Southern Atacama Desert, Chile - Sunday, May 8, 2011 - This is the first of a series of reports from my recent trip to the Atacama Desert. I traveled with a group of scientists, headed by Jacek Wierzchos, of the National Museum of Natural Science in Madrid, who go to the Atacama to study microorganisms that live at the dry limit of life on Earth.

Because the Atacama is so dry, and has been so for millions of years, it is a good test bed for understanding where life on Mars, if it ever existed, may have made its last stand.

The Atacama is a strip of parched land that stretches south from the northern border of Chile for more than 1,000 kilometers (600 miles). It is bounded on one side by the towering cordillera of the Andes and on the other by a lower, older range of coastal mountains that reach down to the Pacific Ocean.

In the hyperarid regions of the Atacama, annual rainfall averages one millimeter (less than one-twentieth of an inch). Rain, when it occurs, falls perhaps once a decade. Some weather stations in the Atacama have never registered rainfall. Not surprisingly, no plants or animals can survive there. It is the very definition of barren: a place of rocks and dirt.

For many years, scientists believed that not even microbial life could survive in the driest parts of the Atacama. But in 2005, in a large salt flat in Yungay, Chile, Wierzchos discovered communities of bacteria living inside stubby, knob-shaped rocks made of pure halite. Table salt. He and others have been studying these halite communities, and other similar "islands of life" in the Atacama ever since.

When I hooked up Saturday night with Wierzchos and his colleagues they had already spent several days exploring sites of interest in the northern Atacama. I joined them in Antofagasta, a port city on the Pacific Ocean about 1100 kilometers (about 700 miles) north of Santiago.

On Sunday, together with two of the group's members, Jocelyne DiRuggiero, a biologist with Johns Hopkins University in Baltimore, Maryland, and Alfonso Davila, an astrobiologist with the SETI Institute who works at NASA Ames Research Center in Mountain View, California, I drove south from Antofagasta about 350 kilometers (about 220 miles), to wetter parts of the Atacama.

"Wetter," when referring to the Atacama, is strictly a relative term, meaning a location that receives perhaps 20 millimeters of rain (three-quarters of an inch) per year, or moisture from coastal fog, or some combination of the two.

Plants were growing at the wettest of the three locations we visited, an uncommon sight in the Atacama, but DiRuggiero was only marginally interested in them. Her focus was on the microbes that inhabit the soil.

Much of the research into microbial life in the hyperarid Atacama has focused on rocks. In other very dry deserts around the world, the Antarctic Dry Valleys and the Negev, for example, microbial communities have been found inside or underneath rocks. Very little life has been detected in soils.

In the hyperarid Atacama, the earliest soil studies concluded that if there were microbes there, they were too few in number to detect. Later, more sensitive studies, however, did reveal trace amounts of DNA in the soil. But little work has been done in this area. "There's a big, big ecosystem which no-one has really looked at, which is the soil," DiRuggiero said. She plans to change that.

The regions where she sampled were based on the locations where another researcher, Kimberley Warren-Rhodes, years earlier had looked underneath thousands of small rocks in the Atacama for microbial colonies similar to those that had been found in Antarctica and the Negev. Warren-Rhodes had studied several sites along a north-south moisture gradient and had good weather data for her sites, data that DiRuggiero wanted to use to "correlate environmental data with microbial populations."

The first task in each of the three areas we drove to was to find a suitable sampling site. This meant driving on main roads to the general region of interest, then veering off on some dirt road into the middle of nowhere, and finally veering off-road even farther into nowhere.

"The main difficulty is to find something that is out of the way of human disturbance," DiRuggiero explained, adding the she "would love to call it pristine," but because mining activity is so widespread, "there is almost nothing pristine in the Atacama Desert."

At each site DiRuggiero would begin by setting out three pink flags to mark off her sampling area. Once the flags were placed, the delineated territory was off-limits. Davila and I were warned against even standing upwind of the sample site, lest DNA from elsewhere that might be clinging to our clothes get blown into her samples and produce false readings.

She then disinfected her tools, put on nitrile gloves and dug a series of three holes, collecting soil samples at 5 cm, 10 cm and 20 cm depth from each hole. Some samples went into small round watertight metal tins, the type typically used for ointments. These will be weighed, then dried out and weighed again, to determine the moisture content of the soil. Other samples were placed in sterile plastic bags and carefully sealed. These will be used to extract DNA.

Why study soils, I wondered, if almost nothing has been found living there? "I'm really curious to know what is the relationship at the genetic level between the microorganisms in the soil and the ones in the rocks," DiRuggiero said.

Each colonized halite rock is a separate "island" of life. And yet similar communities of organisms have been found to inhabit halite rocks at sites separated by hundreds of kilometers. How these microbes find their way into the halite is a mystery.

DiRuggiero suspects that the soil acts as a seeding mechanism for colonization of the rocks, that bacteria are alive in the soil, growing, albeit just barely, "below detection limit," with "maybe a couple of cell divisions in a thousand years."

She suggests, although she doesn't yet fully understand the dynamics of the process, that wind and moisture occasionally do transport these organisms to the halite and other rocks in the desert (quartz, gypsum, rock varnish), where they find a more suitable environment for growth. Comparing in detail the microbial populations in the soil to those in the rocks is one way she hopes to test her theory.

"The question is how do they stay in the soil for so long and do they grow in any way?" she says. "And so far we don't know."

DiRuggiero finished up sampling at the last of her sites just as the sun was setting. That was unfortunate for her. She missed a rare sight in the hyperarid Atacama, a sunset with clouds. Overcast days are a rarity in the core of the Chilean desert: more than 300 days a year the skies are pure blue.

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