The Case for Mars (30 page)

Thus, after a certain number of exploration missions have been carried out, an optimum site for development can be selected and the Mars program will shift from exploration into a second phase, that of base building. While the initial Mars Direct exploration missions will make use of the Martian air to provide fuel and oxygen, in the base building phase this elementary level of local resource utilization will be transcended as the crew of a permanent Mars base masters an increasing array of techniques that will transform Martian raw materials into useful resources. In establishing a sizable Mars base, we will learn how to extract native water and grow greenhouse crops on Mars; how to produce ceramics, glasses, metals, and plastics; how to construct habitats and inflatable structures; and how to manufacture all sorts of useful materials, tools, and structures. While the initial exploration phase can be accomplished with small crews of just four members each operating out of Spartan base camps spread over vast areas of the Martian surface, building a base will require a division of labor entailing a larger number of people, perhaps on the order of fifty individuals, equipped with a wide variety of equipment and substantial sources of power. In short, the purpose of the base-building period is to develop a mastery of those techniques required on Mars to produce food, clothing, shelter, and everything else needed to make colonizing the Red Planet possible.

FOUNDING THE BASE

Under the Mars Direct plan, crews open new territories
on Mars every other year to exploration and possible settlement. Eventually, one of these outposts will be considered the best location for the first permanent Mars base. Once that location is identified, all new crews will land their spacecraft at the designated site. In the Mars Direct plan, the habitat used to house the crew on the outbound leg of the mission is landed and left behind on the planet. Thus, as the sequence of missions proceeds, each mission will add a habitat to the base infrastructure. The habitats landed at the base site (which will be preselected for trafficability) can have wheels attached to their landing gear legs, and then, with the aid of a cable and windlass, be rolled together to be either mated up directly or connected with the aid of inflatable tunnels. Alternatively, second-generation habs can be built whose landing gear legs can articulate not only up and down (as all landing gear must), but also side to side, thus allowing the six-legged habs to walk much as the Martians did in H. G. Wells’s
War of the Worlds!
Either way, using one of these techniques, an initial Mars base of some size can be rapidly built up as an interconnected network of Mars Direct style “tuna can” habitats.

While housing in tuna cans will be sufficient for the iron men and women of the first Mars exploration crews, the prospect is marginal for supporting a large scientific population at a permanent Mars base and utterly hopeless as the basis for a program of Mars colonization. An early task, then, both necessary for the base’s own self-development and for all that will follows op the development of large habitable structures. This will employ the same “live off the land” approach we employed to get to the planet, as these new structures can be assembled out of native materials.

VAULTS OF BRICK

In a series of papers published in the late 1980s, engineer Bruce MacKenzie analyzed this problem in some detail and came to the conclusion that the optimum native material for building the first large structures on Mars is
brick
.
²²
This low-te
ch concept may seem somewhat surprising at fi
rst, but
there’s actually quite a lot of merit to the proposal. Making brick is quite simple. For that reason some of Earth’s first cities were built of brick, and for the very same reason brick may be the literal building block of mankind’s first settlements on Mars. To manufacture brick you simply take finely ground soil, wet it, put it in a mold under mild compression, dry it, and then bake it. High temperatures are not really required—in many parts of the world sun-baked bricks are still used—an oven temperature of 300°C can produce pretty good bricks, especially if some scrap material such as torn parachute cloth is mixed into the mud to add cohesion. (You may recall the biblical description of the Egyptians mixing straw with mud to make brick. This was good engineering, an early example of composites manufacture.) Even the 900°C kiln temperature needed to make first-rate modern bricks can readily be produced on Mars, using either a solar reflector furnace or the waste heat from the base’s nuclear reactor. True, water is needed for the process, but if the oven is constructed correctly, nearly all the water used can be recovered from the steam produced as the brick is dried at 200°C prior to baking. On Mars, excellent raw material for brick manufacture is available nearly everywhere in the form of a finely ground, iron-rich clay-like dust that covers most of the surface to a depth of at least several tens of centimeters. Mixed with water, the same ruddy dust can be used to produce mortar to make the bricks stick together. In fact, in experiments done at Martin Marietta in the late 1980s with Martian soil simulant, chemist Robert Boyd showed that by simply wetting and drying Martian soil, “duricrete” material could be created that is over half as strong as terrestrial concrete.
²³
Viking
results show that Martian soil contains very high amounts of calcium (about 5 percent) and sulfur (2.9 percent), while analysis of SNC meteorites, which are known to have come from Mars, has shown that these are present on the Red Planet in the form of gypsum (CaSO
₄
· 2H
₂
O). On Earth, gypsum is the raw material used to make plaster, and it can be baked to produce lime. This can be added to mortar to produce conventional Portland cement, with a resulting significant improvement in tensile strength.

Structural materials have different kinds of strength, tensile and compressive, reflecting their ability to resist stretching and crus
hing, respectively. A rope or cable can have a great deal of tensile strength, but no compressive strength. A steel girder has plenty of both kinds of strength. Brick walls and columns, on the other hand, have plenty of compressive strength but are quite weak in tension. They are very difficult to crush but are almost useless for holding things together. Nevertheless, brick and mortar structures built three thousand years ago in ancient Egypt still stand today. Constructions made of brick can prove equally durable on Mars provided that Martian architects adopt the central rule governing nearly all ancient architecture: keep brick structures in compression.

