Mercury has no stable atmosphere; the gaseous environment of the planet is best described as an exosphere, a medium so rarefied that the neutral atoms of its constituents never collide. Five elements - oxygen, hydrogen, neon, sodium and potassium - have been detected by Mariner 10 and by ground-based observations.

The atoms that make up the exosphere were forced out of Mercury's crust by impacting high-energy photons and ions from the Sun and by incoming micrometeorites. The study of the exosphere will therefore provide another clue to the chemical composition of the surface.