Eight days after arriving on the rock-strewn Martian plain of Chryse Planitia, Viking 1 sank its stainless steel claw scoop into the rust-tinted soil. It was 3:30 a.m. July 28, 1976. The first search for life on another world had begun.
But far from settling the question of extraterrestrial life, the expedition showed just how difficult it is to answer.
Viking's verdict: Mars is and always was dead. In the years since, however, scientists have questioned whether the spacecraft's instruments were sensitive enough. Others are bothered by baffling inconsistencies in the data it beamed back.
Of the new generation of robots closing in on the Red Planet this month, the British-built Beagle 2 may have the best shot of putting to rest the debate over what, if anything, Viking detected in the Martian dunes that summer day 27 years ago.
Each Viking was equipped with a trio of ingeniously designed biology experiments designed to pinpoint the presence of Martian microbes. Not long after the first scoop of Martian soil tumbled in, the automated biology labs began to signal signs of life.
But with the world watching, Viking scientists were reluctant to declare they had found Martians. So they turned to Viking's gas chromatograph-mass spectrometer. If Mars really harbored life, the spectrometer would see chemical fingerprints of carbon-based organic material.
It didn't. Gerald Soffen, the mission's chief scientist, was puzzled. "All the signs suggest that life exists on Mars, but we can't find any bodies!" he said.
Most scientists now think that Viking's life detectors saw a mirage, detecting chemistry, not biology.
But the absence of organic molecules on Mars remains a nagging mystery, says astrobiologist Christopher McKay of NASA's Ames Research Center in California. Organics, after all, have turned up all over the solar system - including the moon. Why not Mars?
Beagle 2 may finally solve the riddle. Tucked inside its compact clamlike shell, the spacecraft carries a more sophisticated and sensitive version of Viking's organic detector called the gas analysis package, or GAP.
Twelve tiny ovens will cook rock and soil samples to produce carbon dioxide gas. A mass spectrometer will measure the ratio between two isotopes of carbon, Carbon-12 and Carbon-13. On Earth, a higher concentration of Carbon-12 usually signals the presence of life.
