Extraterrestrial Civilizations - Isaac Asimov [35]
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,