The Case for Mars, page 31
It can be seen that while it takes a very long time to reach significant depths, modest depths can be reached rather quickly. So, while it might take thousands of years to penetrate 200 meters to get a full 1,000 millibars out of the regolith, the first 100 millibars can be gotten out in just a few decades.
Once significant regions of Mars rise above the freezing point of water on at least a seasonal basis, the large amounts of water frozen into the regolith as permafrost would begin to melt, and eventually flow out into the dry riverbeds of Mars. Water vapor is also a very effective greenhouse gas, and since the vapor pressure of water on Mars would rise enormously under such circumstances, the reappearance of liquid water on the Martian surface would add to the avalanche of self-accelerating effects all contributing towards the rapid warming of the planet. The seasonal availability of liquid water is also the key factor in allowing the establishment of natural ecosystems on the surface of Mars.
The dynamics of the regolith gas-release process are only approximately understood, and the total available reserves of carbon dioxide won’t be known until human explorers journey to Mars to make a detailed assessment, so these results must be regarded as approximate and uncertain. Nevertheless, it is clear that the positive feedback generated by the Martian carbon dioxide greenhouse system greatly reduces the amount of engineering effort that would otherwise be required to transform the Red Planet. In fact, since the amount of a greenhouse gas needed to heat a planet is roughly proportional to the square of the temperature change required, driving Mars into a runaway greenhouse with an artificial 10° Kelvin temperature rise only requires about 4 percent of the engineering effort that would be needed if the entire 50° Kelvin rise needed to raise the Martian tropics above the freezing point of water had to be engineered by brute force. The question we shall now examine is how such a 10° Kelvin global temperature rise could be induced.
TABLE 9.1
Rate of Outgassing of Atmosphere from the Martian Regolith
METHODS OF ACCOMPLISHING GLOBAL WARMING ON MARS
The three most promising options for inducing the required temperature rise to produce a runaway greenhouse on Mars appear to be the use of orbital mirrors to change the heat balance of the south polar cap (thereby causing its carbon dioxide reservoir to vaporize); the mass production of artificial halocarbon (0;CFC”) gases in industrial facilities on the Martian surface; and the creation of widespread bacterial ecosystems capable of warming the planet through emission of large amounts of strong natural greenhouse gases such as ammonia and methane. We’ll look at each of these in turn. It may be, however, that the synergistic combination of several such methods may yield better results than any one of them used alone.40
Orbiting Mirrors
While the production of a space-based mirror capable of warming the entire surface of Mars to terrestrial temperatures is theoretically possible, the engineering challenges involved in such a task place such a project well outside the technological horizon of this book. A much more practical idea would be to construct a more modest mirror capable of warming a limited area of Mars by a few degrees. As shown by the data in Figure 9.1, a 4° Kelvin temperature rise imposed at the pole should be sufficient to cause the evaporation of the carbon dioxide reservoir in the south polar cap. Based upon the total amount of solar energy required to raise the temperature of a given area a certain number of degrees above the polar value of 150° Kelvin, it turns out that a space-based mirror with a radius of 125 kilometers could reflect enough sunlight to raise the entire area south of 70° south latitude by 5° Kelvin—more than enough. If made of solar sail-type aluminized mylar material with a density of 4 tonnes per square kilometer (about 4 microns thick), such a sail would have a mass of 200,000 tonnes. Many ships of this size are currently sailing the Earth’s oceans. Thus, while this is too large to consider launching from Earth, if space-based manufacturing techniques are available, its construction in space out of asteroidal or Martian moon material is a serious option. The total amount of energy required to process the materials for such a reflector would be about 120 MWe-years, which could be readily provided by a set of 5 MWe nuclear reactors such as might be used in piloted nuclear electric propulsion (NEP) spacecraft. Interestingly, if stationed near Mars, such a device would not have to orbit the planet. Rather, solar light pressure could be made to balance the planet’s gravity, allowing the mirror to hover as a “statite” with its power output trained constantly at the polar region.41 For the sail density assumed, the required operating altitude would be 214,000 kilometers. The statite reflector concept and the required mirror size to produce a given polar temperature rise are shown in Figures 9.8 and 9.9.
If the value of Td is lower than 20° Kelvin, then the release of the polar carbon dioxide reserves by themselves could be enough to trigger the release of the regolith’s reserves in a runaway greenhouse effect. If, however, as seems probable, Td is greater than 20°K, then the addition of strong greenhouse gases to the atmosphere will be required to force a global temperature rise sufficient to create a tangible atmospheric pressure on Mars.
