The mean composition of the atmosphere is shown in Table 2.4 . The minor constituents, including water vapour, carbon monoxide, oxygen, and ozone, have highly variable concentrations. There is some evidence that the atmospheric abundance of water varies from year to year, possibly in association with varying dust storm activity.
Dust is a crucial component of the martian greenhouse effect.
The atmosphere is dusty at all times, with the amount varying a great deal and having an important effect on the atmospheric and surface temperature. The dust haze extends from the surface up to as high as 50 km in altitude and particles sometimes appear to be approximately uniformly mixed in the vertical up to this limit. Optical depths in local and regional dust storms are likely to range up to even higher values. It is quite common for Mars' atmosphere to become saturated with water at some regions and times of day, particularly during northern summer when water evaporates from the polar caps, and in the polar hood regions in autumn in both hemispheres. As a result, haze, clouds, or ground fog are produced.