Extraterrestrial Civilizations - Isaac Asimov [36]
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.
JUPITER
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