The Salar de Uyuni – the contiguous remains of an ancient lake that covered an area of more than 10,000 square kilometers in the Andes of Bolivia, South America – is now a vast and flat white salt pan. During the rainy season, when water collects on the impermeable crystalline layer of the salt pan and forms a large shallow pool, a natural mirror is formed. The resulting natural mirror reflects the sky below as if it were flat – almost perfectly; Thus, space agencies use these flats/pools to test their satellite altimeters. Besides being beautiful, this natural mirror is also located right above a large salt water body with high levels of lithium; Therefore, the Salar de Uyuni symbolizes a unique intersection of a geological phenomenon, an atmospheric phenomenon and today’s technology.
How the Salar de Uyuni in Bolivia became the world’s largest natural mirror
The extreme flatness of the Salar de Uyuni, which has an elevation change of less than 1 meter over its 10,582 square kilometer area, enables it to turn into a completely flat surface. During the rainy season (December to April) the surrounding mountain runoff floods the area, creating a thin layer of uniform water due to the impermeability of the salt layer and the completely flat ground. That is, the water layer is very shallow (a few centimeters thick), forming a liquid lens that reflects the sky and clouds, creating a uniform horizon.
Why are NASA and ESA looking to Bolivia to fix their satellites?
The natural mirror of the Salar de Uyuni is additionally very large and chemically stable. As a result, it is essential to the science of space-based observations of the Earth. For example, NASA and ESA satellite vehicles use the flooded Salar de Uyuni to calibrate the satellite’s radar and laser altimeters. Additionally, because the water surface is an ideal ‘level’, and is highly reflective, scientists can compare satellite measurements against accurately surveyed terrestrial elevations of the flat to verify data accuracy; Therefore, the Bolivian Desert is a very important geologic benchmark for satellite sensors in measuring sea level changes and Earth’s ice sheet changes.
40,000 years of change: how ancient lakes became modern mirrors
The mirror effect is the culmination of 40,000 years of geological history. The Salar de Uyuni was formed by the evaporation of several ancient lakes such as Lake Minchin and Lake Tauca. As these lakes dried up from the high-altitude Andean sun, they left behind large amounts of sodium chloride and other minerals. Beneath this white layer is a huge deposit of lithium-rich salty water. During the rainy season the high concentration of minerals in the water increases the density of the salty water, which suppresses the waves creating a still reflection for visitors today.
Why is thin air ideal for reflection?
Located 3,656 meters above sea level, the dry air of the Altiplano has remarkably low humidity, and this is the key to the mirror’s clarity. With minimal water vapor and pollution in the atmosphere, light can pass through with little scattering due to the absence of aerosols and moisture. When the water is completely calm, a whitening effect emerges, blurring the line between earth and sky.This phenomenon is caused by the specular reflection of sunlight from the smooth surface of water, which is reflected at a consistent angle, creating a celestial dome directly beneath the observer’s feet.