Extraterrestrial Civilizations (11 page)

It is clear, then, that life as we know it is a function of the volatiles and that no world can bear life unless it has at least some volatile matter.

At the temperatures prevailing beyond the orbit of Mars, almost any body, however small, can contain some volatile matter. Every once in a while, for instance, a meteorite falls that is found to contain water, hydrocarbons,
^*
and other volatiles. Not much, only up to 5 percent or so—but they’re there.

Such meteorites, called carbonaceous chondrites, are few indeed compared to the ordinary meteorites that are constructed of metal, or of rock, or of a mixture of the two. Indeed, only about twenty carbonaceous chondrites have ever been located.

This does not really mean that carbonaceous chondrites are rare. They could be very common. However, they tend to be structurally weaker than the rocky and metallic meteorites. The carbonaceous chondrites crumble away more easily in the white-hot passage through the atmosphere, so that very few fragments of any of them survive to strike Earth’s surface.

In recent years, it has turned out that most of the asteroids, particularly those farther from the Sun, have the characteristics (dark color and low density) of the carbonaceous chondrites and therefore have volatile material in them. The two small satellites of Mars are much darker than Mars itself in color and are lower in density, so they must contain some volatile matter.

Then, too, there are the comets, which exist as small solid bodies in that part of their orbit far from the Sun. They are perhaps only a few kilometers in diameter and are largely or almost entirely composed of icy materials.

When they pass through the part of the orbit in the neighborhood of the Sun, some of the ices vaporize and liberate granules of rock or metal that may be mixed with the ices. The whole forms a misty “coma” about the still solid “nucleus.” The Sun constantly emits streams of rapid subatomic particles in all directions (the “Solar wind”) and this sweeps the coma outward in a direction away from the Sun, forming a long, wispy “tail.”

Any objects in the outer Solar system that are larger than asteroids and comets would contain volatile matter almost as a matter of course, we might reason.

Although a lack of volatile materials is a sure sign that the world does not contain life (as we know it), the converse is not true. A world may possess volatile materials and yet not contain life (Venus is an example). If this were not so, we would have to judge that just about every object beyond Mars was life bearing.

After all, volatile materials might be present, yet organic compounds of sufficient complexity to make life possible might not form.

From our vantage point on Earth, however, it is not easy to tell whether a small body beyond the orbit of Mars contains complex organic compounds or not. Short of exacting detail beyond our capacities to do so, is there any way of judging whether life is likely to be present or absent on a distant world?

We can begin by pointing out that we have already said that a liquid medium, like that of water, is required for life.

If, however, a world has sufficient liquid on its surface to make possible the presence of life—not merely as a thin scattering of bacterialike organisms, but in sufficient complexity to allow an approach to intelligence—this liquid would surely vaporize to some extent.

If the world was not capable of holding on to the vapor through its gravitational force, then the liquid would continue vaporizing until it was all gone. If the world
were
capable of holding on to the vapor, then it would have an atmosphere of more than traces of gas; an atmosphere consisting of that vapor at the very least, and possibly of other gases as well.

It follows, then, that a world without an atmosphere cannot bear life (as we know it) above the bacterial level; not because the atmosphere is itself necessarily essential to life, but because sizable quantities of free liquid on the surface are necessary for more-than-bacterial life. Without an atmosphere, what volatiles are present must be in the frozen, solid state, and that is insufficient for life.

With this in mind, let’s consider those objects that lie beyond the orbit of Mars and that are less than 2,900 kilometers (1,800 miles) in diameter.

There are uncounted numbers of these, trillions upon trillions of dust grains, billions of comets, tens of thousands of asteroids, and a couple of dozen small satellites. All can be eliminated. Although a very large proportion of them, perhaps almost all of them over the size of dust grains, contain volatile material, none has a permanent atmosphere or any hope of free liquid. Those comets that approach the Sun have a temporary atmosphere during the approach, but it is very doubtful that they have free liquid even then—and the period of atmosphere makes up a very small fraction of their total stay in orbit.

