The Case for Mars, page 25
Diborane, B2H6, 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, even 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 sources 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.
Photovoltaic panels may potentially be such a technology. As we have seen, the key material for the manufacture of such panels, pure silicon metal, can be produced on Mars, as can the aluminum or copper for their wiring and the plastics needed to insulate the wiring. In efforts to reduce costs, simplified methods of manufacturing solar panels in large single sheets have recently been developed for use on Earth, and such methods, transported to Mars, could well make large-scale local manufacture of photovoltaic systems feasible. It’s somewhat surprising, but it turns out that the performance of a photovoltaic panel on Mars is only modestly degraded when the Martian atmosphere is dusty.31,32 Except during very bad dust storms, the dust levels that typify the northern fall and winter skies scatter most of the Martian sunlight, but block little of it. Photovoltaic panels, unlike solar dynamic reflectors, have no regard for the direction of the incident light. Thus they should work fairly well on Mars all year round. Efficiencies are low, only about 12 percent, and you get no process heat beyond the system’s electric output, but that’s life. The panels’ performance may be significantly degraded by dust precipitating on them. This, however, can be remedied by human crews with brooms, or by manufacturing the panels with a windshield-wiper-type device attached to them.
As a further supplement to base power, wind is a possibility. Windmills have operated on Earth for centuries, and their low-tech nature makes them attractive potential items for Mars base manufacture. It’s true that the great dust storms are quite intermittent, and therefore useless as a real power source. Furthermore, the air is only 1 percent as thick as Earth’s, and the surface winds measured at the Viking sites were only about 5 meters per second (10 mph), implying negligible potential wind power. However, typical winds at altitudes well above the surface measure about 30 m/s (60 mph), which would create the same amount of power per unit of windmill area as a 6 m/s (12 mph) breeze on Earth. This would bquite acceptable for wind power generation. The key, then, to windmill practicality is how high off the surface the windmill must be placed in order to get above the stagnant surface boundary layer. At the present time this is unknown, and the answer is certain to vary locally in any case. However high this should turn out, it should be remembered that on Mars we would be erecting the windmill in a 38 percent gravity field—it may be practical to build windmill towers that Earthlings would consider outlandishly tall.
Generating Geothermal Power
Since about 1930 elementary and secondary boarding schools in the rural areas of Iceland have whenever possible been sited at locations where geothermal energy is available. In these centres the school buildings and living quarters for the pupils and staff are geothermally heated. They are also as a rule equipped with a swimming pool, and are self-supplying with vegetables (tomatoes, cucumbers, cauliflowers etc.) grown in their own hot houses. There are now many such schools in various parts of the country, and quite often they are used as tourist hostels during the summer holidays. Quite often these centres have formed the nuclei of new service communities in the rural areas.
—S.S. Einarson, Geothermal District Heating, 1973
Solar power and wind are means of potentially generating tens or hundreds of kilowatts of electricity with equipment of local manufacture. They are attractive because they can be deployed and set up almost anywhere, thereby allowing for the decentralized generation of power. This will be very useful on Mars, as the provision of some power to widely scattered assets will be necessary, and the infrastructure needed to transmit power over large distances won’t be available for some time. However, the relatively modest total outputs these power sources provide make it desirable to seek a more muscular option. As British scientist Martyn Fogg has pointed out,33 such an option is available on Mars in the form of geothermal power.
Geothermal power is generated by using the high temperatures that exist deep underground to boil a fluid such as water, and then using the steam produced to turn a generator turbine. On Earth, geothermal power is the fourth largest source of power, after combustion, hydroelectric, and nuclear, providing about 0.1 percent of all power used by humanity. The nation of Iceland gets the majority of its power—over 500 MWt—from geothermal heat. A single geothermal power well on Earth will typically generate about 1 to 10 MWe—small by terrestrial power station standards, but large compared to Mars base requirements. On Earth, geothermal power stations of this size can be up and running within six months of the start of drilling, and have a record of being on-line 97 percent of the time, a figure which is exceeded only by hydroelectric power. Furthermore, in addition to supplying large amounts of power, a geothermal station could supply a Mars base with something else equally valuable, a copious supply of liquid water. On Earth, geothermal power suffers the disadvantage that power generating stations must be positioned wherever the Earth in its whimsy has chosen to place a geothermal heat source, and, since we have already chosen the locations of our cities, this frequently presents a problem. On Mars, on the other hand, the cities have yet to be built. Given the value of a geothermal power/water supply, the discovery of such a source would probably dictate the location of the Mars base.
