The Case for Mars (34 page)

Examining
Table 7.1
, we can see that with respect to the large majority of plant soil nutrients, Martian soil is richer than that of Earth. The main question mark is nitrogen, which, due to the limits of its design, the
Viking
X-ray florescence instrument used to analyze the elemental composition of the soil could not assess. Nitrogen is known to>exist in the atmosphere, however, so if the soil should prove nitrate poor, ammonia and other nitrate fertilizers can be synthesized. In fact, the same
Sabatier reactors used to produce methane fuel can also be used to produce ammonia if nitrogen and hydrogen are used as the feedstock. Such reactors are a major source of fertilizer production on Earth. However, based upon our present understanding of planetary formation, Mars should have originated with about the same proportion of nitrogen as Earth, and most of it must still be there, undoubtedly fixed in the soil as nitrates. Natural nitrate beds should be detectable on Mars, which when mined will provide the base with fertilizer by the truckload. The other plant nutrient element in which typical Martian soils appear to be poor is potassium. This can probably be found in high concentrations in salt beds deposited on the now dry shores of Mars’ ancient water bodies.

The physical properties of Martian soil are also likely to be favorable for plant growth, as the globally distributed soil layer appears to be loosely packed and porous, and well adapted mechanically to supporting plants. As discussed earlier, Martian soils are known to contain smectite clays. This is good news for future Martian farmers because smectites are highly effective at buffering and stabilizing soil pH in the slightly acidic range, and also ensure a large reserve of exchangeable nutrient ions in the soil due to their high exchange capacity.

As stated earlier, the Martian greenhouses will be pressurized at 5 psi (340 mbar), or around one third of Earth sea-level pressure. Since Mars’ gravity is one third Earth’s, maintaining this air density will also make insect flight possible, facilitating pollination by bees. Initially the domes will simply be pressurized with Martian air (95 percent carbon dioxide) with a few millibars of artificially generated oxygen included in order to make plant respiration possible. The Martian plants will thus grow in heavily carbon dioxide-rich greenhouse environments, and photosynthetic efficiency should benefit accordingly. On Earth, in a carbon dioxide-poor environment, plants convert sunlight into chemical bond energy with an efficiency of about 1 percent. (The net ecological efficiency of a forest or wild prairie is much lower, perhaps 0.1 percent, but that’s because the dead plants are allowed to decompose. The plants themselves do considerably better, and in an agricultural setting we can take advantage of that by harvesting them before they are disassembled by bacteria.) A good guess for the efficiency of photosynthesis i
n a heavily carbon dioxide-enriched environment might be about 3 percent. Assuming that the 50-meter-diameter dome is a true half-sphere, it would take plants this efficient covering its floor about 310 days to turn virtually all of the enclosed carbon dioxide into oxygen. If a lens-shaped upper dome (a radius of curvature of 50 meters instead of the natural 25) were employed, however, the time would be reduced to only eight days. The oxidant that
Viking
may have detected in Martian soil will be no problem, as it decomposes into reduced material and free oxygen on contact with water. The warm greenhouses will be moist environments, and as the moisture circulates it will quickly cause the greenhouse soils to give off their oxygen.

We’ve all heard arguments advanced by vegetarians that everyone should give up eating meat, because an acre of com can produce far more food for humans than an acre of cattle forage. These arguments are questionable on Earth, because starvation on our planet is not caused by a global food shortage but by lack of cash in the hands of the people who are starving. On Mars, however, an environment in which humans cannot simply
take
tillable land but must
make
it with domes and so forth, about 3 vegetarian thesis has a fair amount of merit. Martian agriculture will have a very strong incentive to be efficient. Employing cattle, sheep, goats, rabbits, chickens, and other warm-blooded herbivores in large numbers as part of the food chain is, in fact, very inefficient. Most of the energy of the plants an animal eats goes into maintaining its body temperature, and very little ever reaches you. (Some years ago a science writer wrote several books in which he popularized the idea of
goats
as the key to future animal husbandry in space They are of convenient size, omnivorous, fast breeding, milkable, and so on. Be that as it may, I’m city born, but have lived the more recent portion of my life in a rural area. I’ve seen what goats can do. Don’t let one near your Kevlar dome. He’ll eat it.) On the other hand, for almost any agricultural plant of interest, at least half of the plant is never eaten by humans. For example, in the case of corn, rice, or wheat, we don’t eat the roots, stems, or leaves. Instead, these parts get plowed back into the soil with the self-consoling thought that we are keeping the ground fertile. But if that were our true objective, we’d plow the whole plant back—really we’re
just wasting energy. So, if we want to be efficient, we need to find a way to use the not-directly edible parts of the plants. Time to bring in the goats? May Be a few, to amuse the kids and keep the base security patrol busy as they leap over three-meter high fences in the light Martian gravity, but there are better ways. One is to use mushrooms. At Purdue University, for example, a NASA-funded space-agriculture research center has isolated species of mushrooms that will grow on the waste portions of plants and turn 70 percent of their material into edible protein that is as high in quality as soy (which is considerably better than goat). The fast-growing mushrooms need no light, just a dark, warm room, the waste corn stalks, and a little bit of oxygen. In other words, you can have a mushroom ranch in a closet. This, by the way, is an example of a technology developed for the extreme demands of space that can have plenty of application meeting basic human needs on Earth. But, if eating mushrooms and beans bores you, there’s still hope. Cold-blooded herbivores, such as tilapia fish, are reasonably efficient in transforming waste plant material into high-quality protein. Fish farms on Mars? Why not? You don’t need a very large tank to grow tilapia, and they won’t escape to eat your dome.

