Martian Winds and Storms
The obvious explanation for the apparent dust suspended in the atmosphere (yellow clouds) is that material is whipped up from the ochre-colored areas by Martian winds. Probably the wind velocities are high, and it is true that fine material can hang in even a tenuous atmosphere for long periods. There is a terrestrual case of the Krakatoa eruption in 1883, when dust and ash sent into the upper atmosphere stayed there for 2 years. On the other hand, it has been suggested that since a wind in the thin Martian atmosphere would have little force, it is rather difficult to see how so much dust could be whipped up in so short a time.
The most obscuring storms of the present century seem to have taken place at or near the time of perihelic oppositions, and this has led to a suggestion that the dust may be due to active vulcanism. If the Martian volcanoes were capable of erupting strongly enough, and spreading material around the upper atmosphere, the quick development and the persistence of the storms would be easier to explain; but this is a minority view and Mariner 9 showed no indication of continuing vulcanism which was confirmed by later missions.
While a major storm is in progress, much of the planed may be covered as in late 1971. This dust must subsequently settle, and after the end of the storm the dark features such as the Syrtis Major reappear, apparently unaltered. Yet if the dust settled over them, one might expect that they would be at least less conspicuouse than before the onset of the storm; and if storms were comparatively frequent, it would be reasonable to suppose that the whole planet would be overlaid with a layer of ochre colored material. But, from the observed behavior of these regions, this is not the case.
It was generally thought that the dark regions must be organic, so that the vegetation could push aside the dust which settled upon it. This was one of the main arguments for believing the some sort of life still existed on Mars. On present evidence such a theory is untenable.
In a way, it was fortunate that Mariner 9 arrived near Mars during a dust storm, beacuase some very valuable photographs of the progress and decline of the storm were sent back. For instance, the arc-like feature of Ascraeus Lacus are certainly due to dusty material, and the same sor of phenomenon was seen in other areas, notably the very high Nodus Gordii. As yet there is no direct knowledge of the sizes of the particles responsible for the obscurations, but they must be small otherwise they would not be elevated by the winds.