Extraterrestrial Civilizations - Isaac Asimov [92]
However, the arguments against the Moon as a captured satellite are not compelling, either. Astronomers have not yet been able to come to a decision in this respect. The Moon may not be a captured satellite, but it may be.
We can be justified, then, in assuming for the sake of argument that the Moon is a captured satellite and see where that leads us.
To begins with, when might it have been captured?
There is no way of telling, really. It could have been captured 4 billion years ago, not long after both bodies were formed and before any life appeared on Earth. It could have been captured 4 million years ago, not long before the first hominids appeared on Earth.
At least there is no way of telling if we consider only the Moon. Suppose, though, we consider the Earth. Is there any sharp revolution in the Earth’s history that might conceivably be correlated with the capture of the Moon and blamed on that capture?
What about the appearance of land life on Earth? The land was colonized oddly later. Whereas life in the ocean began perhaps one billion years after the Earth was formed, life on land did not appear till 4.2 billion years after it was formed. If we equate the 12-billion-year lifetime of Earth as a habitable planet with the 70-year-old lifetime of a human being, sea life began when the Earth was 6 years old and land life when it was 25 years old. Why the difference?
Is it possible that the tides had something to do with the coming of land life?
The periodic progression of water up a shore and then down again would carry life with it. It would leave pools behind in which some forms of life could flourish. There would be water-soaked sands that could become hospitable to life. Adaptations would make it possible for life forms to withstand limited amounts of drying between one high tide and the next, creeping further and further up the shore until finally life was possible without any actual immersion in water at any time.
Could it be that in the nearly tideless ocean of a moonless Earth, the tidal transition between sea life and land life was absent, and that for 3 billion years land life did not develop?
Could the Moon have been captured a little before 600 million years ago and could the tides that suddenly resulted have sufficiently stirred up the forming sedimentary rock to wipe out the earlier traces of fossils and have helped make the appearance of life forms in the Cambrian rocks so seemingly sudden?
And could a couple of hundred millions of years of tides have finally pushed life out onto the land and made intelligence and technology possible?
To be sure, even with the Moon absent, the Earth is not entirely tideless. The Sun produces tides, too, and if the Moon were not in the sky, the tides produced by the Sun alone would be about one-third as high as that produced by the Sun and Moon together now.
One might argue that what the Sun could do wouldn’t be enough and point out, in addition, that what the Moon could do in ages past is more than it could do now.
Because tidal effects are slowing the Earth’s rotation, the Earth is losing angular momentum of rotation. Angular momentum cannot actually be lost; it can only be transferred. In this case it is transferred from the Earth’s rotation to the Earth-Moon revolution. The Earth and Moon slowly recede from each other, make larger sweeps about their mutual center of gravitation, and thus gain angular momentum.
If we look backward in time, we can see that 400 million years ago when the transition from sea life to land life began, the day must have been shorter and the moon must have been closer to the Earth. In actual fact, there is evidence from the growth rings on fossil corals of the period that, at that time, the day was about 21.8 hours long, and the Moon’s period of revolution was 21 days (which meant that it was only 320,000 kilometers, or 200,000 miles, from Earth).
Remembering that the tidal effect varies inversely as the cube of the distance, we can see that the height of the lunar