Navarro-Gonzalez suggested several reasons that Viking could have failed to detect organic materials.
The Viking landers heated Martian soil to scorching temperatures of up to 500 degrees Celsius (932 Fahrenheit) to vaporize its contents.
But, Navarro-Gonzalez stresses, even at such high temperatures many organic compounds are just too stable to be turned into a gas, so they may have been undetectable.
"Another possibility is that the presence of iron oxidizes these organic compounds to carbon dioxide, and this prevents detection by the mass spectrometer," Navarro-Gonzalez explained.
Viking did detect significant levels of carbon dioxidethough most scientists have maintained that it has an inorganic source or was absorbed from the planet's atmosphere.
Scientists Discover Mars-Friendly Microbes
Revisiting Viking's tests could challenge future Mars missions to step up technologies in their search for life.
NASA's Mars Science Laboratory, scheduled to blast off in autumn 2009, should pack a more potent scientific punch.
Paul Mahaffy, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, is principal investigator for the mission's sample and analysis instrument.
He notes that the mission should overcome Viking's detection difficulties and enjoy some huge advantages that will advance the search for life.
Orbiters will provide geological and mineralogical data to pinpoint the most promising surface sites to explore.
"This, combined with the ability of Mars Science Laboratory to rove several kilometers and to core into the interior of rocks, makes the search for organics much more powerful than was possible in the past," Mahaffy said.
Possible Martian life-forms now include a newly discovered class of microorganisms on Earth that can survive and even reproduce at 30 degrees Fahrenheit (minus 1 degree Celsius)below the freezing point of water.
The cold-resistant life-forms fascinate scientists, because frigid planets like Mars are far more common in our galaxy than warmer worlds.
(See Photo in the News: "Ice Lake Found on Mars" [August 2, 2005].)
"Our results show that the lowest temperatures at which these organisms can thrive fall within the temperature range experienced on present-day Mars and could permit survival and growth, particularly beneath Mars's surface," said Neill Reid, an astronomer at the Baltimore-based Space Telescope Science Institute.
"This could expand the realm of the habitable zone, the area in which life could exist, to colder Mars-like planets."
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