The Case for Mars (35 page)

Silicon

In the modern age, silicon has emerged as perhaps the third most important metal after steel and aluminum, as it is central to the manufacture of all electronics. It will be even more important on Mars, because by manufacturing silicon we will be able to produce photovoltaic panels, thereby continually increasing the base’s power supply. The feedstock
for manufacturing silicon metal, silicon dioxide (SiO
₂
), makes up almost 45 percent of the Martian crust by weight. In order to make silicon, you need to mix silicon dioxide with carbon and heat them in an electric furnace. The resulting reaction is:

Once again, we see that the required reducing element, carbon, is a by-product of the Mars base propellant production system. Reaction (7) is highly endothermic, although nowhere near as bad as the alumina reduction reaction (6), and the energy burden involved in reducing silicon is not remotely comparable because the quantities needed tend to be much less.

For some purposes, the silicon product of reaction (7) is good enough for use. For example, you can use it to make silicon carbide, a strong heat-resistant material (it’s used in tiles to protect the Space Shuttle from the heat of reentry). However, it is evident that any hematite impurities present in the reactor feedstock will also be reduced, resulting in iron impurities in the silicon product. To produce hyperpure silicon, then, good enough for computer chips and solar panels, another step is needed. This is accomplished by bathing the resulting impure silicon product in hot hydrogen gas, causing the silicon to turn into silane (SiH
₄
). At room temperature or above, silane is a gas, so it can easily be separated from hydrides of the other metals, all of which are solids. Then, if you want completely pure silicon, all you have to do is pipe the silane to another reactor where you decompose it under high temperatures, thereby producing pure silicon and releasing the hydrogen to make more silane. The silicon can then be doped with phosphorus or other selected impurities to produce exactly the kind of semiconductor device you need.

Alternatively, instead of decomposing the silane, you can liquefy it by refrigerating it down to -112°C. This is only about 20°C below typical Martian nighttime temperatures, so it is easily accomplished, and the resulting fluid can be stored for long periods in insulated tanks without difficulty. Why store liquid silane? Because silane will
burn
in carbon dioxide. Up till now, virtually all the Martian propellant combinations we have discussed, such as methane and oxygen, have required the vehic
le utilizing them to carry both fuel and oxidizer in its tanks. We don’t do things that way on Earth. On Earth, whether it’s burning gasoline in your car or wood in your fireplace, all you do is provide the fuel. The oxidizer comes from the oxygen in the air. Since the oxidizer in general makes up about 75 percent of the reacting mixture, this latter approach is clearly a far more efficient way to go. Well, there is very little free oxygen in the Martian atmosphere; it’s almost all carbon dioxide. Not many things will burn in carbon dioxide, but silane will, in accordance with:

In reaction (8), 73 percent of the propellant mass is carbon dioxide, only 27 percent is silane. Some of the products are solids, so you can’t use this system in an internal combustion engine. But you could use it to fire the boiler of a steam engine, and you could use it in a ramjet engine or for rocket propulsion. Burned in accord with reaction (8), a silane/carbon dioxide rocket engine could produce a specific impulse of about 280 seconds. On the surface this is not that impressive, until you realize that you only need to carry 27 percent of the propellant with you. That is, consider a small rocket hopping vehicle that takes off and lands repeatedly, delivering its telerobot cargo to a series of chosen target sites separated by impassable terrain. It won’t need to carry all its propellant. Instead, it can refuel with carbon dioxide just by running a pump each time it lands. The result is that the effective specific impulse of this system would not be 280 seconds, but 280 seconds multiplied by the ratio of total propellant to silane, which is 3.75. The result—an effective specific impulse of 1,050 seconds, unheard of in chemical rocketry.

Diborane, B
₂
H
₆
, will also burn in carbon dioxide, with a specific impulse of 300 seconds at a mixture ratio of three parts carbon dioxide to one part diborane.
³⁰
A diborane/carbon dioxide rocket hopper could thus have an effective specific impulse of 1,200 seconds, eve
n better than the silane/carbon dioxide system discussed above. However, boron is rare on Mars, while silicon is everywhere, and the processes required to produce diborane are rather complex. So, while small amounts of diborane may be imported to Mars early in the program to permit
high-performance hopper applications (use of a diborane/carbon dioxide system may be the optimal way, for example, of performing a robotic Mars sample return mission), once a base exists capable of producing silane, this locally available product will almost certainly displace diborane.

As an aside, it has frequently been proposed that silicon be manufactured on the Moon to support the production of large quantities of solar panels there. This idea has serious flaws. Yes, it’s quite true that silicon dioxide is as common on the Moon as anyone could ask for, but the carbon and hydrogen necessary to turn it into silicon metal is absent. While in the processes described above these reagents are recycled, in reality such recycling is always imperfect. If you want to produce silicon metal, or
any
other metal, on the Moon, you are going to end up having to import a lot of carbon and hydrogen. On Mars, in contrast, both of these elements are available locally.

