In his groundbreaking book Terraforming: Engineering Planetary Environments, published in 1995, British researcher Martyn Fogg has argued that a synergistic approach will need to be adopted in order to make Mars habitable. In this sense Fogg argues that no one atmospheric heating and thickening process is capable of making Mars fully habitable for free-walking humans, and that multiple processes will have to be run either in parallel or in appropriate order. To date, no detailed synergistic program has been developed for Mars, but Fogg, for example, suggests that a three-phase approach might be followed, with each phase having its own distinct set of goals, the completion of which could be achieved on various long and short timescales. The three main phases for making Mars habitable are:
1. a nonbiological (anaerobic) warming phase
2. a habitability-making phase
Phase 1 will see the initiation of all, or a selection of, the processes already described in this chapter: polar heating with a statite, in situ production of super-greenhouse gases, comet/asteroid impact delivery of greenhouse gas, albedo reduction, and so on. The expression ecopoiesis, meaning the generation of an open, anaerobic biosphere, is often used to describe these first steps of terraforming. The aim of this ecopoiesis phase is to create the atmospheric conditions that provide a surface temperature greater than 273 K over a sizable fraction of the Martian surface, and a surface pressure that will allow liquid water to be stable and, indeed, will foster the growth of a northern hemisphere ocean.
Opinions differ as to whether a full CO2 runaway process should be initiated in order to achieve the goals of the ecopoiesis phase. It is certainly the case that if the CO2 runaway process is allowed to proceed, then with only a small amount of additional heating a warm, wet Mars will be brought about. The problem, however, is that a CO2-rich atmosphere will have been produced, and while various plants and algae could thrive under such conditions, the atmosphere would be lethal to humans (and any other higher life forms). Fogg suggests that rather than allowing the CO2 runaway process to dominate the Phase 1 heating phase, an atmosphere with the following partial pressures might be aimed for: P(CO2) = 350 millibars, P(N2) = 10 millibars, P(O2) = 20 millibars, and P(H2O) = 25 millibars. Such an atmosphere, while still not breathable by humans, would result in an equatorial temperature of about 275 K and above-freezing temperatures over a 10°-band centered on the Martian equator (based upon the model described in the Appendix of this book). The point that Fogg is making is that eventually, if Mars is ever going to move beyond a cloistered city world (as described in Chapter 2) upon which human beings might freely move about, then the atmospheric CO2 partial pressure will have to be brought below 10 millibars. It is in this sense that the smaller the quantity of CO2 that is released in order to achieve the end of Phase 1, the better.
By necessity, the first Martian colonists will be constrained to live in self-contained, self-sufficient, and self-regulating housing (see Figure 6.18). Such quarters, however, will be natural extensions of the systems that already exist on, for example, the International Space Station (ISS). In contrast to living in low-Earth orbit, as the ISS crew do, Martian colonists will need to grow all their own fresh food. Indeed, creating the conditions suitable for a crop management cycle will be of paramount importance during the Phase 1 terraforming process.
Preliminary studies have already been conducted with respect to growing cereal crops in CO2-rich atmospheres. Indeed,
researchers at the university of Guelph, in Ontario, Canada, in conjunction with the Canadian Space Agency and NASA, are studying advanced life-support systems and greenhouse technology (these are real greenhouses, not the gases) on Devon Island, a Mars analog site located off the Arctic coast of Canada.
The biological fertility of Martian soil has also been studied, by Michael Mautner (Lincoln University, New Zealand). Mautner finds, in fact, that asparagus and potato tissue cultures can be successfully grown in pulverized Martian meteorite soil. He also finds that the biological fertility of Martian basalts is greater than that of terrestrial basalts. Not only can plants thrive in laboratory analogs of Martian regolith, but so, too, can micro-bacteria, a result that prompted Mautner to write in his 2004 book Seeding the Universe with Life—Securing our Cosmological Future, that ''Microorganisms pioneered life on Earth, and similarly, they can pioneer life on new planets and establish ecosystems suitable for humans.''
Not only can plant cultures grow in soil composed of Martian meteorites, but they can also grow, although with a lower fertility yield, in soils made from pulverized carbonaceous chondrite meteorites (see Figure 3.9). This latter result may turn out to be particularly useful, since the two moons of Mars, Phobos (see Figure 6.13) and Deimos, are both rich in carbonaceous material, and this makes them ideal as mining sites to produce a rich fertilizer for the early Martian colonists. Remember also, as discussed earlier, that something will have to be done about Phobos and Deimos anyway, since they are currently destined to crash into the Martian surface in several hundred million years' time. If these moons are not utilized as part of the regolith devolitization process, then they might usefully produce soils for the first Martian crops. Some crops might also be produced on the moons themselves and then transported to the Martian surface.
