text: The Moon stands out as being very depleted in volatiles.
  The Moon not only lacks water and atmospheric gases, but also lacks moderately volatile elements such as K, Na, and Cl. The Earth's K/U ratio is 12,000, while the Moon has a K/U ratio of only 2,000. This difference suggests that the material that formed the Moon was subjected to temperatures considerably higher than the Earth.
  The prevailing theory is 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.  This is called the Giant-impact hypothesis.
  It is hypothesized that most of the outer silicates of the colliding body would be vaporized, whereas a metallic core would not. Hence, most of the collisional material sent into orbit would consist of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision probably would escape the Solar System, whereas silicates would tend to coalesce.
  The ratios of the Moon's volatile elements are not explained by the giant-impact hypothesis. If the giant-impact hypothesis is correct, they must be due to some other cause.