The fact that neutral atoms are never colliding with each other is one of the proofs that the atmosphere of Mercury is virtually non-existent. It is more appropriate to speak about exosphere. Thanks to the American spacecraft Mariner 10 and to the observations performed from Earth, five composition elements have been detected : oxygen, hydrogen, neon, sodium and potassium.

The atoms that make up the exosphere are ejected from it by impact of photons at very high energies, ions from the Sun and micrometeorites. The study of exosphere will allow as well a better knowledge of the composition of the surface of the planet.