Extraterrestrial Civilizations - Isaac Asimov [34]
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