Earth’s Moon

1. Earth’s Moon

2. PHYSICAL PROPERTIES • DISTANCE: 356,000 - 407,000 km (59.5 RE) • DIAMETER: 3,476 km (0.27 DE) • MASS: 7.35 x 1022 kg (0.0123 ME) • DENSITY: 3.34 g/cm3 (0.61 ρE) • GRAVITY: 0.17 g • PERIOD: 27.322 days • ATMOSPHERE: Essentially none

3. 0 The History of the Moon Moon: small; low mass→ rapidly cooling off; small escape velocity → no atmosphere→ unprotected against meteorite impacts. Moon must have formed in a molten state (“sea of lava”); Heavy rocks sink to bottom; lighter rocks at the surface No magnetic field→ small core with little metallic iron. Surface solidified ~ 4.6 – 4.1 billion years ago. Alan Shepard (Apollo 14) analyzing a moon rock, probably ejected from a distant crater. Heavy meteorite bombardment for the next ~ ½ billion years.

4. Moon’s Interior

5. Implications of Size • Smaller worlds cool off faster and harden earlier • Larger worlds remain warm inside, promoting volcanism and tectonics • Larger worlds undergo erosion because their gravity retains an atmosphere

6. Question The diameter of the Moon is about ¼ the diameter of the Earth. Assume both were heated to about the same temperature when they formed. How much faster would the Moon cool down compared to the Earth? ___ times faster. (Hint: Remember … heat loss time is proportional to ….?) ¼ 1/16 4 16 64

7. Processes that Shaped Surface • Impact cratering • Impacts by asteroids or comets • Volcanism • Eruption of molten rock onto surface • Tectonics • Breakup of crust by internal stresses • Erosion • Surface changes made by wind, water, or ice

8. 0 Impact Cratering Impact craters on the moon can be seen easily even with small telescopes. Ejecta from the impactcan be seen as bright rays originating from young craters Craters are about 10 times wider than object that made them. Small craters greatly outnumber large ones.

9. History of Impact Cratering Rate of impacts due to interplanetary bombardment decreased rapidly after the formation of the solar system. Most craters seen on the Moon’s (and Mercury’s) surface were formed within the first ~ ½ billion years.

10. Impact Craters Meteor Crater (Arizona) Tycho (Moon)

11. Question • The rate of cratering of planets and moons mostly died out about 4 billion years ago when _______. • the last dinosaur was struck on the head and killed by the last meteorite • the Sun ignited and started generating heat • no more comets were left in the Oort cloud • the planets in the solar system were fully formed and much of the debris in orbit around the Sun was almost fully depleted • none of the above

12. Cratering of Moon • Some areas (light highlands … older regions)… heavily cratered … • … Some are not (dark maria… younger regions) … flooded by lava after heavy cratering period

13. Highlands and Maria Mare (dark colored) Highlands (light colored)

14. Cratering of Moon Cratering map of Moon’s entire surface

15. 0 Formation of Maria Impacts of heavy meteorites broke the crust and produced large basins that were flooded with lava

17. Apollo 11 — July 1969

18. Water on the Moon?

20. Rotation and Revolution are ‘synchronous’ … 1:1 … caused by ‘tidal locking’

24. Tidal Forces Create ‘Bulges’

29. Question The gravitational pull of the Sun on the Earth is stronger than the gravitational pull of the Moon on the Earth. Which object, Sun or Moon, is more important in causing tides? Neither—tides are caused by Earth’s rotation. The Sun has stronger pull and clearly raises tides. Earth’s orbit is stable because the gravitational effects of the Moon and the Sun just balance. Thus they are equally important in causing tides. Neither—whales swim twice a day from the Antarctic to the equator and this causes the tides. The Moon—because the difference between the near-side and far-side gravitational forces on Earth caused by the Moon is greater than the difference between the near-side and far-side gravitational forces caused by the Sun.

30. Formation Hypothesis #1 Capture - The Earth captures a moon adrift in space • Pros:well, it's not impossible • Cons: • it's very unlikely • kinetic energy must be carried away by a 3rd body

31. Formation Hypothesis #2 Accretion - The Earth accretes planetesimals in early solar system to form Moon. • Pros: • Lots of material around in the early solar system to accrete • Cons: • Why would the Earth and Moon have different compositions • (i.e., why is the moon deficient in Iron)? • Earth-Moon system has too much ‘angular momentum’ • compared to other planets.

32. Formation Hypothesis #3 Fission - A fast spinning Earth "calves" the Moon. • Pros: • Density of Moon similar to that of the outer layers of the Earth • Cons: • Moon should be orbiting along Earth's equator; it's not. • Composition of Moon rocks dissimilar to that of Earth's crust. • Earth would have to have been spinning extremely fast.

33. Formation Hypothesis #4 • Impact─ A large planetesimal hits Earth, ejecting material which forms the Moon

34. Impact Simulation • Ejecta consists mostly of crust and mantle material • Lacks iron • Lacks volatiles

35. Collisional Hypothesis • Pros: • Collisions happen (but this is a whopper: Mars sized or bigger!) • Explains lack of volatiles in Moon • Explains lack of an Iron core • Explains same Oxygen isotope composition • Simulations:confirm the possibility • Bonus:explains tip of the Earth's axis! • ─ Cons: • … none really …

36. Question • The Moon was most likely formed by ________ since its average density is similar to that of Earth’s outer layers and Moon rocks are deficient in iron and volatile elements. • capturing a giant asteroid that passed to close to Earth • co-formation along with the Earth • matter that accreted after being ejected from Earth’s crust and mantle by the impact of a large, Mars-sized asteroid • breakup of a much larger planetary-sized object • material captured from the solar wind