Producing Halocarbons on Mars
The most obvious way to increase the temperature on Mars is simply to set up factories there to produce the strongest greenhouse gases known to man, the halocarbons or CFCs that many consider to be currently threatening the Earth with a dangerous greenhouse effect, and then release them into the atmosphere. Here on Earth CFCs are also blamed for the destruction of the ozone layer. However, if we choose our halocarbon geenhouse gases carefully and employ varieties lacking in chlorine, we can actually build up an ultraviolet-shielding ozone layer in the Martian atmosphere. One good candidate for such a gas would be perfluoromethane, CF4, which also has the desirable feature of being very long-lived (stable for more than 10,000 years) in an upper atmosphere. In Table 9.2 we show the amount of halocarbon gases needed in Mars’ atmosphere to create a given temperature rise, and the power that would be needed on the Martian surface to produce the required CFCs over a period of twenty years. If the gases have an atmospheric lifetime of one hundred years, then approximately one-fifth the power levels shown in the table will be needed to maintain the CFC concentration after it has been built up. The industrial effort associated with such a power level would be substantial, producing about a trainload of refined material every day and requiring the support of several thousand workers on the Martian surface. Power levels of about 5,000 MWe might be needed, which is about as much power as is used today by a large American city, such as Chicago. A total project budget of several hundred billion dollars might well be required. Nevertheless, all things considered, such an operation is hardly likely to be beyond the capabilities of the mid twenty-first century.
FIGURE 9.8
Solar sails of 4 tonnes/km2 density can be held stationary above Mars by light pressure at an altitude of 214,000 km. Wasting a small amount of light allows shadowing to be avoided.
FIGURE 9.9
Solar sail mirrors with radii on the order of 100 km and masses of 200,000 tonnes can produce the 5°K temperature rise required to vaporize the CO2 in Mars’ south polar cap. It may he possible to construct such mirrors in space.
The Biological Solution
The level of effort required by humans to greenhouse Mars could be reduced substantially, however, if biological assistants can be employed. This approach to terraforming has been championed by Carl Sagan ever since the 1960s, when he initiated the field of scientific terraforming speculation by suggesting that Venus might be made more habitable by seeding its atmosphere with algae that would consume the carbon dioxide in its atmosphere, thereby diminishing that planet’s hellish greenhouse effect.42 That idea probably will never work, but in more recent Mars studies, Sagan and his collaborator James Pollack have pointed out that bacteria exist that can metabolize nitrogen and water to produce ammonia.43 In addition to its minority presence in the atmosphere, nitrogen can likely be found on Mars in substantial amounts in regolith nitrate beds. Other bacteria can synthesize water and carbon dioxide into methane. Now, while not as good as halocarbons, both ammonia and methane are excellent greenhouse gases, thousands of times more powerful on a molecule-for-molecule basis than carbon dioxide. If an initial greenhouse condition were to be created by polar mirrors or CFC manufacture, thereby putting some liquid water into circulation, it may be possible that a bacterial ecology could be set up on the planet’s surface that would accelerate the process by producing large amounts of ammonia and methane. In fact, if 1 percent of the planet’s surface were to be covered by such bacteria, and we assume that they operate at about 0.1 percent efficiency in converting the energy of sunlight into chemical compounds, then around one billion tonnes of methane and ammonia would be produced each year. This is enough to warm the planet 10° Kelvin in about thirty years.
TABLE 9.2.
Greenhousing Mars with CFCs
As an added benefit, ammonia and methane will shield the planet’s surface against solar ultraviolet radiation. In the process, though, the ammonia and methane will be continuously destroyed, with a typical molecule having an atmospheric lifetime of several decades. The bacteria will constantly replace them, however. Also, as the planet warms and carbon dioxide outgasses from the regolith, Mars’ ozone layer will thicken, providing extra UV shielding to both the surface and the ammonia-methane greenhouse gases in the atmosphere. (Carbon dioxide contributes to ozone formation. In fact, Mars currently has an ozone layer about 1/60th as thick as Earth’s, which is pretty good when you consider that its atmosphere is only 1/120th as thick.)
In a matter of several decades, using a combination of these approaches Mars could be transformed from its current dry and frozen state into a relatively warm and slightly moist planet capable of supporting life. Humans could not breathe the air of the transformed Mars, but they would no longer require space suits and instead could travel freely in the open wearing ordinary clothes and a simple SCUBA type breathing gear. In addition, because the outside atmospheric pressure will have been raised to human tolerable levels, it will be possible to have enormous habitable areas for humans under huge dome-like inflatable tents containing breathable air. (The domes could be of unlimited size because, unlike the pressurized domes employed during the base-building phase, there would be no pressure differential between their interior and the outside environment.) On the other hand, simple hardy plants could thrive in the carbon dioxide-rich outside environment and spread rapidly across the planet’s surface. In the course of centuries, these plants would introduce oxygen into Mars’ atmosphere in increasingly breathable quantities, opening up the surface to advanced plants and growing numbers of animal types. As this occurred, the carbon dioxide content of the atmosphere would be reduced, which would cause the planet to cool unless greenhouse gases were introduced capable of blocking off those sections of the infrared spectrum previously protected by carbon dioxide. Providing these matters are attended to, however, the day would eventually come when the domed tents would no longer be necessary.
