Extraterrestrial Civilizations (18 page)

There is no way, at the moment, in which we can tell whether either alternative is true. We have no recourse but to adhere to “the principle of mediocrity” and to assume that this one case we know
of—that of Earth—is not atypical, but is about average in its nature.

We must, therefore, cling to a 5-billion-year lifetime on the main sequence as an essential minimum for the development of civilization.

A star that is 1.4 times as massive as the Sun and is of spectral class F2 remains on the main sequence for 5 billion years, and we can therefore come to the conclusion that any star more massive than 1.4 times the mass of the Sun will not serve as an appropriate incubator for life. There may indeed be life on a planet circling such a too-massive star, but the chance that it will exist long enough to reach the appropriate pitch of complexity to produce an extraterrestrial civilization is small enough to ignore.

This means that the bright stars we see in the sky, which are (at least, most of them are) considerably more massive than the Sun, are unsuitable incubators. Sirius, for instance, will remain on the main sequence for 500 million years altogether, Rigel for only 400 million years. We can ignore such stars.

As it happens, however, it is precisely these massive short-lived stars that are fast-rotators and were therefore not included by me in the number of stars possessing a planetary system. Their exclusion from further consideration is thus doubly justified.

Let’s try the other extreme, now, and consider a star with 1/16 the mass of the Sun and one-millionth the luminosity. (Any object less massive than that would probably not be massive enough to ignite the nuclear fires at the center and would not, therefore, be a true star.)

A midget star with 1/16 the mass of the Sun would be 65 times as massive as the planet Jupiter, but would surely be much more dense and might not be much larger than Jupiter in size. It might perhaps be 150,000 kilometers (93,000 miles) in diameter.

Next, suppose that Earth were 300,000 kilometers (186,000 miles) from the center of such a star and therefore circling it at a height of 150,000 kilometers (93,000 miles) above its surface. Earth would circle that star every 1.1 hours.

Earth would receive as much total energy from that very nearby
midget star as the Earth now does from the Sun. The fact that the midget star would be barely red hot would be made up for by the fact that from the distance of the planet its apparent size would be 3,000 times that of the Sun as we see it from Earth.

To be sure, the nature of the energy received from the midget star would be different from that of the Sun. The midget star would deliver virtually no ultraviolet radiation and, in fact, very little visible light. Most of its energy would be in the form of infrared light.

This would be very inconvenient from our standpoint. To our own eyes, everything would seem very dim and unpleasantly deep red in color. We could imagine, however, that life on such a planet would have developed a sense of sight that would be sensitive to red and infrared, and perhaps see sections of it in different colors. To such life, the light might well appear white and sufficiently bright.

Red and infrared are less intensively energetic than the remainder of the visible light spectrum, and there would be many chemical reactions that yellow, green, or blue light could initiate that red and infrared could not. However, life is not based on photochemical reactions, except for photosynthesis and that is initiated by red light. No doubt we would not have to stretch matters intolerably to imagine life on such a world—so far.

Let us, however, take up a new issue:

The gravitational field of any object decreases in intensity with the square of the distance. If distance is doubled, the intensity falls to ¼ of what it was; if the distance is tripled, it falls to 1/9 and so on.

This affects the manner in which the Moon and the Earth attract each other.

The average distance between the center of the Moon and the center of the Earth is 384,390 kilometers (238,860 miles). This varies somewhat as the Moon moves about its orbit, but that doesn’t affect the line of argument.

Not all parts of the Earth are, however, at the same distance from the Moon. When the center of the Earth is at its average distance from the center of the Moon, the surface of the Earth that directly faces the Moon is 6,356 kilometers (3,950 miles) closer. The surface of the Earth that faces directly away from the Moon is 6,356 kilometers (3,950 miles) farther.

This means that while the surface of the Earth directly facing the Moon is at a distance of 378,034 kilometers (234,910 miles)
from the Moon’s center, the surface of the Earth facing directly away from the Moon is at a distance of 390,746 kilometers (242,810 miles) from the Moon’s center.

