A Prehistoric Lakebed
Salar de Uyuni’s exceptional flatness is a direct result of its geologic history. The entire 10,582-square-kilometer salt pan is the final remnant of a massive prehistoric lake called Lake Minchin, which covered this portion of the Andean Altiplano between 30,000 and 42,000 years ago. Over millennia, this and subsequent lakes like Paleo Lake Tauca evaporated, leaving behind thick, layered deposits of salt and lacustrine mud.
The process was simple but effective. As water evaporated, dissolved minerals precipitated and settled on the lakebed. Seasonal flooding would dissolve the top layer of salt, and as this shallow brine settled and re-evaporated, it smoothed out any irregularities. This natural leveling process, repeated over thousands of years, created a surface with an average elevation variation of less than one meter across its entire expanse. The salt crust itself varies in thickness, reaching up to 10 meters in some areas.
The Satellite Calibration Standard
The unique combination of large area and extreme flatness makes Salar de Uyuni an ideal location for calibrating the altimeters of Earth-observing satellites. Instruments like the laser altimeter on NASA's ICESat (Ice, Cloud, and land Elevation Satellite) and the radar altimeter on ESA's Sentinel-3 require a stable, predictable surface to verify their altitude measurements. The salt flat is approximately five times more effective for this purpose than the surface of an ocean, which is constantly affected by tides and currents.
Scientists use ground-based GPS surveys to create a precise digital elevation model (DEM) of the salt flat. One such survey, conducted by crisscrossing a large section of the salar with GPS-equipped vehicles, found that the highest and lowest points over more than 50 kilometers were separated by just 77 centimeters. When a satellite passes overhead, its altimeter readings are compared against this known surface elevation. This allows engineers to identify and correct for biases in the satellite's instruments, ensuring accuracy to within centimeters.
During the wet season, typically from December to March, a thin layer of rainwater transforms the salar into the world's largest natural mirror. This reflective, uniform surface is also used for the radiometric calibration of optical satellite sensors, like those on the Landsat satellites, ensuring they accurately measure the intensity of light reflecting off the Earth's surface. Beneath the solid crust lies a massive brine deposit, which contains one of the world's largest reserves of lithium.