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Planetology 101

[Author's Note: This article is a look at world-building from a chemical point of view. This is a consolidated and somewhat updated version of material that appeared on the Traveller Mailing List in October 1998.]

The physical conditions on a planet, the cosmic abundances of the elements, and the laws of chemistry practically dictate not only what kind of envelope (atmosphere and hydrosphere) a planet has, but what kinds of life are most likely to develop.

The principal physical conditions are size and temperature. Small planets, moons, and so forth, are likely to be more abundant than large ones. Temperature is principally governed by how close the planet is to its star and what kind of star that is, although it it is also influenced by the atmosphere.

The abundances of the elements are also important. Hydrogen is the most abundant element in the galaxy, followed by helium, oxygen, carbon, and nitrogen in that order. Helium is a noble gas and will be neglected. The others are the Big Four of planetary chemistry. Sulfur is the only element that comes anywhere near the Big Four in importance to life. Halogens such as chlorine and fluorine are both much less important and more likely to be locked into chemical combination; the noble gases can be dismissed, and the rest of the elements are chiefly rock-formers and metals.

The physical conditions shape the chemical composition of the planet's atmosphere. Substances that have a low molecular weight are most volatile and are easily driven off small or high temperature planet. If they freeze, liquefy, or enter into chemical combination, they may be retained on the planet's crust and not affect its atmosphere. The average speed of a molecule depends on its molecular weight and the atmospheric temperature: while escape velocity depends on the worlds gravity. Molecules with greater average speed than the world's escape velocity will only be present in trace amounts. Those with average speeds over about 20% of escape velocity are very likely to be lost over geologic time periods.

Escape Velocity = sqrt(2 * gravitation * world radius)
(use 1 g = 10 m/sec^2 and world radius in meters)

Most probable molecular speed = 105 * sqrt(absolute temperature/molecular weight)

Randomly generated Traveller UPP's don't quite fit with this closer analysis. You may have to tinker with either the numbers you generate or with planetary history and conditions to get a believable world. YMMV: natural conditions often surprise us anyway.

Hydrogen

Hydrogen rules: it is more abundant by far than any of the other elements.

It also has the lowest molecular weight of any ordinary gas, and the lowest boiling point of any except helium. This makes it the most volatile. The smaller or hotter a planet is, the faster it will lose hydrogen from its atmosphere.

Compounds of hydrogen and other elements, for instance water, methane, ammonia, and hydrogen sulfide, also tend to be light and to have low boiling points. These are also easily lost from small or hot planets.

Finally, hydrogen may be driven from its compounds by ultraviolet radiation. The hydrogen escapes, leaving the heavier element to combine with something else. Any planet smaller than a gas giant is likely to lose hydrogen as it ages.

All this means that hydrogen content makes a good yardstick for comparison of planets.

At the high end of the scale are the gas giants, which are mostly hydrogen. As a matter of fact, any planet than can retain substantial free hydrogen is likely to belong in the gas giant class. Small gas giants, such as Neptune and Uranus, become progressively more enriched in the less volatile elements and compounds.

At some point smaller than these but somewhat larger or cooler than the earth, there is a cutoff point, where a planet cannot retain its hydrogen but can retain its heavier volatiles. These are "middle hydrogen" planets, and are the most likely places for life to originate.

At some point only a little smaller or hotter than the earth, a planet loses all its hydrogen-containing compounds unless they are in a frozen state. These are hydrogen poor planets, though some are poorer than others. The earth itself is a borderline case: although it has retained substantial water, like the middle hydrogen planets, most of it is liquid, a condensed state. This has little to do with atmospheric density. For example, Mars and Venus are both hydrogen poor, but one has a very thin, and the other a very dense atmosphere.

Helium

Helium is next most abundant, but since it is chemically unreactive, it will be ignored. It is very nearly as volatile as hydrogen, but since it does not combine chemically, it is virtually absent from any but gas giants.

Oxygen

The next most abundant substance is oxygen. There is enough oxygen in the galaxy to combine with all less abundant elements with some left over. For the most part, oxygen also combines with these other elements better than hydrogen does, and its compounds are much less volatile. Rock is chiefly oxides of the metallic elements. Hydrogen-poor usually means oxygen-rich. Free hydrogen and free oxygen are never found together in nature for very long. Given a few microseconds to a few thousand years, depending on conditions, they combine to form water. The good news for us water-drinkers is that water is the single most abundant compound in the universe. The bad news is the old spaceman's lament...water, water, everywhere, and not a drop to drink.

