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Dosya Açıklaması : . Introduction From the perspective of biology, planetary engineering is the ability to alter the environment of a planet so that terrestrial organisms can survive and grow (McKay, 1982). The feasibility of altering planetary environments is clearly demonstrated by mankinds activities on the Earth (Levine, 1991; Fogg, 1995a) and it is increasingly apparent that in the near term future mankind will gain the technological capability to engineer the climate of Mars. Current thought experiments/proposals for the planetary engineering of Mars differ in their methodology, technical requirements, practicality, goals and environmental impact (reviewed and discussed by Fogg, 1995b). The planetary engineering of Mars may be divided into two distinct mechanistic steps, ecopoiesis followed by terraforming. Ecopoiesis, a term derived by Haynes (1990) which, when applied to Mars, can be viewed as the creation of a self-regulating anaerobic biosphere. On the other hand, terraforming refers to the creation of a human habitable climate (discussed in Fogg 1995b). Whether the creation of such biospheres are possible is not known (Fogg, 1989; Pollack and Sagan, 1993; Fogg, 1995b). However, the majority of these planetary engineering models invoke the use of biological organisms, both during alteration of the planetary environment and in the regulation of the resulting biosphere. This article will briefly review the implications of the current Martian environment and assets for biology and then discuss the relationship between biology and planetary engineering. II. Current Martian environment and implications for biology At present the Martian surface environment is effectively sterilizing for all forms of terrestrial organisms (Rothschild, 1990; Mancinelli and Banin, 1995; Dose et al. 1995), although some protected niches may exist above and below the surface of Mars (Friedmann, 1986; Thomas and Schimel, 1991; Boston et al. 1992; Rothschild, 1990, 1995). The properties of the Martian environment that would preclude the survival and growth of terrestrial organisms are as follows (but see also McKay (1982); Rothschild (1990); Banin and Mancinelli, (1995); Mancinelli and Banin (1995)): 1. Low pressure. The atmospheric pressure on Mars (Table 1), mostly due to carbon dioxide, varies from approximately 7.4 to 10 millibar (mbar) (Hess et al. 1980). Extremely low pressure damages organisms and can affect efficient DNA repair (Ito, 1991; Koike et al. 1991). 2. Low temperature. The average diurnal temperature ranges from approximately 170 K to 268 K. During the Martian summer the temperature perhaps rises above the freezing point of water at some equatorial latitudes. From temperature requirements alone, organisms would not be able to survive on present day Mars for a number of reasons: First, the temperatures would completely freeze any organism and depending on the freezing process would cause cellular damage through the formation of ice crystals. Second, such low temperatures would raise the activation energy for enzyme catalyzed processes and thus inhibit biochemical/metabolic reactions. Third, biochemical reactions occur in solution and the transport of metabolites would not occur efficiently in a ice crystals. 3. Water. Liquid water which is a prerequisite for life (McKay, 1991; McKay and Stoker, 1989), under the current Martian atmospheric pressure is unstable. Such extreme dry conditions would cause dehydration, for example damaging DNA (Dose et al. 1995) and leading to mutation and cell/organism death. 4. Radiation. The main source of radiation at the Martian surface is ultraviolet (UV) radiation between the wavelengths of 190 and 300 nm. UV-radiation can be lethal. It is absorbed by nucleic acids (i.e. DNA) and activates the chemical formation of various adjuncts that inhibit replication and transcription of DNA. In the absence of an ozone layer, organisms can only escape the lethal affects of UV-radiation by living in protected habitats. Even those surface organisms which have efficient DNA and cellular repair enzymes would probably perish. 5. Oxidants. Due to the continuous bombardment of the Martian surface with UV-radiation the topmost layer of the regolith is thought to contain strong oxidants which are damaging for cellular components. 6. Carbon dioxide. As mentioned previously the major atmospheric component is carbon dioxide (Table 1). In organisms the relatively high concentration of carbon dioxide would probably cause a low intracellular pH. i.e. acidosis which may be damaging for cellular proteins, cellular components and metabolism (Hiscox and Thomas, 1995). 7. No organic material. Because of the continuous bombardment of UV-radiation and oxidizing conditions, no organic material will be present on the Martian surface (Bullock et al. 1994 and references there in). Table 1. Mars-atmospheric composition and partial pressure of the most abundant gases. (Data from Fogg 1995c, Hiscox 1995 and references therein). III. Biologically useful Martian resources Undoubtedly the current Martian environment is extremely hostile for terrestrial life. However, Mars does contain sufficient volatiles to enable some form of colonization and perhaps planetary engineering to render environmental conditions more clement for terrestrial life to survive and grow (Meyer and McKay, 1984, 1989; McKay et al. 1991a; Fogg, 1995c; Zubrin, 1995). Analysis of Martian soil and shergottites, nakhlites and chassignittes (SNC) meteorites (believed to have been ejected from Mars (Mustard and Sunshine, 1995 and references therein)) has shown