Salt flat in Atacama Region, Chile
Salar de Pajonales is a playa in the southern Atacama Region of Chile and the third-largest in that country, behind Salar de Punta Negra and Salar de Atacama. It consists mostly of a gypsum crust; only a small portion of its area is covered with water. During the late Pleistocene, Salar de Pajonales formed an actual lake that has left shoreline features.
The climate in the region is arid and windy, with high insolation and low atmospheric pressure. The extreme environmental conditions have drawn comparisons to Mars, and Salar de Pajonales has been used as an analogue for Martian environments.
Geography and hydrology.
The salar lies at the margin of the hyperarid Atacama Desert, on the Altiplano-Puna high plateau at elevation. It covers an area of , making it the third-largest in the region behind Salar de Atacama and Salar de Punta Negra. It has roughly the shape of a square, with three peninsulas jutting in on the northeastern and southeastern corners; this shape may be due to fault activity. Several islands are found in the north-central and south-central area of the salar. Only about of Salar de Pajonales are actually covered with water in the form of ponds and lagoons, mostly on the margins and at the centre of the salar. A road crosses the salar.
The principal salt has variously been stated to be either halite or gypsum in the form of selenite. There are minor amounts of calcium chloride, elemental sulfur and ulexite. The salts form a thick, brine-filled salt formation with domes, mounds, ridges, polygonal cracks, tumuli (an area covered with numerous domes is called "Dome Field") and a rough crust. A raised area at the eastern end of the salar features small holes draped in gypsum that appear to be produced by degassing. Water at Salar de Pajonales is extremely salty, with calcium chloride and calcium sulfate being the main components. There is evidence of ongoing dissolution of the salt, which has left deep pits and cavities reaching height. The dissolution may be caused by salt draining into groundwater. Salt deposition is influenced by the activity of cyanobacteria and sulfur-oxidizing bacteria, which precipitate gypsum that may entomb microorganisms.
Its catchment covers an area of , but lacks permanent surface drainages. Its waterbodies, which are inhabited by flamingos, are sustained by underground water. In the north, there is intermittent discharge in the San Eulogio River, Quebrada La Pena and in other nameless streams from Cerro La Pena, Cerro Pajonales and Pampa San Eulogio. The terrain is formed mostly by volcanic rocks consisting of andesite and rhyolite; there are active hot springs. The Lazufre uplift, an area where the ground is rising, lies in close proximity to Salar de Pajonales.
Prehistoric lake.
Around the last glacial maximum, water levels in Salar de Pajonales rose by about , covering an area of about . The lake left clear shorelines with terraces, river deltas and wavecut platforms and notches. Islands in the salar were cut in or even flattened by wave erosion. This lake level highstand may be correlated to the Lake Tauca episode on the Bolivian Altiplano, which took place shortly after the last glacial maximum when water levels rose in the regional lakes. Archaeological sites found at Pajonales and neighbouring Aguas Calientes may be correlated to a former lake highstand. The present-day gypsum deposits formed underwater during this highstand. The climatic conditions may have resembled these of Mars during the Hesperian period.
Climate and vegetation.
The mean temperature at Salar de Pajonales is and annual precipitation ranges . Annual potential evaporation reaches . Precipitation occurs mostly during summer from the east and in winter from the west. 
The vegetation of the area consists of matorral formed by "Fabiana bryoides" and "Phacelia pinnatifida" and steppe dominated by "Stipa frigida". Other plant species recorded are "Cistanthe minuscula" and "Cristaria andicola".
Microbialites and stromatolites grow in the salar, and former stromatolites form mounds and small domes on the crust of Salar de Pajonales. Maximum development has variously been found either in the central parts of the salar and the deeper lagoons, or around the islands. There are also diatoms at Salar de Pajonales.
Human use.
Salar de Pajonales lies in the southern part of the Llullaillaco National Park. There are prehistoric shelters, "pircas", in the area. The European Large Southern Array observatory project placed instruments at Salar de Pajonales in 1997, before moving them to Llano de Chajnantor the following year.
Owing to its extreme conditions, like the low atmospheric pressure, extreme salinity and aridity, large day-night temperature differences and high UV radiation, Salar de Pajonales has been used as an analogue for environments on Mars where life may persist to this day. Traces of such life can be preserved by sediments and sulfate-chloride salts. Models of how life-bearing environments may appear from remote imaging have been developed on the basis of the appearance of Salar de Pajonales.
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