By Philippe Bourseiller

The Atacama Desert is a plateau in South America, covering a 1000-km (600-mi) strip of land on the Pacific coast, west of the Andes mountains. It is the driest nonpolar desert in the world. According to estimates, the Atacama Desert occupies 105,000 km2 (41,000 sq mi), or 128,000 km2 (49,000 sq mi) if the barren lower slopes of the Andes are included. Most of the desert is composed of stony terrain, salt lakes (salares), sand, and felsic lava that flow towards the Andes.

Geographically, the aridity of the Atacama is explained by it being situated between two mountain chains (the Andes and the Chilean Coast Range) of sufficient height to prevent moisture advection from either the Pacific or the Atlantic Oceans, a two-sided rain shadow.

The Atacama Desert is commonly known as the driest nonpolar place in the world, especially the surroundings of the abandoned Yungay town (in Antofagasta Region, Chile). The average rainfall is about 15 mm (0.6 in) per year, although some locations, such as Arica and Iquique, receive 1 to 3 mm (0.04 to 0.12 in) in a year. Moreover, some weather stations in the Atacama have never received rain. Periods up to four years have been registered with no rainfall in the central sector, delimited by the cities of Antofagasta, Calama, and Copiapó, in Chile. Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971.

The Atacama Desert may be the oldest desert on earth, and has experienced extreme hyperaridity for at least 3 million years, making it the oldest continuously arid region on earth. The long history of aridity raises the possibility that supergene mineralisation, under the appropriate conditions, can form in arid environments, instead of requiring humid conditions. Geological research suggests that in some sections of the Atacama Desert, such as in today's Chile, hyperaridity has persisted for the last 200 million years (since the Triassic), competing only with Africa's Namib Desert for such a title.

The Atacama is so arid that many mountains higher than 6,000 m (20,000 ft) are completely free of glaciers. Only the highest peaks (such as Ojos del Salado, Monte Pissis, and Llullaillaco) have some permanent snow coverage.

The region may be unique on Earth in this regard, and is being used by NASA to test instruments for future Mars missions.

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