In 1877, Giovanni Schiaparelli produced the first "modern" map of Mars, on which he showed a system of what he called canali. Although canali in Italian means "channel", without the implication of being an artificial feature, the word was commonly translated into English as "canal".
Schiaparelli's map of Mars (1888)
In 1910, Percival Lowell captured the imagination of the public with his book Mars As the Abode of Life. Based on his extensive visual observations (and as we know today, an active imagination) Lowell painted a compelling portrait of a dying planet, whose inhabitants had constructed a vast irrigation system to distribute water from the polar regions to the population centers nearer the equator.
Despite its appeal to the public, the astronomical community never gave serious credence to the details of Lowell's theory. The failure of many observers to confirm the existence of the canals eventually led scientists to suspect that their colleagues had been fooled into seeing the canals, by the difficulty in resolving fine detail from Earth and their own desire to believe. (This map, constructed from Viking orbiter images in the same format as Schiaparelli's -- south is up -- shows no sign of the canals, though a few features may have been interpreted as such.)
Mars from Viking
But the Lowell-inspired idea of an Earthlike Mars proved more durable. At the dawn of the space age, Mars was considered to have an atmosphere about a tenth the density of Earth's, water ice polar caps that waxed and waned with the seasons, and an annual "wave of darkening" that was often interpreted as growing plant life.
In the 1960s, observations from Earth and flyby spacecraft signalled the beginning of the end for Lowell's Mars. The Mariner 4, 6, and 7 missions returned images of a moonlike, heavily-cratered surface. The atmosphere was found to be almost pure carbon dioxide (CO2), only a hundredth the density of Earth's, and the polar caps proved to be almost entirely frozen CO2. The first global views of Mars, returned by the Mariner 9 orbiter in 1972, revealed that the planet was far more complex than the earlier flyby missions had shown, with huge volcanoes, an enormous canyon system, and evidence of running water at some point in the past. But the wave of darkening was shown to be the result of seasonal redistribution of windblown dust on the surface, the atmosphere's composition and density were confirmed, and most of the evidence for an Earthlike Mars was swept away.
But despite all these blows, the possibility of organisms on the surface could not yet be ruled out. For this reason, in 1976 the Viking landers carried a sophisticated instrument to look for possible life forms on the martian surface.
Viking Biology Experiment
In addition, independent of the biology experiments, Viking carried a
Gas Chromatograph/Mass Spectrometer (GCMS) that could measure the
composition and abundance of organic compounds in the martian soil.
(It should be noted that
The LR experiment moistened a 0.5-cc sample of soil with 1 cc of a nutrient consisting of distilled water and organic compounds. The organic compounds had been labeled with radioactive carbon-14. After moistening, the sample would be allowed to incubate for at least 10 days, and any microorganisms would hopefully consume the nutrient and give off gases containing the carbon-14, which would then be detected. (Terrestrial organisms would give off CO2, carbon monoxide (CO), or methane (CH4).)
The GEX experiment partially submerged a 1-cc sample of soil in a complex mixture of compounds the investigators called "chicken soup". The soil would then be incubated for at least 12 days in a simulated martian atmosphere of CO2, with helium and krypton added. Gases that might be emitted from organisms consuming the nutrient would then be detected by a gas chromatograph -- this instrument could detect CO2, oxygen (O2), CH4, hydrogen (H2), and nitrogen (N2).
Of the three Viking biology experiments, only the PR experiment approximated actual martian surface conditions and did not use water. In this experiment, a 0.25-cc soil sample was incubated in a simulated martian atmosphere of CO2 and CO labeled with carbon-14. A xenon arc lamp provided simulated sunlight. After 5 days, the atmosphere was flushed and the sample heated to 625 degrees C (1157F) to break down, or pyrolyze, any organic material, and the resulting gases were passed through a carbon-14 detector to see if any organisms had ingested the labeled atmosphere.
The total absence of organic material on the surface made the results of the biology experiments moot, since metabolism involving organic compounds were what those experiments were designed to detect. However, the results from the biology experiments were sufficiently confusing to be worth examining.
To reduce the chance of false positives, the biology experiments not only had to detect life in a soil sample, they had to fail to detect it in another soil sample that had been heat-sterilized (the control sample). Had terrestrial life been tested with the Viking biology instrument, the following results would have been expected:
response for response for sample heat-sterilized control GEX oxygen or CO2 emitted none LR labeled gas emitted none PR carbon detected noneIf life was completely absent from Mars, as the GCMS results suggested, these should have been the results from the biology experiments:
response for response for sample heat-sterilized control GEX none none LR none none PR none noneIn highly simplified form, these were the actual results from Mars:
response for response for sample heat-sterilized control GEX oxygen emitted oxygen emitted LR labeled gas emitted none PR carbon detected carbon detectedThe fact that both the GEX and PR experiments produced positive results even with the control sample indicates that non-biological processes are at work. Subsequent laboratory experiments on Earth demonstrated that highly-reactive oxidizing compounds (oxides or superoxides) in the soil would, when exposed to water, produce hydrogen peroxide. Oxidized iron, such as maghemite, could act as a catalyst to produce the results seen by the PR experiment.
Only the LR experiment appears to have met the criteria for life detection, and it does this rather ambiguously. When the nutrient was first injected, there was a rapid increase in the amount of labeled gas emitted. Subsequent injections of nutrient caused the amount of gas to decrease initially (which is surprising if biological processes were at work) but then to increase slowly. No response was seen in the control sample sterilized at the highest temperature (160C, 320F.) While there is still some controversy, the consensus opinion is that the LR results can also be explained non-biologically.
However, we have seen evidence that Mars may have been significantly wetter, perhaps with a denser atmosphere, earlier in its history. If so, there is the possibility that life arose on Mars, only to die out as conditions on the planet worsened. Therefore, some researchers have suggested that future searches for life on Mars be shifted to focus on extinct, rather than extant, life.
On Earth, such extinct life can be found in the form of microfossils and stromatolites. Such forms, as found in western Australia, are the oldest evidence of life on Earth, dating from 3.5 billion years ago. Microfossils are individual fossilized organisms (typically algae), as much as a few millimeters in diameter. Stromatolites are formed when layers of microbial organisms in shallow lakes or pools are covered with sediments. The organisms migrate toward the light after being covered, and the remaining organic material forms a characteristic layered or domed structure.
Stromatolites are important because they may be large enough to be seen by lander (or perhaps even high-resolution orbiter) cameras, and so some researchers have suggested searching for them near features that appear to be ancient lakes or bays. While definitive proof of biological origin would require microscopic imaging or sample return, the discovery of such features would lend credibility to the idea of extinct life.
However, the search for life on Mars thus far has been unsuccessful. Some portion of the scientific community feels that further searches are a waste of time, while another portion remains neutral or guardedly optimistic. In principle, it's simple to prove that there is life on Mars -- all one need do is find an example. Proving there isn't life on Mars is much harder. Even a prolonged negative search can be countered with the suggestion of yet another, more inaccessible place in which to look.
In the case of Mars, the issue has been complicated by the emotional belief in an Earthlike Mars, which has largely been shown to have been a myth. Mars is a spectacular place, and will remain so even if it is finally proved to be lifeless. Today, we don't know for sure if there is or ever was life on Mars. But one thing is certain -- one day, there will be.