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Planetology 102

In the previous article, I discussed the "big four" elements (hydrogen, oxygen, carbon, and nitrogen) and their relationship to life. In this series, I look at the principal rock-formers and their relationship to planets.

For reference and to allow world-builders to experiment, here is a list of solar system abundances of the elements. These solar system abundances are thought to be reasonably close to close to cosmic proportions, but, as I will show at the end, minor differences on a cosmic scale may be very important on a planetary scale.

Solar System Abundances
of the 45 most common elements
(atoms per 1.0e6 Si atoms)
Element

Abundance

Element

Abundance

Element

Abundance

H

27.9e9

Cr

14.0e3

Ga

38.0e0

He 2.7e9 P 10.0e3 Sc 34.0e0
O 23.8e6 Mn 9.6e3 Sr 24.0e0
C 10.1e6 Cl 5.2e3 B 21.0e0
Ne 3.4e6 K 3.8e3 Br 11.8e0
N 3.1e6 Ti 2.4e3 Zr 11.0e0
Mg 1.1e6 Co 2.2e3 Rb 7.1e0
Si 1.0e6 Zn 1.3e3 As 6.6e0
Fe 0.9e6 F 843.0e0 Te 4.8e0
S 515.0e3 Cu 522.0e0 Xe 4.7e0
Ar 101.0e3 V 293.0e0 Y 4.6e0
Al 85.0e3 Ge 119.0e0 Ba 4.5e0
Ca 61.0e3 Se 62.0e0 Sn 3.8e0
Na 57.0e3 Li 57.0e0 Pb 3.2e0
Ni 49.0e3 Kr 45.0e0 Mo 2.6e0

The Noble Gases

Helium and Neon are important industrial gases. They are comparatively rare and expensive at lower TLs, since they are light and easily lost from terrestrial planetary atmospheres, but they make up some 25% (by weight) of a gas giant's atmosphere. The same noble gases that are contaminants in the jump fuel are essential in the superconducting magnets in the fusion reactor and weapons systems. Skimming and separation facilities located in orbit or on a convenient moon of gas giant at TL 9+ worlds should be major suppliers of these gases.

Oxides

There is more than enough oxygen in the cosmos to combine with all elements less abundant than carbon, with plenty left over. Oxygen in some form is hard to escape. Most elements have one or more compounds with oxygen, and all but a few are solids at temperatures comfortable to humans. The number one oxide and compound in the cosmos is water, but pressures below about 2% of earth's atmosphere, it does not exist as a liquid. Carbon dioxide can be liquefied at high pressure and low temperature, but the combination of a sufficiently cold and thick atmosphere is more typical of gas giants in the outer system than rocky planets in the inner system.

Silicates

Silicon is chemically related to carbon, but there are important differences. Although the pure element has important uses as in semiconductor technology, it is never found in nature, and it is always an energy-intensive process to extract it. While carbon atoms combine with hydrogen or oxygen with approximately the same ease, silicon has a strong preference for combination with oxygen. Although silanes analogous to the hydrocarbons can be made, they are (unfortunately for speculative xenologists) unstable and not found in nature, since they react with water to form silicon dioxide and hydrogen. (not to mention burning in oxygen).

Silicon dioxide, also called silica, forms the basis for the rock, the most common solid material. Unlike carbon dioxide, which does not combine with itself and remains gaseous, silicon dioxide molecules readily combine with each other. The highly stable and high-melting chains, sheets, and 3D networks formed by silicon dioxide create the large, complex family of silicate minerals which are the primary rock formers everywhere. The basic unit of silicate minerals is a tetrahedron with a silicon atom surrounded by 4 oxygen atoms, each free to combine with another silicon, or some other variety of atom. There is also a less common high pressure form. The variety of atoms that can fit into the fundamental silicate network gives them a variety second only to the complexities of organic chemistry. The silicates have a more continuous range of compositions and are harder to work with, but high-tech materials science includes a wide variety of special purpose ceramics and derivatives.