To build a pressumns, on td structure out of bricks on Mars, you excavate a trench and then within it build a Roman-style vault, or better yet, a series of Roman-style vaults or even a Roman-style atrium as shown in
Figure 7.1
. The vaults are covered with soil, thereby putting a large downward load upon them, and only then are pressurized with breathable air (produced either by the oxygen-making chemical units described in Chapter 6 or by the greenhouses described later in this chapter). How much soil covering is needed depends upon how much air pressure is used. If we stick with our proposed Martian standard of 5 psi (3.5 psi oxygen and 1.5 psi nitrogen, as in Skylab), the vaults will experience a pressure force trying to explode them upward of about 3.5 tonnes per square meter. Assuming that Martian soil has an average density four times that of water, this would mean that a layer of dirt about 2.5 meters deep on top of the vault would be enough to keep the whole structure compressed. (Remember gravity on Mars is only 0.38 that of Earth. If terrestrial gravity held sway we could get away with just one meter.) A dirt layer this thick would also provide massive radiation shielding, reducing the cosmic-ray dose experienced by those living in such a subsurface structure to roughly terrestrial levels. In addition, the soil would provide excellent thermal insulation, causing the large temperature swings experienced on the surface during the Martian day-night cycle to go essentially unnoticed by those below and greatly reducing total power requirements to heat the habitat. The brick and soil construction would probably leak air, albeit very slowly. This can be remedied, however, by using a thin layer of plastic sealant either sprayed on the walls or attached to them in the form of “wallpaper.” Slow leaks should tend to seal themselves over time, however, as the relatively moist air leaking from the structure causes leak-blocking permafrost or ice to form in the diffusion paths of the surrounding soil. As can be seen in
Figure 7.1
, using these relatively simple, fundamentally ancient techniques, pressurized structures the size of shopping malls can be constructed on Mars.

FIGURE 7.1
Roman style vaults either singly or in series (a) can he used to construct large subsurface pressurized habitats, including even spacious atriums (b) on Mars. (Designs by MacKenzie, 1987.)

AT HOME IN A DOME

Habitation in a subsurface shopping mall is a big impr
ovement over living in Mars Direct-style tuna cans (my teen-age daughter Sarah would probably jump at the chance to live in a mall), but ultimately on Mars we can do better. We don’t have to burrow underground to protect ourselves from radiation (as on the Moon) because the Martian atmosphere is dense enough to shield people living on the surface against solar flares. The planet’s surface will be open to us, and, even during the base-building phase, large inflatable structures made of transparent plastic protected by thin, hard-plastic ultraviolet- and abrasion-resistant geodesic domes could be readily deployed, rapidly creating large domains for both human habitation and eventual crop growth. It should be noted in passing that even without the problems of solar flares and a month-long diurnal cycle, such simple transparent surface structures would be impractical on the Moon as they would create unbearably high temperatures inside. On Mars, in contrast, the strong greenhouse effect created by such domes would be precisely what is necessary to produce a temperate climate inside.

During the base-buildding phasee, domes of this type up to 50 meters in diameter and containing the 5 psi atmosphere necessary to support humans could be deployed. If made of high-strength plastics such as Kevlar (with a fabric yield stress of 200,000 psi—twice as strong as steel) such a dome one millimeter thick would be three times as strong as it needs to be to resist bursting and weigh only about 8 tonnes (including its subsurface hemisphere) with another 4 tonnes required for its unpressurized Plexiglas shield. (A habitation dome made of rip-stop Kevlar fabric is unlikely to fail catastrophically. Even if someone shot a large-caliber bullet through a 50-meter diameter dome, it would take over
two weeks
for the air to leak out, leaving plenty of time for repair.) In the early years of settlement, such domes could be imported prefabricated from Earth. Later on they could be manufactured on Mars, along with larger domes (with the mass of the pressurized dome increasing as the cube of its radius, and the mass of the unpressurized shield dome increasing as the square of the radius: 100-meter domes would mass 64 tonnes and need a 16-tonne Plexiglas shield, etc.).

The key problem with using domes is their foundations. The natural shape for a pressure-containing flexible container to assume is a sphere, as this spreads the load out everywhere equally. While a spherical shape is simple and robust, it does pose a daunting problem when used as the basis for a dome shelter, for you must undertake an enormous amount of excavation work to construct the dome. Imagine partially burying a beach ball in the sand such that its lower hemisphere is buried and its upper hemisphere exposed. To do so, you have to dig out a pit equal in size to the lower hemisphere. While that may seem trivial at the beach, it certainly wouldn’t be a trivial amount of excavation on Mars when you’re planning to erect a 50-meter dome. In the latter case, you would again excavate a pit and put your sphere in place, but you would then shovel the excavated material back inside to fill up the lower half of the sphere’s interior. The result would be a grand space 50 meters across and 25 meters from its dirt floor to the top of the dome (
Figure 7.2a
)—beautiful, but a lot of work because it would have entailed digging out and then replacing about 260,000 tonnes of material. Finding a natural crater of about the right size would give you a big head start, but it’s very unlikely that nature will provide you with one, let alone two or more, at your desired base site.