ESA Science & Technology30-Jun-2005 14:20:29

Martian Atmosphere

Dust and Circulation

If the atmosphere is dusty when Beagle 2 lands, there will be no clouds to witness. Dust heats up the atmosphere so that water vapour cannot condense to form clouds. Beagle 2, similarly to NASA's Pathfinder spacecraft, may well encounter dust devils, swirling winds that whip dust up into funnels high above the ground. They are common on Earth in desert regions such as the Sahara. On Mars, however, they seem to occur particularly frequently and often grow to sizes similar to earthly tornadoes.

Much of Mars' surface is covered with dust. In the desiccated environment, winds can stir it up despite the low density of the atmosphere. When prevailing winds are weak, dust devils occur during the day when air, heated by the relatively hot ground, rises rapidly and becomes turbulent. When prevailing winds are strong, they can cause violent regional storms, which sometimes become global during southern summer.

Dust Storms on Mars

Mars' southern hemisphere is tilted towards the Sun when the planet is closest to the Sun in its orbit and the northern hemisphere is tilted towards the Sun at furthest approach. Consequently, the southern summer is considerably hotter than the northern summer. This extra heat pouring into the southern hemisphere increases the circulation of the atmosphere and drives stronger winds. "Global dust storms originate in the southern hemisphere because of the greater heating, the stronger trade winds, and the amplification of the wind due to the hilly ground," says Forget.

However, global storms do not occur every southern summer, which is surprising. Forget and colleagues at LMD and the University of Oxford, England, have developed a global atmospheric circulation model for Mars along the same principles as those developed for Earth. "The models simulate the storms, but they do not tell us why we see big storms some years and not others," he says.

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