Extraterrestrial Civilizations - Isaac Asimov [32]
The inner Solar system out to the orbit of Mars is a comparatively small structure. Beyond Mars is the “outer Solar system,” which is far vaster and within which giant planets orbit. There are no less than four such giants out there: Jupiter, Saturn, Uranus, and Neptune. Each dwarfs Earth, particularly Jupiter, which has over 1,000 times the volume of Earth and over 300 times its mass.
Why should the inner Solar system contain pygmies and the outer Solar system giants? Consider—
The cloud out of which the Solar system was formed would naturally have been made up of the same kind of substances that make up the Universe generally—more or less. Astronomers have, through spectroscopy, determined the chemical structure of the Sun and of other stars, as well as of the dust and gas between the stars. They have therefore come to some conclusions as to the general elementary makeup of the Universe. This is given in the accompanying table:
Element Number of Atoms
for every 10,000,000
Atoms of Hydrogen
Hydrogen 10,000,000
Helium 1,400,000
Oxygen 6,800
Carbon 3,000
Neon 2,800
Nitrogen 910
Magnesium 290
Silicon 250
Sulfur 95
Iron 80
Argon 42
Aluminum 19
Sodium 17
Calcium 17
all other elements combined 50
As you see, the Universe is essentially hydrogen and helium, the two elements with the simplest atoms. Together hydrogen and helium make up nearly 99.9 percent of all the atoms in the Universe. Hydrogen and helium are, of course, very light atoms, not nearly as heavy as the others, but they still make up about 98 percent of all the mass in the Universe.
The fourteen most common elements given in the table above make up almost the entire Universe. Only one atom out of a quarter million is anything else.
Of the fourteen, the atoms of helium, neon, and argon do not combine either with each other or with the atoms of other elements.
Hydrogen atoms will combine with other atoms after colliding with them. In view of the makeup of the Universe, however, hydrogen atoms will, if they collide with anything at ail, collide with other hydrogen atoms. The result is the formation of hydrogen molecules, made up of two hydrogen atoms each.
Oxygen, nitrogen, carbon, and sulfur are made up of atoms that are likely to combine with hydrogen atoms when the latter are present in overwhelming quantity. Each oxygen atom combines with two hydrogen atoms to form molecules of water. Each nitrogen atom combines with three hydrogen atoms to form molecules of ammonia. Each carbon atom combines with four hydrogen atoms to form molecules of methane. Each sulfur atom combines with two hydrogen atoms to form hydrogen sulfide.
These eight substances—hydrogen, helium, neon, argon, water, ammonia, methane, and hydrogen sulfide—are all gases at Earth temperatures or, in the case of water, an easily vaporized liquid. We can lump them all together as “volatiles” (from a Latin word for to fly since, as gases or vapors, they are not held firmly to matter, but tend to diffuse or fly away).
Silicon combines with oxygen much more easily than with hydrogen. Magnesium, aluminum, sodium, and calcium combine readily with the silicon-oxygen combination, and these six elements together make up the lion’s share of the rocky materials (“silicates”) that we are familiar with.
As for iron—that tends to be present in rocks, but is sometimes present in considerable excess so that much of it remains in metallic form. To the iron are added the similar but less common metals nickel and cobalt.
The atoms and molecules of rocks and metals cling together, bound by strong chemical forces, so that they remain solid up to white-hot temperatures. They do not require gravitational forces to hold them together so that atoms in tiny grains of rock or metal, where the gravitational forces are utterly negligible, nevertheless hold firmly together.
Of the original material composing the primordial nebula out of which the Solar system was formed about 99.8 percent of the mass were volatiles, and only 0.2 percent were solids.
In the inner Solar system, the heat of the