Extraterrestrial Civilizations (6 page)

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 that
matter, 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 across the line into the region of no evidence at all I will not do.

Therefore,
until evidence to the contrary is forthcoming
, I must conclude that, on the basis of what we know of life (admittedly limited), a world without liquid is a world without life. Insofar then as the Moon seems to be a world without liquid, the Moon would seem to be a world without life.

We might be more cautious and say that a world without liquid is a world without life-as-we-know-it. It would be tiresome, however, to repeat the phrase constantly, and I will say it only now and then to make sure you don’t forget that that is what I mean. In between, please take it for granted that in this book I am speaking of life-as-we-know-it, whenever I speak of life. Please remember also that there is not one scrap of evidence, however faint or indirect, that speaks for the existence of life-not-as-we-know-it.

Even now, we may be rushing to a conclusion too rapidly. The astronomers at their early telescopes could see clearly that there was no water on the Moon in the sense that there were no seas, great lakes, or mighty rivers. As telescopes continued to improve, no sign of “free water” on the surface ever showed up.

Yet might there not be water present in minor quantities, in small pools or bogs in the shadow of crater walls, in underground rivers and seepages, or even just in loose chemical combination with the molecules making up the Moon’s solid surface?

Such water would surely not be observable through a telescope, and yet it might be enough to support life.

Yes, it might—but if life had its origin through chemical reactions taking place randomly (and we will discuss this in a later chapter), then the larger the volume in which those random processes take place, the greater the chance that they would finally succeed in producing something as complicated as life. Furthermore, the larger the volume in which the process took place, the more room there would be for the kind of prodigal outpouring of death and replacement that serves as the power drive for the random process of evolution.

Where only small quantities of water exist, the formation of life becomes very unlikely; and if it does form, its evolution is very slow. It simply passes the bounds of likelihood that there would be time and opportunity for a complex life form to form and flourish, certainly not one complex enough to develop intelligence and a technological civilization.

Consequently, even if we admit the presence of water in quantities not visible through the telescope, we can at best postulate only very simple life. There is no way in which we can imagine the Moon to be the home of extraterrestrial intelligence—assuming it has always been as it is now.

Again I say that it is
not
the concept of extraterrestrial intelligence that is hard to grasp. It is the reverse notion that meets with resistance. Telescopic evidence (in the Moon’s case) to the contrary, it remained hard to imagine dead worlds.

In 1686, the French writer Bernard Le Bovier de Fontenelle (1657–1757) wrote
Conversations on the Plurality of the Worlds
, in which he speculated charmingly on life on each of the then known planets from Mercury to Saturn.

And though the case of life on the Moon was already dubious in Fontenelle’s time and grew steadily more dubious, it proved quite possible to hoodwink the general public with tales of intelligent life on the Moon as late as 1835. That was the year of the “Moon Hoax.”

This took place in the columns of a newly established newspaper,
The New York Sun
, which was eager to attract attention and win readers. It hired Richard Adams Locke (1800–1871), an author who had arrived in the United States three years before from his native England, to write essays for them.

Locke was interested in the possibility of life on other worlds and had even tried his hand at science fiction in that connection. Now it occurred to him to write a little science fiction without actually saying that that was what it was.

He chose for his subject the expedition of the English astronomer John Herschel (1792–1871). Herschel had gone to Capetown in southern Africa to study the southern sky.

Herschel had taken good telescopes with him, but they were not the best in the world. Their value lay not in themselves but in the fact that since all astronomers and astronomical observatories were at that time located in the northern hemisphere, the regions near the South Celestial Pole had virtually never been studied at all. Almost any telescope would have been useful.

Locke knew well how to improve on that. Beginning with the August 25, 1835 issue of the
Sun
, Locke carefully described all sorts of impossible discoveries being made by Herschel with a telescope capable (so Locke said) of such magnification that it could see objects on the Moon’s surface that were only eighteen inches across.

In the second day’s installment, the surface of the Moon was described. Herschel was said to have seen flowers like poppies and trees like yews and firs. A large lake, with blue water and foaming waves, was described, as were large animals resembling bisons and unicorns.

One clever note was the description of a fleshy flap across the forehead of the bisonlike creatures, a flap that could be raised or lowered to protect the animal “from the great extremes of light and darkness to which all the inhabitants of our side of the moon are periodically subject.”

Finally, creatures with human appearance, except for the possession of wings, were described. They seemed to be engaged in conversation: “their gesticulation, more particularly the varied action of their hands and arms, appeared impassioned and emphatic. We hence inferred that they were rational beings.”

Astronomers, of course, recognized the story to be nonsense, since no telescope then built (or now, either) could see such detail from the surface of the Earth, and since what was described was utterly at odds with what was known about the surface of the Moon and its properties.

The hoax was revealed as such soon enough, but in the interval the circulation of the
Sun
soared until, for a brief moment, it was the best-selling newspaper in the world. Uncounted thousands of people believed the hoax implicitly and remained eager for more, showing how anxious people were to believe in the matter of extraterrestrial intelligence—and indeed in any dramatic discovery (or purported discovery) that seems to go against the rational but undramatic beliefs of realistic science.