Copper

As a final example of producing a key industrial metal at a Mars base, let us consider copper. Copper, which is absent on the Moon, has been detected in SNC meteorites at about the same concentrations that it is found in soil on Earth. This is quite low, however, about 50 parts per million. If you want to obtain useful quantities of copper, you don’t extract it from soil. Instead, you must find places where nature has concentrated it in the form of copper ore. Commercially, the most important sources of copper ore on Earth are copper sulfides. As we have seen, sulfur is much more common on Mars than on Earth, and it is probable that copper ore deposits are available on Mars in the form of copper sulfide deposits formed at the base of lava flows. Once found, copper ore can easily be reduced by smelting or leaching, as has been practiced on Earth since ancient times.

The example of copper drives home the fact that, in general, the only way of accessing geochemically rare elements is by ming local concentrations of high-grade mineral ore. However, you will find ores only where complex hydrologic and volcanic processes have occurred that can concentrate these elements into local ore deposits, and, within our solar system, only Earth and Mars have experienced such processes. Because
these processes have occurred on Mars, we should be able to find concentrated ore of nearly every metal, rare or common, necessary to build a modern civilization.

THE QUESTION OF POWER

It should be evident that the availability of large amounts of both thermal and electrical power is the key to being able to conduct the manufacturing processes to develop a significant Mars base. It may be unpopular to say it, but by far the best way to provide this power during the early years of base development is by importing nuclear reactors produced on Earth. On Earth today, the main sources of power for our civilization are hydroelectric, fossil fuel and wood combustion, and nuclear. Geothermal heat provides a distant fourth source of energy, and way behind it are solar power and wind, which play very minor roles. On Mars, hydroelectric dams and fossil fuel combustion are not power source options. In the long run, the prospects for generating thermonuclear fusion power on Mars are excellent, because the ratio of deuterium (the heavy isotope of hydrogen needed to fuel fusion reactors) to ordinary hydrogen found on Mars is five times as high as it is on Earth. Unfortunately, fusion reactors don’t currently exist. That leaves nuclear power as the only option for the initial source of large-scale power. A nuclear reactor producing 100 kWe and 2,000 kilowatts of thermal process heat twenty-four hours a day for ten years would weigh about 4,000 kilograms—just four tonnes—making it light enough to import from Earth. In contrast, a solar array that could produce the same round-the-clock electrical output (but only one twentieth the thermal output) for about the same lifetime would weigh about 27,000 kilograms and would cover an area of 6,600 square meters (about two-thirds of a football field). If we wanted the same
thermal
output (for brick making and water processing), the solar array needed would weigh 540,000 kilograms and cover thirteen football fields. This is obviously far too much material to import from Earth. The advantage of nuclear power for opening Mars is enormous—so much so that the efforts of the Clinton administration to shut down the American space nuclear power research and development
program can only be condemned in the harshest terms. If we give up space nuclear power, we will give up a world.

While the initial base power supply will need to be nuclear, once the base is well established, the equations could change. It should be possible at some point to construct solar power systems out of indigenous materials on Mars. If you are living on Mars, hundreds of tonnes of local materials may be much easier to come by than four tonnes of equipment imported from Earth.

Harnessing the Sun and Wind

There are two kinds of solar power systems that can be manufactured on Mars, dynamic and photovoltaic. Solar dynamic systems are low-tech; they work by using a parabolic mirror to concentrate sunlight on a boiler, where a fluid is heated and expanded to turn a generator turbine. These systems can be fairly efficient (about 25 percent efficiency), but to date they have not found much favor in the space program, as the fact that they rely on moving parts has caused many to consider them to be unreliable. However, at a permanently staffed Mars base, people would be on hand to maintain systems and adjust failing equipment. The reliability argument against dynamic systems becomes considerably lessorceful in the context of a Mars base. Moreover, because they are low-tech assemblages of mirrors, boilers, and similar gear, it is relatively easy to see how such systems could be manufactured on Mars. The mirrors could be made of inflatable plastic, for example, covered with a very thin layer of aluminum for reflectivity. The pipes, boilers, and turbine shaft and blades could all be made of steel. To actually attain the 25 percent efficiency level, the turbines must be manufactured to tolerances that are too exact to be realistic for a Mars base, but this is hardly a show stopper. If necessary, lower tolerances and 15 percent efficiencies could easily be accepted. In addition to these advantages, the dynamic cycles also offer the attractive feature of producing a goodly amount of useful process heat, perhaps equal to four to six times their electrical output.

Solar dynamic cycles, however, require clear skies. In order for the parabolic mirrors to effectively concentrate light, the light must all come from the same place, directly from the Sun. It cannot come from diffuse sourc
es of scattered light all over the Martian sky. Based upon
Viking
data, the kind of clear skies needed for effective solar dynamic operation can only be expected during the northern spring and summer. During the other half of the year, the solar dynamic concentrators are likely to put out very little power. Such a seasonal swing in power availability might be acceptable for some purposes. You don’t necessarily have to be making metals all year long. But if solar power is to become the primary source of base power, a more dependable technology is needed.