When NASA’s Mars Science Laboratory (MSL) lands its rover, Curiosity, on Mars [it landed yesterday], it will be the latest in a series of missions to the red planet that began more than three decades ago. The MSL mission is not a search for life. Curiosity will sniff for chemicals that could be relevant to life, but it will not be looking for biological organisms as such.
Why is this? Given the huge public interest in life on Mars, why doesn’t NASA just go and look for it directly? To understand, you have to go back to 1976, and NASA’s trail-blazing Viking mission. Two identical spacecraft landed on Mars to look for microbes in the topsoil. Several experiments were performed. One consisted of adding a nutrient broth to soil to see if anything consumed it and gave off carbon dioxide. To scientists’ surprise, something did — repeatedly — on both spacecraft. When the soil was heated, the response stopped. To this day the designer of the experiment, Gilbert Levin, insists he found life on Mars.
Few scientists agree. The other Viking experiments did not give such clear-cut results, and NASA’s official position is that the mission did not detect life. So what caused the broth to emit gas? Nobody really knows. It may indeed have been life, but it may also have been complex soil chemistry. Conditions on the surface of Mars are very harsh. Radiation is intense. Water exists, but almost never in liquid form. Reactive chemicals such as oxidants can accumulate over immense durations without being washed away or neutralized. So it is perhaps no surprise that adding liquid broth made the soil fizz.
Because of the inconclusive Viking results, the direct search for life on Mars has effectively stalled, as it is hard to know precisely what to look for. What would be an unambiguous signature of life anyway? Scientists cannot even agree on a definition of life as we know it, let alone a possibly different form of life. And when the soil chemistry is unfamiliar, the problems are compounded.
However, clarification could come by seeking out Mars-like surfaces on Earth and studying what, if anything, lives there. Mars is very cold and very dry, but, of the two, the dryness is the more serious obstacle: water is crucial to known life. The driest place on Earth is the Atacama desert in Chile, and for years astrobiologists have been sifting the soil there, looking for hardy microbes able to eke out an existence in the hyper-arid terrain. For a while it looked as if no life could withstand the desiccating conditions of the Atacama’s core, but then in 2006 a visiting chemist from the University of Lleida in Spain, Jacek Wierzchos, made a discovery.
Projecting out of the parched dusty surface of the desert are countless natural sculptures made of common salt. Wierzchos broke one open and was puzzled to find a distinctive dark layer inside. He dissolved the salt rock and found the coloration was caused by several new species of microbe living inside.
How do Wierzchos’s bizarre microbes survive? It seems that enough light penetrates the salt to permit photosynthesis. However, what supplies the all-important water? This was the most astonishing part. A distinctive property of salt, known as deliquescence, is its ability to suck in moisture directly from the air. The microbes scavenge this sparse resource, ingesting tiny quantities of water from microscopic pores inside the crystalline matrix. Although the fierce desert sun bakes the water out of the salt in the daytime, there is enough humidity in the air at night to replenish it by deliquescent absorption. So even if it never rains, life can go on.