atmosphere contains 2.7 per cent nitrogen, an essential requirement for life, although when surface samples were analysed in a search for organic compounds, at sites several thousand kilometres apart, they revealed no signs of life. This may suggest that the chemistry of the atmosphere rules out organic development.
There were three Viking experiments (Smith, 1989). The first involved two cameras mounted on the lander to take photographs of any large plant or life-form in the vicinity. None was recorded. The second experiment involved a gas chromatograph/mass spectrometer which searched for organic molecules in the soil. No signs of life or its precursor was found. There was no evidence of existing or fossil life. However, the dried-out Martian lake beds might be a good place to look for fossils, and plans are underway for future landings in the vicinity of Martian lakes. There may be fossils in the sedimentary deposits of these ancient lakes that existed over 3.8 billion years ago. Organic material has been found in meteorites and Mars has been bombarded by them, so it might be concluded that something on Mars destroys organic molecules – the most widely accepted cause is held to be the ultraviolet flux from the Sun.
The third Viking experiment consisted of tests on samples of Martian soil to look for metabolic processes like those used by bacteria, green plants and animals. This search also proved inconclusive. The main reason given for the absence of life is that Mars lacks oceans of water in full view of the Sun.
Despite the inconclusive Viking experiments the case against life on Mars has not been proved beyond doubt. Biologists are familiar with exotic examples where life has been developed and sustained in extremely hostile environments. Life may exist in isolated ‘oases’, near the equator or in deep depressions. It may be that deeper soil samples, or samples taken from nearer the Martian ice-caps, may reveal organic compounds. If there is life on Mars, it is most likely to be found in the form of bacteria buried deep in the planet’s permafrost, or lichens growing within rocks (Kiernan, 1994). It has also been alleged that there are water springs which may harbour life in some remote parts of the planet, and observations from the Mars Global Surveyor lent support to the belief that the Martian polar caps may be covering water lakes. The Hubble space telescope has shown that there is water-ice on Mars, not only in the polar regions but on high altitudes and in the atmosphere. But conditions are not believed to be stable enough for the survival of liquid water on the Martian surface.
One current view is that the surface itself could be covered with residual chemical signs of life which previous missions, such as the Viking Landers, were not equipped to detect. Since Viking it has been assumed that the planet’s exposure to the Sun’s ultraviolet rays has oxidized organic molecules on its surface. But several scientists have suggested that ultraviolet radiation need not destroy all organic molecules as the hydroxyl radicals may react with some organic molecules to form stable compounds and carbolic acids (Knight, 2000: 11). The Viking Landers were not equipped to detect carbolic acids, but future missions could be designed to bring the untreated sample of soil back to Earth
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where more sophisticated experiments could determine the presence or absence of organic compounds.
Clues regarding the kind of life-forms to seek could be found by searching for life-forms in the Earth’s colder regions where conditions are more similar to Mars, although these clues would not count as evidence. Lichens have been found within Antarctic rocks, which protect them from the cold and absorb water. Bacteria have also been found in the Siberian permafrost, proving that life can exist in extreme circumstances (Kiernan, 1994). One of the most extreme examples of life is the discovery of terrestrial bacteria which live hundreds of metres below the ground, existing on a diet of rock and water and without any energy input from the Sun. Instead they derive their energy from chemical reactions between rock and water. These bacteria were found in deep aquifers near the Columbia River by Todd Stevens and Jim McKinley of the US Government’s Pacific Northwest Laboratory in Richmond, Washington. They called them a ‘subsurface lithoautotrophic microbial ecosystem’, or SLIME. This form of life could be present on Mars, where SLIME could survive if they are there. The problem is that we have no idea how they originated on Earth. But if they are precursors of more complex forms of life there would be a strong case for a search for Martian SLIME (O’Hanlon, 1995: 19).
In August 1996, the belief that Mars was once inhabited by primitive life-forms was revived when NASA scientists announced in a blaze of fanfare that the meteorite ALH84001 contained microscopic fossils. NASA researchers had examined this meteorite, which weighed 1.9 kilograms and was collected in Antarctica. It is said to have been chipped off Mars following a collision with an asteroid or comet. About 13,000 years ago this rock came close enough to be drawn into the Earth’s gravitational field. After detailed studies with high-powered microscopes and laser-based techniques of chemical analysis, it was claimed that certain markings resembled the outlines of tiny cells, and there were tiny beads of magnetite similar to those excreted by terrestrial bacteria.
The announcement that primitive life once existed on Mars, together with speculation that some forms of life may still survive there, stimulated a flurry of interest and support for Martian voyages. US President Clinton said: ‘I am determined that the American space programme will put its full intellectual power and technological prowess behind the search for further evidence of life on Mars.’ Announcements of planned missions were also made by Japan and Russia.
Sceptics were quick in their denunciations, suggesting terrestrial contamination or inorganic processes that mimicked signs of life. In reply it was argued that after crashing in the Antarctic the rock was swept along ice-floes, perfectly refrigerated and uncontaminated, until it was collected by scientists in 1984. However, while early analysis seemed to suggest that the fossil findings originated as early Martian life-forms, the issue was far from decisive and a very confusing picture emerged. Some opponents argued that the meteorite had been contaminated during the examination process or by components from the
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