ACTIVATING THE HYDROSPHERE
The first steps required in the terraforming of Mars, warming the planet and thickening its atmosphere, can be accomplished with surprisingly modest means using in-situ production of halocarbon gases supplemented by helpful bacteria. The oxygen and nitrogen levels in the atmosphere, however, would be too low for many plants, and, if left in this condition, the planet would remain relatively dry, as the warmer temperatures would take centuries to melt Mars’ ice and deeply buried permafrost. It is in this second phase of terraforming Mars when the hydrosphere is activated, the atmosphere is made breathable for advanced plants and primitive animals, and the temperature is increased further, that space-based manufacturing of large solar concentrators is likely to assume an increasingly important role.
The use of orbiting mirrors provides a potentially rapid method for hydrosphere activation. For example, if the 125-kilometer radius reflector discussed earlier for relain vaporizing the pole were to concentrate its power on a smaller region, 27 terrawatts would be available to melt lakes (one terrawatt, or TW, equals one million megawatts). This is enough to melt 2 trillion tonnes of water per year (a lake 200 kilometers on a side and 50 meters deep). A single such mirror could also drive vast amounts of water out of the permafrost and into the nascent Martian ecosystem very quickly. The more rapidly water gets into circulation, the more action of denitrifying bacteria in breaking down nitrate beds to increase the atmospheric nitrogen supply and the spread of plants to produce oxygen will be accelerated. Activating the hydrosphere will also serve to destroy the oxidizing chemicals in the Martian regolith (which Viking showed are unstable in the presence of water), thereby releasing some additional oxygen into the atmosphere in the process. Thus, while the engineering of such mirrors may be somewhat grandiose, the benefits to terraforming of being able to wield tens of terrawatts of power in a controllable way can hardly be overstated.
OXYGENATING THE PLANET
The most technologically challenging aspect of terraforming Mars will be the creation of sufficient oxygen in the planet’s atmosphere to support animal life. While bacteria and primitive plants can survive in an atmosphere without oxygen, advanced plants require at least 1 mbar and humans need 120 mbar. While Mars may have super-oxides in its regolith or nitrates that can be heated to release oxygen and nitrogen gas, the process would require enormous amounts of energy, about 2,200 TW-years for every millibar produced. Similar amounts of energy are required for plants to release oxygen from carbon dioxide. Plants, however, offer the advantage that once established they can propagate themselves. The production of an oxygen atmosphere on Mars thus breaks down into two phases. In the first phase, brute-force engineering techniques supplemented by pioneering cyanobacteria and primitive plants are employed to produce sufficient oxygen (about 1 millibar) to allow advanced plants to propagate across Mars. Assuming three 125-kilometer radius space mirrors active in supporting such a program and sufficient supplies of suitable target material on the ground, such a goal could be achieved in about twenty-five years. Alternatively, a 1 millibar oxygen content could be added to the atmosphere in about a century through the action of photosynthetic bacteria. Either way, once an initial supply of oxygen is available, and with a temperate climate, a thickened carbon dioxide atmosphere to supply pressure and greatly reduce the space radiation dose, and a good deal of water in circulation, plants that have been genetically engineered to tolerate Martian regoliths and to perform photosynthesis at high efficiency could be released together with their bacterial symbiotes. Assuming that global coverage could be achieved in a few decades and that such plants could be engineered to be 1 percent efficient (rather high, but not unheard of among terrestrial plants) then they would represent an equivalent oxygen-producing power source of about 200 TW. By combining the efforts of such biological systems with perhaps 90 TW of space-based reflectors and 10 TW of installed power on the surface (terrestrial civilization today uses about 13 TW) the required 120 millibars of oxygen needed to support humans and other advanced animals in the open could be produced in about nine hundred years. If more powerful artificial energy sources or still more efficient plants (or perhaps truly artificial self-replicating photosynthetic machines) were engineered, then this schedule could be accelerated accordingly, a fact that may well prove a driver in bringing such technologies into being. It may be noted that thermonuclear fusion power on the scale required for the acceleration of terraforming also represents the key technology for enabling piloted interstellar flight. If terraforming Mars were to produce such a spinoff, then the ultimate result of the project will be to confer upon humanity not only one new world for habitation, but myriads.
A GIFT TO THE FUTURE
Witness this new-made World, another Heav’n
From Heaven Gate not farr, founded in view
On the clear Hyaline, the Glassie Sea;
Of amplitude almost immense, with Starr’s
Numerous, and every Starr perhaps a World
Of destined habitation ...
