Trying to define life is not just a philosophical exercise. We need to understand what separates living creatures from non-living matter before we can claim to find life elsewhere in the Universe.
In 1944, the physicist Erwin Shrodinger defined living matter as that which "avoids the decay into equilibrium." This definition refers to the Second Law of Thermodynamics, which says that entropy always increases. Entropy is often referred to as chaos or disorder, but really it is the spreading out of energy towards a state of uniformity. This law can be seen in a cold glass of water that slowly grows warmer until it is the same temperature as the surrounding air. Because of this trend toward equilibrium, the Universe eventually will have a complete lack of structure, consisting of evenly spread atoms of equal warmth.
But living things, said Shrodinger, are able to postpone this trend. Consider: while you are alive your body maintains its structure, but once you die your body begins to break down through bacterial action and chemical processes. Eventually the atoms of your body are evenly spread out, recycled by the Earth. To die is to submit your body to the entropy of the Universe.
Living things resist entropy by taking in nutrients. This biochemical process of taking in energy for activities and expelling waste byproducts is known as a "metabolism." If metabolism is a sign of life, scientists can look for the waste byproducts of a metabolism when searching for life on other worlds.
After the nutrient was injected, there was a rapid increase in carbon dioxide (CO2) gas. Because this gas had the radioactive label, scientists at first concluded that organisms in the Martian soil were eating the nutrient and releasing the CO2 as a waste byproduct. However, the Martian soil turned out to have a unique soil chemistry that could produce a metabolic-like reaction. Although the test remains inconclusive, most scientists believe that non-living, chemical processes in the Martian soil caused the "metabolic" reaction. The Viking experiments showed that while metabolism may be a quality of life, it is not a narrow enough guideline to search for life elsewhere.
Another quality of all life on Earth is a dependence on water. Since water plays such a crucial role in all known life forms, many scientists believe that water-use will be a quality universal to all life. But Benton Clark, an astrobiologist with the University of Colorado and Lockheed Martin, says that water is really a side issue.
"Water doesn't define life, it is just an aspect of our environment," says Clark.
Life on Earth evolved with water, and so today life on Earth is dependent on that resource. But we cannot say that without water, life is impossible. On Earth, life has been able to adapt to the harshest environments, so it is possible that life may have found a way to survive on worlds that have no liquid water.
Steven Benner, an astrobiologist with the University of Florida, agrees that water is not necessarily a universal quality of life.
"We can conceive of chemistries that might occur in sulfuric acid as a solvent - as on Venus - or in methane-ammonia mixtures - as on Jupiter," says Benner. "Discovering these would have a profound impact on our view of life, however, as well as the way that NASA looks for it."
A recent definition of life (often created to Gerald Joyce of the Scripps Research Institute) doesn't mention either metabolism or water. This definition says that life is "a self-sustaining system capable of Darwinian evolution."
But Clark says most life forms technically are not self-sustaining. Animals feed on plants or other animals, plants need microorganisms at their roots to take up nutrients, and bacteria often live inside other organisms, relying on the internal environment of their host. He says the only truly self-sustaining organisms are chemolithotrophs and photolithotrophs, and they are relatively rare.
Clark says that Darwinian evolution is another problematic criteria. How could you tell if something has undergone Darwinian evolution? The time scales involved are enormous - scientists would need a complete understanding of an organism's fossil history before being able to declare that the object is, indeed, alive.
According to Clark, living organisms exhibit at least 102 observable qualities. Adding all these qualities together into a single - if exceedingly long - definition still does not capture the essence of life. But Clark has picked out three qualities from this list that he considers universal, creating a new definition of life. This definition says that "life reproduces, and life uses energy. These functions follow a set of instructions embedded within the organism."
The instructions are the DNA and RNA "letters" that make up the genetic code in all organisms on Earth. A wild fire, one might say, reproduces and uses energy. So do crystals and various chemical reactions. In fact, Benner says that, "every spontaneous chemical process must expend free energy, living or not."
Not all living things are capable of reproduction, however. Mules are born sterile. Most honeybees do not reproduce: only the Queen bee has that honor. Many human beings live their entire lives without producing offspring, and no one would argue that such people were not therefore alive.
But Clark says that reproduction and energy-use need not both occur for life to exist. He divides life into two categories: "organisms" and "Lifeforms." Organisms channel energy according to embedded instructions, and this energy allows the organism to perform certain activities. A Lifeform, says Clark, is a broader category that encompasses organisms and makes reproduction possible.
"What I am proposing is that the individual physical entities should be called 'organisms,' but it sometimes takes a collection of organisms, the 'Lifeform,' to achieve reproduction," says Clark.
There have been many definitions of life created over the years, but there has yet to be a single definition accepted by all. Every definition has had to face down challenges to its validity. According to Carol Cleland of the University of Colorado, this is because definitions are concerned only with language and concepts; they can't expand our understanding of the world. We can only define things we already understand.
Cleland says that scientists in the seventeenth century had the same problem trying to define water. There are many descriptions of water - it's wet, thirst-quenching, it freezes and turns into vapor - but other substances also have these qualities. Once scientists discovered molecular chemistry, they could define water to everyone's satisfaction as one oxygen atom coupled with two hydrogen atoms (H2O). Perhaps we need a similar revolution in scientific thought in order to define life.
Could we use Clark's definition to find life on other worlds? The Viking Lander already looked for energy-use in the form of a metabolism, and the results were inconclusive. To search for this criterion as a means for finding life, we would need to consider other ways life could use energy.
The problem with searching for life forms with embedded instructions, says Clark, is that the criteria may be too specific. The only instructions we know of are DNA and RNA - there may be other genetic systems possible in the Universe that do not resemble the system found here on Earth.