Although pure silicon dioxide is important in industry (a source of silicon for semiconductors to name only one), it is not especially common in this form. Silica is usually mixed or compounded with something else. Silica bearing mixtures are as common as ordinary rock, and mostly about as valuable. Commercial-grade deposits are found where natural separation and concentration process work, mostly on cooler planets with significant water, carbon dioxide, and weather. Also unlike carbon dioxide, silica is almost insoluble in water. It does forms an extremely dilute solution of silicic acid; (mostly H4SiO4). The solubility is enhanced in hot alkaline solutions. As soon as it becomes concentrated, however, the silicon tetrahedra start to recombine by eliminating water molecules between them. There are terrestrial organisms (such as some plankton and sponges) which incorporate dissolved silica into their skeletons, and besides the fact that such skeletons are brittle, inflexible, and heavy if they are dense, there is no fundamental reason higher forms of life can't do the same.

Hydroxides

Since water is more abundant than many elements and forms an important part of many compounds, discussion of the various minerals is incomplete without mentioning hydroxide minerals. Hydroxide compounds may be treated as the reaction product of an oxide with water. Silicic acid, already mentioned, and carbonic acid, formed when carbon dioxide dissolves in water, are examples. These are only found in an excess of water, but many other hydroxides are insoluble minerals. Elements which combine with more than one oxygen may have partial hydroxides; the silicates again as an example. Other minerals are much more abundant, but since they are closely related to the oxides and water is prevalent on planets interesting to humans, I mention them earlier than their abundance merits.

Hydroxides are low temperature minerals (as minerals go, that is), and ice works as a source of water for them. Water may be driven off from them by heating, so they are probably rare in the inner zone. On the other hand, water does dissolve in molten rock, so few planets are entirely without hydroxide minerals in their interior. With high tech levels, a closed environment, and tight water discipline, enough water to support a human colony can be extracted from quite barren rockballs. It may be still be cheaper to import it, though.

Magnesium

This is the most abundant of the metals. Magnesium belongs to the chemically reactive alkaline earth family, and like silicon, it is not found free in nature. It is an important industrial metal because it is lightweight, and fairly easy to separate from its compounds. On the downside, the pure metal is not very strong.

Magnesium does not naturally combine with hydrogen. It combines quite well with oxygen forming the oxide MgO. (TL 5-6 photography uses it for flashbulbs). The hydroxide Mg(OH)2 (brucite) occurs in hydrothermal deposits, since it is mostly insoluble in water; but it reacts too well with acids for there to be much of it. (Milk of magnesia is a suspension of magnesium hydroxide.)

Even the oxide is chemically reactive. Nearly all of it is found in combination with silica as the minerals enstatite (Mg2Si2O6, 2 units MgO, 2 units SiO2), and forsterite (Mg2SiO4, 2 units MgO, 1 unit SiO2). There is in fact slightly more magnesium than silicon, more than enough to combine with all of it as enstatite, but not nearly enough to saturate it as forsterite. These minerals tie up most of the silica and nearly all the magnesium in the solar system.

Although magnesium silicates are common throughout the solar system, they are not especially abundant on the earth's continental crust for reasons partly to be discussed later. One reason is that they are (slowly) attacked by water. First, they are converted to hydroxide minerals such as serpentine and talc, and then the magnesium dissolves, leaving silica behind. There is an inexhaustible supply of magnesium dissolved in seawater, so there is no need to go to space looking for it. (Seawater is already the major commercial source, weathering can replace it faster than human industry can extract it, and there's a whole planet full of it.) In vacuum, dry, or high temperature environments, the magnesium silicates remain stable.

