It is true that some heat was formed when the planets coalesced, but it is not possible that all this heat has been retained the three billion years they have been in existence. In the earth's case this can be checked by measuring the rate at which the internal heat diffuses through the crust. The calculation show that the earth could be nowhere near three billion years old if the internal heat was formed when the earth coalesced. Also the continuing contraction theory of heat formation is no longer tenable because the earth would have contracted to a very much smaller object than it is now in three billion years.
Safely rejecting these two hypotheses, we are again left with the question posed above: Where does the energy come from? The answer is that the earth uses the natural radioactivity of radium, uranium, and thorium to produce the energy by radioactive decay. It is true that there are only small amounts of the natural radioactive elements in the earth, but the non-conductivity of the earth's rock keeps the heat at its present level - there is an equilibrium between the amount of heat lost and that being newly formed by radioactive decay.
This hypothesis seems logical enough, and in view of the theory mentioned earlier which said that the composition of the planets is fairly constant throughout the universe, it is safe to assume that it is correct. Accepting this, I will base further extrapolation on it, although it is impossible to verify this hypothesis any further at present.
The metallic core seems to be a necessary part of the planet. While it does not affect the life on the planet's surface directly, it is essential to the stability of the planet. A planet without a dense core would be more oblate than Jupiter and probably could not stand large tidal strains.
The existence of the metallic core is explained by saying that when the planets were forming, the heavier elements naturally collected at the center of gravity. This would hold true whether the planet was forming from cold dust or incandescent gasses. As iron appears to be the commonest heavy element, we can assume that the cores of most planets will consist of it. But whether iron or not, the planets will have heavy cores, and this is important because it ensures the fact that the minor planets will have densities and, therefore, gravities not varying greatly from the earth's.
In the earth the rock layer which surrounds the core is peridotite, which is an iron or magnesium silicate, (Fe, Mg)2 SiO4, and there is strong evidence that this is a common substance, at least in the solar system, in the fact that 75% of the meteorites found consist of this material. They are commonly called stony meteorites. If the meteors were the remnants of a destroyed planet beyond Mars, then this would fit in nicely with our theories, as 75% of the earth is also that material. The other 25% of them are nickel-iron, as is the rest of the earth, it's believed.
This rock layer is important because the crust floats on it; it is responsible for most of the geological changes taking place on the surface, like mountain building and volcanic action. I will go into these effects in more detail later on.
This layer accounts for the largest part of the planets by volume and also by weight. Its formation is explained similarly to the core's, both being a gravitational formation. When the heavy core formed, these materials, being the next most dense, packed around the core. This idea is born out in the earth, as we find that there are variations in the layer's density, and yet the heavier layers are always deeper. Naturally, with the increasing depth there is also an increase in pressure, which would increase the density of the lower material, but this is no argument against the hypothesis because the deeper materials have naturally higher densities, even without compression, than the higher level material.
The final solid part of the planets, and the one with which life is most immediately and vitally concerned, is the crust. It was formed at the same time the planet formed, and its formation is not difficult to visualize with the facts we now have.
As the planet contracts during its formation, heat is formed by the process explained earlier. And there is plenty of oxygen present. Oxygen is one of the most active elements there is; it reacts with nearly every element at normal temperatures, and if the temperature is slightly above normal, the reactions will proceed violently. Therefore, when the condensation of the planet has proceeded far enough to raise the temperature to about 100°C. - and by this time the formation almost complete - the free oxygen begins to combine with every element it can.
Most of the hydrogen had been lost before the temperature got high enough for the hydrogen to react with oxygen, but that which remained did combine. There was so little water formed, relatively, that nearly all of it was used up in further reactions with other compounds and elements. After the crust had formed probably very little water, if any, was left uncombined. Then how is the presence of large oceans explained? I will answer that question later, as it is another story.
These oxides and compounds which formed made up a relatively light mixture, compared with the rock layer, so they floated on the surface of the heavier layer and eventually cooled and solidified into the hard material of the final stage. The final product isn't all oxides, by any means; it contains silicates and other compounds, with small amounts of uncombined, noble metals and dissolved gasses.
