There is currently no adopted Reference Atmosphere Model for Mercury. The atmosphere is more accurately described as an exosphere, where individual atoms and molecules reside mainly on the surface, until released into ballistic orbits by a combination of excitation by solar photons and energetic particles (again, primarily of solar origin). The composition is not precisely known, but O, H, He, Na, Ca and K have been detected. The temperature reflects the Mercury surface temperature, the surface pressure is of order 10-12 bar, with a number density of order 1011 m-3