Carbonates

As already mentioned, carbon dioxide dissolves in water to form carbonic acid. Many oxides are alkaline (releasing OH- ions) when dissolved in water. These will react with carbonic acid to form carbonate minerals. Many carbonate minerals are only slightly soluble in water. The result is that on planets with liquid water and carbon dioxide atmosphere, silicate rocks will be weathered and release the more soluble alkali metals. These will then combine with carbonic acid and form slightly soluble carbonate minerals. As these precipitate from the seas and form solid minerals, they extract carbon dioxide from the atmosphere. A great deal of the magnesium in the earth's crust, for instance, is found in magnesium-bearing carbonates.

Most of the carbonates are (as minerals go) soft, low-temperature materials that release carbon dioxide when heated. Like water, carbon dioxide (and monoxide, formed by conversion of methane) dissolves in molten rock, so some carbonate minerals should be found in most planets. They will probably be scattered throughout the crust of worlds that have never had liquid water and not form extensive deposits as they do on earth, and they will most likely disappear from the surface of the hot worlds of inner systems.

Iron

Iron is only slightly less abundant than silicon and magnesium. It is by far the most important industrial metal. The earth has plenty of iron, and only the most highly concentrated deposits have been exploited. For example, today ore containing 1% copper is considered a real find. An ore containing only 10% iron is worthless. Iron can also be found in elemental form in asteroid belts as a result of planetary formation processes (to be discussed later) that separate and concentrate it.

Unlike the other elements so far considered, iron has two stable ionic forms, Fe+2, and Fe+3. This produces two distinct oxides: FeO, (Ferrous oxide, wuestite), and Fe2O3 (Ferric oxide, hematite). There is also a compound of these, Fe3O4 (magnetite). These last two compounds are the principal forms found in the earth's crust. Iron and the ferrous compounds are more stable in vacuum, dry, and low oxygen environments. On the earth's surface, the generally reddish colored ferric compounds predominate.

Iron hydroxides and partial hydroxides are also known. Iron rust, which only forms in the presence of both water and oxygen, is a mixture of these. Ferric hydroxide is thought to be responsible for the pronounced reddish color of Mars. These compounds are barely soluble at all; more than silica, but less than magnesium.

From a cosmic point of view, the most important forms of iron are the silicate minerals ferrosilite (Fe2Si2O6, 2 units FeO, 2 units SiO2) and fayalite (Fe2SiO4, 2 units FeO, 1 unit SiO2). It is actually quite difficult to separate magnesium and iron silicates. Forsterite (the principal magnesium silicate) and fayalite (the principal iron silicate) mix with each other in all proportions, as a solid solution. Mixtures of different proportions are given different mineral names. Collectively they are called olivine.

Although there is not enough magnesium to saturate the combining capacity of silicon, it turns out that inclusion of ferrous oxide makes up the difference, so that the greatest share of all three elements becomes tied up in olivine and related minerals. This accounts for why these are the principal components of stony meteorites and probably planetary mantles, including the earth's. The combination is not complete and other materials combine better with silicon than iron does, so that there is a little iron left over. Away from the earth's crust, most of the small surplus is in the form of ferrous oxide. Iron silicates are weathered by water in somewhat similar fashion to magnesium silicates, so olivine does not last long (geologically speaking) where there is plenty of water. There is a ferrous carbonate (FeCO3) which can be found especially in older sedimentary deposits, but the small ferric ion found mostly in an oxygen atmosphere with its charge of +3 and the large carbonate ion with its charge of -2 make an awkward combination. Hematite and magnetite are more stable.

Sulfur

This chemical relative of oxygen acts like it in many ways, but unlike it in many others.

The uncombined element is not particularly common in the cosmos, but it is found under certain conditions; the earth has huge deposits in various places. Unlike oxygen, it has a tendency to form rings and chains. Unfortunately for the speculative xenobiologist, it does not have the rich branching possibilities or the stability of carbon chemistry and it is much less common. Sulfur forms a volatile compound with hydrogen, H2S, analogous to water, H2O. In a huge excess of hydrogen as in most of the universe, this should be its normal form. On very small bodies, this is lost with the other volatiles. Much like ammonia, hydrogen sulfide is not stable in an oxygen atmosphere. Oxygen claims the hydrogen and leaves free sulfur.

