A region of soluble salt
The Atacama Desert in northern Chile contains the largest surface nitrate deposits on Earth. These deposits, known as "caliche," form a nearly continuous belt approximately 700 kilometers long and 20 kilometers wide. The caliche is a complex mineral bed, typically 0.2 to 3 meters thick, cemented with highly soluble salts. The primary components are sodium nitrate (NaNO₃), also known as Chile saltpeter, and other salts like chlorides and sulfates. The ore also holds the world's most significant deposits of natural iodine, found in minerals such as iodates and periodates.
The existence of these vast, water-soluble mineral beds shows the region's extreme and prolonged hyper-aridity. Geologic records indicate these dry conditions have persisted for at least 10 to 15 million years. In the desert's hyper-arid core, average rainfall is less than 2 millimeters per year, and measurable rain may only occur once every 5 to 20 years. This near-total absence of water has allowed the salts to accumulate over millions of years without being washed away, a process that would happen quickly in almost any other climate. The origin of the nitrates has long been debated, with Charles Darwin first proposing they formed in an ancient inland sea. Current scientific consensus points to a multi-source model. Isotope analysis confirms a significant portion of the nitrate originates from atmospheric deposition over immense timescales, supplemented by components from ancient groundwater that leached minerals from marine and volcanic rocks.
An economic boom and a planetary analog
From the 1830s until the early 20th century, these nitrate deposits, also called "white gold," were a globally important resource. Mined extensively for use in fertilizers and explosives, the saltpeter trade fueled the Chilean economy and was a primary cause of the War of the Pacific (1879-1883), in which Chile fought and defeated a combined force from Peru and Bolivia. The victory gave Chile control over the valuable nitrate-producing regions, including Antofagasta, permanently altering the political map of South America. Over 200 mining towns, or oficinas, sprang up in the desert, creating unique communities of workers from Chile, Bolivia, Peru, Europe, and Asia. The industry collapsed after German chemists Fritz Haber and Carl Bosch developed a process for synthesizing ammonia in 1909, making industrial fertilizer production cheaper and rendering the Chilean mines obsolete.
Today, the Atacama's extreme environment has a new scientific value as one of Earth's best analogs for Mars. The combination of hyper-aridity, high ultraviolet radiation, and oxidizing, and saline soils mimic the conditions found on the Martian surface. For this reason, space agencies like NASA use the desert to test rovers and life-detection instruments. The Atacama Rover Astrobiology Drilling Studies (ARADS) project, for example, uses the desert to trial drilling technology designed to search for biosignatures deep beneath the surface, informing the search for past or present life on Mars.