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
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
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