The propagation of plant species and the introduction of microorganisms will, with little doubt, play an important role in the Phase 1 (and after) alteration of Mars. With respect to the initial plant colonization phase, the process will mimic that observed in the Earth's mountainous regions. Specifically, as terraforming proceeds, so the Martian regolith will experience a gradual warming and increase in surface pressure; this combined change is similar to that experienced in descending a tall mountain on the Earth (recall Figure 5.4). At the top of a high mountain, the nival region, nothing can grow, but below about 3,000 m a tundra region begins to open up where hardy lichens and a selection of cold-adapted microorganisms can thrive.
As on the Earth, so too on Mars the lichens will be one of the first pioneer species. Importantly, lichens are highly resistant to UV radiation (which on the Phase 1 Mars will be high due to the lack of any ozone layer), and they excrete acids that dissolve rock minerals, an action that will aid in the generation of an organically rich surface soil on Mars. Lower down the mountain slopes, we encounter the analog to the Phase 2 terraforming stage with the appearance of more complex and diverse microorganisms, along with flowering plant and various conifer and deciduous tree species.9
Phase 2 is concerned with the process of making Mars habitable for a wide range of biota and free-moving human beings over larger and larger regions of the planet's surface. This will primarily require the production of an atmosphere rich in nitrogen and oxygen. Phase 2 will also see the final generation of two large bodies of standing water: the northern boreal ocean and a near-circular southern hemispheric ocean located in the mighty Hellas Planitia (see Figure 6.8, top image).
To make the Phase 2 atmosphere of Mars breathable for humans, the Phase 1 abundances will require considerable alteration. The partial pressure of oxygen will need to be increased by a factor of about 5, while those of nitrogen and carbon dioxide will need to be increased and reduced, respectively, by a factor of about 30. The increased oxygen and nitrogen abundances are required for human respiration as well as allowing for the growth of a temperate ecosystem containing flowering plants, grassland regions, forests, multiple microorganisms, and diverse animal life forms. The oxygen increase will be driven by microbial activity, such as that due to cyanobacteria, and through planet photosynthesis. Usefully at this stage of the terraforming process, the oxygen is being produced in a nonreducing environment, and it will therefore go directly into the atmosphere. The generation of oxygen will not only be important for making the atmosphere of Mars breathable, it will also produce an ozone layer that will act to reduce the surface UV flux (just as it does on the Earth).
Mars currently supports a nitrogen partial pressure of 0.15 microbars, and this will have to be significantly increased during the Phase 2 stage of terraforming. It seems highly likely that Mars' initial inventory of atmospheric nitrogen was much higher than it is today, its loss being precipitated through oxidation to produce nitrate (NO3), which currently resides in the regolith. The denitri-fication of the regolith can be brought about through bacterial action in an aqueous environment, whereby the nitrate is initially converted into nitrite (NO2). Other bacteria then reduce the nitrite to nitric oxide (NO) and nitrous oxide (N2O), with the eventual release of nitrogen gas (N2). Furthermore, other microorganisms can assimilate nitrate to produce ammonia (NH3), and then the nitrate in the regolith can be reduced and the partial pressure of nitrogen increased. Importantly, once the partial pressure of nitrogen exceeds 5 microbars, then nitrogen fixation can begin, and a closed biochemical nitrogen cycle will become established.
Although the Phase 2 stage of nitrogen production will most likely proceed through biotic activity, the initial Phase 1 increase might require the direct importation of nitrogen. Usefully, the atmosphere of Saturn's largest moon, Titan (Figure 3.6) is nitrogen rich, and transport and extraction problems aside, this exotic satellite might play an important role in making Mars habitable. The reduction in atmospheric CO2, a vital part of the Phase 2 stage, might also proceed biologically through the growth of bryophytes, such as mosses. Indeed, their role will be to sequester carbon dioxide in decay-resistant organic compounds in Martian peat lands. Conditions compatible with the eventual appearance of trees will further enhance the biological reduction of atmospheric CO2.
Since the CO2 abundance must be reduced during Phase 2, alternate super-greenhouse gases will need to be added to the atmosphere in order to keep the surface temperatures above the freezing point. It is not beyond the realms of possibility that microorganisms capable of producing the desired greenhouse gases might be genetically engineered and released into the Martian regolith. The insolation might also be increased at this stage by the placement of large or multiple statite mirrors in close Martian proximity, or by the introduction of dark, surface-growing lichens and microbes.
The onset of the stewardship, Phase 3, stage of Martian terra-forming will in some sense mark the end of the terraforming process. Once this stage begins, Mars will be fully, or at least mostly, habitable. The planet will be able to support larger and larger numbers of people in step with the large-scale growth and development of surface agriculture and manufacturing industries. A long-lived, zero-maintenance, life-supporting atmosphere on Mars, however, will never be fully realized, and a terraformed Mars will require continuous monitoring and stewardship. We will not acquire the new Eden on Mars for free, and this will place great responsibility upon our descendants. Indeed, they will need to avoid all of the pitfalls and fallacies of the industrio-political ideologies that have dominated the workings of the world in recent (if not historical) times.
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