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 itwould 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 asymmetricand 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
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 itwould 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 asymmetricand 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
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