Stars are far too hot for water to exist as a compound. It's present in gas giants, but since it's denser than hydrogen, you can expect to find it mostly in their lower layers. (These are no place for beings with ideas of space travel to go looking for it). If you don't mind it being strictly a mineral, there's plenty of it in the outer reaches of most star systems. If you want to melt it, find a place with some atmospheric pressure. In vacuum, water behaves like dry ice and sublimes instead of melting. Since water vapor is a volatile, fairly lightweight gas, it's easy for asteroids, moonlets, and anything else that gets close enough to a star to lose its supply to space. (There's plenty of it there too, if you have a few thousand years to spare collecting it from a few cubic AUs of excellent vacuum.)

Thus far, we can expect:

Carbon

We can expect most life in this galaxy to be carbon based, for two reasons. First, carbon is better suited for forming complex organized structures than most other elements. Second, it is thousands of times more abundant than any of its potential competitors.

On Hydrogen rich worlds, carbon is likely to be found in the form of methane. Methane has a lower atomic weight than water and a much lower melting and boiling point, so it is more volatile. It can be expected most in the atmospheres of gas giants or in the ices of outer systems. Only the larger or cooler of middle hydrogen worlds can be expected to retain much methane, and even there it's problematic.

Carbon and oxygen react better with each other better than either does with hydrogen. A planet that can't hold its hydrogen will start losing it from methane as well.

  1. methane + energy = carbon + hydrogen
  2. carbon + water = carbon monoxide + hydrogen.
  3. water + energy = hydrogen + oxygen.
  4. carbon monoxide + oxygen = carbon dioxide + energy

The first can be accomplished by ultraviolet light. Small bodies such as cometary nuclei end up with carbon coatings and much of their carbon in monoxide form.

Methane, carbon, carbon monoxide, and carbon dioxide fall on a scale, and where a planet falls on this scale depends on the ratio of hydrogen to oxygen. High-hydrogen environments, such as gas giants, are on the methane end of the scale. Samples of intermediate environments include carbonaceous meteoroids and cometary nuclei. Most terrestrial planets are driven to the low-hydrogen end of the scale. Methane is not particularly stable in these environments and will have to be resupplied by continuing geological or biological action. Geologically young planets may have more carbon monoxide than earth does, but any sufficiently hydrogen-poor planet will have most of its carbon in the form of carbon dioxide. In a planet with a surplus of oxygen in the atmosphere, carbon monoxide will be rapidly oxidized (at least in geological times) to carbon dioxide.

The stable carbon dioxide molecule is much the heaviest yet considered, and carbon dioxide will be retained in a planet's atmosphere better than any other common gas. It is likely to dominate the atmospheres of hydrogen-poor worlds unless there is some means of removing it. It may escape, be frozen out, (as almost, on Mars) or get locked in the crust (as on Earth). This last option works best when there is liquid water to help dissolve it and the metallic ions it can combine with. Dry worlds (like Mars and Venus) get stuck with it in the atmosphere.

A complex series of hydrocarbons and carbohydrates may also occur as intermediates in these processes. As long as there is water available, life can be supported:

  1. methane + water + energy = complex hydrocarbons + hydrogen
  2. carbon monoxide + water + energy = complex hydrocarbons
  3. carbon dioxide + water + energy = complex hydrocarbons + oxygen

The first of these equations suggests that the methane breathers of science fiction are actually plants! The animals would be hydrogen breathers. The odds are stacked against methane-hydrogen ecologies, except on gas giants. Not only is methane one of the more volatile gases, but an extensive methane-consuming, hydrogen-generating plant life would tend to drive a planet's evolution toward the hydrogen-poor end of the scale. However, it is possible. On earth, there is a group of terrestrial bacteria (methanogens) that use cheaper sources of hydrogen than water and convert complex hydrocarbons into methane and water, extracting the energy for their own use. Higher forms of life are imaginable.

The second of these equations occurs even in interplanetary space, but it is difficult for envision forms of life that can flourish there, since in vacuum environments, water does not form a liquid. Conditions are either freezing or boiling with no middle ground. But on a terrestrial-type planet with free water, such reactions are relatively easy, and it is thought that terrestrial life originated with them.

The last reaction, using carbon dioxide instead of monoxide and is familiar to us. Terrestrial organisms run it both ways. Water is still required, but life processes can still take place on otherwise hydrogen-poor worlds.

Nitrogen

Ammonia lovers will find that the universe is biased against them.

At least in this region, nitrogen is less than a tenth as abundant as carbon. On gas giant types, it is commonly found in combined form, as ammonia. If there were no oxygen or carbon to be considered, this would force a ammonia-in-excess hydrogen or ammonia-in-excess nitrogen split, just as oxygen does. Nitrogen has an odd atomic number and odd number of combining electrons, which makes its combinations with oxygen a bit complicated and those with carbon even worse.

