A landscape sterilized by time
The Atacama Desert is the oldest and driest nonpolar desert on Earth. Some parts of its hyperarid core, particularly the area around the abandoned mining town of Yungay, have likely been arid for 150 million years. This region exists in a double rain shadow, blocked from moisture by the Andes to the east and the Chilean Coast Range to the west. Average rainfall in the driest zones is just 1 to 3 millimeters per year, and some weather stations have never recorded rain at all. The soil is also exposed to the highest levels of surface ultraviolet radiation on the planet, with summer UV Index values reaching over 20 in high-elevation areas.
This combination of extreme desiccation and intense radiation creates a soil so seemingly sterile that it has become a primary analog for the surface of Mars. In the 1970s, NASA's Viking landers failed to detect organic molecules in Martian soil. Decades later, when instruments designed for future Mars missions were tested on soil from the Yungay region, they also found nothing. The soil composition is strikingly similar to Mars, containing highly oxidizing substances like perchlorates, which can destroy organic matter when heated—an important process used by the Viking instruments. These tests suggested that if Viking had landed in the Atacama, it would have concluded this part of Earth was lifeless.
Life finds a way in salt
Despite the surface sterility, life persists in a remarkable refuge: the interior of halite (rock salt) nodules. In the driest parts of the desert, where no plants or surface microbes can survive, communities of cyanobacteria and other microorganisms thrive inside these translucent salt rocks. A specific species, similar to Chroococcidiopsis, forms a distinct green or gray layer just 3 to 7 millimeters below the rocks' surface.
This unique habitat provides a solution to the Atacama's two greatest challenges. The salt crystals scatter the intense UV radiation, protecting the microbes' DNA. The salt provides water. Through a process called deliquescence, the sodium chloride actively draws scarce water vapor directly from the atmosphere when relative humidity rises above 70-75%, creating a concentrated brine inside the rock that the microbes can use for metabolism. This allows them to remain metabolically active for several consecutive days, a feat impossible on the desert floor. These endolithic (rock-dwelling) communities are entirely self-sufficient, one of the most tenacious ecosystems known. This survival strategy—sourcing water from the air and seeking shelter just below the surface—informs how astrobiologists might search for life on Mars. NASA's Atacama Rover Astrobiology Drilling Studies (ARADS) project specifically tested rovers and drills here to practice looking for buried biosignatures.