Nestled in the heart of the vast Atacama Desert, Yungay is the driest place on Earth. This utter desiccation is its draw. Not only is it one of Earth's most extreme environments, it is also one of the most Mars-like.

This month, an international team of scientists, headed by Chris McKay of NASA Ames Research Center, will set up camp for a couple of weeks in Yungay. For the past several years, McKay and his colleagues have traveled here each year, at the start of the Chilean winter, to find out what type of ecosystem can survive in such a barren place. The short answer: none.

It doesn't rain in Yungay. Precipitation there is measured not in inches per year, but in millimeters - perhaps quarter of an inch - per decade. Yungay lies in a double rain shadow. To the west, moisture from the Pacific is blocked by a range of coastal mountains. To the east lies the towering cordillera of the Andes, a massive barrier to precipitation from the Amazon Basin.

So perhaps it is no surprise that Yungay is lifeless. Life needs water, and Yungay doesn't have any.

Other deserts are teeming with life. Even the driest regions of Death Valley are home to cacti, desert scrub, succulents, insects, birds, lizards and kangaroo rats. Death Valley, receives an average of two inches of rain per year - a virtual flood compared to Yungay. In the Dry Valleys of Antarctica, plants and animals cannot survive, but microbes can. Not on the surface, just below it. Inside rocks.

Pick up the right rock there - a certain type of sandstone - break it open, and down about a quarter of an inch you'll find a series of thin, colored layers: one green, one white, one black.

The green layer is colonized by cyanobacteria, which derive their color from chlorophyll. Like plants, cyanobacteria use chlorophyll to perform photosynthesis, to convert sunlight into food. The white and black layers are lichens.

Sandstone is porous enough to trap tiny bits of moisture inside, enough for the organisms to live on. But not all sandstone is the right type. Its surface has to be translucent, so that some light can get through to drive the photosynthesis. The rock also acts as a natural sunscreen, blocking UV radiation that could damage the microorganisms.

Yungay is too dry for cyanobacteria. In parts of the Atacama closer to the coast, although there's still no rainfall, fog drifts up from the Pacific Ocean, and provides enough moisture for life to gain a foothold. There, you can find microbial life; even succulents can survive.

Thus, the transition between the Yungay region of the Atacama and the slightly moister region to the west represents, literally, the threshold of life. Researchers want to understand precisely where that boundary lies, what combination of factors defines it.

Understanding life's threshold on Earth will help planetary scientists like McKay design strategies to search for life on Mars. By understanding which conditions enable life and which don't, by analyzing the mechanisms life uses to survive under the harshest of conditions, and by being able to detect the difference between a lifeless environment and one that has the merest trace of life, scientists will gain knowledge that will help them search for evidence of life on another world.

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NASA Ames Research Center