When the crust had finally cooled as much as it was going to, there were many mountain ranges and irregularities caused by its contraction during cooling. These, added to its original unevenness, produced a very rugged terrain. Young planets probably have surfaces resembling our best mountains all over them.
After the crust had gone through its final stages of formation, the energy from radioactive decay had built up enough; it had to escape somehow. It must establish an equilibrium between the heat - energy - lost and amount forming. To do this it helps build mountains, causes earthquakes, and produces the volcanoes. As a rule, these phenomena would be directly proportional to the size of the planet, as they are dependent on the amount of radioactive matter for their magnitude - larger planets having more, smaller less.
Other factors also affect mountain building; a planet without an atmosphere would have practically none of it because it is dependent on the weathering action of the atmosphere. Planets like the moon or Mercury are probably much the same as when they formed.
Two requirements must be fulfilled before mountain building can take place; they are a constant source of energy and a constant weathering action which will produce continual sedimentation.
All planets will have the first requirement because of the radioactive matter in them, and practically every planet with an atmosphere will meet the second. So it is safe to assume that mountain building will be common to most minor planets. I'll explain the process briefly.
First, and most important to an understanding of mountain building, is the principle of isostacy. It states that two equal sections of a planet must have the same mass, regardless of their height. This means that if you were to cut two sections of equal diameter from the earth, one from the Himalayan Mountains and the other from the Mindanao deep, they would weigh the same, although there is a difference in height of about 70,000 ft. this can be explained if you remember the fact that the sedimentary rocks making up the continents are less dense than the basalt of the ocean floor.
Here I'll leave isostacy for a while and look at the other most important factor in mountain building; as I have said, the radioactive matter, decaying in the earth, releases large amounts of heat. This heat escapes in some way from the interior, maintaining the equilibrium. How? There are three ways in which heat can be transferred: conduction, convection, and radiation. Of these, one, radiation, is obviously impossible inside the earth or any planet. The other two are possible. Practically everyone is familiar with conduction, but convection is not so well known.
Convection is the movement of a hot mass to another, cooler location. The turbulence in a pot of non-boiling water is the result of convection: the hot water, being less dense than the cool, raises to the surface, while the cool sinks to the bottom of the pot.
Because we are familiar only with surface rocks, it does not seem possible that convection could take place in a planet which is almost solid rock. This is definitely not the case; convection does the major share of the heat transference in the planets for several reasons. One of these reasons is that the rocks inside the planets are very poor conductors indeed; another is that due to the enormous pressure and temperature found in the planet's interiors, the rocks have a consistency much like that of putty and can be made to move around fairly easily, though slowly.
Now imagine a great column of plastic rock which is extremely hot at its base and relatively cool at its top; therefore, the base is less dense than the top. The hot base wants to float and the cool top wants to sink; the whole mass begins to rotate slowly. In practice, there are usually two counter-rotating masses next to each other. These huge, rotating columns exert great frictional attraction on the crust of the planet; in fact, they actually pull the crust down with them. Places where such convection currents - for that is what they actually are - produce a sinking in the crust called geosynclines. A geosyncline is the first indication that a new mountain range will form at a certain place.
The development of a geosyncline goes very slowly, as all geological phenomena do, taking millions of years to complete the full cycle because of the extremely slow rotation of the convection currents and the very inelastic quality of the crust. Fortunately, the sinking does not produce a huge declivity in the immediate area of the geosyncline; in fact, it is not even noticeable because the area is continually being filled by sediments from nearby mountain ranges as rapidly as it sinks, giving birth to the layers of folded sediments you can see in the mountains.
When the geosyncline has stopped sinking, it means that the convection currents have reached their new equilibrium, with the hot rocks on top and the cool rocks on the underneath. This is the stable position for the rocks, so they cease moving. However, all the sediments dragged down with the currents are now causing a violation of the principle of isostacy. The huge amounts of light, sedimentary rocks reduce the density of the area around the geosyncline considerably. Hence, the final stage of mountain building occurs to correct this infraction, the uplifting of the sediments to form the new mountain range from a one-time valley.