If the distance of the Earth’s near side from the Moon’s center is set at one, the distance of the Earth’s far side is 1.0336. This difference, only 3.36 percent of the total distance from the Moon, does not seem like much. However, the gravitational pull of the Moon falls off over that small distance by an amount equal to 1/1.0336
²
and is only 0.936 at the far side as compared with 1.000 at the near side.

The result of this difference in the Moon’s pull at the near and far sides of the Earth is that the Earth is stretched in the direction of the Moon. The near surface is pulled toward the Moon more forcibly than the center is, and the center is pulled toward the Moon more forcibly than the far surface is. Both the near and far surface bulge, the former toward the Moon, the latter away from the Moon.

It is a matter of a small bulge only, half a meter or so. Still, as the Earth rotates, each part of its solid matter bulges up when it turns toward the side facing the Moon, reaching its greatest height when it passes under the Moon, then settling back. The solid matter bulges as it turns toward the side away from the Moon, reaching another peak when it is directly opposite the position of the Moon, then receding.

The water of the ocean bulges up also, to a greater extent than the solid land does. This means that as the Earth turns, the land surface passes through the higher bulge of water and, as it does so, the water creeps up the shore and then back down. It does so as it passes through both bulges of water, one on the side facing the Moon and one on the side away from it. This means the water rises and falls along the shore twice a day; or, we can say, there are two “tides” a day.

Because this difference in gravitational pull causes the tides, it is referred to as a tidal effect.

Naturally, the Earth also exerts a tidal effect on the Moon. Since the Moon is smaller than the Earth, the Moon’s diameter being 3,476 kilometers (2,160 miles) as compared with Earth’s diameter of 12,713 kilometers (7,900 miles), the drop in gravitational pull across the Moon is smaller than the drop across the Earth.

The width of the Moon is only 0.90 percent of the total distance between the Earth and the Moon, so that the gravitational pull on
the far side is 98.2 percent of the force on the near side. The tidal effect on the Moon would be, in this respect, only 0.29 times what it is for the Earth,
but
the Earth’s gravitational field is 81 times that of the Moon, since the Earth is 81 times as massive as the Moon. If we multiply 0.29 by 81, we find that the tidal force of the Earth on the Moon is 23.5 times that of the Moon on the Earth.

Does this difference matter? Yes, it does.

As the Earth turns and bulges, the internal friction of the rock as it lifts up and settles down, and the friction of the water moving up the shore and back, consumes some of the energy of Earth’s rotation and turns it into heat. As a result, tidal action is slowing the Earth’s rotation. However, the Earth is so massive and the energy of its turning is so huge that the Earth’s rotation is slowing very slowly indeed. The length of the day is increasing by one second every 100,000 years.
^*
This isn’t much on the human time scale, but if the Earth has been in existence for 5 billion years and this rate of day lengthening has been constant throughout, the day has lengthened a total of 50,000 seconds or nearly 14 hours. When the Earth was created, it may have been rotating on its axis in only 10 hours—or less, if the tides were more important in early geologic times than they are now, as they well might have been.

What about Earth’s tidal effect on the Moon?

The Moon has a smaller mass and therefore, very likely, a smaller rotational energy to begin with. Furthermore, the tidal effect on the Moon is 23.5 times that on the Earth. The stronger effect, working on the smaller mass, has a greater slowing effect. As a result, the Moon’s rotational period has slowed until it is now equal to exactly one revolution about the Earth. Under those conditions, the same side of the Moon always faces the Earth, the tidal bulge is always in the same spot on its surface, so that different parts of its body no longer have to heave up and settle back as it turns. There is no further slowing (at least as far as Earth’s tidal effect on the Moon is concerned) and the Moon’s rotational period is now stable.

As a result of tidal effect, small bodies would always be expected to turn only one face to the large bodies they circle. (This was first suggested by Kant in 1754.) Not only does the Moon turn only one face to the Earth, the two Martian satellites turn only one face to Mars, the five innermost satellites of Jupiter turn only one face to Jupiter, and so on.

In that case, though, why doesn’t the Earth turn only one side toward the Sun?

