Extraterrestrial Civilizations - Isaac Asimov [90]
The unusual factor is Earth’s satellite, the Moon.
I have already said that the Earth-Moon combination is the nearest approach in the Solar system to a double planet because of the Moon’s extraordinary size in relation to the world it circles. * The Moon has 1/81, or 0.0123, the mass of the Earth. The following table gives the total satellite mass for each planet of the Solar system, excluding Pluto, in terms of the mass of the planet itself.
Earth (1 satellite) 0.0123
Neptune (2 satellites) 0.0013
Saturn (10 satellites) 0.00025
Jupiter (13 satellites) 0.00024
Uranus (5 satellites) 0.00010
Mars (2 satellites) 0.00000002
Pluto (no satellites)
Venus (no satellites)
Mercury (no satellites)
Taking the mass of every satellite relative to the mass of the planet it circles, the Moon is, so to speak, 6.5 times as massive as all the other satellites in the Solar system put together, excluding Charon.
From that point of view, the Moon is a most unusual satellite, and it makes the picture of a forming Earth utterly different from the other planets as they formed.
All the sizable planets but Earth would seem to have formed about a central condensation point with at best several inconsiderable knots of matter at the outskirts, so small in comparison to the central condensation point that they could scarcely be thought to affect the manner in which the main planet is formed.
In connection with Earth, however, there seem to have been two condensations—one considerably larger than the other, but not overwhelmingly so.
Consider Venus and Earth, then, so alike in mass and constitution, yet so different in present surface conditions. Is it possible that this present difference can, at least in part, be explained by the fact that Venus formed in one condensation and Earth in two? Did the Moon’s formation somehow draw off material in a crucial way that acted to change the chemical or physical state of the Earth so as to initiate a different geological evolution as compared to Venus? Did that difference, slight to begin with perhaps, diverge until Earth became a cool planet with an ocean and a comparatively thin atmosphere, while Venus became a hot planet, with no liquid water at all, and a very thick atmosphere?
It might be that the double condensation that formed the Earth-Moon double planet is an exceedingly rare occurrence; so that in assuming that one out of every two planets in the ecosphere of a Sunlike star would be an Earthlike planet, we would be wrong. It would be an Earthlike planet only if it had a Moonlike satellite and that might virtually never happen. In the absence of a Moonlike satellite we would get only a Venuslike planet at best.
If that were so, we would have to conclude there were virtually no habitable planets in the Universe and that Earth was an incredibly fortuitous freak. Naturally, there would then be no extraterrestrial intelligences, or virtually none, and there would be no reason to be surprised that space is quiet and that we haven’t heard from them.
Yet having argued in this fashion, can we find the argument compelling? Just what is the influence of the Moon’s formation on that of the Earth? What could it have done in forming to decrease Earth’s atmospheric density, increase its water supply, prevent a runaway greenhouse effect?
There is no reasonable answer to that as yet.
Finally, we can point out a way of rationalizing the differences between Venus and Earth that seems more probable than anything to do with the Moon.
Venus is closer to the Sun than Earth is and by a considerable amount. The process of photolysis, whereby the Sun’s ultraviolet radiation breaks up the water molecules to hydrogen and oxygen would