Extraterrestrial Civilizations by Isaac Asimov

body, even right out at the known limits of the Solar system, cannot collect them.
    Frozen water is, of course, ice. The solid forms of the other volatiles resemble ice in physical appearance so that the solid volatilesmay be referred to as ices. To distinguish the original ice, frozen water, we may call it water-ice.
TITAN
    Let us see, then, how little we can know about a world in the outer Solar system, and still be able to judge at once that it cannot bear life (as we know it).
    We have already decided that organic compounds are essential for life. Organic compounds consist of molecules made up of chains and rings of carbon atoms to which are invariably added hydrogen atoms, with lesser admixtures of nitrogen atoms, oxygen atoms, and sulfur atoms. These five types of atoms make up 99 percent or more of all the atoms in organic compounds. These atoms also make up five of the eight volatile substances. (The atoms of the other three—argon, neon, and helium—undergo no combinations and play no role in life.)
    It is clear, then, that life as we know it is a function of the volatiles and that no world can bear life unless it has at least some volatile matter.
    At the temperatures prevailing beyond the orbit of Mars, almost any body, however small, can contain some volatile matter. Every once in a while, for instance, a meteorite falls that is found to contain water, hydrocarbons, * and other volatiles. Not much, only up to 5 percent or so—but they’re there.
    Such meteorites, called carbonaceous chondrites, are few indeed compared to the ordinary meteorites that are constructed of metal, or of rock, or of a mixture of the two. Indeed, only about twenty carbonaceous chondrites have ever been located.
    This does not really mean that carbonaceous chondrites are rare. They could be very common. However, they tend to be structurally weaker than the rocky and metallic meteorites. The carbonaceous chondrites crumble away more easily in the white-hot passage through the atmosphere, so that very few fragments of any of them survive to strike Earth’s surface.
    In recent years, it has turned out that most of the asteroids, particularly those farther from the Sun, have the characteristics (dark color and low density) of the carbonaceous chondrites and therefore have volatile material in them. The two small satellites of Mars are much darker than Mars itself in color and are lower in density, so they must contain some volatile matter.
    Then, too, there are the comets, which exist as small solid bodies in that part of their orbit far from the Sun. They are perhaps only a few kilometers in diameter and are largely or almost entirely composed of icy materials.
    When they pass through the part of the orbit in the neighborhood of the Sun, some of the ices vaporize and liberate granules of rock or metal that may be mixed with the ices. The whole forms a misty “coma” about the still solid “nucleus.” The Sun constantly emits streams of rapid subatomic particles in all directions (the “Solar wind”) and this sweeps the coma outward in a direction away from the Sun, forming a long, wispy “tail.”
    Any objects in the outer Solar system that are larger than asteroids and comets would contain volatile matter almost as a matter of course, we might reason.
    Although a lack of volatile materials is a sure sign that the world does not contain life (as we know it), the converse is not true. A world may possess volatile materials and yet not contain life (Venus is an example). If this were not so, we would have to judge that just about every object beyond Mars was life bearing.
    After all, volatile materials might be present, yet organic compounds of sufficient complexity to make life possible might not form.
    From our vantage point on Earth, however, it is not easy to tell whether a small body beyond the orbit of Mars contains complex organic compounds or not. Short of exacting detail beyond our capacities to do so, is there any way of