How to explain the absence of organic molecules? Some process either destroys them as quickly as they are formed (a torch burns up the confetti as it comes from the machine), or isolates them so that they escape detection (the detector lands in the next field and finds only an occasional scrap blown over by the wind). The most plausible attempts to account for such processes involve the actions of Martian organisms.
For example, these valuable molecules may simply be eaten up by living creatures in the soil. On earth, areas where food is scarce often support a large population of scavengers. (Vultures abound in desert regions.) The Martian environment is very harsh, and food may be hard to find. Thus, a Martian organism would have to be adept at searching out and quickly devouring every available scrap of organic material. This kind of efficient recycling would cut down on the buildup of waste products, and perhaps leave too little for the GCMS to detect, even if the soil was teeming with a large variety of active, healthy creatures.
Another possibility is that life exists on Mars, but only in certain pockets, or "microenvironments," as Carl Sagan and Joshua Lederberg have suggested. In other words, Mars may be a vast desert with only a few isolated oases, where life can thrive under special conditions in a small area—for example, where water has accumulated because of geothermal heating (the process which causes "hot springs" on earth). In that case, a small number of seeds or spores might be transported all over the planet by the winds, but would grow and reproduce only in these few select spots. Thus, Viking would detect a few spores, but no accumulation of organics.
This conclusion has strong support from the fact that of all the Earth soils which have been tested by the pyrolitic-release experiment, the one that comes the closest to reproducing the Martian results is Antarctic soil, which includes spores transported from elsewhere by the wind, but no active life.
Another theory is that Martian organisms may have developed hard shells to protect them from the deadly ultraviolet radiation, and that these shells might prevent the organic remains from reaching the GCMS and being detected. In short, if life exists on Mars, the apparent conflict between biology results and the organic chemistry analysis, may be easier to explain. If no life exists, the contradiction is much more difficult to resolve.
Some say the strong ultraviolet radiation which constantly bombards the Martian surface explains the supposed paradox. In theory, organic molecules might be destroyed by the UV almost as fast as they were being created, and therefore would never have chance to pile up. This theory received a major setback when the GCMS ran a test on soil from underneath rock, where it had presumably been shielded from ultraviolet radiation for millions of years. Since this test, like the previous ones, failed to detect any organics, destruction by UV seems not to be the answer.
A great deal of evidence has accumulated from the Viking mission to support the conclusion that Mars is a biologically active planet. According to Dr. Gilbert Levin, of the biology team, "The accretion of evidence has been more compatible with biology than chemistry. Each new test result has made it more difficult to come up with a chemical explanation, but each new result has continued to allow for biology."
All of the life-seeking tests showed reactions, which, says Dr. Levin, "if we had seen them on earth, would unhesitatingly have described as biological. All of them have been confirmed by control experiments. All of them have been successfully repeated at both of the Viking landing sites.
We may have set our sights too low in the Viking experiments, however. We simply weren't optimistic enough about the possibility of life on Mars, and therefore concentrated on looking for the simplest and most primitive forms of life—the ones most likely to exist.
We found what we were looking for, but we may have been examining bacteria in the midst of a rich assortment of flora and fauna. We have not eliminated any possibilities about the extent or the level of intelligence of Martian life. All we've done is to raise the bottom line: at the very least, a substantial population of microorganisms almost certainly exists.
This possibility is clearly illustrated by an exhaustive investigation conducted by Dr. Sagan. He carefully examined thousands of satellite photographs of Earth which had a limit of resolution (the ability to distinguish detail), similar to those that have been taken of Mars by the Mariner 9 and the Viking orbiters. His conclusion: At that resolution, one can find no visible evidence for the existence of any kind of life on this planet, in 99 percent of the photographs.
In fact, he determined that in order to be sure of getting unambiguous evidence for intelligent life on Earth, one would need a resolution 10 times greater than we have yet achieved in photographing most of the Martian surface. Even if an advanced civilization exists on Mars, even if large cities can be found there, we simply wouldn't have been able to see them yet. We haven't looked closely enough.
But even at the present limits of resolution, some surprising formations have been seen, the most inexplicable of which are the three-sided pyramids found on the plateau of Elysium. Scientists have tried to find a natural geological process that would account for the formation of these pyramids, some of which are two miles across at the base, but as yet their origin is far from being explained. Such tantalizing mysteries may not be fully solved until astronauts are able to make direct observations on the Martian surface.
Some argue that the harshness of the Martian environment—the low temperatures, the absence of liquid water, the ubiquitous lethal ultraviolet radiation—make it extremely unlikely that advanced forms of life would ever have evolved there. However, considerable evidence now suggests that Mars used to be a much more pleasant place to live.
For example, the Mariner photographs showed a vast network of sinuous channels covering most of the planet. Scientists agree that these must almost certainly be the dried-up beds of vast rivers which once coursed across the Martian landscape.
This has enormous implications for the past climate of Mars, and therefore for the evolution of Martian life. First of all, water is a basic prerequisite for all life that we know of. The presence of vast amounts of water flowing across the face of Mars in some ancient era, therefore, greatly increases the likelihood that advanced and even quite familiar forms of life have been able to develop there. But it also implies some other differences in the Martian environment of the past. In order for water to exist in a liquid state on the surface of Mars, which is impossible today, the atmosphere must have been much denser and the temperatures much higher.
On Mars today, the air is so thin that ice turns directly to steam without ever going through a liquid state—just as "dry ice" (frozen carbon dioxide) does on Earth. However, if a large amount of water vapor accumulated in the Martian atmosphere (for example, by the melting of part of the polar ice cap), this would increase the air pressure to the point where water could exist as a liquid, and rain would begin to fall. Rivers would begin to flow. Seas would begin to fill.
At the same time, a substantial "greenhouse effect" would occur, in which the warmth of the sun would trapped by the atmosphere faster than it could dissipate again into space, and the temperature of the entire planet would be raised—according to Sagan's calculations, perhaps by 60 degrees or more. The net result of all this would be a very comfortable, Earth-like planet, capable of supporting an array of familiar creatures. Indeed, if Sagan's reconstruction is correct, during these warmer epochs of Martian climate, a man might have been able to walk around on Mars in his shirtsleeves in perfect comfort.
Many scientists, including Dr. Sagan, believe that this sequence of events has happened not only once but repeatedly throughout Martian history, in a continuing cycle—similar, perhaps, to the cycle of ice ages, on our planet.
And if some epochs were comfortable enough to foster the development of advanced forms of life, at least some of those forms then might have been able to adapt to the changing conditions, and might still be around today—perhaps in a dormant state.
The discovery of any kind of life on Mars, however primitive it may prove to be, would profoundly increase the probability of finding other living beings elsewhere in the universe.
As long as Earth was the only life-bearing planet that we knew of, the chance existed that life might have been produced by some fluke, some unique set of circumstances which had not occurred on any other planet—a rare and exotic process so unlikely that we would never find another example.
But if the process is not unique, the universe may be teeming with all manner of beings at every imaginable stage of development. And we may someday establish communication with another race of intelligent creatures.The knowledge to be gained from such a contact can scarcely be imagined.