The driest life on Earth
In the Salar de Atacama, the driest non-polar desert on Earth, life exists where it was once thought impossible. The surface of this vast salt flat appears sterile, receiving less than two millimeters of precipitation annually. Yet, hidden just millimeters beneath the surface of translucent halite (rock salt) crystals, entire microbial communities thrive. These organisms are endoliths—creatures that live inside rocks. This habitat contains a mix of cyanobacteria, archaea, and other bacteria that have adapted to an existence defined by extreme dryness and intense solar radiation.
Their survival depends on the salt itself. Halite is hygroscopic, meaning it naturally absorbs atmospheric moisture. Each morning, as marine fog rolls in and relative humidity briefly rises above 75%, the salt crystals pull enough water vapor from the air to create microscopic films of liquid brine within their structure. This process, deliquescence, provides the water necessary for the endolithic community to activate its metabolism for a short time. The salt crystal also shields, filtering out the harsh ultraviolet radiation that bombards the desert surface, while allowing just enough light (about 0.02% of incident PAR) to penetrate for photosynthesis.
Pushing the limits of biology
The organisms within the Atacama's salt crusts represent life at its absolute water limit. They exist in a state of near-constant desiccation, only becoming metabolically active when deliquescence provides temporary brine. Studies have documented diverse communities, including cyanobacteria of the genus Halothece and various salt-loving archaea from the class Halobacteriales. These communities cycle carbon on timescales ranging from years to decades, depending on the scarce availability of water.
This remarkable survival strategy has significant implications for astrobiology. The Atacama Desert is a the most important Mars analog environments on Earth because of its extreme aridity and soil chemistry. Scientists use this natural laboratory to test life-detection instruments and drilling technologies for future missions to Mars. The discovery that life can persist within salt crystals by drawing water from the air shows that if life ever existed on Mars, its biosignatures might be preserved in similar evaporite deposits. NASA's Atacama Rover Astrobiology Drilling Studies (ARADS) project specifically investigates these environments to refine strategies for searching for life on other planets.