That is, it does if oxygen is in limited supply. On low-hydrogen worlds, sulfur forms two principal oxides, sulfur dioxide SO2, and sulfur trioxide SO3. These are among the heaviest of the gaseous compounds, and may remain on a planet when the more abundant lighter gases are lost (such as on Io). It is tricky to make a biosphere out of these; not so much because they are chemically unsuitable, but because at the temperatures and pressures where they may remain liquid solvents, a planet is more likely to keep the more abundant volatiles, especially water. The sulfur oxides dissolve in water to form sulfurous acid, H2SO3, and sulfuric acid, H2SO4. Sulfur dioxide is more soluble in water, but in an oxygen atmosphere, sulfurous acid and its compounds are readily oxidized to sulfuric acid and its compounds.

Sulfides, sulfates, and planetary formation

Sulfur combines with metallic elements as oxygen does, but not as well. Magnesium and silicon, for instance, prefer the oxide so much that sulfur is excluded. Iron forms a couple of sulfides, mostly FeS (triolite), but also FeS2, iron pyrite, "fool's gold". The presence of hydrogen complicates this a little. Given equal amounts of oxygen and sulfur, the combination H2O + FeS would be preferred to H2S and FeO, but amounts are nowhere near equal. The preference is slight enough that overall, sulfur combines with more than its fair share of the iron not absorbed by silicates, but oxygen gets most of it.

Enough of the building blocks have been discussed that it is time to pause and present a somewhat speculative but chemically plausible scenario for the formation of rocky planets. The starting point is mineral grains coated with ices. These collide, melting the surfaces, and stick together, forming larger and larger grains and lumps. At a certain size, the heat produced by collisions (or close proximity to the sun) begins boiling off some of the ices, concentrating the minerals. When larger asteroid-sized lumps collide, they boil off more volatiles and begin to melt the minerals as well. Under these conditions, methane and ammonia begin to react with water to form hydrogen, nitrogen gas, and carbon monoxide.

Sulfur comes into play because ferrous sulfide does not mix well with molten silicates and forms a separate phase, like oil and water. In the meantime, ferrous oxide reacts with the excess hydrogen and the carbon monoxide to give water, carbon dioxide and free iron, which then concentrates in the sulfide phase. Above a certain size and temperature, gravity begins to concentrate the much denser and lower-melting iron and sulfide in the center. This process releases energy and heats the mixture, and since iron conducts heat better than rock, the heat tends to concentrate in the core. Heat from radioactive decay also concentrates in the core. Planets and moons larger than a few hundred km in size tend to develop a thick ferromagnesian silicate mantel and a smaller iron-iron sulfide core. The remains of failed and shattered moonlets and asteroids in the asteroid belt rain meteorites down to the earth that show this process frozen at various stages.

Few chemical separations are complete, and small amounts of ferrous sulfide remain in the mantle and crust. The trapped and dissolved water assures that some gets converted back to ferrous oxide and hydrogen sulfide which gets released to the surface by volcanoes. Volcanic gases are composed of water, carbon monoxide and dioxide, nitrogen, hydrogen sulfide, sulfur dioxide, and a few other volatiles, depending on the particular volcano. It is thought that such volcanic bases (probably richer in hydrogen 4+billion years ago) formed earth's early atmosphere and much of its oceans, and that similar mixtures are typical of the early stages of planetary formation. Although this seems to be the general picture, planetary astronomers and geologists seldom summarize and simplify it. They do vigorously debate the details, relative importance, and timing of the various events and processes involved. For planets with water and carbon dioxide but little free oxygen, there is no reason organic sulfide chemistry cannot form a basis for metabolism and life. For many "primitive" and anaerobic bacteria on earth, it does. There are various hints that terrestrial life may have begun with sulfur chemistry and shifted later to an oxygen base. Once such a shift is well underway, sulfur begins to concentrate in the crust as sulfates begin to replace sulfur and sulfides. Sulfuric acid, already mentioned, is even more effective at combining with alkaline metallic hydroxides than carbonic acid is. Only the lesser abundance of sulfur keeps it from dominating mineral composition. Magnesium forms a sulfate MgSO4, epsomite, AKA epsom salt. Ferrous sulfates are also known, but the same conditions that work for sulfates work against the ferrous state of iron, so they are not really abundant. Ferric sulfates have the same problems as carbonates, and again the oxides are preferred.

