Nitrogen is third among the Big Four, and is likely to be found just about anywhere. Just as oxygen does, nitrogen combines with hydrogen. The product is ammonia, and this is nitrogen's major source.
Because of their combining tendencies, if you only had those two elements, you would a choice of ammonia with excess hydrogen, or ammonia with excess nitrogen. Nitrogen, like oxygen, is gaseous.
Nitrogen does not have strong combining tendencies with oxygen. Nitrogen oxides can be formed, but they require an input of energy. They are not major components of natural atmospheres under anything like earth-like conditions of temperature and pressure, and especially not in thin atmospheres.
Part of the bias is that nitrogen and ammonia with it are less abundant in the first place than oxygen and water are. If that wasn't bad enough, ammonia has a smaller molecular weight than water, so water sticks to planets better. Even worse, if cool your planet down, water condenses and freezes out first, while ammonia is still floating around in the upper atmosphere. Worst of all, it is more easily broken up by ultraviolet light than water is leaving only the nitrogen gas behind.
Once you try introducing free oxygen into the system, however, the bias really begins to bite. First of all, you can forget about ammonia oceans with oxygen atmospheres. The oxygen gobbles up ammonia's hydrogen leaving you with water and nitrogen. (Sound familiar, Terrans?)
This leaves you with a limited set of choices for three-element planetary envelopes. Type 1 is water-and-ammonia in hydrogen. Type 2 is water-and-ammonia in Nitrogen. Type 3 is water in nitrogen-and-oxygen.
In the first two, you can guess which component, water or ammonia, is most likely to dominate. If you guessed ammonia, try again.
Overall, then, the universe is not kind to ammonia-lovers. However, all is not lost. One of the Big four isn't vanquished all that easily. Terra itself had an early atmosphere, circa the orgin of life, without free oxygen, which suggests a type 1 or type 2 atmosphere: Various lines of evidence suggest type 2 as more likely. Make the earth a little bit bigger, and further out, or turn down the sun a tad...
Furthermore, ammonia is quite soluble in liquid water, which could help preserve it. Also, water and ammonia have different temperature ranges where they are liquid: If the water freezes out, ammonia oceans become possible. These temperature ranges are also sensitive to atmospheric pressure, and dissolved substances: but I am not so certain about the details.
The nitrogen gas present in type 2 and 3 environments is chemically quite sluggish, and there are two major ways organisms can incorporate it.
There are Terran bacteria which metabolize ammonia, releasing nitrogen gas and water, taking advantage of the reaction discussed in Nitrogen I. If this reaction were reversed by connecting it to an energy-generating mechanism like that of photosynthesis, it might provide a substitute for the oxygen-powered combustion reactions used in terran life.
The other process is used by terran life, and involves using nitrogen oxides. Although these require energy to produce, Lightning provides a natural source. Also, some plants are capable of fixing nitrogen biochemically, though at some cost. Nitrogen oxides react with water to form nitric acid and nitrous acide, giving nitrates and nitrites.
Terran life uses these to store nitrogen in a more tractable form than nitrogen gas, but in an environment where free oxygen is in short supply, the nitrates and nitrites make an excellent way to store oxygen. The chemical energy and oxygen stored in nitrates are in fact used by some terran bacteria to support their life processes. Plants which make use of nitrates and nitrites have ways to controllably oxidize them to the amines they use in proteins, and these reactions are in principle reversible.
A plausible basis for an alien biochemistry would be for plantlike organisms to photosynthetically produce ammonia and oxygen from nitrogen and water, and instead of releasing the oxygen, incorporate it with nitrogen into nitrates.
Animals eat the plants containing ammonium nitrates, and slowly release its explosive energy, to power their life processes, returning nitrogen and water to the environment.
Variant are possible, depending on whether ammonia must be generated or is available in the environment, and whether nitrates are available in the environment or must be generated.
Would this biochemistry be alien? yes. Impossible? Not necessarily. Its components are known to exist. Rare, or abundant? Insufficent data.
On type 1 worlds, life is under the necessity to expel the excess hydrogen from both water and ammonia; a considerable handicap for life.
However attractive this possibility might be, a combination of hydrogen, oxygen, and nitrogen alone cannot give us the large and varied molecules and macromolecules that are characteristic of life. For this, we have to have something like the last of the big four... carbon.
Carbon is mostly found in combination with hydrogen, as would be expected from the saturation of the universe with hydrogen; the combination is methane.
A surplus of hydrogen in a system gives methane in a hydrogen atmosphere. Since methane is the lightest of the series, methane, ammonia, and water, it is the most likely to be lost from a planet.
However, if breakup by ultraviolet light is the primary cause of loss of hydrogen, then carbon is likely to be retained. Dehydrogenation of carbon compounds gives rise to increasing numbers and varieties of hydrocarbon compounds that are progressively heavier, wiht higher melting and boiling points.
However, almost no systems are pure carbon and hydrogen, and given the existence of hydrogen poor worlds, it makes sense to consider carbon-oxygen systems. Carbon and oxygen, like the big four and hydrogen, but unlike nitrogen and oxygen, form stable compounds with a release of energy, and will not coexist for long as the pure elements.
Carbon forms two important compounds with oxygen: carbon monoxide, and carbon dioxide. A range of proportions exists, but we can consider environments to be oxygen-rich, which gives carbon dioxide and a surplus of oxygen, and oxygen-poor, which will have varying amounts of carbon monoxide.
Given the cosmic abundances of oxygen and carbon, you can expect most worlds to be oxygen-rich in this sense. On hydrogen-poor worlds, you can expect significant amounts of carbon dioxide, for at least four reasons. The lack of substantial formation of nitrogen oxides leaves a carbon-oxygen combination as the next in line. The general oxygen-richness of the universe indicates dioxide. Of all the gases considered so far, carbon dioxide is by far the heaviest, and most likely to stick to a planet. It will stick when even molecular oxygen and nitrogen are lost.
The general progression goes something like this. On high-hydrogen worlds, where there is a surplus of hydrogen, there is not much opportunity for hydrogen-saturated oxygen and water to react. However, as the hydrogen content of a system decreases into the middle hydrogen range, there is increased opportunity for water to react with unsaturated hydrocarbons. among the first and simplest of these are the alcohols. At a further stage, there are the aldehydes and at a further stage, the organic acids. Just as ammonia does not tolerate the presence of free oxygen, neither do hydrocarbons in large quantities. They rapidly undergo combustion to Carbon Dioxide and water.
The reaction of carbon and nitrogen is much like that of oxygen and nitrogen: they can coexist almost indefinitely without reacting much. Nevertheless, many compounds containing both carbon and nitrogen can be formed with a modest input of energy. Binding with carbon has a stabilizing effect on a great many nitrogen compounds. Some of the most important, such as the nucleic and amino acids, have vital nitrogen components.
The dominant reactions of life, Photosynthesis: water + CO2 = oxygen + carbohydrates, and respiration, its reverse, should be too familiar to need an extended discussion.
For now, a closing summary will suffice.
It should be noted that there can be a great variety of worlds in any given class, depending on temperature, atmospheric density, and details of the composition. A planet is classed primarily by its atmosphere, with liquid (if any) given secondary importance, and solids least.
This classification cuts across the UWP physical profile, and so it is of somewhat limited use. However, for world builders who would like design their alien environments with some care, and not rely strictly on random dice rolls, or those artists who desire a semblance of reality, this should provide something of a guide.
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Joseph Heck
(joe@mu.org)
21 August 2000