Mars's Salt Thermos ✛
Every Martian night, salt crystals on the planet's surface soak up moisture from the thin air, much like a sponge draws water from a damp cloth. By morning, a microscopic brine accumulates inside them, and during the day, as temperatures rise, that brine finds itself trapped in tiny cavities. Thermal expansion closes the cracks through which the moisture entered, turning the crystal into a miniature thermos holding liquid inside.
Calculations show that when heated from 150 to 300 K (from −123 to +27 °C), the volume of an NaCl crystal increases by 1.64%. For gypsum, the change is smaller: 0.87%; for magnesium sulfates, about 0.5%. Calcium chloride solutions expand more strongly: from 3.75% to 6.9%, depending on concentration. These changes are enough to open and close microcracks in the crystal.
The mechanism has not yet been tested experimentally, and data on the thermal expansion of hydrated perchlorates are lacking. But if it is confirmed, salt crystals would become priority targets in the search for potentially habitable microenvironments on present-day Mars. In the Atacama Desert, one of the driest places on Earth, cyanobacteria already live inside halite crystals, obtaining moisture through nocturnal deliquescence of the salt.
In the Atacama Desert, one of the driest places on Earth, cyanobacteria live inside halite crystals and obtain moisture precisely through salt deliquescence at night, when air humidity rises.