Unlike hydrogen compounds and carbon dioxide, nitrogen oxides require energy to form: they are not normally present in large amounts. If they do form, they tend to spontaneously decompose to a mixture of oxygen and nitrogen. (Sound familiar, Terrans?) The nitrogen oxides can be stabilized somewhat by reacting with water (as in nitric and nitrous acids) or with hydrocarbons.

Nitrogen can combine with carbon, but not easily. A little bit of hydrogen stabilizes the compounds, as in hydrogen cyanide. These are fairly easily oxidized to nitrogen and carbon dioxide if oxygen is present.

Ammonia is somewhat more volatile than water, (lighter and lower-boiling) but somewhat less than methane. This puts it more on the high-hydrogen end of the scale.

As with methane, a planet that can't hold its hydrogen is likely to start losing it from its ammonia as well, with nitrogen as the remainder (as on Titan and Earth). Ammonia is somewhat closer to water in its properties, so it might stick around a bit better than methane.

Another difficulty is that a planet that can keep its ammonia is even more likely to keep its water, which is up to a hundred times more abundant in the first place. Ammonia dissolves quite well in water, and dissolved ammonia acts as an antifreeze. This could extend the life zone into colder regions. On the other hand, once dissolved, there are plenty of negative ions for ammonia to react with: some of it could get locked up in the crust.

Ammonia-oxygen atmospheres are unstable: they rapidly evolve to nitrogen-water instead. (@#$ Terrans again)

Given that the carbon compound-water reactions are the basis of life, the inclusions of nitrogen and ammonia broadens the possibilities somewhat. The reaction

Nitrogen + water + energy = nitrates + ammonia

is an interesting possibility. There are terrestrial bacteria which run modifications of this reaction in both directions: Ammonium nitrate is a potent explosive, but living organisms specialize in slow release of energy from combustible mixtures. If ammonia is present, it's likely that some amount of cyanides are, also.

Nitrogen atmospheres are familiar to us earth-dwellers and probably come next after carbon dioxide atmospheres in importance on hydrogen-poor worlds. Nitrogen remains a medium-weight gas even when the hydrogen containing compounds and carbon dioxide freeze out.

Sulfur

This is even less abundant than nitrogen, and is a major component of planetary envelopes only in exceptional cases. Sulfur has a volatile compound with hydrogen: hydrogen sulfide. In a hydrogen poor environment, Sulfur does tend to form chains as carbon does, but it cannot form the side branches and multiple rings that make carbon chemistry interesting and complex.

In an oxygen rich environment, sulfur combines with oxygen to form sulfur oxides. These, like carbon dioxide, can be mineralized and locked up in the crust. When other volatiles are gone, sulfur and sulfur oxides are next in line. In Sol system, there is Jupiter's satellite Io with its sulfur volcanoes, or the sulfuric acid clouds which have scavenged what little water remains on Venus.

Considerable variation on chemical composition is possible, but the following patterns are what we see in Sol system. With some variation, we can expect similar patterns in others. YMMV.

  1. Hydrogen rich

    In order of decreasing volatility, Hydrogen, methane, ammonia, water.
    Typical of gas giants. The principal characteristic is a reducing atmosphere, with plenty of free hydrogen.

  2. Middle hydrogen--Life bearers

    In order of decreasing volatility; Methane, ammonia, water, nitrogen, carbon monoxide, carbon dioxide.
    Heavier hydrocarbons and carbohydrates may also be present. Neither strongly reducing nor strongly oxidizing. Good support for many varieties of biochemistry. Early earth was probably on the lower end of this scale. Titan might qualify if it weren't so cold there.

  3. Hydrogen poor---common.

    nitrogen, carbon monoxide, oxygen, carbon dioxide, sulfur dioxide
    Water may be present, principally in frozen or liquid state. Principal characteristic is an oxidizing atmosphere. In most cases, carbon dioxide dominates, Nitrogen is next most likely. If water is present and the planet is life bearing, free oxygen may be present. If water is not present, sulfur dioxide or sulfuric acid may be minor components.

The basic composition and chemistry of this galaxy make it biased in favor of carbon-based, water drinking, oxygen-breathing organisms. But for those who like the exotic, there is room to add organisms which can tolerate or even depend on compounds poisonous to humans: such as carbon monoxide, ammonia, nitrogen oxides, hydrogen cyanide, hydrogen sulfide, and sulfur dioxides, and dozens of fairly simple organic compounds, without losing too much credibility.

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