What about the objects beyond the orbit of Mars that have diameters between 2,900 and 6,500 kilometers (1,800 and 4,000 miles)?

There are exactly six of these, the satellites, Io, Europa, Ganymede, Callisto, Titan, and Triton. (Until 1978 it was thought the planet Pluto was a seventh, but very recent information makes it appear a surprisingly small body.)

Of these six bodies, the four satellites Io, Europa, Ganymede, and Callisto circle Jupiter and are the nearest to the Sun. None has anything better than trace atmospheres.

Io, which is the closest to Jupiter, must have been exposed to considerable warmth in the early days of planetary formation when Jupiter itself, as it formed, radiated heat strongly. At any rate, judging from its density Io is very much like our Moon and includes little if any volatile material in its structure.

The farther satellites have progressively lower densities and must, therefore, contain more and more volatiles. These volatiles must be chiefly water, together with smaller quantities of ammonia and hydrogen sulfide. Methane is a gas even at temperatures as low as those that prevail in the neighborhood of Jupiter, and its molecules are too nimble to be held by the small gravitational pulls of the satellites.

Europa, the second of the large satellites, probably has a thin layer of water-ice on its surface. The third and fourth of the large satellites, Ganymede and Callisto, have much thicker layers of volatile materials around a rocky core. The layers may even be hundreds of kilometers thick. On the surface, there is a layer of water-ice but underneath, warmed by internal heat, there may be a layer of liquid water. Can life have developed on these two satellites in a region of eternal darkness, sealed away from the rest of the Universe by an unbroken miles-thick layer of ice? As yet, we can’t say.

If Jupiter’s satellites are the nearest of the six bodies we are discussing, Pluto lies beyond all six. Pluto is so far from the Sun and is at such a low temperature that even methane is frozen. Recent observations of the light it reflects indicate, in fact, that it is covered with a layer of frozen methane. It might conceivably have a thin atmosphere of hydrogen, helium, and neon, but there is as yet no indication of that. Even if it did, however, this would not help it have any free liquid on its surface, since at Pluto’s temperature, hydrogen, neon, and helium are gases and everything else is solid. Furthermore, in 1978 it was found that Pluto was not one body, but two. It has a satellite, now named Charon, and each—the planet and the satelliteis smaller than our Moon. Neither can bear life.

The next-farthest world is Triton, a satellite of the planet Neptune. Very likely it is in Pluto’s case, with a coating of solid methane and a very thin atmosphere of hydrogen, neon, and helium, but as yet that is only a presumption.

The remaining world in this size range is Titan, the largest satellite of Saturn. It is farther from the Sun and colder than the four satellites of Jupiter. It is closer to the Sun and warmer than Triton, Charon, and Pluto.

Titan’s temperature is about –150° C (–207° F), 15 Centigrade degrees lower than that of Jupiter’s satellites. At Titan’s temperature, methane is still gaseous, but it is pretty close to the point where it
would liquefy (–161.5° C or −233.1° F) and its molecules are sluggish indeed. They could be held by Titan’s gravitational pull, even though that pull is only two-thirds as intense as that of our Moon.

It follows that Titan could conceivably have a methane atmosphere and, in 1944, Gerard Kuiper actually detected such an atmosphere. What is more, the atmosphere is a substantial one, very likely denser than that of Mars.

Titan is the only satellite in the Solar system known to have a true atmosphere. It is also the smallest body in the Solar system to have a true atmosphere, and it is the only body of any size to have an atmosphere that is primarily methane.

Methane, with a molecule consisting of one carbon atom and four hydrogen atoms, is the smallest organic compound. Thanks to the peculiar properties of the carbon atom and the readiness with which it will hook onto other carbon atoms, it is easy for methane molecules to combine into larger ones containing two carbon atoms, or three or four, with some appropriate number of hydrogen atoms also attached. The Sun, although very distant from Titan, would nevertheless supply enough energy to drive such reactions.

It may, therefore, turn out that Titan’s atmosphere has as minor constituents a complicated mix of vapors of higher hydrocarbons and it may be this mix that causes Titan to appear distinctly orange in color when viewed through the telescope.

