It is pointed out that ions and free radicals will exist in the high atmospheres of the planets and that these ions will absorb and fluoresce in the visible and near ultra-violet radiations. Applications of this effect to the problem of the colour of Venus, the blue haze of Mars, the variation of brightness of Jupiter and the haze of Mercury are discussed. The presence of a tenuous atmosphere on Mercury is justified by its equilibrium with the local interplanetary gas.