Meteorology and Atmospheric Dynamics

The global pattern of atmospheric circulation on Mars shows many superficial similarities to that of Earth, but the root causes are very different. Among these differences are the atmosphere's ability to adjust rapidly to local conditions of solar heat input; the lack of oceans, which on Earth have a large resistance to temperature changes; the great range in altitude of the surface; the strong internal heating of the atmosphere because of suspended dust; and the seasonal deposition and release of a large part of the Martian atmosphere at the poles.

Near-surface winds at the Viking and Pathfinder landing sites were usually regular in behaviour and generally light. Average speeds were typically less than 2 metres/sec (4.5 miles/hour), although gusts up to 40 metres/sec (90 miles/hour) were recorded. Other observations, including streaks of windblown dust and patterns in dune fields and in the many varieties of clouds, have provided additional clues about surface winds.

Global circulation models, which incorporate all the factors understood to influence the behaviour of the atmosphere, predict a strong dependence of winds on the Martian seasons because of the large horizontal temperature gradients associated with the edge of the polar caps in the fall and winter. Strong jet streams with eastward velocities above 100 metres/sec (225 miles/hour) form at high latitudes in winter. Circulation is less dramatic in spring and fall, when light winds predominate everywhere. On Mars, unlike on Earth, there is also a relatively strong north-south circulation that transports the atmosphere to and from the winter and summer poles. The general circulation pattern is occasionally unstable and exhibits large-scale wave motions and instabilities: a regular series of rotating high- and low-pressure systems was clearly seen in the pressure and wind records at the Viking lander sites.

Smaller-scale motions and oscillations, driven both by the Sun and by surface topography, are ubiquitous. For example, at the Viking and Pathfinder landing sites, the winds change in direction and speed throughout the day in response to the position of the Sun and the local slope of the land.

Turbulence is an important factor in raising and maintaining the large quantity of dust found in the Martian atmosphere. Dust storms tend to begin at preferred locations in the southern hemisphere during the southern spring and summer. Activity is at first local and vigorous (for reasons yet to be understood), and large amounts of dust are thrown high into the atmosphere. If the amount of dust reaches a critical quantity, the storm rapidly intensifies, and dust is carried by high winds to all parts of the planet. In a few days the storm has obscured the entire surface, and visibility has been reduced to less than 5 percent of normal. The intensification process is evidently short-lived, as atmospheric clarity begins to return almost immediately, becoming normal typically in a few weeks.

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