Soil in one of the oldest and driest deserts in the world is a close match to the rocky red soil of Mars, say scientists.
By Helen Briggs
BBC News Online
The discovery of Martian-like soil in Chile's Atacama Desert could shed light on the landings of the Viking spacecraft on Mars in the 1970s.
Sunset over the Atacama Desert
Dry and largely devoid of organic material or microbial life, the desert earth has similar chemical properties to the Red Planet's.
The finding could explain why tests conducted by the Mars Viking missions raised false hopes in the search for life on another planet.
In 1976, the world was gripped by excitement when a robotic spacecraft touched down on Mars for the first time in history.
A biology experiment on Viking 1 detected strange signs of activity in the Martian soil - akin to microbes giving off gas.
Before announcing the news that life had been found on another planet, the US space agency (Nasa) carried out more tests to look for evidence of organic matter.
The Viking experiments failed to find carbon - the basic building block of life - and it was concluded that Mars was a dead planet. Nasa put the aberrant results down to the presence of powerful oxidising agents in the Martian soil.
Some observers have never accepted this explanation and have called for the experiments to be repeated.
This has now happened, not on Mars but in the Atacama desert. In some ways it settles the debate over the Viking experiments but also raises further questions.
"The commonly held interpretation of the Viking results is that the soil was chemically, but not biologically active," said Dr Richard Quinn of the Nasa Ames Research Center in the United States.
The barren landscape of the Atacama Desert
"Our work has identified a site on Earth where similar chemical processes may be occurring and as a consequence inhibit life."
The soils from the core of the Atacama desert are extremely dry. The driest sites are similar to the areas of Mars investigated by the Viking missions.
Low water levels combined with sunlight seem to have created a chemical cycle that decomposes organic materials and inhibits life.
When the researchers repeated the Viking experiments on the Atacama soils, they came up with similar results.
But they did find minute traces of organic matter, at levels lower than the Viking experiments would have been able to detect on Mars.
So, the possibility remains that organic matter was there all the time.
"Currently, the unanswered question is whether Viking failed to detect certain types of organic compound on Mars," Dr Quinn, a member of the international research team, told BBC News Online.
"Our work suggests that this may have been the case. However, new experiments would need to be done on Mars to answer this question."
In the meantime, the Atacama soil may provide a valuable testing ground for instruments and experiments designed for future Mars missions.
Two new instruments are in development - one to characterise chemical processes on Mars (Mars Oxidant Instrument) and the other to detect organic compounds (Mars Oxidant Detector).
"We hope to use these instruments on Mars to identify soils that may have contained life at some point in the planet's history," said Dr Quinn.
The research is published in the journal Science.