Extraterrestrial Civilizations - Isaac Asimov [31]
No type of atom other than carbon can form chains and rings with anything like this facility.
Furthermore, it is difficult to imagine that so complex and versatile a phenomenon as life can make do with anything less complex than the molecules with which we are familiar in Earthly organisms.
This does not seriously limit the infinite variability of life. It is enormously variable here on Earth, in form, in structure, in behavior, in adaptation, yet it is all based on organic compounds, which are in turn based on chains and rings of carbon atoms.
What is more, the number of conceivable variations on the structure of the organic compounds is so enormous as to be far beyond expression in any comprehensible manner. The number of organic compounds used by terrestrial life compared to all the organic compounds there can conceivably be is far less than the size of an atom compared to the size of the entire Universe.
In summary, then, the number of complex compounds based on carbon atoms is virtually limitless, and in comparison the number of complex compounds that do not contain the carbon atom is virtually zero. We can assume, therefore, that if a world lacks organic compounds, it lacks life.
Again, it would be well not to hasten on too rapidly. Can we be sure that under certain conditions of a type with which we are not familiar, elements or combinations of elements other than carbon might not produce complicated compounds? Can we be sure that under certain conditions life might not be built up out of relatively simple compounds?
We can’t be. Considering how little we know of the details of other worlds, and of the finer points of life other than what we can glean from our own example, we can’t be sure of anything.
But we can ask for evidence. There is no evidence whatever of the possible existence of molecules as complex, delicate, and versatile as organic compounds, built up of any element but carbon, or of any combination of elements that excludes carbon. Nor is there any evidence that something as complex as life could be built up out of relatively simple compounds.
Therefore, until evidence to the contrary is forthcoming, we can only assume that if organic compounds are not present, life is not present.
As it happens, the analysis of Martian soil by Vikings 1 and 2 indicates the absence of organic compounds.
This leaves the matter of life on Mars ambiguous. The evidence is clearcut neither for nor against and must await further and better testing. Nevertheless, if life is present, there seems very little chance that it is anything more than very primitive in nature—no more than on the level of bacterial life on Earth.
Such simple life would be quite sufficient to excite biologists and astronomers, but as far as the search for extraterrestrial intelligence is concerned, we are left with what is overwhelmingly likely to be zero.
We must look elsewhere.
*There may be small amounts of water in the solid state (ice) held to the asteroids and other small worlds by chemical bonds that don’t depend on gravitational forces for their efficacy. Frozen water, however, is not suitable for life and even on Earth the frozen ice sheets of Greenland and Antarctica are life free in their natural state.
*He was the father of John Herschel, who a half-century later was to be victimized by the Moon Hoax.
* Today, we know of some exceptions.
* They can also be formed in the laboratory. In addition, uncounted thousands of such compounds, not quite like any to be found in living organisms or their residues, have also been synthesized by chemists. But then, chemists are living organisms so that even the synthetic molecules that “are not found in nature” are the result of the actions of living organisms.
CHAPTER 4
The Outer Solar System
PLANETARY CHEMISTRY