Astroecology of Asteroid and Martian Soils

Samples of carbonaceous asteroids and Martian rocks are available in meteorites. We found that microorganisms can use organic compounds in the carbonaceous meteorites/asteroids as carbon sources. Chemical analysis also found bio-available mineral nutrients: N (Nitrate), (P (phosphate), S (sulfate), K (potassium), and micronutrients in these materials, that were found to support algae, potential colonizing organisms. Further experiments showed that potato and asparagus cultures grow in these materials. Therefore, asteroid and Martian soils sustain a viable soil ecology and provide food for space colonies and Martian terraforming.

Further details are presented in the paper below and in the references therein.
 

Links

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Technical Paper on Resources for Life in Space:  

 Planetary Resources and Astroecology - Implications for Space Populations and Panspermia

Astrobiology 2002,2,59-76

Michael N. Mautner, Soil, Plant and Ecological Sciences Division, Lincoln University, Lincoln, New Zealand and Department of Chemistry, University of Canterbury, Christchurch 8002, New Zealand m.mautner@solis1.com

 
Table of Contents

Abstract
Introduction
Materials and Methods
Results: Meteorite Extracts
Results: Microbiology
Planetary Microcosms
Origins of Life and Past Microbes in Asteroids
Microbial Populations in Meteorite Solution
Asteroids as Space Resources
Natural and Directed Panspermia
Space Agriculture
Summary and Conclusions
References

Abstract

Planetary microcosms were constructed using extracts from meteorites that simulate solutions in the pores of carbonaceous chondrites. The microcosms were found to support the growth of complex algal and microbial populations. Such astroecology experiments demonstrate how a diverse ecosystem could exist in fluids within asteroids and in meteorites that land on aqueous planets. The microcosm solutions were obtained by extracting nutrient electrolytes under natural conditions from powders of the Allende (CV) and Murchison (CM2) meteorites at low (0.02 g/ml) and high (10.0 g/ml) solid/solution ratios. The latter solutions, which simulate natural extractions of asteroids and meteorites by water during aqueous alteration, were found to contain >3 mol/L electrolytes and 1 moI/L organics, concentrated solutions favorable for pre biotic synthesis. The solutions and wet solids, inoculated with diverse microbial populations from a wetland, were found to support complex self-sustaining microbial communities for long periods (>8 months), with steady-state populations on the order of 4 x 105 CFU/ml algae and 6 x 106 CFU/ml bacteria and fungi. Planetary microcosm experiments based on meteorite materials can assist in assaying the fertilities of planetary materials and identifying space bioresources, targeting astrobiology exploration, modeling past and future space-based ecosystems, and evaluating sustainable populations in the Solar System. The results also suggest that protoplanetary nebulae can be effective nurseries for microorganisms and useful targets for directed panspermia.

Introduction

Carbonaceous objects in the Solar System include meteorites, asteroids, comets and interplanetary dust particles (IDPs). Under aqueous conditions, these objects form internal solutions that may originate and sustain microbial life. To assess these roles, it is necessary to understand the chemistry and biology of these materials. The present series of studies applies microcosm simulations, based on actual extraterrestrial materials in meteorites, to elucidate these properties (Mautner et al., 1995; Mautner 1997a; Mautner et al., 1997; Mautner, 2002; Mautner and Sinaj, 2002). The questions of interest are: What are the chemical properties of solutions formed when these materials are subjected to various aqueous environments? Can these solutions sustain complex microbial populations? The present study addresses these questions in relation to the potential roles of carbonaceous chondrites in early and future space ecosystems.

1. Organics on early planets

Meteorites, comets and in particular interstellar dust particles (IDPs) imported large amounts of organics to the early Earth, and presumably to Mars and the other planets (Delsemme 1995, Oro et al., 1995). The rate of infall of organic carbon to Earth during the intense bombardment period was of the order 10⁸ - 10⁹ kg yr^-1  by IDPs, 10⁵ - 10⁶ kg yr^­-1 by comets and 10³ - 10⁴ kg yr^-1 by meteorites (Chyba and Sagan, 1992).

