalesced to form the moon Most of the debris came from the impactor

18. Planet accumulation Formation of planets during the proto- planetary disk phase of the solar system relates closely to Giant Impact Dust particles coalescing to form planetesimals lead to planetesimals colliding to form planets In the final stages of planetary formation, the angles of collisions and size of collided objected determined whether or not a moon formed

19. Time frames Collision: 30 minutes –Accounts for initial collision and rebound Moon formation: ~10 years –Extreme heat; molten debris; magma ocean –Objects potentially half the size of the original moon may have accreted during this phase »Half hour after impact

20. Five hours later

21. Further support of theory The magma ocean surrounding the early moon could explain some of the discolored basins on the moon, as impacts would have scrambled the crust Rocks of various compositions could be added to the moon from such collisions Explains certain unexplained lunar features such as discoloration and inhomogeneous composition

22. On a related note… The magma ocean theory discredits the Capture Theory, as capture would not heat the moon enough to create a molten surface Collisions on the moon would not have disrupted the mantle in the same way

23. Things to test/calculate… Initial temperature of Earth versus Moon Mantle composition Time of core formation

24. Problems… Much evidence is destroyed in the course of planetary evolution Insufficient data Imprecise scientific measurements Variances in Earth composition based on depth, pressure, temperature, amount of available oxygen and sulfur Difficult experiments to conduct Due to several parameters, a model can be tweaked to fit the data

25. More problems… Large impacts to the Earth would leave large areas made of different materials No knowledge of upper mantle versus lower mantle composition on Earth

26. Even so… The main problems associated with the Giant Impact Hypothesis are shortcomings in science rather than holes in the theory This hypothesis is the only one proposed that can explain the compositional and structural characteristics of the Moon Ockham’s Razor –We could come up with more complex theories, but there would be no reason to embrace them

27. Conclusion The Giant Impact Hypothesis has yet to be proven wrong, or even strongly argued against Other theories simply have too many holes International agreement on plausibility of theory

28. Bibliography Origin of the Earth and Moon,1998, LPI Contribution No. 957, Lunar and Planetary Institute, Houston. Hartmann, W. K. and Ron Miller 1991. The History of Earth, (New York: Workman Publishing Co.) The Origin of the Moon website at the Planetary Science Research Institute.The Origin of the Moon Building Planets at PSI: the Origin of the Solar System website at the Planetary Science Research Institute.Building Planets at PSI: the Origin of the Solar System http://www.knowledgerush.com/kr/encyclopedia/Giant_impact_theory/ http://www.astronomynotes.com/solarsys/s13.htm http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question38.html http://www.psi.edu/epo/moon/moon.html