Brief history of the Martian 'violet haze' problem.

A brief but thorough survey of the literature on the Martian 'violet haze' problem is presented. It is evident that both the normal lack of contrast of the surface features in violet light and their occasional appearance are phenomena intrinsic to Mars. Models involving simple uniform layers of scattering or absorbing materials are inadequate to account for the observations. We suggest that the role of haze has historically been misinterpreted. The blank disk of Mars in violet light occurs when the atmosphere is relatively free of haze. The formation of optically thin white hazes over the bright areas increases the contrast and produces 'blue clearing.'