As the Moon’s deadness became more and more apparent, however, hope remained that this was an unusual and an isolated case; and that the other worlds of the Solar system might be inhabited.

When the English mathematician William Whewell (1794–1866), in his book
Plurality of Worlds
published in 1853, suggested that some of the planets might not bear life, this definitely represented a minority opinion at the time. In 1862, the young French astronomer Camille Flammarion (1842–1925) wrote
On the Plurality of Habitable Worlds
in refutation, and this second book proved much the more popular.

Soon after the appearance of Flammarion’s book, however, a new scientific advance placed the odds heavily in Whewell’s favor.

In the 1860s, the Scottish mathematician James Clerk Maxwell (1831–1879) and the Austrian physicist Ludwig Edward Boltzmann (1844–1906), working independently, advanced what is called the kinetic theory of gases.

The theory considered gases as collections of widely spaced molecules moving in random directions and in a broad range of speeds. It showed how the observed behavior of gases under changing conditions of temperature and pressure could be deduced from this.

One of the consequences of the theory was to show that the average speed of the molecules varied directly with the absolute temperature, and inversely with the square root of the mass of the molecules.

A certain fraction of the molecules of any gas would be moving at speeds greater than the average for that temperature, and might exceed the escape velocity for the planet whose gravitational attraction held them. Anything moving at more than escape velocity, whether it is a rocket ship or a molecule, can, if it does not collide with something, move away forever from the planet.

Under ordinary circumstances, so tiny a fraction of the molecules of an atmosphere might attain escape velocity—and retain it through inevitable collisions until it reached such heights that it could move away without further collision—that the atmosphere would leak away into outer space with imperceptible slowness. Thus, Earth, for which the escape velocity is 11.3 kilometers (7.0 miles) per second, holds on to its atmosphere successfully and will not lose any significant quantity of it for billions of years.

If, however, Earth’s average temperature were to be substantially increased, the average speed of the molecules in its atmosphere would also be increased and so would the fraction of those molecules traveling at more than escape velocity. The atmosphere would leak away more rapidly. If the temperature were high enough, the Earth would lose its atmosphere rather quickly and become an airless globe.

Next, consider hydrogen and helium, which are gases that are composed of particles much less massive than those making up the oxygen and nitrogen of our atmosphere. The oxygen molecule (made up of 2 oxygen atoms) has a mass of 32 in atomic mass units, and the nitrogen molecule (made up of 2 nitrogen atoms) has a mass of 28. In contrast, the hydrogen molecule (made up of 2 hydrogen atoms) has a mass of 2 and helium atoms (which occur singly) a mass of 4.

At a given temperature, light particles move more rapidly than massive ones. A helium atom will move about three times as quickly as the massive and therefore more sluggish molecules of our atmosphere, and a hydrogen molecule will move four times as quickly. The percentage of helium atoms and hydrogen molecules that would be moving more rapidly than escape velocity would be much greater than in the case of oxygen and nitrogen.

The result is that Earth’s gravity, which suffices to hold the oxygen and nitrogen molecules of its atmosphere indefinitely, would quickly lose any hydrogen or helium in its atmosphere. That would leak away into outer space. If the Earth were forming under its present condition of temperature and were surrounded by cosmic clouds of hydrogen and helium, it would not have a sufficiently strong gravitational field to collect those small and nimble molecules and atoms.

It is for this reason that Earth’s atmosphere does not contain anything more than traces of hydrogen and helium, although these two gases make up by far the bulk of the original cloud of material out of which the Solar system was formed.

The Moon has a mass only 1/81 that of the Earth and a gravitational field only 1/81 as intense. Because it is a smaller body than the Earth, its surface is nearer its center, so that its small gravitational field is somewhat more intense at its surface than you would expect from its overall mass. At the surface, the Moon’s gravitational pull is 1/6 of the Earth’s gravitational pull at its surface.

This is reflected in escape velocity as well. The Moon’s escape velocity is only 2.37 kilometers (1.47 miles) per second. On Earth, a vanishingly small percentage of molecules of a particular gas might surpass its escape velocity. On the Moon, a substantial percentage of molecules of that same gas would surpass the Moon’s much lower escape velocity.

Then, too, because the Moon rotates on its axis so slowly as to allow the Sun to remain in the sky over some particular point on its surface for two weeks at a time, its temperature during its day rises much higher than does the Earth’s temperature. That further increases the percentage of molecules with speeds surpassing the escape velocity.

The result is that the Moon is without an atmosphere. To be sure, even the Moon’s low gravity can hold some gases if their atoms or molecules are massive enough. The atoms of the gas krypton, for instance, have a mass of 83.8 and the atoms of the gas xenon, a mass of 131.3. The Moon’s gravitational field could hold them with ease. However, these gases are so uncommon in the Universe generally, that even if they occurred on the Moon and made up its atmosphere,
that atmosphere would be only a trillionth as dense as the Earth’s atmosphere, if that, and could at best be described as a “trace atmosphere.”

To all intents and purposes, as far as the problem of extraterrestrial life is concerned, such a trace atmosphere is of no consequence and the Moon can still fairly be described as airless.