Extraterrestrial Civilizations (12 page)

After
Pioneer 10
had triumphantly passed Jupiter, a second probe,
Pioneer 11
, a close duplicate of the first one, was approaching the planet. It had left Earth on April 5, 1973, and passed Jupiter at a distance of 42,000 kilometers (26,000 miles) from its surface on December 2, 1974. It passed over Jupiter’s north polar region, which human beings cannot see from Earth.

Both probes sent back photographs and other useful information. From that information, astronomers feel that rock and metal
make up a very small quantity of Jupiter’s total structure. Apparently, Jupiter would seem to consist chiefly of hydrogen, with a small admixture of helium, and traces (in comparison) of the other volatiles. Just as Earth is essentially a spinning ball of rock and metal, so Jupiter is a spinning ball of hot liquid hydrogen. (Ordinarily, liquid hydrogen boils at extremely low temperatures, but under the enormous pressures within Jupiter it apparently reaches far higher temperatures.)

The outermost skin of Jupiter’s ball of liquid is cold, but the temperature rises rapidly with depth. At 950 kilometers (600 miles) below the visible cloud surface, the temperature is already 3,600° C (6,500° F).

In the uppermost cool layer of the planet there is water, ammonia, methane, and other volatiles, including small percentages of hydrocarbons with two or three carbon atoms in the molecule.

Naturally, there is probably circulation in the planetary liquid of Jupiter as there is in Earth’s oceans. There may be vast columns of the Jupiter-liquid sinking and warming, while other columns, equally vast, are rising and cooling.

Here the arguments for life are intriguing. Water is certainly present in the fluid, and while it may be present in small percentages, on vast Jupiter even a small percentage is a large quantity in absolute terms. Even though the water is completely overwhelmed by the hydrogen, there could easily be more water by far on Jupiter than on Earth.

Then, too, there is methane and ammonia in addition to water, and the three could combine to form the kind of organic molecules we associate with life. It would take energy to force the combination, but considering Jupiter’s enormous internal heat, that would be no problem.

We could easily imagine living cells, and perhaps complicated multicellular animals, living in the Jovian ocean, maintaining themselves at a level of comfortable temperature, swimming up in a descending column or down in an ascending column, or perhaps switching from one to another when necessary.

It doesn’t seem hard to believe, really, and it would even be life-as-we-know-it; though, of course, we couldn’t really be certain until we could figure out some way of actually exploring the Jupiter-ocean.

Although we have not yet explored any of the other outer giants
as we have Jupiter (though several probes are
en route
to Saturn after having passed Jupiter), there seems no reason to doubt that what might be true for Jupiter might also be true for the others.

There might be four worlds, then, in the outer Solar system, that could be far richer in life than Earth.

Yet life on these outer planets would be ocean life, for planets that are largely made up of volatiles with a preponderance of hydrogen must be purely liquid. There is no way in which we can expect continents or even islands.

The life forms on the outer planets would, therefore, be very likely to be streamlined for getting rapidly through a medium more viscous than Earthly air and would, in consequence, be very apt to lack manipulative organs.

And even if they could manipulate the environment, could they develop the use of a convenient form of inanimate energy equivalent to our fire? (To be sure, there is no free oxygen on a planet like Jupiter, but there is free hydrogen, and oxygen-rich compounds might burn in a hydrogen atmosphere.)

Somehow, it seems rather likely that if life developed on the giant planets and evolved to the point of intelligence, it would be the intelligence of the dolphin rather than that of the human being. It would be an intelligence that might lead to a better way of life, but it would not involve the building of a technology based on ever more elaborate and sophisticated tools, with which the intelligent creature might directly manipulate the environment more and more subtly.

This would also be true of life developing, against the odds, in a possible water layer beneath the surface crust of Ganymede or Callisto.

In other words, there might be life on Jupiter and the other giant planets, even intelligent life—but it doesn’t seem likely that there would be technological civilizations in our sense.

Having gone rather exhaustively through the Solar system, it would appear that although there may be life on several worlds other than Earth, even conceivably intelligent life, the chances are not high. Furthermore, the chances would seem to be virtually zero that a technological civilization exists, or could exist, anywhere in the Solar system but on Earth.

Nevertheless, the Solar system is by no means the entire Universe. Let us look elsewhere.

We might imagine life in open space in the form of concentrations of energy fields, or as animated clouds of dust and gas, but there is no hint of evidence that such a thing is possible. Until such evidence is forthcoming (and naturally the scientific mind is not closed to the possibility), we must assume that life is to be found only in association with solid worlds at temperatures less than those of the stars.

The only cool, solid worlds we know are the planetary and subplanetary bodies that circle our Sun, but we cannot assume from
this that all such bodies in the Universe must be associated with stars.
^*
There may be clouds of dust and gas of considerably smaller mass than that from which our Solar system originated, and these may have ended by condensing into bodies much smaller than the Sun. If the bodies are sufficiently smaller than the Sun, say with only 1/50 the mass or less, they would end by being insufficiently massive to ignite into nuclear fire. The surfaces of such bodies would remain cool and they would resemble planets in their properties, except that they would follow independent motions through space and would not be circling a star.

All our experience teaches us that of any given type of astronomical body, the number increases as the size decreases. There are a greater number of small stars than of large ones, a greater number of small planets than large ones, a greater number of small satellites than large ones, and so on. Might we argue from that, that these substars, too small to ignite, are far greater in number than those similar bodies that are massive enough to ignite? At least one important astronomer, the American Harlow Shapley (1885–1972), has very strongly advanced the likelihood of the existence of such bodies.