From what I have stated previously, the processes which produce mountains are likely to occur on any earth-like planet, so we can safely predict that most minor planets with water vapor in their atmospheres will have topographical features similar to earth's, the surfaces of these planets should be mainly sedimentary rocks again like earth's. I mentioned above that water vapor is necessary for mountains, and as it depends on oceans, I'll discuss oceans next.
The oceans, which are undoubtedly the place where life forms, are, as far as astronomers can tell, lacking on all other planets in the solar system. Does this mean that they will be uncommon in the rest of the universe? No, I think not, but it might mean that they aren't as common as we might like them to be.
We owe our existence here now to the oceans; in our veins and arteries still flows a substitute for the sea we have left, and our bodies are almost 60% water. If a planet without any oceans could develop life, it would indeed be very alien to ours. Such a planet would have, if revolving, great variation in its temperature between night and day; its surface would be arid landscape, much like the Sahara desert. All in all, it would be a planet very inimical to our forms of life. If oceans are uncommon, then there is little hope for life forms similar to ours developing anywhere else; however, what evidence we now have does not support this view; rather, it prefers the opposite view to a large extent.
To understand the ocean's formation we must go back again to the early periods in the planet's development, just after it had condensed into a solid mass. At this time the oxygen had already combined with al the hydrogen that had not escaped as yet, and the water that was formed was in turn used in other reactions. None was left from the oceans, and little, if any, was in the atmosphere. How then did the oceans form? The answer to that is found in volcanic action.
Volcanic action was taking place then as now, only a little more violently. When a volcano erupts, it expels lava and gasses. These gasses are what we are interested in; they consist mainly of water vapor, carbon dioxide, and sulfur dioxide. Fortunately, the bulk of them is water vapor and carbon dioxide. When the concentration of water vapor in the atmosphere gets high enough, precipitation takes place, and so the oceans are slowly built up by this process.
The size and rate of growth of the oceans depend on the mass of the planet. Plant life prevents the atmosphere from becoming saturated with carbon dioxide as fast as the water vapor. If a planet does not develop plant life early in its existence, the carbon dioxide content of its atmosphere will prevent, in all likelihood, any life from developing on it.
Here we have a case where planetary conditions depend on organic factors. A minor planet without life will probably be much like Venus. Venus has very heavy clouds of carbon dioxide in its atmosphere, and very little else, from what we can observe.
That discussion of Venus' atmosphere brings us to the final point which I'll consider in this article - the atmospheres of the planets in general. Possibly the atmosphere is the most important single factor for the existence of life - at least as we know it - on a planet. Will atmospheres exist on most planets? Will they be similar to earth's? Or will they contain deadly poisons, etc? I'll attempt to answer these and other pertinent questions as well as it is possible.
The determining factor which governs whether a planet will have an atmosphere is its mass. If a planet's mass is large enough to give it a gravitational attraction greater than the average velocity of most gasses at average temperatures, it will have an atmosphere, under ordinary circumstances.
A planet with large mass like Jupiter will retain all elements, so its atmosphere will be made up of the lighter gasses such as ammonia, methane, hydrogen, and perhaps helium. The very small planets, around the moon's size, will not retain any type of atmosphere, unless they are very cold. Planets like Mars will hold only thin atmospheres varying in density with their masses. Temperature is also a factor which effects a planet's atmosphere; the lower the mean temperature the easier it is for that planet to retain an atmosphere, and vice-versa.
We can divide atmosphere into two general categories, those which consist of gaseous elements and those which consist of gaseous compounds. Of course, all atmospheres will contain gasses of both types, but one kind will usually predominate. I will take up the former type first simply because more is known about it.
There are relatively few gaseous elements in the temperature range we are considering; hence, the possibilities are not large to begin with, and they can be cut down even more. All the inert gasses (6) would qualify, but they are too rare for us to expect enough of them would be found on any planet to constitute its atmosphere. More likely they will be present in all atmospheres in minute quantities. Nitrogen, which makes up the bulk of the earth's atmosphere, is also inert. It will make suitable filler material though, and as it is a common element, it will probably be found in many atmospheres in fair amounts. Of course, no inert gas can support life.
