8. Mars: the red planet
The winds of Mars
Why the winds blow
The Martian atmosphere responds to both the daily and seasonal temperature differences by generating winds that blow from hot to cold regions, transporting heat and trying to equalize the temperatures. And since a rise in temperature is equivalent to an increase in pressure, it is high-pressure air that rushes toward low pressure. This rush is the wind. The sharper the temperature or pressure difference, the stronger the wind.
Windblown dust and sand
The icy Martian winds have swept up vast dunes of sand and fine-grained dust. Rippled dunes have piled up in basins, craters and chasms, but the dunes cover only a small percentage of the land on Mars, probably less than 1 percent. Both dark and light sand dunes are found on Mars. Starkly beautiful patterns are created when the polar caps warm up in local spring and summer, exposing dark sand dunes. Extensive dunes form a dark collar around the north polar cap in the local summer, while global winds tend to blow fine dust from the south to the north.
Dust devils and global dust storms
As the powerful winds roar on an otherwise silent world, they occasionally stir up small, local dust storms, in much the same way that winds sometimes whip the terrestrial soils into towering columns called dust devils. The local dust storms form when the ground heats up during the day, warming the air immediately above the surface. The warm air rises in a spinning column that moves across the landscape like a miniature tornado, sweeping up dust that makes the vortex visible and leaving a dark streak behind. They have scratched tangled paths across some parts of Mars, often crossing hills and running across large sand dunes and through fields of house-sized boulders.
Large dark areas, such as the elevated plateau Syrtis Major, apparently develop when the surface rocks are scoured by powerful, seasonal winds. On close inspection, these dusky areas dissolve into swarms of elongated light and dark streaks, often tens of thousand of meters long, pointing in the direction of strong prevailing winds. The light streaks consist of fine dust deposited on a darker terrain by the prevailing winds, on the downwind, or leeward, side of craters and hills. The dark streaks result from the removal of dust by strong winds to expose the underlying rock. When all the steaks in a given area are integrated and superimposed by the human eye or at the detector of a telescope, they form the larger, global features visible from Earth, in much the same way as dots in newsprint combine to make a picture.
Strong winds carry dust from the surface high into the atmosphere, forming yellow dust clouds that have been reported by telescopic observers for centuries. Numerous fleeting and localized dust storms occur each Martian year. They can occur at any season, but are more frequent in southern spring and summer. Small dust storms can form simultaneously at several points on the planet, and then coalesce with each other, producing dust storms larger by far than any seen on Earth. They sometimes grow and spread across the planet, engulfing the entire globe and shrouding it in an opaque yellow cloud.
Only a handful of these global dust storms have been observed, beginning with telescopic observations during the oppositions of 1922 and 1956. Mariner 9 arrived at Mars during the slow decay of one of them; until the dust settled, only the summits of volcanoes were visible to the spacecraft cameras. Two planet-encircling dust storms were observed during the Viking missions, when the sky above the landers turned dark red and the Sun was greatly dimmed. A global dust storm next hid the surface of Mars from view during the summer of 2001.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University