Extraterrestrial Civilizations - Isaac Asimov [91]
Many details remain to be worked out, but it is much easier to believe the difference between Venus and Earth rests in the difference in distance from the Sun than the difference in the nature and existence of a satellite.
Pending further evidence, then, there seems no way of denying the existence of many habitable, life-bearing planets. Even so, granting that, we have not yet done with the peculiarity of the Moon’s existence.
OUR CAPTURED SATELLITE?
So odd is the Moon’s existence as a satellite of the Earth, that some astronomers have suggested that it did not form as a satellite, but was captured by the Earth. If so, this, too, might have a conceivably fatal effect on our hope for the existence of civilizations elsewhere.
In favor of the possibility of the Moon’s being a captured body, there is the fact that the Moon is as large as it is and as far from the Earth as it is. Moreover, its orbit is in a plane close to that in which the planets generally revolve about the Sun, and is considerably less close to the Earth’s equatorial plane, where experience tells us a satellite is more likely to revolve. All that might lead one to believe it had been a small planet to begin with, rather than a satellite.
Then, too, the Moon is somewhat different in composition from the Earth. It has only three-fifths the density of Earth and lacks a metal core. In this, it much more closely resembles the structure of Mars. Could it be that the Moon was formed out of that portion of the original cloud of dust and gas from which Mars was formed, rather than Earth?
Further, the Moon is much more lacking than Earth in those solid elements that melt at a not too high temperature and that may, therefore, have boiled away from the Moon. Again, bits of glassy materials, formed by rocky substances that have melted and resolidified, are common on the Moon, though rare on the Earth. Both these characteristics of the Moon seem to indicate that it was at one time exposed, for a considerable period, perhaps, to temperatures higher than those to which Earth (or the Moon itself) are now exposed.
Could it be, then, that the Moon, formed in the same process that formed Mars, had for some reason a highly eccentric orbit? Perhaps it swooped in nearly as close to the Sun as Mercury does at one end of its orbit and receded almost as far as Mars does at the other end. That would account for its Mercurian surface and Martian interior.
Then, at one time, something happened that made it possible for Earth to capture the Moon at one of the latter’s close approaches.
To be sure, none of these arguments for the Moon’s status as a captured body is compelling. Its large size is not a convincing argument, for those satellites in the Solar system that astronomers are certain are captured are all tiny. The Moon’s distance from the Earth could be the result of tidal action; the eccentricity of its orbit is not as great as that of other surely captured satellites; the inclination of its plane of revolution to its planet’s equatorial plane is not as great as that of Neptune’s satellite, Triton.
As for the difference in composition, it might be that the metals condensed first and that when the Moon began to condense at a distance from the primary condensation site, the cloud out of which it formed was predominantly rocky. To account for the great heat to which its surface was exposed, we need only remember that the Moon, unlike the Earth, lacks an atmosphere and an ocean to serve as a buffer against solar radiation.
Worst of all, the mechanics by which the Earth would be able to capture a body the size of the Moon are very tricky, and astronomers have not been able to suggest a