Orchards will also be desirable to produce fruit. Therefore wood will eventually be available as well. This wood can be used as such, to make furniture and so forth. Alternatively, it, together with other cellulose waste from agriculture, will be fed into the plastics industry, where it will significantly increase the variety of plastics that can be produced.

MARTIAN METALLURGY

The ability to manufacture metals is fundamental to any technological civilization. Mars provides abundant resources to support their production. In fact, in this respect, Mars is considerably richer than the Earth.

Steel

By far the most accessible industrial metal present on Mars is iron. The primary commercial ore of iron used on Earth is hematite (Fe
₂
O
₃
). This material is so ubiquitous on Mars that it gives the Red Planet its color, and thus indirectly, its name. Reduc
ing hematite to pure iron is straightforward, and, as mentioned both in the Old Testament and in Homer, has been practiced on Earth for some three thousand years. There are at least two candidate processes suitable for use on Mars. The first, as discussed earlier in this chapter, uses waste can monoxide—reaction (1), above—produced by the base’s RWGS reactor as follows:

The other uses hydrogen produced by the electrolysis of water.

Reaction (4) is slightly exothermic and reaction (5) is mildly endother-mic, so after heating the reactors to startup conditions, neither will require much power to run. In the case of reaction (5), the hydrogen needed can be obtained by electrolyzing the water waste product, so the only net input to the system is hematite. Carbon, manganese, phosphorus, and silicon, the four main alloying elements for steel, are very common on Mars. Additional alloying elements such as chromium, nickel, and vanadium, are also present in respectable quantities. Thus, once the iron is produced, it can readily be alloyed with appropriate quantities of these other elements to produce practically any type of carbon or stainless steel desired.

The widespread availability of carbon monoxide at the Mars base, due to its production as waste from RWGS reactors, opens up some interesting possibilities for novel kinds of low-temperature metal casting techniques on Mars. For example, carbon monoxide can be combined with iron at 110°C to produce iron carbonyl (Fe(CO)5), which is a liquid at room temperature. The iron carbonyl can be poured into a mold, and then heated to about 200°C, at which time it will decompose. Pure iron, very strong, will be left in mold, while the carbon monoxide will be released, allowing it to be used again.
You
can also deposit the iron in layers by decomposing carbonyl vapor, allowing hollow objects of any complex shape desired to be made. Similar carbonyls can be formed between carbon monoxide and nickel, chromium, osmium, iridium, ruthenium, rhenium, cobalt, and tungsten. Each of these carbonyls decomposes under slightly different conditions, allowing a mixture of metal carbonyls to be separated into its pure component
s by successive decomposition, one metal at a time.
²⁹

Aluminum

On Earth, after steel, the second most important metal for general use is aluminum. Aluminum is fairly common on Mars, comprising about 4 percent of the planet’s surface material by weight. Unfortunately, as on Earth, aluminum on Mars is generally present only in the form of its very tough oxide, alumina (Al
₂
O
₃
). In order to produce aluminum from alumina on Earth, the alumina is dissolved in molten cryolite at 1,000°C and then electrolyzed with carbon electrodes, which are used up in the process, while the cryolite is unharmed. On Mars, the carbon electrodes needed could be produced by pyrolyzing methane produced in the base’s Sabatier reactor, as described in Chapter 6. This process can be written:

Aside from its complexity, the main problem with employing reaction (6) to produce aluminum is that it is very endothermic. It takes about 20 kWh of electricity to produce a single kilogram of aluminum. That’s why on Earth aluminum production plants are located in areas where power is very cheap, such as the Pacific Northwest. On Mars during the base-building phase, power is not going to be cheap. At 20 kWh/kg, a 100 kWe nuclear reactor could only produce about 123 kilos of aluminum per day. Thus steel, not aluminum, will be the primary material used to build high-strength structures, although due to lower gravity, steel on Mars will weigh about the same as aluminum does on Earth! Aluminum will be saved for special applications, such as electrical wiring or flight system components, where its high electrical conductivity and/or light weight put a premium on its use.