The case for mars, p.23
The Case for Mars, page 23
So, if we are wise, we will position our base near where water is likely to be found. On Mars, this probably means the northern hemisphere. If you look at Mars today, you’ll see a large region of depressed topography in the Martian Arctic that contains very few craters. It is believed that in Mars’ early history, this vast basin was filled with water, which shielded the surface from meteor impact during the planet’s first billion or so years. The last remnant of this ancient ocean is the northern polar cap, which is made of water ice (about two million cubic kilometers of it, by current estimates24). In addition, we see from orbital images that the north boasts many more dry river beds and outflow channels than the south. It is likely that when these channels flowed their last, deposits of ice or permafrost were left at their mouths. These deposits may still exist, hidden from our view by a layer of dust. Measurements of atmospheric humidity taken from orbit also leave no doubt that the northern hemisphere is wetter than the southern, with the wettest time of the year being the northern spring. The existence of much larger amounts of water in the northern hemisphere’s past is also significant to future Martian colonists for another reason; hydrological activity is key to the formation of a large variety of mineral ores. If Horace Greeley had lived on Mars, his advice to young Martians seeking their fortune would have been simple—Go north.
There are a number of possible ways to get water on Mars. The first, most attractive, but most problematical method is simply to find it. As discussed in Chapter 6, there may be subsurface, geothermally heated pools of liquid water on Mars. Such pools could be detected within a kilometer of the surface by rover crews equipped with ground penetrating radar. The rover crews won’t have to search randomly. Low-resolution radar investigations conducted from orbit or from balloon-borne probes can identify in advance the best places to look. If we find such a pool and drill down to it, the hot pressurized water should come shooting out of the ground like a Texas oilfield gusher. Once it hits the low-pressure, cold Martian air, the water won’t stay warm for long. Depending upon its speed of ejection, it will probably freeze into ice crystals and fall back to the ground before it has gone a hundred meters. In no time at all a snow volcano could form, possibly of considerable size. Extracting the water in such a spectacular way would be rather wasteful though, because such a hydrothermal well could also represent a significant source of power. But as far as water access is concerned, next to siting the base over a hot artesian well, this is about as good as it gets.
Of course, things might not work out so well. Subsurface liquid water within drilling range may not be found. What then? Well, the next best thing would be to find brines. Saturated salt solutions can be liquid at temperatures as low as -55°C, which means that even without geothermal heat, such liquid brines, protected from evaporation by a modest layer of soil or ice, could exist on Mars today very close to the surface. In addition to being a good source of water, brines would be of great interest as candidate sites for finding extant Martian life. No brines have been identified on Mars as yet, but salts certainly have, and some scientists believe that light-colored features surrounding certain basins imaged on Mars may represent large salt deposits left behind on the shore lines of vanished Martian seas.
After brines, the next most interesting source of water on Mars would be ice. There are large deposits of water ice on Mars’ north polar cap, but that’s not where we are going to build our base. We see no large permanent deposits of ice south of 75° north latitude, but theory indicates that poleward of 40° N, underground ice should be stable within a meter of the surface. There may also be local anomalies. Where I live in Colorado, it can be winter on the north side of the house while it is summer on the south side, and it is not uncommon even on a blistering, mid-August day to come across snow nestled in a shady depression of a hill’s northern side. Without a doubt in some cold crevice, lava tube, or cavern on the north face of some hill on Mars there is ice to be found, and in regions where planetary-scale climate models say it can’t be. If you want to harvest some, though, bring dynamite. Ice at Martian temperatures can be pretty tough stuff. Still, a pure ice deposit in a nonpolar region would be a rare find. It’s much more likely that Martian explorers would come across permafrost, or frozen mud. Permafrost can be very strong. In fact for some applications it’s the ideal material for construction on Mars. A permafrost brick would be much stronger than a hot fired red clay brick, and you don#8217;t need an oven to make one or use mortar to get one brick to adhere to the next. Instant rock, just add water. Bring lots of dynamite.
