 The Atacama Desert
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Nov. 7, 2003 — Tests carried out in the deadest, driest desert on Earth suggest that chemical reactions — not biological activity — can explain the ambiguous experimental results from the 1976 Viking spacecraft investigation into life on Mars.
When the Viking experiments were repeated in the barren soils at the heart of the Atacama Desert, in Chile, very similar results were obtained, said an international team led by Rafael Navarro-Gonzáles at the Universidad Nacional Autónoma de Mexico in Mexico City, today in the journal Science.
"We study this desert because it is an analogue of Mars," Navarro-Gonzáles told ABC Science Online. "The Atacama Desert is probably 10 to 15 million years old, making it the oldest desert in the world."
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The deserts ultra-dry core — which is over 30 miles in diameter and virtually never gets rain — supports no life, Navarro-Gonzáles said. Even bacteria carried in on the wind do not survive long.
Ahead of the forthcoming Mars space missions, the researchers set out to replicate the famous experiments performed by a robotic laboratory carried on the first of two Viking landers in 1976. A probe scooped up a handful of Martian soil and analyzed it for signs of life. Scientists still do not agree on how to interpret the results.
A minority, including a prominent Viking researcher, Gilbert Levin, have since argued that at least one experiment did find evidence of life. In that experiment, when a soup of nutrients was added to the soil the resultant reaction suggested that living microbes might be responsible.
Navarro-Gonzáles' team contends, however, that the nutrients should have been separated into biological and non-biological ones — depending on whether the chemicals that make them up are left- or right-handed in their atomic structure (many molecules can exist in mirror-image in either form).
"Life on Earth likes to eat amino acids that are left-handed, not right-handed," he said. "Life on Earth likes to eat sugars that are right-handed, not left-handed. So we could make up a soup which life likes, and then we could make up an anti-soup that organisms won't eat."
When the nutrients were added to soils from the relatively wetter regions of the Atacama Desert, the biologically compatible soup was consumed much faster than the anti-soup — revealing the presence of bacterial life.
But both the soup and anti-soup were consumed at equal rates in the desert's ultra-dry core: "We think that's definitely evidence of chemistry," Navarro-Gonzáles said.
The team still does not fully understand what kind of chemistry, but they believe it is a harsh oxidizing reaction — like bleaching — and that it is closely linked to sunlight and an absence of rain. The reaction not only kills off living things but also fully decomposes their remains.
If a similar process is happening on Mars, shallow surface probes do not seem promising as ways of detecting any life — or former life — on that planet. Navarro-Gonzáles cautions that even at a depth of 6 1/2 feet the driest Atacama soils show no sign of present or past life.
Yet although the Viking landers detected no organic compounds, and hence no clear evidence of life, when Navarro-Gonzáles' team used a lower temperature setting in their analysis they did find minute traces of organics at levels lower than the Viking experiments would have been able to detect.
But if the mystery oxidant on Earth is also found on Mars, the next generation of Martian probes will need to look deeper still — or in different surface conditions — or use other methods to look for evidence of life.
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