The Origin of the Moon

''The evolution of the world can be compared to a display of fireworks that has just ended; some few red wisps, ashes and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanishing brilliance of the origin of the worlds.''  
-- Georges Lemaitre

Begin with the origin of the solar system:

• The solar nebula was a giant ball of gas and dust
• Over the eons, the inexorable force of gravity collapsed it in size
• The Sun formed as an immense central bulge
• The planets, satellites, asteroids and comets formed from the debris (why didn't they fall into the Sun?)

Consider what we learned from the manned missions to the Moon:

• The Moon is ancient, but not primordial
• Around 4.5 billion years ago, the Moon was so hot that the surface melted, forming a magma ocean
• The Earth and the Moon show evidence of a common ancestry
• The Moon lacks iron and volatile elements (which would form water, and atmospheric gases)
• The oldest rocks on Earth are as old as the youngest rocks found on the Moon

Is the Moon ...

• A sister planet to the Earth? (formed from similar raw materials in the early solar system nebula)
• A captured asteroid? (requiring a near-perfect alignment of orbital trajectories and velocities at approach)
• A spun-off fragment? (perhaps a chunk of mantle from the Pacific Ocean basin)
• A relic of a near-catastrophic collision? (heated by the force of impact)

What are the key pieces of data, and how well do our four hypotheses explain them?

• Similarities to Earth:
  • The Earth and the Moon have the same fractional amounts of various oxygen isotopes, suggesting that they formed from materials found at a similar orbital radius (the same distance from the Sun)

• Differences from Earth:
  • The Moon has a significantly lower overall density than the Earth (3.3 versus 5.5 grams per cubic centimeter), too low for the Moon to contain a heavy iron core
  • Surface samples contain fewer volatile elements, suggesting that they were baked out (or evaporated under heat, akin to heating a towel in the clothes dryer to remove water)

Images of the Earth and Moon, and astronaut Buzz Aldrin's footprint in the lunar dust. [NASA]