Extraterrestrial Civilizations by Isaac Asimov Read Free Book Online

mixtures are always homogeneous, that is, all components are well mixed. Any chemical reaction that takes place in one part can equally well take place in another part and therefore spreads from one part of the system to the other with explosive rapidity. It is difficult to see how the carefully controlled and regulated reactions, which seem essential to something that is as complicated and finely balanced as living systems would appear to be, can exist in a gas.
    Then, too, the molecules making up gases tend to be very simple. The complicated molecules that we can assume would be needed (ifwe are expected to witness the varied, versatile, and subtle changes that must surely characterize anything as varied, versatile, and subtle as life) are, under ordinary circumstances, in the solid state.
    Some solids can be converted into gases by being heated sufficiently, or by being put under very low pressure. The complicated molecules characteristic of life would break up into small fragments if heated, however, and would be useless. If placed under even zero pressure, the complicated molecules will produce only insignificant quantities of vapor.
    We conclude, then, that we cannot have life in the gaseous state.
    In solids, the component molecules are in virtual contact, and can exist to any degree of complication. What’s more, solids can be, and usually are, heterogeneous; that is, the chemical makeup in one part can be quite different from the chemical makeup in another part. In other words, different reactions can take place in different places at different rates and under different conditions.
    So far, so good, but the trouble is that the molecules in solids are more or less locked in place, and chemical reactions will take place too slowly to produce the delicate changeability we associate with life. We conclude, then, we cannot have life in the solid state.
    In the liquid state, the component molecules are in virtual contact, and the possibility of heterogeneity exists, as in the solid state. However, the component molecules move about freely, and chemical reactions can proceed quickly, as in the gaseous state. What’s more, both solid and gaseous substances can dissolve in liquids to produce extraordinarily complicated systems in which there is no limit to versatility of reaction.
    In short, the kind of chemistry we associate with life would seem to be possible only against a liquid background. In Earth’s case that liquid is water, and we will have something to say later in the book as to whether there is the possibility of any substitute.
    A world, then, that is without water (and without any other liquid that might substitute) would seem to be surely incapable of supporting life.
    Or am I still being too narrow minded?
    Why can’t life, with chemical and physical properties completely different from terrestrial life, nevertheless develop and even evolve intelligence? Why can’t there be a very slow, solid life form (too slow, perhaps, to be recognized as life by us) living on the Moon or, for thatmatter, here on Earth? Why not a very rapid and evanescent gaseous life form, literally exploding with thought and experiencing lifetimes in split seconds, existing on the Sun, for instance.
    There have been speculations in this direction. Science fiction stories have been written that postulated enormously strange life forms. The Earth itself has been considered as a living being, as have whole galaxies, and as have clouds of dust and gas in interstellar space. Life consisting of pure energy radiation has been written about and life existing outside our Universe altogether and therefore indescribable.
    There is no limit to speculation in this respect, but in the absence of any evidence, they can only remain speculations. In this book, however, I will move only in those directions in which there is at least some evidence to guide me. Fragmentary and tenuous that evidence may be, and the conclusions shaky enough—but to step