Extraterrestrial Civilizations (10 page)

In 1975, two probes,
Viking 1
and
Viking 2
, the former launched on August 20, the latter on September 9, were sent to Mars. They were to land on the planet and observe it in various ways. In particular, they were to test the planet for signs of life.

They landed safely in the summer of 1976 in two widely separated places. They analyzed the Martian soil and found it to be not too different from Earth’s, but richer in iron and less rich in aluminum.

Three experiments were conducted that might detect life. All three gave results of the kind that might be expected if there were living cells in the soil.

There was, however, a fourth experiment that cast doubt upon the first three. To understand that, we will have to consider the nature of the molecules most characteristic of living organisms as we know it.

Against the background of water, there is in living organisms a rapid and never ending interplay involving complex molecules made up of anywhere from a dozen to a million atoms. These are found, in nature, only in living organisms and in the dead remnants of once living organisms.
^*
For that reason, such complex molecules are called organic compounds.

Organic compounds have something in common—the element carbon. Carbon atoms have a unique facility for combining with each other in complex chains, both straight and branched, and in rings or collections of rings to which chains of atoms, either straight or branched, can be attached. Also attached on the outskirts of the carbon chains and rings are atoms and combinations of atoms of other elements, chiefly those of hydrogen, oxygen, and nitrogen, plus occasional atoms of sulfur, phosphorus, and so on. Sometimes, one of these other atoms may actually be incorporated into the body of the carbon chain or ring.

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.

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:

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 nearby Sun raised the temperature high enough to keep the atoms and molecules of the volatiles moving fast enough to be too nimble to be caught gravitationally. The planets in the inner Solar system ended up composed of rocks and metals that required no gravitational force to be held, but that also made up only a very small part of the nebular material. That is why the inner planets are small.

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.

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.)