The Case for Mars (36 page)

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 th
e northern fall and winter skies scatter most of t
he 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
s
ites 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 sub
surface 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 dis
tant. 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.

The terrain on Mars is far rougher than even Antarctica. To have true long-range mobility on Mars it is necessary to be able to fly. While balloons and perhaps subsonic aircraft can be used to loft small robotic packages across the windy Martian skies, the only systems trustworthy enough for human transport will be rocket powered vehicles that can punch through any weather. These could be either purely ballistic vehicles that leap out of the Martian atmosphere to travel from one side of the planet to the other, or winged rocketplanes capable of supersonic flight. Both of these types of systems are by their nature propellant hogs, and the operation of either would be unthinkable unless the large amounts of propellant required are manufactured on Mars.

For example, consider a Mars piloted ballistic hopper with a mass of 10 tonnes powered by methane/oxygen rocket engines with a specific impulse of 380 seconds. Let’s say we want to fly it 2,600 kilometers (which is 45 degrees of latitude or longitude across Mars), land, and then eventually return with no added cargo. In order to perform this maneuver the vehicle will need a mass ratio of about seven so the total propellant needed will be 60 tonnes. If we were to do it with a 15-tonne rocketp
lane (its wings will make the plane heavier) with a supersonic glide lift-to-drag (L/D) ratio of four, the mass ratio would be about five, so again 60 tonnes of propellant would be needed. It’s clear that there is no way these sorts of vehicles are going to be doing much flying on Mars if their methane/oxygen propellant, or even the hydrogen feedstock required to manufacture it, must be transported from Earth.

The need to carry enough propellant for both the outbound and return legs of the exploratory sortie constrains the maximum range of chemical-propelled rockets on Mars to about 4,000 kilometers. These limits can be overcome if the vehicle can make its own propellant after it lands. Chemical bipropellants don’t allow this because their manufacture requires too much power (about 5 kWh per kilogram), and therefore too large a power supply to make such a system flight-mobile. A few years ago, however, I came up with a vehicle concept I called a “NIMF” (for nuclear rocket using indigenous Martian fuel) that can overcome this problem.
³⁴
,
³⁵
In the case of the NIMF, raw carbon dioxide from the Martian air
is used
as a propellant, and is heated by an onboard nuclear thermal rocket (NTR) engine to create a hot rocket exhaust. Because NTRs don’t convert their heat into electricity, all the power conversion gear that actually forms most of a nuclear power reactor’s mass is eliminated, allowing these systems to be small and lightweight. Because the propellant is simply raw carbon dioxide, which can be acquired at low energy cost (less than 0.3 kWe-hrs/kg) through direct compression out of the atmosphere, not much onboard electric power is needed, and all the chemical synthesis gear is eliminated as well. Hot carbon dioxide is not a high-class rocket propellant, and specific impulses of about 260 seconds are all you can expect. But a prospector needs a mule that can eat mountain scrub; high-strung race horses that will eat only gourmet fodder are of little use in the hills. The NIMF is the ideal exploration craft because it can live on what it finds in the field. Rocket vehicles equipped with this type of propulsion system would give Mars explorers complete global mobility, allowing them to hop around the planet in a craft that can refuel itself each time it lands. NIMF ballistic hoppers and rocket-planes are depicted
in the plates.