How does life survive in the Atacama Desert, and what does it teach us about Mars?

Life survives in the Atacama by hiding. The Atacama Desert in Chile is the driest non-polar desert in the world, with about 15 millimetres of rain a year on average and some weather stations that have never recorded rain. Yet some locations get a marine fog, the camanchaca, that gives enough moisture for algae living under rocks, as well as lichens and even a few cacti. Hypoliths, communities of photosynthetic organisms that live underneath rocks, are protected by the rock from harsh ultraviolet light and wind. The rock can trap moisture, and if it is translucent it lets light in, so the microbes get shade, water and sunlight in one place.

That is also why the Atacama matters for Mars. Its soil and near-total absence of microbial life have made it a testing ground for Mars mission instruments. In 2003 researchers duplicated the tests that the Viking landers used in 1976 to look for life on Mars and could detect no signs of life in Atacama soil from the region of Yungay. But they also found that those tests missed present signs of life in soils from several dry places, including the Atacama and Antarctic dry valleys.

So the lesson of the Atacama is a warning about how hard it is to prove that a place is dead. A test that finds nothing in dry soil may have simply missed tiny communities tucked inside and underneath rocks.