Aluminum

Aluminum, like magnesium and silicon, is chemically far too reactive to be found as the free metal, and it is always an energy-intensive process to extract it. (Both aluminum and silicon react easily with the oxygen in the air, but the thin oxide surface layer protects the rest of it.) This lightweight industrial metal is used extensively where light weight is an important design criterion.

Much like magnesium and silicon, aluminum combines with oxygen so well that it excludes practically everything else. There are various forms of the oxide (Al2O3, or alumina). One of them, corundum, forms a basis for several of the precious gemstones and is one of the harder minerals (a distant second to diamond on the usual scale). Aluminum also forms a hydroxide Al(OH)3, which is poorly soluble in water, and a partial hydroxide AlO(OH). These are the principal components of bauxite, the preferred ore for aluminum.

The pure aluminum silicate Al2SiO5 sillimanite is fairly uncommon. The vast majority of aluminum in the cosmos is found substituting for silicon in the huge aluminosilicate subclass of the silicate minerals. Other metallic ions, including magnesium and iron, are usually incorporated into these minerals to make the ionic charges balance. On the earth, one of the chief components of soil is clay, which is chiefly impure hydrated aluminum silicates.

Aluminum hydroxide may be either alkaline or acid, depending on conditions, but generally speaking it is difficult to form and no more soluble than silica. Depending on local conditions, either silica or alumina may be extracted from rocks and recrystallized elsewhere.

Calcium and Sodium.

Calcium belongs to the same chemical family as magnesium, but is more reactive. Sodium is the most common of the very reactive alkali metals. Both of these combine well with oxygen, and even their oxides are chemically active. Their oxides, hydroxides, and carbonates are collectively known as soda and lime. Sodium compounds are more soluble in water, so sodium concentrates in the oceans. Calcium carbonate is less soluble in water, and there are huge deposits of calcium carbonate (calcite, in impure form limestone) in the earth's crust. Calcium, more than magnesium, is responsible for removing carbon dioxide from the atmosphere. Most of the magnesium extracted by weathering from the magnesium silicates is found in combination with calcium as calcium-magnesium carbonate, dolomite. There is also plenty of calcium sulfate (gypsum).

Silicon dioxide forms a glass, but it is viscous, high melting, and hard to work with. With the addition of sodium and calcium compounds, it becomes much easier to work, and ordinary glass is soda-lime glass, or sodium-calcium silicate. Another reason these two elements are considered together is that they are found together as the natural companions of aluminum in aluminosilicate rocks, their primary source. These are slowly weathered by water, more rapidly by acid water, to form the more soluble calcium and sodium hydroxides, carbonates, and sulfates; clay minerals; and silica.

