Extraterrestrial Civilizations Page 5

  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.

  MOON HOAX

  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.

  AIRLESSNESS

  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 mad
e 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.

  All this has meaning with respect to a liquid such as water. Water is “volatile,” that is, it has a tendency to vaporize and turn into a gas. At a given temperature, there is a countertendency for the gaseous water vapor to recondense into liquid. At any particular temperature, liquid water is therefore liable to be in equilibrium with a certain pressure of water vapor, provided that water vapor is not removed from the vicinity as, for instance, by a wind.

  If the water vapor is removed, equilibrium pressure is not built up and more of the liquid water vaporizes, and still more, till it is all gone. We are all familiar with the way in which the water left behind by a rainstorm evaporates until it is finally all gone. The higher the temperature, the faster the water evaporates.

  Naturally, the water vapor is not removed from the Earth altogether. If it does not condense in one place, it condenses in another as dew, fog, rain, or snow, and thus the Earth holds on to its water.

  If there were liquid water on the Moon, the vapor that would form would leak out into space, for the mass of the water molecule is but 18 and the Moon’s gravitational field would not hold it. The liquid water would continue to vaporize and eventually the Moon would dry up altogether. The fact that there is no air on the Moon means there is no air pressure to slow the rate of water evaporation, and the water, if it had been present, would have been lost all the more quickly.

  The Moon, therefore, must be without water as well as without air. What’s more, any airless world would be a lifeless world—not because air is necessarily essential to life, but because an airless world is a waterless world, and water is essential.

  Even the kinetic theory of gases leaves loopholes, however. The possibility remains that scraps of water, even air, can exist underground on the Moon, or in chemical combination with molecules in the soil. In that case, the small molecules would be prevented from leaving by forces other than gravity—by physical barriers or chemical bonding.

  Then, too, there may have been a time early in the history of the Moon when it had an atmosphere and an ocean, before it lost them both to space. Perhaps in those early days, life developed, even intelligent life, and it may have adapted itself, either biologically or technologically, to the gradual loss of air and water. It might, therefore, be living on the Moon in caverns, with a supply of air and water sealed in.

  As late as 1901, the English writer H. G. Wells (1866–1946) could publish The First Men on the Moon and have his heros find a race of intelligent Moon beings, rather insectlike in character and highly specialized, living underground.

  Even that much seems doubtful, however, since calculations show that the Moon would have lost its air and water (if any) quite rapidly. It would have retained them for many times the lifetime of a human being, of course, and if we were living on the Moon when it still had an atmosphere and ocean we could live out our life normally. The atmosphere and ocean would not last long enough, however, to allow life to develop and intelligence to evolve from zero. It wouldn’t even come close to doing that.

  And we seem to be at a final answer now. On July 20, 1969, the first astronauts landed on the Moon. Samples of material from the Moon’s surface were brought back on this and later trips to the Moon. Apparently the Moon rocks all seem to indicate that the Moon is bone dry; that there is no trace of water upon it, nor has there been in the past.

  The Moon would seem to be, almost beyond conceivable doubt, a dead world.

  * It was the first science fiction story to be written by a professional scientist—but not, by a long shot, the last.

  CHAPTER 3

  The Inner Solar System

  NEARBY WORLDS

  Once Galileo began to study the sky with his telescope, he could see that the various planets expanded into tiny orbs. They appeared as mere dots of light to the unaided eye merely because of their great distance.

  What’s more, Venus, being closer to the Sun than Earth is, showed phases like the Moon, as it should under such conditions if it were a dark body shining only by reflection. That was proof enough that the planets were also worlds, possibly more or less Earthlike.

  Once that was established, it was taken for granted that all of them were life bearing and inhabited by intelligent creatures. Flammarion maintained this confidently, as I said in the previous chapter, as late as 1862.

  The kinetic theory of gases, however, ruled out not merely the Moon as an abode of life, but any world smaller than itself. Any worlds smaller than the Moon could scarcely be expected to possess air or water. They would lack the gravitational field for it. Consider the asteroids, the first of which was discovered in 1801. They circle the Sun just outside the orbit of Mars and the largest of them is but 1,000 kilometers (620 miles) in diameter. There are anywhere from 40,000 to 100,000 of them with diameters of at least a kilometer or 2, and every last one of them lacks air or liquid water* and are therefore without life.

  The same is true for the two tiny satellites of Mars, discovered in 1877. They are in all likelihood captured asteroids, and have neither air nor liquid water.

  Within the orbits of the asteroids lies the “inner Solar system” and there we find four planetary bodies larger than the Moon. In addition to the Earth itself, we have Mercury, Venus, and Mars.

  Of these, Mercury is the smallest, but it is 4.4 times as massive as the Moon and its diameter is 4,860 kilometers (3,020 miles), which is 1.4 times that of the Moon. Mercury’s surface gravity is 2.3 times that of the Moon and nearly 2/5 that of the Earth. Might it not manage to retain a thin atmosphere?

  Not so. Mercury is also the closest of the planets to the Sun. At its nearest approach to the Sun it is at only 3/10 the distance from it that the Earth is. Any air it might have would be heated to far higher temperatures than the Earth’s atmosphere. Gas molecules on Mercury would be correspondingly speedier in their motion and harder to hold onto. Mercury, therefore, would be expected to be as airless and waterless—and as lifeless—as the Moon.

  In 1974 and 1975, a rocket probe, Mariner 10, passed near Mercury’s surface on three occasions. On the third occasion, it passed within 327 kilometers (203 miles) of the surface. Mercury was mapped in detail and its surface was found to be cratered in a very Moonlike way, and its airlessness and waterlessness is confirmed. There is no perceptible doubt as to its lifelessness.

  Venus looks far more hopeful. Venus’s diameter is 12,100 kilometers (7,520 miles) as compared with Earth’s 12,740 kilometers (7,920 miles). Venus’s mass is about 0.815 times that of the Earth and its surface gravity is 0.90 times that of the Earth.

  Even allowing for the fact that Venus is closer to the Sun than Earth and would therefore be hotter than Earth, it would seem that Venus should have an atmosphere. Its gravitational field is strong enough for that.

  And, indeed, Venus does have an atmosphere, a very pronounced one, and one that is far cloudier than ours. Venus is wrapped in a planet-girdling perpetual cloud cover, which was at once taken as adequate evidence that there was water on Venus.

  The cloud cover does, unfortunately, detract from the hopeful views we can have of Venus, since it prevents us from gathering evidence as to its fitness for life. At no time could astronomers ever catch a glimpse of its surface, however good their telescopes. They could not tell how rapidly Venus might rotate on its axis, how tipped that axis might be, how extensive its oceans (if any) might be, or anything else about it. Without more evidence than the mere existence of an atmosphere and clouds it was difficult to come to reasonable conclusions about life on Venus.