the moon - Why is sodium such a common ion for 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.