These various silica-rich minerals are less dense than the than the much more abundant ferromagnesian silicates discussed earlier. Generally speaking, they have higher melting points as well and are the first to solidify. The final major component of rocky planets is a thin mostly aluminosilicate crust. About 60% of the earth's crust is composed of feldspar (chiefly calcium and sodium aluminosilicates) and 12% silica minerals. The more complex aluminosilicates, ferromagnesian silicates, and calcium-magnesium carbonates and sulfates account for most of the rest. On earth, oceanic crust is thin and richer in the ferromagnesian silicates. Continental crust is thicker and richer in the silica minerals and aluminosilicates. The difference in these minerals affects volcanic activity dramatically. Volcanoes in the thin oceanic crust tend to have comparatively frequent, gentle eruptions of flowing lava that forms dark colored, dense, basalt rock, rich in olivine and other ferromagnesian silicates. The midoceanic ridges and Hawaiian Island volcanoes are typical of this type of volcanic activity. In contrast, volcanoes in the thicker continental crust tend to have less frequent but more violent eruptions of viscous lava that flows poorly and cools rapidly to form light colored rocks rich in feldspar minerals and quartz, a form of silica. The eruption of Mount St. Helens and more recently Pinatubo are typical of this type of activity. This kind of magma is less likely to surface and erupt at all; it may slowly cool and crystallize beneath the surface to form granite and related rocks.

Nickel

I mention this metal chiefly for its industrial uses and to round out the cosmic top fifteen. This tough, corrosion-resistant, magnetic companion of iron is usually found along with it. It makes up about 5% of iron meteorites, and probably planetary cores as well. It combines with oxygen rather poorly and not much better with anything else, but it makes a fine catalyst for various chemical reactions. It is also used in coinage (although, oddly enough, not in the US 5 cent piece.). Its association with iron means that it turns up in iron-bearing silicate rocks, but the resistance to oxidation means that it tends to stay there. Nickel is far less concentrated in the crust than in extraterrestrial sources. It is probable that belters are looking for iron asteroids, and more for the nickel in them than for the iron.

Variations and Conclusion.

At the beginning, I also promised a summary of how small differences in solar system composition could produce radical differences in the character of planets. Prediction is a risky business, and nature has fooled every planetologist there ever was with some effect he neglected. But given this limitation, here are some possibilities.

In Sol system, abundances of magnesium, iron, and silicon in particular are so nearly balanced that differences of only a few percent can make a significant difference in the size and composition of the core, mantle, and crust of planets.

Larger size, thick crusts, thick mantle, and less volcanic activity all mean slower cooling, so volcanic activity should last longer. Smaller size, thin crusts, thin mantle, and more volcanic activity mean faster cooling, and activity dies off faster. Smaller cores may mean weaker planetary magnetic fields.

In magnesium rich system, magnesium should tend to displace iron from the ferromagnesian silicates, and may produce larger cores and more free iron in the system. In magnesium poor systems, mantle composition will shift to include more iron, at the expense of the cores.

Iron rich systems may put excess iron in the cores rather than displacing magnesium; iron poor systems take it from them. These differences will probably show up in the asteroid belt as well.

Silicon rich systems should have thicker mantles and crusts as well. Silicate and aluminosilicate crusts are good insulators, so perhaps less volcanic activity, more violent when it does occur. Volcanic activity might be concentrated in would be concentrated in hot spots or cracks, with less mobility of tectonic plates. Silicon poor systems should force more iron to the core, with a thinner mantle and crust. These would have more but gentler tectonic activity, with more and more mobile tectonic plates.

High sulfur means probably more of it remaining in the crust and mantle. Life should still be possible, but the planet inhospitable to humans. Low sulfur may mean life has a harder time getting started, so the planet never evolves to satisfy human tastes.

Aluminum-rich systems ought to have thicker crusts; there isn't enough of it to affect the mantle much. Aluminum-poor systems can be expected to have thin crusts, much like those that are silicon-poor, and will probably see more natural calcium and sodium silicates.

Differences in calcium and sodium will have their most visible effects in the composition of the sedimentary class of rocks, and in the composition and salt balance of oceans. Terrestrial life is highly sensitive to the balance of salts in body fluids, which in most cases is rather similar to that in terrestrial seawater. On a world with different mineral proportions, Terran transplants (including humans) could fare very poorly except in limited areas.

At this point, I have covered the chief aspects of planetology and will end this particular series. Most of the other elements add only details and refinements to this basic character. Hopefully, this series will allow world builders to add a little color and realism to their worlds.

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