So much for the heroic forms of Martian water-prospecting and mining. Let’s take a look now at some more mundane, industrial-style methods.
Martian soil has some water in it. We know that for a fact because at both Viking landing sites, random samples of soil scooped from the top 10 centimeters of the surface emitted about 1 percent of their weight in water when heated to 500°C. That’s not too bad, but in fact the test was unfairly skewed, because surface soil is the driest there is; the samples were heated for only 30 seconds; and furthermore, the samples were held in an unsealed vessel at 15°C for days before the test. Since 15°C is much warmer than average for Mars, the odds are very high that a significant amount of water was lost from the samples via outgassing prior to the test. On the basis of the Viking results, it would be a good bet that average Martian soil is at least 3 percent water. But some soils are likely to be much wetter than this average. For example, there are salts on Mars that typically contain up to 10 percent chemically bound water that can be released by heating to appropriate temperatures. Clays, which are common on Mars, also have excellent water adsorption capacities. For example, smectite clays have been found in SNC meteorites. Smectite clay is known commonly as “swelling clay,” because it can absorb several tens of percent of its weight in water and will swell in the process. The mineral gypsum (CaSO4 · 2H2O) has been also been found in many SNC meteorites. It’s likely that gypsum is quite common on Mars, because sulfur and calcium concentrations measured at both Viking landing sites were much higher (forty and three times, respectively) than their averages in soils on Earth. Gypsum can be over 20 percent water by weight.
Whether 3 percent or 20 percent, to get this water out of the soil all that is needed is heat. This can be done in one of two ways—either you bring the soil to the heater or the heater to the soil. The first of these methods is illustrated in Figure 7.3. A truck loaded with some relatively wet soil dumps its load onto a conveyor belt leading into an oven. The oven heats the soil to 500°C or so, causing the adsorbed water to outgas. The steam so produced is collected in a condenser, while the dehydrated dirt is dumped. The resulting “slag heap” is an inconvenience, but the energetics of this system aren’t too bad. If soil with a 3 percent water content is used as a feedstock, the energy required to run the system is about 3.5 kWh (kilowatt hours) of heat for every kilogram of water produced.25 At that rate a 100 kWe (kilowatt-electric) reactor could produce 700 kg/day of water if its electricity is used to power the oven, or up to 14,000 kg/day of water if the reactor’s waste heat is used to bake the dirt. (Thermoelectric generators used on today’s space nuclear power sources are only 5 percent efficient at turning their power into electricity; the other 95 percent comes out as “waste heat.”)
Alas, but there is that annoying waste pile of dried dirt. We could make 14,000 kg/day of water, but we’d be piling up 462,000 kg/day of desiccated “slag.” That might be acceptable—it’s only about 120 cubic meters, or six truckloads, of material. May Be we can use the slag for something; may be we can just dump it in a nearby crater.
FIGURE 7.3
Truck, oven, and slag pile system or extracting water from Martian soil. (Artwork by Michael Carroll.)
But, if you don’t want to move all that dirt around, the alternative is to bring the heater out to the field. One way that’s been suggested to do this is to have a mobile oven that wheels along, ingesting soil, baking it, condensing the steam, and ejecting the desiccated dirt as it travels.20 You probably wouldn’t want to use a nuclear reactor for such a system, but, instead, a radioisotope thermoelectric generator (RTG) such as has been used on Voyager, Viking, Galileo, and other outer solar system spacecraft. The standard RTG puts out 300 watts of electricity, enough to move the cart, along with 6 kW of waste heat, sufficient to produce 42 kilograms of water a day from 3 percent grade feedstock. Such a unit would be quite handy to small crews operating out in the field, or as an adjunct piece of equipment for early exploration missions (42 kilograms of water produced daily over the course of a single, 500-day Mars Direct mission surface stay adds up to 21,000 kilos of water), but its output is quite small relative to the needs of a large developing Mars base. Of course, we could produce all the water we need by operating a multitude of them, but all those RTGs would be expensive, and anyway we’d still be moving a lot of dirt, pebbles, and rocks around, with all the wear and tear on the involved equipment that implies. Is there a gentler approach?
