Superfície de Marte

Deserts are classified by their geographical location and dominant weather pattern as trade wind, midlatitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in nonarid environments are paleodeserts, and extraterrestrial deserts exist on other planets.

Trade wind deserts

The trade winds in two belts on the equatorial sides of the Horse Latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds. The world's largest desert, the Sahara of North Africa, which has experienced temperatures as high as 57° C, is a trade wind desert.

The Sahara of Africa is the world's largest desert. It contains complex linear dunes that are separated by almost 6 kilometers. (Skylab photograph).

Midlatitude deserts

	Midlatitude deserts occur between 30° and 50° N. and S., poleward of the subtropical highpressure zones. These deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America is a typical midlatitude desert.
A rare rain in the Tengger, a midlatitude desert of China, exposes ripples and a small blowout on the left. Winds will shortly cover or remove these features.

Rain shadow deserts

	Rain shadow deserts are formed because tall mountain ranges prevent moisture-rich clouds from reaching areas on the lee, or protected side, of the range. As air rises over the mountain, water is precipitated and the air loses its moisture content. A desert is formed in the leeside "shadow" of the range.
This Landsat image shows the Turpan Depression in the rain shadow desert of the Tian Shan of China. A sand sea is in the lower center on the right, but desert pavement, gray in color, dominates this desert. The few oases in the desert and the vegetation in the mountains at the top are in red. A blanket of snow separates the vegetation in the Tian Shan from the rain shadow desert.

Coastal deserts

Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. Winter fogs, produced by upwelling cold currents, frequently blanket coastal deserts and block solar radiation. Coastal deserts are relatively complex because they are at the juncture of terrestrial, oceanic, and atmospheric systems. A coastal desert, the Atacama of South America, is the Earth's driest desert. In the Atacama, measurable rainfall--1 millimeter or more of rain--may occur as infrequently as once every 5-20 years.

 

Crescent-shaped dunes are common in coastal deserts such as the Namib, Africa, with prevailing onshore winds. Low clouds cover parts of the Namib in this space shuttle photo.

 

High dunes of the Namib desert near Sossus Viei (photograph by Georg Gerster).	Morning fog moistens the dunes of the Namib coastal desert (photograph by Georg Gerster).

Monsoon deserts

"Monsoon," derived from an Arabic word for "season," refers to a wind system with pronounced seasonal reversal. Monsoons develop in response to temperature variations between continents and oceans. The southeast trade winds of the Indian Ocean, for example, provide heavy summer rains in India as they move onshore. As the monsoon crosses India, it loses moisture on the eastern slopes of the Aravalli Range. The Rajasthan Desert of India and the Thar Desert of Pakistan are parts of a monsoon desert region west of the ranqe.

The Indus River floodplain, lower left, is the western border of the Thar Desert. This Landsat image of the monsoon desert shows small patches of sand sheets in the upper right, with three types of dunes; some dunes are almost 3 kilometers long.

Polar deserts

Polar deserts are areas with annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10° C. Polar deserts on the Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. Sand dunes are not prominent features in these deserts, but snow dunes occur commonly in areas where precipitation is locally more abundant. Temperature changes in polar deserts frequently cross the freezing point of water. This "freeze-thaw" alternation forms patterned textures on the ground, as much as 5 meters in diameter.

The Dry Valleys of Antarctica have been ice-free for thousands of years (courtesy of USGS Image Processing Facility. Flagstaff. Arizona).

Paleodeserts

Data on ancient sand seas (vast regions of sand dunes), changing lake basins, archaeology, and vegetation analyses indicate that climatic conditions have changed considerably over vast areas of the Earth in the recent geologic past. During the last 12,500 years, for example, parts of the deserts were more arid than they are today. About 10 percent of the land between 30Š N. and 30Š S. is covered now by sand seas. Nearly 18,000 years ago, sand seas in two vast belts occupied almost 50 percent of this land area. As is the case today, tropical rain forests and savannahs were between the two belts.

Fossil desert sediments that are as much as 500 million years old have been found in many parts of the world. Sand dune-like patterns have been recognized in presently nonarid environments. Many such relict dunes now receive from 80 to 150 millimeters of rain each year. Some ancient dunes are in areas now occupied by tropical rain forests.

The Nebraska Sand Hills is an inactive 57,000square kilometer dune field in central Nebraska. The largest sand sea in the Western Hemisphere, it is now stabilized by vegetation and receives about 500 millimeters of rain each year. Dunes in the Sand Hills are up to 120 meters high.

 

This aerial photograph of the Nebraska Sand Hills paleodesert shows a well-preserved crescent-shaped dune (or barchan) about 60 to 75 meters high. (Photograph by Thomas S. Ahlbrandt)	A dry community of vegetation grows among the dunes of the Nebraska Sand Hills. (Photograph by N.H. Darton)

Extraterrestrial deserts

Mars is the only other planet on which we have identified wind-shaped (eolian) features. Although its surface atmospheric pressure is only about one-hundredth that of Earth, global circulation patterns on Mars have formed a circumpolar sand sea of more than five million square kilometers, an area greater than the Empty Quarter of Saudi Arabia, the largest sand sea on our planet. Martian sand seas consist predominantly of crescent-shaped dunes on plains near the perennial ice cap of the north polar area. Smaller dune fields occupy the floors of many large craters in the polar regions.

 

This Viking spacecraft image of Mars shows alternating layers of ice and windblown dust near the north polar cap. Annual and other periodic climatic changes due to orbit fluctuations may occur on Mars (courtesy of USGS Image Processing Facility, Flagstaff, Arizona).

 

One of the first images taken at the Viking 2 landing site on Mars shows the pink sky over Utopia and the desert pavement on the ground (courtesy of NASA).	Some of the crescent-shaped dunes in this Viking image of Mars are more than a kilometer wide. The dark material that streaks from the horn-shaped features may be dust recently blown from the dunes (courtesy of NASA).

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