Besides clouds, another interesting dynamic feature within the Martian atmosphere are dust storms. Martian dust storms are thought to occur every Martian year and begin in the southern hemisphere during the summer. During the southern spring, many small local storms can be seen in areas where high winds develop. The picture on the right shows a local storm near the South Polar Region (Figure 1.4). This dust storm was several hundred kilometers across. The storm was created when the frost particles on the ground were lifted into the atmosphere by the polar high winds. The more dust in the atmosphere, the more quickly the atmosphere heated up during the day, resulting in an increased temperature difference between day and night in the upper atmosphere. The increasing temperature differences in turn produced large winds that picked up more dust particles such that the storms fed on themselves and spread rapidly over the planet.

Prior to the Viking missions, each dust storm season was thought to be much the same. However, the year when Viking landed on Mars, two full-scale dust storms were detected. The following year, dust storms turned out to be very mild and never reached a global scale. In figure 1.5 the southern hemisphere was completely engulfed in a north-spreading dust storm. How long can global dust storms last? It is thought that a global dust storm will end when so much dust is in the atmosphere that sunlight is blocked out, causing surface temperatures to fall. Wind speeds would then drop and dust would settle out from the air. It usually takes about three months for the atmosphere to become clear again. Recent observations by the Martian Global Surveyor confirm that global dust storms on Mars remain an active geological process on the planet.