Extraterrestrial Civilizations - Isaac Asimov [33]
The smallest, in fact, contain no volatiles at all. Mercury is made up of a sizable metal core, surrounded by a rocky mantle. (We know this is so because Mercury’s density is so high that much of it must be the high-density metal and only the rest of it medium-density rock.) The Moon is made up of rock only. Its density is too small to allow any metal core of significant size. Both Mercury and the Moon lack volatiles.
Mars, like the Moon, is of rock only. Earth and Venus, like Mercury, are made up of rock over a metal core. These three, however, are all large enough to be able to retain some volatiles by gravitational attraction.
Beyond the orbit of Mars it becomes easier to accumulate volatiles at a given level of gravitational intensity. For one thing, at lower temperatures, all molecules move more slowly and are less likely to exceed escape velocity. For another, the volatiles solidify one by one as the temperature drops, and solid volatiles will cling together by chemical attraction and no longer be dependent on gravitational pull.
The freezing points, under terrestrial conditions, of the eight volatiles are given in the accompanying table:
This means that anywhere beyond the orbit of Mars even small bodies can collect not only metal and rock, but also such volatiles as water, ammonia, and hydrogen sulfide in solid form. If the small bodies are sufficiently far from the Sun to have temperatures very low, then methane and argon can also be collected in solid form. Neon, hydrogen, and helium freeze at so low a temperature that a small 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 volatiles may 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