Of the active elements, we can eliminate hydrogen immediately from consideration because it is too light and would not be retained in a minor planet's atmosphere. There are now left on the list of active, gaseous elements oxygen, fluorine, sulphur - only at extremely high temperatures - and chlorine. Sulphur can be largely disregarded because of its high vaporization point, 444° C, and only planets very close to their sun with very large masses could maintain this or higher temperatures and retain a sulphur atmosphere. Fluorine can be eliminated on the grounds that it is too active; it will combine with everything except inert gasses. It will never exist uncombined when there is something for it to combine with. As it forms gaseous compounds when it combines, any planet with a large amount of fluorine on it would have an atmosphere consisting mainly of flourides. And since flourine is rather rare, it is unlikely that many such planets exist.
Oxygen and chlorine are the two remaining elements. Which is the more common in the universe? The question can not be answered definitely. However, chlorine has an atomic weight twice that of oxygen, and the rule seems to be that the lighter elements are more common than the heavier ones. Chlorine is much less common than oxygen on earth. Therefore chlorine atmospheres should be nowhere near as common as oxygen ones on this basis of relative abundance. Still, they should be more common than fluorine atmospheres.
Unlike fluorine, chlorine could support an oxidation-reduction process, as it is not nearly so reactive as fluorine. On the other hand, it has a valence of only minus one, so it would not be as attractive to life maintenance as di-valent elements such as oxygen. However, I still think that there is a good possibility that chlorine dependent life exists.
The gaseous elements that are likely to be found in the atmospheres of extraterrestrial planets are nitrogen, oxygen, chlorine, and the inert gasses in that order of abundance. The nitro-oxy atmosphere is probably the commonest among planets with life on them. Nitrogen with carbon dioxide is a possibility for those without life. Nitro-chlorine atmospheres might be fairly common also, while fluorine compound ones might exist on a very few. Sulphur is a slight possibility in planets with very high temperature and large masses.
As for the other category, I don't think the possibility of finding a planet with an atmosphere composed mainly of hydrogen cyanide or nitrous oxide is very good, although little is known on the subject for sure. Carbon dioxide is the only compound on which anything can be said with any assurance of being correct. We know that it constitutes the largest part of Venus' upper atmosphere. Adel and Slipher found that they could duplicate the reflected spectrum of Venus by passing sunlight through forty-five meters of carbon dioxide which was under forty-seven atmospheres pressure - one atmosphere equals approximately fifteen pounds - except that even this spectrum was slightly less intense than Venus'.
Undoubtedly, as there are thousands of gaseous chemical compounds, some planets have atmosphere with one or more of them constituting an important, if not the largest part, of the atmospheres. Some compounds could even support life. It is hard to make any definite predictions as to which compounds will be more common, except with carbon dioxide, and it, at least, should be rather common.
There are reasons why I don't think such atmospheres will be anywhere near as frequent as the gaseous element type. Firstly, many of the lighter compounds are composed of hydrogen, and hydrogen prefers to combine with oxygen before any other element. So long as there is any oxygen present in goodly amounts, all the hydrogen will be locked up in the stable water molecules. Secondly, many of the gas compounds are composed of elements which are too rare to be present in amounts large enough to constitute a major part of an atmosphere. Thirdly, only a few of these gasses are stable enough to exist for very long; they all react with other compounds easily. Finally, most of the compounds will form only in elaborate laboratory conditions and are not likely to be found in nature.
While there may be some gaseous compound atmospheres, I still think that they will be rare and gaseous elements will be the most likely constituents of most planets' atmospheres. Only carbon dioxide should be the exception, and probably only on planets without any life. The only other slight possibility is fluorine compound atmospheres, and these could not support life either.
In the course of this discussion we have covered practically every major aspect of the planets from their numerical prominence to their upper atmosphere. It is not unreasonable to expect that there are numerous planets that could support life. In my opinion, there is probably a lot of life on those planets; some of the forms may be very similar to us. And undoubtedly alien civilizations of our level or a calibre many times ours exists somewhere. We have never met yet, but someday we might.
Data entry and page scans provided by Judy Bemis
Data entry by Judy Bemis and Tony Parker
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