In Northern Chile, a pick-up truck bumps along dusty old mining roads toward the Atacama Desert. A team of scientists is driving from the coastal town of Antofagasta, and they occasionally pass other vehicles on the road -mostly prospectors searching for metals and minerals. After an hour, they arrive at a lonely meteorological station situated in the driest part of a very dry desert.
The scientists have come to the Atacama to investigate how much water life needs to survive. Water is necessary for life, but water is so scarce in the Atacama that it is a wonder anything can live there at all. Some parts of the desert have not seen rain for centuries. The desert itself is thought to be between 10 to 15 million years old, making it the oldest desert on Earth.
The Atacama is probably also the driest desert in the world. While some areas of the Atacama along the coast have succulent plants like cacti, the more arid parts of the desert have no vegetation. These parched regions do not even have cyanobacteria - green photosynthetic microorganisms that live in rocks or under stones in most other deserts.
Over the course of five expeditions to the Atacama, the scientists have been searching for any traces of microscopic life in the desert rocks.
"What we are studying is, 'What is the limit of life?'" says Imre Friedmann, a microbiologist with Florida State University and the NASA Ames Research Center. "At the moment, we do not know the limit of life along the aridity gradient. Organisms may have several strategies to cope. Some organisms become desiccated; others can live with very little water. Some organisms, like the common mold, can take up water from the atmosphere if the humidity is above a certain level. But these strategies aren't very well explored, and the conditions are very difficult to replicate in the lab."
The best way to study such life is to go outside and observe Nature's laboratory - a far more complex structure than any man-made lab. While this is possible in the Atacama, at the moment such a field trip is impossible for the planet Mars.
But by studying life in the Atacama, scientists hope to learn more about the possibility of life on Mars. Mars has two major factors affecting life: the planet is very cold - with an average temperature of minus 69 degrees C (minus 92 F) - and it is very dry. Although channels and basins seem to give testament to liquid water on the surface of Mars, they probably were created long ago. Today, any water on Mars will be in the form of ice - an essentially "dry" medium.
"When is dry too dry for life?" asks Chris McKay, a planetary scientist with NASA Ames. "In the Atacama, we think we have crossed that threshold."
The Atacama, however, is not as dry as Mars. It is not as cold as Mars, either, although it is cooler than most other deserts on Earth. The Atacama enjoys a temperate, Mediterranean-like climate with temperatures ranging between 0 and 23 degrees C (32 to 73 F).
"The problem with having an Earth analog of Mars is that Mars is both extremely cold and very dry - on Earth, both conditions don't often occur together," says Friedmann. "In Antarctica, we studied the affect of cold on life. Atacama will help us answer the other question about Mars: the affect of dryness on life."
The Antarctic studies showed scientists that cold itself is not necessarily fatal to life. Life can tolerate very low temperatures and the process of freezing. But problems occur when low temperatures persist for long periods of time. All organisms on Earth need a period every now and then when they can awaken from their winter hibernation. They use this time to gather energy, repair any DNA damage their cells may have incurred from UV radiation, grow in size, and reproduce.
"There are places in Antarctica that are 'dead' because there are not enough hours of warmer temperatures," says Friedmann. "Even asleep, organisms need energy. The dead areas are where organisms must have used more energy than they could produce."
Antarctica makes a good Mars analogue because the region is both cold and essentially dry. But Antarctica experiences seasonal fluctuations when the ice in the microorganism's environment melts, so it can not answer questions about the water limits of life. To learn more about the "dry" part of the Mars question, Friedmann, McKay, and others are now focusing on the Atacama.
"The soils in the extreme arid core region of the Atacama appear to be the most lifeless and Mars-like on Earth," says McKay.
The large deposits of nitrate in the Atacama indicate the area has very little life. The nitrate is produced by electrical discharges (lightning): during storms, the lightning causes nitrogen and oxygen in the atmosphere to combine to form nitrate. This nitrate precipitates down to the desert floor, but in such small amounts it is not discernable.
"You cannot see or feel the tiny amounts of falling nitrate, and it doesn't collect in your clothes," says Friedmann. "After a year, though, you can show by chemical tests their presence in a rock."
Normally, bacteria and plants immediately use up any nitrate produced by lightning. The accumulation of nitrate - such as the deposits found in the Atacama - usually means there is no life present.
While cyanobacteria appear to be absent, the scientists did find small numbers of heterotrophic bacteria in some Atacama desert soils. Instead of producing their own energy through photosynthesis - as cyanobacteria do - heterotrophic bacteria gather their energy by feeding on other organisms. The scientists don't know what these Atacama bacteria are eating, or how they get their water. They are not even sure why some spots of the desert had the heterotrophic bacteria, while other areas seemed to be completely lifeless.
"It may be awhile before we understand the water limits of life," says Friedmann. "The Antarctic studies took us 20 years before we felt we had an answer about the limits of low temperature."
In addition to water limits, the scientists are also hoping to determine how organisms develop defenses against UV radiation. Such radiation would be yet another limit to life on Mars, which experiences more intense radiation than Earth.
Friedmann cautions that while the Atacama studies may go a long way toward improving our understanding of the limits of life, they will not fully answer our questions about life on Mars.
"All our Earth analogies are much weaker than the conditions that are present on Mars," says Friedmann. "Mars is much drier, much colder, with much more intense UV radiation."
"Our main focus right now is to understand these organic-free, sterile, and presumably oxidizing soils that we find in the core of the Atacama Desert," says McKay.
For instance, by combining measurements from nature, theoretical equations, and lab experiments, the scientists hope to determine what sort of climate microorganisms may experience in the soils of the Atacama.
"Microorganisms live in a different climate than what we live in," says Friedmann. "They live between particles in the soil, in compact places with very small distances. We're trying to measure the climate in these tight places."