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?
"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.
Extending @Aaron: Which also does matter, that it is relatively common in the universe ( https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements ). For example, rubidium has a much lower ionization energy, but it can only be found in traces, while sodium is one of the most frequent elements in the stony planets.