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Lecture 5: Formation of the Moon


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Lecture 5 of the course Our Moon: From Imagination to Exploration. In this lecture, I discuss various formation ideas of the Moon including capture, co-accretion, fission, the "canonical" giant impact model, and a number of the modified giant impact models.

Please see the presenter notes section for what I said about each slide in class.

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Lecture 5: Formation of the Moon

  1. 1. Our Moon Lecture 5 Formation of the Moon
  2. 2. Basic Properties Images: NASA 5.5 g/cm3 3.3 g/cm3 Sizes to scale Distance not to scale
  3. 3. Capture Model Issue Very hard to make this work for a Moon-mass object Earth Moon
  4. 4. Co-Accretion Model Image: NASA/JPL-Caltech Issues Why is the Moon significantly less dense? Angular momentum of the system*
  5. 5. Fission Model Issues Feasibility of Earth spinning that fast Earth day ≈ 3 hours! George Darwin Son of Charles Darwin Pickering (1907) Image: ESO/L. Calçada
  6. 6. Basic Properties Images: NASA 5.5 g/cm3 3.3 g/cm3 Angular momentum… Sizes to scale Distance not to scale
  7. 7. Total Angular Momentum of Planetary Systems
  8. 8. Angular Momenta of Planetary Systems
  9. 9. Apollo 15 Image & Video: NASA
  10. 10. Some Constraints Most of the angular momentum is in the Moon’s orbit Deep (1000 km?) magma ocean The Moon is less dense than the Earth The Moon seems to have less volatiles than the Earth Lunar Magma Ocean (LMO) 𝜔 5.5 g/cm3 3.3 g/cm3 vs.
  11. 11. “Canonical” Giant Impact Model Simulation: R. Canup These models are typically done using a technique called Smoothed-Particle Hydrodynamics (SPH)
  12. 12. “Canonical” Giant Impact Model Miki Nakajima
  13. 13. From: Zhang et al. (2012) The Earth’s and Moon’s Titanium Isotopes are Similar
  14. 14. “Hit and Run” Model
  15. 15. “Equal Mass” Model
  16. 16. “Fast Spinning Earth” Model
  17. 17. “Multiple Impacts” Model Rufu et al. (2017)
  18. 18. “Synestia” Model Lock et al. (2018)
  19. 19. Hosono et al. (2019) “Terrestrial Magma Ocean” Model
  20. 20. Part 1: How did the Moon form? Likely in the aftermath of a giant impact but details not definitive as of 2019
  21. 21. Timing: Hafnium—Tungsten Chronometer 182Hf → 182W Half-life ≈ 9 million years Hf is lithophilic (rock loving) W is siderophilic (iron loving) Fe Fe Fe Fe Hf Hf Hf Fe Fe ‘Early’ core formation ‘Late’ core formation Hf Hf Hf Fe W W W W W Fe W W
  22. 22. Early Timeline of the Moon Connelly et al. (2012) Moon formation 62 (+90/ -10) Myr after CAIs Touboul et al. (2009) LMO solidification ~30 Myr Perera et al. (2018) Alibert et al. (1994) Borg et al. (1999) Borg et al. (2015) Borg et al. (2015)
  23. 23. What are five important clues that help us understand the origin of the Moon? What are some other giant impact scenarios? Describe the “Canonical” Giant Impact Model Summary Angular momentum, magma ocean, volatile depletion, density difference & isotope similarity “Hit and run” model, “equal mass” model, “multiple impacts” model, … Moon formation due to a Mars-sized object impacting the proto-Earth