One way might be for the cart to use a microwave device to heat the soil below it. This would cause the water in the soil to vaporize and rise up as steam. The cart would carry a kind of canopy with a flexible skirt brushing the ground all around it. This skirt would act as a sufficiently good seal to hold the water vapor until most of it frosted out on the canopy roof, after which it could be collected for use. The advantage of this scheme is that no digging is required, and furthermore, microwaves can be tuned so that they put most of their energy into heating the water molecules, instead of wasting power by heating water and dirt indiscriminately. Unfortunately, the rising vapor will transfer heat to the soil, so that in the end much of the heat ends up wasted anyway (although not as much as in a purely thermal heating system). The problem, however, is that the microwave power input must be electrical, not thermal. The 6,000 W of waste heat the RTG produces can’t be used to drive the system, only the unit’s 300 W of electrical output. Thus, even if one watt of microwave power should prove twice as efficient as thermal power in driving water out of soil, you still come out with only one tenth the output because thermal power is twenty times more available. If the water concentration was very high, however, and the ground too strong to break (as would be the case with permafrost) this system might work better than a mobile digger, though its output would still be rather low. For example, let’s assume we operate such a system over a permafrost deposit that is 30 percent water by weight. We estimate that about 1 kWe-hr would be needed to extract each kilogram of water. So, over the course of a Martian sol (24.6 terrestrial hours), the microwave cart driven by a 300-watt RTG could extract about 7.4 kilograms of water. The only way to improve on this performance would be to apply a lot more power, perhaps by connecting the cart by a long cable to the base’s nuclear reactor and applying 100 kWe. In that case, 2,200 kilos of water per day could be produced, but mobility would be lost.
FIGURE 7.4
Mobile methods of extracting water from Martian soil: (a) soil eater on wheels; (b) mobile microwave system with shirt; (c) port greenhouse dome with condenser. (Artwork by Michael Carroll.)
I think a better solution would be to put a transparent tent over a selected area of Martian terrain and warm the inside via the greenhouse effect that would occur naturally within. The greenhouse heating could be augmented by positioning large, lightweight reflectors around the tent, and moving them with the Sun to maximize the solar heating of the enclosed area. Inside the tent, the soil would be heated, not to 500°C certainly, but far above its average temperature. This would cause a fraction of the adsorbed water the soil contains to out-gas, and the moisture released could be captured as frost on a cold plate kept refrigerated in one corner of the tent (just like the frost build-up in your freezer). To see how effective such a system might be, consider that the average solar incidence on Mars is 500 watts per square meter (W/m2). If the tent is a hemisphere 25 meters in diameter, and the tent greenhouse plus reflector arrangement causes the equivalent of an extra 200 W/m2 of heating to occur within the tent, the total effective power of the system would be 98 kW This is enough to release 224 kilograms of water from 3 percent-grade soil in the course of an eight-hour day. This amount of water would be available within the first half centimeter of soil within the tent. Made of 0.1 mm thick polyethylene, the tent would have a mass of only 100 kilograms (and therefore weigh 38 kilograms on Mars), so it could be carried about by rover crews to a new position every day. After the tent moved on, the mined surface soil would rehydrate itself naturally, allowing the same field to be repeatedly “farmed” for water.
