Extraterrestrial Civilizations (7 page)

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.

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.

Mars’s, on the other hand, is at once less hopeful and more hopeful.

It is less hopeful because it is distinctly smaller than Earth. Its diameter is only 6,790 kilometers (4,220 miles) and its mass is only 0.107 that of the Earth. With a mass only 1/10 that of Earth it is not exactly a large world, but on the other hand it is 8.6 times as massive as the Moon, so it is not exactly a small one, either. It is, in fact, twice as massive as Mercury.

Mars’s surface gravity is 2.27 times that of the Moon and is just about that of Mercury. Mars, however, is four times as far from the Sun as Mercury is, so that Mars is considerably the cooler of the two. Mars’s gravitational field need deal with considerably slower molecules for that reason.

It follows that although Mercury is without an atmosphere, Mars may have one—and it does. Mars’s atmosphere is a thin one, to be sure, but it is distinctly there. Mars is presumably drier than the Earth, for its atmosphere is not as cloudy as Earth’s (let alone Venus’s), but occasional clouds are seen. Dust storms are also seen, so there must be sharp winds on Mars.

The more hopeful aspect of Mars is that its atmosphere is sufficiently thin and cloud free to allow its surface to be seen (rather vaguely) from Earth. For centuries, astronomers have done their best
to map what it was they saw on that distant world. (At its closest, Mars can approach as closely as 56,000,000 kilometers [34,800,000 miles] to Earth, a distance that is 146 times as far away from us as the Moon.)

The first to make out a marking that others could see as well was the Dutch astronomer Christiaan Huygens (1629–1695). In 1659, he followed the markings he could see as they moved around the planet and determined the rotation period of Mars to be only a trifle longer than that of Earth. We now know Mars rotates in 24.66 hours compared to Earth’s 24.

In 1781, the German-English astronomer William Herschel (1738–1822)
^*
noted that Mars’s axis of rotation was tilted to the perpendicular, as Earth’s was, and almost by the same amount. Mars’s axial tilt is 25.17° as compared with Earth’s 23.45°.

This means that not only does Mars have a day-night alteration much as Earth has, but also seasons. Of course, Mars is half again as far from the Sun as we are, so that its seasons are colder than ours. Furthermore, it takes Mars longer to complete its orbit about the Sun, 687 days to our 365¼, so that the seasons on Mars are each nearly twice as long as ours.

In 1784, Herschel noted that there were ice caps about the Martian poles, as there were about Earth’s poles. There was one more point of resemblance in that the ice caps were assumed to be frozen water, and therefore to serve as proof there was water on Mars.

Mars and Venus both looked like hopeful possible abodes of life, certainly far more hopeful than the asteroids or the Moon or Mercury.

In 1796, the French astronomer Pierre Simon de Laplace (1749–1827) speculated on the origin of the Solar system.

The Sun rotates on its axis in a counterclockwise direction when viewed from a point far above its north pole. From that same point,
all the planets known to Laplace moved about the Sun in a counterclockwise direction, and all the planets whose rotations were known rotated about their axes in a counterclockwise direction. Added to that was the fact that all the satellites known to Laplace revolved about their planets in a counterclockwise direction.

Finally, all the planets had orbits that were nearly in the plane of the Sun’s equator and all the satellites had orbits that were nearly in the plane of their planet’s equator.

To account for all this, Laplace suggested that the Solar system was originally a vast cloud of dust and gas called a nebula (from the Latin word for
cloud
). The nebula was turning slowly in a counterclockwise direction. Its own gravitational field slowly contracted it, and as it contracted it had to spin faster and faster in accordance with something called the law of conservation of angular momentum. Eventually, it condensed to form the Sun, which is still spinning in the counterclockwise direction.

As the cloud contracted on its way to the Sun and as it increased its rate of spin, the centrifugal effect of rotation caused it to belly out at its equator. (This happens to the Earth, which has an equatorial bulge that lifts points on its equator 13 miles farther from the center of the Earth than the north and south poles are.)

The bulge of the contracting nebula became more and more pronounced as it shrank further and speeded up further, until the entire bulge was thrown off like a thin doughnut around the contracting nebula. As the nebula continued to shrink, additional doughnuts of matter were shed.

Each doughnut, in Laplace’s view, gradually condensed into a planet, maintaining the original counterclockwise spin, and speeding up that spin as it condensed. As each planet formed there was a chance it might shed smaller subsidiary doughnuts of its own, which became the satellites. The rings around Saturn are examples of matter that has been given off (according to Laplace’s nebular hypothesis) and has not yet condensed to a satellite.

The nebular hypothesis explains why all the revolutions and rotations in the Solar system should be in the same direction.
^*
It is because all participate in the spin of the original nebula.

It also explains why all the planets revolve in the plane of the Sun’s equator. It is because it is from the Sun’s equatorial regions that they were originally formed; and it is from the planetary equatorial regions that the satellites formed.

The nebular hypothesis was more or less accepted by astronomers during the nineteenth century, and it added detail to the picture that people drew of Mars and Venus.

As the nebula condensed, according to this theory, it would seem that the planets would form in order from the outermost to the innermost. In other words, after the nebula had condensed to the point where it was only 500,000,000 kilometers (310,000,000 miles) across, it gave off the ring of matter that formed Mars. Then, after considerable time taken up in further contraction, it gave off the matter that formed the Earth and the Moon, and after another unknown length of time, the matter that formed Venus.

By the nebular hypothesis, therefore, it would seem that Mars was considerably older than Earth, and that Earth was considerably older than Venus.