Figure 36.14 Dust storm on Mars.
This Viking 2 Orbiter image shows a large dust storm over the Thaumasia region on Mars. This large disturbance grew into the first global dust storm observed by the Viking Orbiter. This image was taken near perihelion (closest approach of Mars to the Sun), when heating of Mars is at a maximum. Dust storms are common during perihelion, when the planet receives 40 percent more sunlight than during aphelion (when Mars is farthest from the Sun). This annual variation in sunlight results in a 20°C increase in temperature during perihelion. The increased temperature causes continental-scale dust storms at the planet's surface. The dust is swept aloft to altitudes of tens of kilometers, where it spreads globally, absorbs light from the Sun, and heats the entire atmosphere by about another 15°-28°C. Mars also has a distinctive aphelion climate. The cold atmospheric conditions during aphelion stimulate the formation of planetwide belts of water ice clouds surrounding the planet at altitudes of 3 to 10 km. Surface dust raised by low dust storms is confined to low altitudes (about 10 km) and is eventually swept to the ground by the water ice clouds. The clouds further reduce atmospheric temperatures by forming around the dust. Without sunlight, the dust freezes and falls to the ground. This strong competition between dust heating and cloud cooling drives sweeping annual and short-term regional changes in Mars's climate. (NASA/JPL)