that all of the elements necessary for carbon based life on Earth are present on Mars (Dreibus and Wanke, 1987; Gooding, 1992; Banin and Mancinelli, 1995). It is evident that Mars once possessed a more clement climate and many observable surface features have been attributed to the presence of liquid water and a dense carbon dioxide atmosphere (Carr, 1986; 1987). Many planetary engineering scenarios (see Fogg, 1995c and references there in) propose that it may be possible to return Mars to an earlier such climate using planetary engineering techniques (with the proviso that such volatiles are still present). Fogg (1995c) suggests that unless impact erosion (Melosh and Vickery, 1989) "blasted" the atmosphere into space then huge quantities of volatiles are still likely to reside on the planet. Over geological history Mars may have lost more volatiles than it gained. For example, water may also have been lost by hydrodynamic escape, atmospheric spluttering and other mechanisms (refer to Carr, 1987; Jakosky, 1991; Kass and Yung, 1995). Therefore returning Mars to a past climatic state may not be possible, and clearly given the climatic history of Mars such a climate maybe geologically unstable and undesirable for the extreme long term habitability of the planet. A number of compounds and elements are absolutely required for life; liquid water, the so called CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorous and sulfur) are the main elements which constitute amino acids (which make up proteins) and nucleotides (which make up DNA and RNA) and various minerals are also required. All of these elements/compounds are believed to be present on Mars (Banin and Mancinelli, 1995). The amount and location of these resources on Mars is briefly reviewed below. For a more in depth reviews refer to Fogg (1995b,c); Meyer and McKay, 1989, 1991a; and Banin and Mancincelli (1995). 1. Water. Currently, the surface of Mars is devoid of liquid water and the atmosphere only contains minute amounts of water vapor (Table 1)(Carr, 1987). The two main sources of remaining water on Mars are thought to be the north polar cap and the regolith. The quantity of water on Mars is uncertain, and estimates range in order of magnitudes, equivalent to a layer of water over the planet 13 meters (m) to 100 m (Squyres and Carr, 1986). The north polar cap is composed mainly of water ice (Kieffer et al. 1976). The equatorial regions of Mars appear to be ice poor whereas the heavily cratered terrain pole-ward of ± 30° latitude appears to be ice rich (Squyres and Carr, 1986), with perhaps a conservative estimate of the equivalent of 17 m of ice spread over the surface of Mars (Jankowski and Squyres, 1993). How much liquid water would be necessary, or indeed liberated by either ecopoiesis and/or terraforming has not been determined. However, based on current data, a detailed model for the hydrological cycle on Mars has been proposed (Clifford, 1993) and perhaps this could be adapted for modeling the hydrological cycle during ecopoiesis/terraforming. Mars will probably never be a wet planet as it might have been in the past (Carr, 1986; 1987), although the view that Mars was "warm and wet" is uncertain and perhaps "cold and icy" may be more appropriate (Kasting, 1991; Squyres and Kasting, 1994). However, there will probably be sufficient water for some type of a biosphere to be established. For certain, the water requirement for ecopoiesis will be several orders of magnitude less than that for a terraformed biosphere. Ultimately, it may be possible to import water onto Mars, for example by the redirection of ice asteroids into the Martian atmosphere to release their volatile components (see Fogg, 1995b). However, although such proposition might be technically feasible, the number of asteroids needed to be diverted is very large. 2. Buried organic material. Bullock et al. (1994) estimate that organic material, either deposited by meteorites and/or remains from an earlier biosphere, maybe between 3 and 40 meters from the surface or perhaps be present in polar regions (Bada and McDonald, 1995). These deposits could therefore be utilized by plants that have long root systems and/or by subsurface microorganisms. However, such scenarios depend on how long it would take thermal waves to penetrate through the ground during planetary engineering. 3. Carbon. On first inspection the two main sources of "trapped" carbon dioxide are as a solid in the polar caps and adsorbed in the regolith. These sources are thought to exchange between 10 and 100 times the current atmospheric pressure of CO2 via the atmosphere and are thus thought to regulate climate change on Mars (Fanale et al. 1982). The permanent cap at the south pole is thought to contain at the most around 10 mbar of CO2 (Fanale and Cannon, 1979) (however this figure is uncertain). Due to the uncertainty in the extent of the Martian regolith, the total mineral surface area exposed to the Martian atmosphere is not known. However, laboratory simulations of the simultaneous adsorption of H2O and CO2 (Zent and Quinn, 1995), where palagonite is used as an analogue of the Martian regolith (Zent et al. 1987), would appear to confirm that the current absorbed inventory of CO2 is 30-40 mbar. An even greater source of CO2 may be combined in the form of carbonate. Carbonates would have been formed by CO2, present in the early Martian atmosphere, dissolving in water and combining with cations such as Ca2+, Fe2+ and Mg2+ and subsequent precipitates forming carbonates (refer to McKay and Nedell, 1988 and references there in). Warren (1987) suggests that the regoliths low .......