Naturally, since they do not shine, they remain undetected and we are unaware of them. But if they exist, we might reason that there exist substars in space through an entire range of sizes from super-Jupiters to small asteroids. We might even suppose that the larger ones could have bodies considerably smaller than themselves circling them, much as there are bodies circling Jupiter and the other giant planets within our own Solar system.

The question is, though: Would life form on such substars?

So far I have suggested that the irreducible requirements for life (as we know it) are, first, a free liquid, preferably water, and, second, organic compounds. A third requirement, which ordinarily we take for granted, must be added, and that is energy. The energy is needed to build the organic compounds out of the small molecules present at the start, small molecules such as water, ammonia, and methane.

Where would the energy come from in these substars?

In the condensation of a cloud of dust and gas into a body of any size, the inward motion of the components of the cloud represents kinetic energy obtained from the gravitational field. When the motion stops, with collision and coalescences, the kinetic energy is turned into heat. The center of every sizable body is therefore hot. The temperature at the center of the Earth, for instance, is estimated to be 5,000° C (9,000° F).

The larger the body and the more intense the gravitational field that formed it, the greater the kinetic energy, the greater the heat, and the higher the internal temperature. The temperature at the center of Jupiter, for instance, is estimated to be 54,000° C (100,000° F).

It might be expected that this internal heat is a temporary phenomenon and that a planet would slowly but surely cool down. So it would, if there were no internal supply of energy to replace the heat as it leaked away into space.

In the case of Earth, for instance, the internal heat leaks away very slowly indeed, thanks to the excellent insulating effect of the outer layers of rock. At the same time, those outer layers contain small quantities of radioactive elements such as uranium and thorium, which, in their radioactive breakdown, liberate heat in large enough quantities to replace that which is lost. As a result, the Earth is not cooling off perceptibly, and though it has existed as a solid body for 4,600,000,000 years, its internal heat is still there.

In the case of Jupiter, there seem to be some nuclear reactions going on in the center, some faint sparks of starlike behavior, so that Jupiter actually radiates into space three times as much heat as it receives from the Sun.

This long-lasting internal heat would be more than ample to support life, if living things could tap it.

We could fantasize life as existing within the body of a planet where nearby pockets of heat might have served as the energy source to form and maintain it. There is, however, no evidence that life can exist anywhere but at or near the surface of a world, and until evidence to the contrary is obtained, we should consider surfaces only.

Suppose, then, we consider a substar no more massive than the Earth; or a body that massive that is circling a substar somewhat more massive than Jupiter but yielding no visible light.

Such an Earthlike body, whether free in space or circling a substar, would tend to be a world like Ganymede or Callisto. There would be internal heat, but, thanks to the insulating effect of the outer layers, very little would leak outward to the surface; any more than Earth’s internal heat leaks outward to melt the snow of the polar regions and mitigate the frigidity of Earth’s temperatures.

To be sure, on Earth there are local leaks of considerable magnitude, producing hot springs, geysers, and even volcanoes. We might imagine such things on Earth-sized substars as well. In addition, there could be energy derived from the lightning of thunderstorms. Still, whether such sporadic energy sources would meet the requirements for forming and maintaining life is questionable. There is also the point that a world without a major source of light from a nearby star may be unfit for the development of intelligence—a subject I will take up later in the book.

The Earth-sized substar would be composed of a much larger percentage of volatiles than Earth itself, since there would have been no nearby hot star to raise the temperature in surrounding space and make the collection of volatiles impossible. Therefore, again as on Ganymede and Callisto, we might imagine a world-girdling ocean, probably of water, kept liquid by internal heat, but covered by a thick crust of ice.

Substars still smaller than the Earth would have less internal heat and would be even more likely to be icy, have less in the way of sporadic sources of appreciable energy, have smaller internal oceans or none at all.

If a body were small enough to attract little or no volatile matter even at the low temperatures that would exist in the absence of a nearby star, it would be an asteroidal body of rock or metal or both.

What about substars that are larger than Earth and therefore possess greater and more intense reservoirs of internal heat? Such a larger body is bound to be Jupiterlike. A large substar is certain to be made up largely of volatile matter, particularly hydrogen and helium; and high internal heat will make the planet entirely liquid.

Heat can circulate much more freely through liquid by convection than through solids by slow conduction. We can expect ample heat at or near the surface in such large substars and the heat may remain ample for billions of years. However, again the most we can
expect on a large substar is intelligent life of the dolphin variety—and no technological civilization.

In short, the formation of substars would rather resemble the formation of bodies in the outer Solar system, and we may expect no more of the former than of the latter.

For a technological civilization, we need a solid planet with both oceans and dry land, so that life as we know it can develop in the former and emerge on the latter. To form such a world there must be a nearby star to supply the heat that would drive away most of the volatile matter, but not all. The nearby star would also supply the necessary energy for the formation and maintenance of life in a copious and steady manner.

In that case, we must concentrate our attention on the stars. These, at least, we can see. We know they exist and need not simply assume the probability of their existence as in the case of the substars.

If we turn to the stars and consider them as energy sources in the neighborhood of which we may find life, possibly intelligence, and possibly even technological civilizations, our first impression may be heartening, for there seem to be a great many of them. Therefore, if we fail to find life in connection with one, we may do so in connection with another.

In fact, the stars may well have impressed the early, less sophisticated watchers of the sky as innumerable. Thus, according to the Biblical story, when the Lord wished to assure the patriarch Abraham that, despite his childlessness, he would be the ancestor of many people, this is how it is described:

“And he [God] brought him [Abraham] forth abroad, and said, ‘Look now toward heaven, and tell the stars, if thou be able to number them’; and he [God] said unto him [Abraham], ‘So shall thy seed be.’ ”