The more complicated a hydrocarbon molecule, the higher the temperature at which it liquefies. Though the higher hydrocarbons may exist as vapors in the atmosphere, the major portion will be in liquid form on the surface. Since cigarette lighter fluid is made up of molecules of hydrocarbon with five or six carbon atoms, we might visualize Titan as possessing lakes and seas of cigarette lighter fluid, with still more complicated molecules dissolved in them, or forming sludges along the shores of those lakes and seas.

Thus, Titan would have free liquid in quantity
and
organic compounds in quantity as well.

This represents the minimum requirement for life, but there is a serious question as to whether hydrocarbons can substitute for water as the basic liquid against which the pattern of life can be constructed.

Water is a “polar liquid.” That is, its molecules are asymmetric
and there are tiny electric charges at the opposite ends. These tiny electric charges set up attractions and repulsions that play an important part in the chemical changes characteristic of life. Hydrocarbon molecules are “nonpolar liquids,” however, with symmetrical molecules and no tiny electric charges. Can nonpolar liquids serve as an adequate background for life?

Can
any
liquid other than water serve as a background to life? The only liquids that have any reasonable chance to do so are those that are present in large quantities in the Universe generally and that are indeed liquid at planetary temperatures. In addition to water and hydrocarbons there are only two other candidates, ammonia and hydrogen sulfide. Ammonia is a polar liquid, but not as polar as water, and hydrogen sulfide is less polar still.

With sufficient ingenuity we can work out chemistries that use these liquids as background and have life in the foreground, but those are just exercises in speculation. We have no evidence whatsoever that any common liquid will substitute for water.

Until such evidence is forthcoming, at least some tiny scrap of it, we must remain conservative and count on water life only. For that reason, although Titan will offer us a fascinating chemical world if we can ever study it in some detail, we cannot bet very heavily on it as an abode of life.

In the cold reaches beyond Mars, it might happen that a world as it formed would pick up enough in the way of icy materials (in addition to what rock and metal might be available) to develop a gravitational field strong enough to hold on to helium and neon. The added mass would intensify the gravitational field and make it possible, perhaps, for it to hold on to hydrogen, which is present in greater amounts than any other substance.

Every bit of hydrogen added makes it that much easier to gather more hydrogen, so that there is a snowball effect that quickly empties surrounding space of its material and produces a giant planet, leaving only enough material behind to make small bodies such as satellites and asteroids.

There are four planets in the outer Solar system that have been formed in this way: Jupiter, Saturn, Uranus, and Neptune.

Of these, the largest is Jupiter, with a diameter of 143,200 kilometers (89,000 miles) or 11.23 times that of Earth. The smallest is Neptune, with a diameter of 49,500 kilometers (30,800 miles) or 3.88 times that of Earth. The volumes range from 1,415 times that of Earth for Jupiter to 58 times that of Earth for Neptune.

Because these outer giants are made up so largely of the volatiles, which are of low density, their overall density is considerably smaller than that of Earth. The densest of the giants is Neptune, which has an average density 1.67 times that of water. The least dense is Saturn, with an average density 0.71 times that of water. (Saturn would float on water if there were an ocean big enough and if Saturn would remain intact in the process.) Compare this with Earth’s average density of 5.5 times that of water.

Since the outer giants are so low in density, their mass (the quantity of matter they contain, roughly speaking) is lower than one might think from their size. The most massive is Jupiter, with 318 times the Earth’s mass; and the least massive is Uranus, with 14.5 times the Earth’s mass.

From such considerations alone, it is clear that the properties and nature of the outer giants is enormously different from Earth’s. Is life conceivable on them?

On March 2, 1972, a probe,
Pioneer 10
, was launched for a rendezvous with Jupiter. On December 3, 1973, it passed Jupiter at a distance of only 135,000 kilometers (85,000 miles) from its surface.

During the four days it took
Pioneer 10
to fly by Jupiter, its instruments picked up radiation, counted particles, measured magnetic fields, noted temperatures, and analyzed sunlight passing through Jupiter’s atmosphere.