Mars. Photo: NAS/JPL

In 1976, two superbly designed robotic emissaries from the earth soft-landed on the rock-strewn surface of another world in search of life. After conducting three biology experiments, the American Viking missions to Mars concluded that they could neither confirm nor refute the presence of biological entities. Moreover, the instruments on board the spacecraft failed to detect even the faintest presence of organic matter - the stuff of life - in the Martian soil. But now, a quarter of a century on, new research raises fresh doubts about a seminal conclusion reached by these historic experiments.

For over a century, Mars has captured the imagination of the general public as a likely abode for life beyond the earth. And though astronomers' knowledge has come a long way from the days of Percival Lowell's Canals, many scientists still entertain hopes of finding some life on the Red planet. But when the Vikings went to Mars over a quarter of a century ago, they dealt the Martian life hypothesis a venomous blow.

Using a powerful scientific instrument called a gas chromatograph-mass spectrometer (GC-MS), samples of Martian soil were subjected to heating at temperatures between 200o C and 500o C for 30 seconds, after which the vapourised molecules were identified on the basis of their molecular mass and electrical charge.

Though the Viking landers were positioned nearly 8,000 kilometres apart, neither detected any organic matter. But that result was just plain weird. For one thing, the Red planet is dusted with some 240 thousand kilograms of organic material each year via meteorites - a concentration that could easily have been detected by the Viking GC-MS experiments. The answer, chemists soon realised, was that the Martian soil is highly oxidising, more than capable of sundering even the most robust organic species into smithereens. This result, above all others, was and remains strong evidence against the possibility of life on Mars - at least at the surface.

Now, in a paper published in March 14 edition of Proceedings of the National Academy of Sciences, USA, a team of scientists headed by Charles Benner at the University of Florida believe that a sizeable store of organic material may have been undetectable with the instruments placed on board Viking. In particular, they have outlined an elaborate sequence of chemical reactions in which organic molecules could have been modified by highly reactive peroxides and radicals in the Martian regolith, rendering their products difficult to detect using GC-MS. The failure of the Viking experiments to find organics should not, they insist, be taken as a strong argument against the presence of all organic matter on the Red Planet. The mystery continues.