Extraterrestrial Civilizations (27 page)

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.
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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.

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 be accelerated; the hydrogen would escape rapidly thanks to the higher temperatures caused by the nearness of the Sun; the oxygen would combine with any methane present to form water and carbon dioxide. The process would continue, leading eventually to a thick atmosphere consisting chiefly of carbon dioxide, which would accelerate the greenhouse effect and bring about the Venus we know.

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.

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 credible way in which it could have been done in actual fact.

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 tides 400 million years ago was 1.66 what it is today and 1.44 what the lunar and solar tides together are now. With tides roughly one and a half times the height they are now, and moving up and down at a speed 10 percent greater than at present (thanks to the shorter day back then), the push toward land life could have been considerably more effective then than it would be today.

We
might
conclude, then, that the Earth, in accomplishing the very ticklish task of capturing the Moon (so difficult a task that astronomers can’t figure out just how it might have happened) made it possible for land life to exist.

When we calculated how many myriads of habitable planets there were, we left out of account how few of them might have succeeded in capturing a large satellite that just happened to be there, and how few might therefore have developed land life and in that way have the kind of intelligence and technology that we are looking for.

And yet this argument in favor of Earth’s being unique in possessing land life, and therefore intelligence and technology, is also
not compelling. We don’t need a captured Moon to explain the coming of land life. During the billions of years that life existed in the sea and not in the land, the Moon’s tides, however high, probably could not have brought about a transfer of life to the land.

During most of Earth’s existence after all, the Earth’s atmosphere did not contain more than a very small percentage of free oxygen, if any at all. This meant that there was no ozone layer in its upper reaches, and ultraviolet radiation from the Sun could reach the Earth’s surface in large quantites.

The energetic ultraviolet radiation is inimical to life since it tends to break up the complicated molecules on which life depends. This would not affect life in the ocean, however, which could drift just far enough under the ocean surface to receive enough of the ultraviolet energy without receiving too much.

On land, however, it is not that easy to escape the deadly radiation of the Sun, so the land remained dead, sterilized by sunlight.

Even at the beginning of the Cambrian, 600 million years ago, the Earth’s atmosphere was not quite 5 percent oxygen. The oxygen content was now increasing rapidly, however, and an ozone layer was forming and growing denser. The ultraviolet was increasingly blocked by the forming ozone layer and by 400 million years ago, it no longer reached the Earth’s surface in deadly quantity. Now for the first time, fragile living tissue pushed up on the shore by the tides was not killed at once. Slowly, the land was colonized.