Why is sodium such a common ion for in ion tails? I was doing research about ion tails of planetary bodies and noticed that ion tails composed of sodium were common. For example, Mercury and the Moon both have ion tails made of sodium. Why is sodium, as opposed to other ions, commonly seen in ion tails?
From http://science.nasa.gov/science-news/science-at-nasa/2000/ast26oct_1/:


  "When a Leonid meteoroid hits the Moon it vaporizes some dust and
  rock," explains Jody Wilson of the Boston University Imaging Science
  Team. "Some of those vapors will contain sodium (a constituent of Moon
  rocks) which does a good job scattering sunlight. If any of the impact
  vapors drift over the lunar limb, we may be able to see them by means
  of resonant scattering. They will glow like a faint low-pressure
  sodium street lamp."


Also, relative to other elements, sodium has a low ionization energy.