In short, geothermal power supplies would be enormously advantageous to Martian settlers. The question is, do ty exist? Perhaps somewhat surprisingly, the answer is almost certainly yes.
Large-scale volcanic features exist on Mars, for example in Tharsis, that have been dated to less than 200 million years old. About 4 percent of the planet’s surface (about 5 million square kilometers, mostly in the northern regions of Elysium, Arcadia, and Amazonia, as well as the equatorial Tharsis region) is classified by Mars geologists as “Upper Amazonian,” which means that it has been resurfaced by either volcanic action or flooding sometime within the past 500 million years. Now ages of 200 to 500 million years may seem like ancient history, but given Mars’ 4 billion year age, they actually qualify as “the present.” From a geological point of view on Mars, 200 million years ago is “today.” If volcanoes were active then, they are just as likely to be active now.
Furthermore, as we have seen, Mars possesses large supplies of water, with a liquid water table probably existing within a kilometer of the surface at least in some places. If an area was geothermally active in the recent past, this water could be hot enough to represent a practical power source.
If we consider only the upper Amazonian territories as viable candidates, and spread their formation equally in time across the 500 million years of that era, we find that 10 percent, or 0.5 million square kilometers, is probably less than 50 million years old; 1 percent, or 50,000 square kilometers, is probably less than 5 million years old; and 0.1 percent, or 5,000 square kilometers, has probably been active within the past 500,000 years.
You don’t have to extract geothermal power from a region that is actually volcanically active now. The ground stays hot a long time after activity has subsided. In his seminal paper on Mars geothermal power, Fogg presented calculations of the temperature profiles of Martian land as a function of the time since the region was active. His results are summarized in Table 7.2.
As a point of reference, the current state of the art of terrestrial drilling technology is to be able to drill down to about 10 kilometers. On Mars it should be easier to drill deeper, because the lower gravity will compact the soil less forcefully. It can be seen that the amount of territory that has had associated geothermal activity within the past five million years is quite large, and for these territories, wells just a few kilometers deep should be adequate to bring up very hot water. Once brought to the surface, the water would be flashed to steam and used to power a turbine to generate electric power. This will work even more efficiently on Mars than it does on Earth, because the low atmospheric pressure will allow the steam to be much more fully expanded before it is condensed. Some of the “waste water generated by this process will be tapped off to supply the base with as much water as it needs. The rest will be channeled back down into the well to replenish the subsurface aquifer.
TABLE 7.2
Characteristics of Mars Geothermal Fields
Geothermal energy cannot be generated on the Moon, it cannot be generated on asteroids. Of all extraterrestrial bodies in our solar system, only Mars has the potential to produce such a bountiful source of power to support human settlement.
The options for developing solar and wind power for outlying installation power, together with the use of geothermal energy for the main base load, indicate that once given a fair start with a nuclear reactor, a Mars base that has mastered an appropriate array of local resource utilization technologies can continue to expand its own power supply on the basis of its own efforts. The more power it has, the faster it will grow; the faster it grows, the more power it will have. Once it is possible to produce solar, wind, and especially geothermal power on Mars, the growth of the base will become exponential.
USE OF THE BASE TO SUPPORT LONG-RANGE MOBILITY ON MARS
While all this development is going on at the base, will our global exploration of Mars cease? Far from it. However well the base location is chosen, it is certain that some essential resources needed for its development will be available only at sites tens, hundreds, or thousands of kilometers distant. Global exploration for and transport of these resources will be an essential capability necessary for the growth of the base. In a symbiotic relationship, it will be the base itself that will create the capability for precisely such long-range mobility.
The situation is somewhat analogous to the development of human exploration of Antarctica. Prior to the International Geophysical Year in 1957, Antarctic exploration was conducted via a series of sorties, with each exploration party generally using its own ship as its base. Starting that year, however, a decision was made and implemented to build up a large permanently staffed base at McMurdo Sound. Today, this base provides facilities that allow the support of mechanized vehicles, helicopters, and aircraft that give Antarctic researchers access to every portion of the continent. By concentrating resources at a single point, a capability was created that affords much broader penetration than ever would have been possible by continuing a tradition of dog sled and ski sorties from individual exploration ships.