A completely different approach would entail extracting water from the Martian atmosphere. The problem here is that the air on Mars is very dry—under typical conditions you have to process one million cubic meters of Martian air to acquire one kilogram of water. In a classic paper, engineer Tom Meyer and Mars scientist Chris McKay proposed a mechanical compressor system capable of doing just that.26 The authors found that every kilogram of water produced would require about 103 kWh of electrical energy. Comparing this result to the figures for the soil-based water-extraction systems described above (about 3.5 kWh of thermal energy per kilogram), it certainly seems unattractive, although it should be pointed out that the compressor system will also produce a lot of useful argon and nitrogen from the atmosphere for base life support. More recently, however, Adam Bruckner, Steven Coons, and John Williams of the University of Washington undertook a study in which instead of compressing the air, they simply employed a fan to blow it against a zeolite sorption bed.27 Zeolite is an extreme desiccant, and can be used to reduce atmospheric water vapor concentrations to a few parts per billion, well below even Martian humidity. At Martian temperatures, zeolite will adsorb up to 20 percent of its weight in water. Once the zeolite is saturated, you can bake the water out at an energy cost of about 2 kWh of thermal energy per kilogram, after which the now desiccated zeolite can be used again. Since all you have to do is move the air, not compress it, the mechanical fan power is much less than the pump power needed by the Meyer and McKay system, perhaps requiring another 2 kWh of electrical energy per kilogram of water processed. Energy costs are thus comparable to the systems based upon soil water extraction. The main problem with any atmosphere-based water-extraction system on Mars, however, is that it must be rather large to achieve a useful level of output. For example, a system pairing an intake ductth a 10 square meter cross-sectional area and a fan capable of generating an intake air speed of 100 meters per second (close to 200 miles per hour) would produce just 90 kilograms or so of water per day. However, since the machine does not need to be mobile, the 8 kWe needed to run the fan could readily be provided by the base power supply. This, taken together with the facts that no digging or prospecting is required, the system is susceptible to complete automation, and the raw material, air, is infinitely renewable, may ultimately make such atmospheric water extraction systems quite attractive.
All in all, while there may not be enough water available on Mars to support Lowell’s visions of water-bearing canals criss-crossing the planet, there is certainly enough available to support a Mars outpost. No doubt much of the water tapped from Mars’ arid environs will go to adding a touch of green to the Red Planet.
GREEN THUMBS FOR THE RED PLANET
Given the costs of interplanetary transportation, it is obvious that if significant human populations are to settle on other worlds, they will eventually have to grow their own food. In this respect, Mars stands at an enormous advantage to the Earth’s Moon and every other known extraterrestrial body. Of the four main elements comprising organic matter—hydrogen, carbon, nitrogen, and oxygen—all are readily available on Mars. It’s been argued that asteroids are likely to contain carbonaceous material, and some evidence has been presented from the Clementine mission indicating that the Moon may harbor ice deposits in permanently shaded areas near its south pole. But these arguments miss the point, because the biggest problem with the Moon, as with all other airless planetary bodies and proposed artificial free-space colonies (such as those proposed by Gerard O’Neill28) is that sunlight is not available in a form useful for growing crops. This is an extremely important point and it is not well understood. Plants require an enormous amount of energy which can only come from sunlight. For example, a single square kilometer of cropland on Earth is illuminated with about 1,000 MW of sunlight at noon, a power load equal to an American city of one million people. Put another way, the amount of power required to generate the sunlight responsible for the crop output of the tiny country of El Salvador exceeds the combined capacity of every power plant on Earth. Plants can stand a drop of perhaps a factor of five in their light intake compared to terrestrial norms and still grow, but the fact remains: The energetics of plant growth make it inconceivable to raise crops on any kind of meaningful scale with artificially generated light. That said, the problem with using the natural sunlight available on the Moon or in space is that it is unshielded by any atmosphere. (The Moon has an additional even more intractable problem with its twenty-eight-day light/dark cycle, which is completely un acceptable to plants.) Thus, plants grown in a thin-walled greenhouse on the surface of the Moon or an asteroid would be killed by solar flares. In order to grow plants safely in such an environment, the walls of the greenhouse would have to be made of glass 10 centimeters thick, a construction requirement that would make the development of significant agricultural areas prohibitively expensive. Using reflectors and other light-channeling devices would not solve this problem, as the reflector areas would have to be enormous, essentially equal in area to the crop domains, creating preposterous engineering problems if any significant acreage is to be illuminated.
