What is life? Trying to answer this question often leads people to a dead end. The impasse begins with the fact that in any language there are many definitions of the word "life". For example, here are three uses with different meanings.
Is there life on Mars?
Is there life in this organism?
Is life worth living?
The definition of "life" in these three cases is fundamentally different. In the first case, life means a collective phenomenon, in the second it refers to the ability of an individual organism to metabolize and grow, and in the third, life refers to the activity that an organism experiences during its life. The first two are usually directly related to astrobiology.
The usual definition of life, used in the first case, means a system of material objects that can evolve, reproduce, mutate and select. This is what we are looking for on Mars and other worlds. We would be interested if such a life had an independent origin from ours.
It is often noted that the definition of life as a system capable of evolution suggests that isolated individuals do not represent “life”. This is nonsense that confuses the first and second examples of life.
Many experts are of the opinion that effectively seeking life on other worlds requires us to agree on what to call life. Yet many agree that once we understand life, we can produce a completely mechanistic and predictable theory of life. For example, take the same water. Water is generally defined by two hydrogen atoms and one oxygen atom. But life is not a simple substance like water, but rather a process like fire.
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Dr. Chris McCoy
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Fire has no simple definition. If life is like fire, then even with a purely mechanistic and predictable theory, we will not be able to define a simple closed form of fire. The search for life on other planets may be based on what life does, rather than its definition. For example, it can build large, specialized molecules like DNA and proteins.
Viking, the only mission to find life on other worlds, focuses on the second example. Viking biologists are looking for something alive in the sample. It is assumed that if something lives, then it consumes organic matter and gives off gases; has a metabolism. Thus, the working definition of "life" in Viking experiments is based on the ability to metabolize under experimental conditions.
But there are a number of problems with this definition. First of all, there are many non-biological processes during which organic matter is consumed and gases are released. Secondly, our earthly experience has shown that many microorganisms are picky eaters and do not grow in laboratory conditions on food additives.
Perhaps the biggest problem with the Viking approach is that it cannot detect dead organisms, which, unfortunately, are likely to be in this state on Mars, Europa or Enceladus. The search for life in our solar system should generally begin with the definition of death.
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What does it mean to be dead? This means that the organism once lived and consists of organic molecules that are specific to life - DNA, ATP, proteins. These are biomarkers that could provide compelling evidence that an organism was once alive and is a product of a system of life that has passed an evolutionary path.
The search for such biomarkers formed the basis of the methods for the search for life that are being developed now. The challenge is to develop tools that can search for biomarkers of terrestrial life, and can also recognize biomarkers of unknown alien life.