Consider what would happen if the Moon receded from the Earth. As it receded, Earth’s gravitational pull would decrease as the square of the distance. Also as it receded, the fraction of the total distance represented by the diameter of the Moon would decrease in proportion to the distance. The tidal effect would decrease for both reasons, and if both are taken into account it means that the tidal effect falls off as the
cube
of the distance.

The Sun is 27 million times as massive as the Moon. If both Sun and Moon were at an equal distance from the Earth, the Sun’s tidal effect upon the Earth would be 27 million times that of the Moon’s tidal effect upon the Earth.
^*
The Sun, however, is 389 times as far from the Earth as the Moon is. The Sun’s tidal effect is weakened by an amount equal to 389 × 389 × 389, or 58,860,000. Divide 27 million by 58,860,000 and we find that the Sun’s tidal effect on Earth is only about 0.46 that of the Moon. If the Moon’s tidal effect has not sufficed to slow the Earth’s rotational period very much as yet, the Sun’s certainly would not.

Mercury is closer to the Sun than the Earth is, and that would be a factor that would tend to increase the tidal effect of the Sun. On the other hand, Mercury is smaller than the Earth, and that would tend to decrease it. Taking both factors into account, it turns out that the Sun’s tidal effect on Mercury is 3.77 times that of the Moon’s tidal effect on the Earth, and only 1/6 Earth’s tidal effect on the Moon.

The Sun, therefore, slows Mercury’s rotation more effectively than the Moon slows Earth’s, but less effectively than the Earth slows the Moon’s. We might suspect, then, that Mercury rotates slowly but not so slowly as to face one side only to the Sun.

In 1890, Schiaparelli (who reported the canals on Mars thirteen
years before) undertook the task of observing Mercury’s surface. This is a very difficult thing to do, since Mercury is farther from us than Mars, usually; since Mercury shows only a crescent phase, usually, whereas Mars is always full or nearly full; and since Mercury, unlike Mars, is usually close enough to the brightness of the Sun to make comfortable viewing unlikely. Nevertheless, from what faint spots Schiaparelli could make out on the surface of Mercury, he decided that it rotated only once in each revolution of 88 days, and that it faced only one side to the Sun.

In 1965, however, radar waves that were emitted from Earth were bounced off Mercury’s surface. The echo, received on Earth, told a different story. The length of the radar waves changes if they strike a rotating body, and the change varies with the speed of rotation. From the nature of the reflected radar waves, it turns out that Mercury’s period of rotation is 59 days, or just ⅔ of its period of revolution. This is a comparatively stable situation, not as stable as having its rotation equal to its period of revolution, but stable enough to resist further change through the Sun’s insufficiently strong tidal effect.

Now we can return to the imaginary situation of our midget star, with Earth circling it at a distance of 300,000 kilometers (186,000 miles) from its center. This distance is only 1/500 that of our Earth from the Sun, and even allowing for the fact that the midget star had only 1/16 the mass of the Sun, its tidal effect on Earth would be 150,000 times that of the Earth’s tidal effect on the Moon.

There is no question, then, but that if Earth were close enough to a midget star to be within its ecosphere, the powerful tidal effect of the star would slow its rotation, and quite early in its lifetime cause it to face one side forever toward the star and one side forever away.

On the side facing always toward the star, the temperature would go up past the boiling point of water. On the side facing always away from the star, the temperature would drop far below the freezing point of water. There would be no liquid water on either side.

One could imagine that there might be a “twilight zone” on the boundary between the forever-lit and the forever-dark hemispheres, in which the conditions would be mild. This would be so only if the orbit of the planet were nearly circular. Even then, the temperature on the hot side might be hot enough to result in the slow loss of the
atmosphere, so that the planet would be airless and the twilight zone no more habitable than any other part.

As we imagine a larger and larger star, the ecosphere would be farther and farther from it. A planet within the ecosphere would be subjected to a smaller and smaller tidal effect. Eventually, if the star were large enough, the tidal effect will no longer be large enough to render the planet unfit for life as we know it.