2. Biogenesis on early planets

 After infall to planets the IDPs, comets and meteorites can be exposed to water. The interiors of these objects are capable of forming concentrated solutions of organics and salts in the presence of mineral catalysts. Meteorite organics, as well as phosphate and other essential inorganic biological components such as Ca, Mg,  Na, K, chloride and sulphate were shown to be extractable under planetary conditions (Mautner et al., 1995). The soluble meteorite organics include amino acids and adenine, as well as membrane-forming components and polycyclics that can affect energy conversion (Deamer, 1985; Deamer, 1992).

The interiors of IDPs (Kruger and Kissell, 1989; Maurette et al., 1995), cometary ponds (Clark, 1988), and the pores of meteorites on early Earth (Mautner, 1997a; Mautner et al., 1997) could allow prebiotic synthesis and the origins of life. Of these objects, meteorite pores have the advantage of trapping the chemicals and allowing continuing chemical and microbial evolution. In this respect, it has been shown that various organics can be extracted under planetary conditions and some of these components can form vesicles (Mautner et al., 1995).

3. Biogenesis, lithopanspermia and directed panspermia in the Solar Nebula 

Similar solutions can form in the interiors of carbonaceous chondrite parent asteroids during early aqueous alteration and in cometary nuclei during perihelion passes (Bunch and Chang, 1980; Tomeoka and Buseck 1985; Komle et al., 1991; Brearley and Jones 1998; Shearer et al., 1998). Both were suggested as potential sites for biogenenesis (Chyba and McDonald, 1995) and for transporting microorganisms (Hoyle and Wikramasinghe, 1978). Similarly, it was suggested that solar nebulae and young solar systems in star-forming interstellar clouds can be seeded with microorganisms, possibly using solar sailing or comets as vehicles (Mautner and Matloff, 1979; Mautner, 1995; Mautner, 1997b).

With respect to life in early solar systems, the survival of microorganisms on carbonaceous chondrite materials is of interest. In this respect, algae growing on meteorite dust in Greenland were observed as early as 1870 (Leslie, 1879; Maurette et al., 1986). The nutrient values of organic planetary materials were demonstrated on tholin, a synthetic analogue of organics formed under reducing Jupiter conditions (Stoker et al., 1990).  Actual carbonaceous chondrite materials were examined in Murchison extracts were observed to support various soil microorganisms such as the oligotrophs Flavobacterium oryzihabitans and Nocardia asteroides, and experiments with Pseudomonas fluorescens showed that meteorite organics can serve as a sole carbon source (Mautner et al., 1995; Mautner et al, 1997). Indications were also found that the Murchison materials support the anaerobic thermophile eubacterium Thermotoga maritima and the aerobic thermophile Thermus aquaticus (H. W. Morgan, quoted in Mautner et al., 1997). In contrast, the Allende meteorite was observed to inhibit biological growth in some cultures (Mautner, 1997a).  Recently, various carbonaceous chondrites  were found to contain diverse microorganisms from terrestrial contamination (Steele et al., 2000). Plant tissue cultures of Asparagus officinalis and Solanum tuberosum (potato) can also uptake nutrients from the meteorite materials (Mautner, 1997a; Mautner et al., 1997).

 4. Terraforming and space colonization.

In the future, asteroids may be used as resources for space colonization (O'Neill, 1974; O'Leary, 1977; Lewis, 1993). Carbonaceous chondrite materials from Phobos and Deimos may be used as fertilizers in Martian terraforming (Lewis, 1997). Relating to these applications, the Murchison CM2 meteorite was observed to have soil fertility parameters comparable to productive terrestrial soils (Mautner, 1997b; Mautner, 1999).

The microcosm simulations below will use realistic solid/solution ratios, and complex soil microbial communities that extend the previous work on isolated species (Mautner et al, 1995; Mautner et al, 1997). 

Materials and Methods

1. Aqueous extractions

Solid samples of the Allende and Murchison meteorites were obtained from the Smithsonian Institute and from commercial sources. The mineralogies of the meteorites are well-established (Fuchs et al., 1973; Barber, 1981; Komacki and Wood, 1984). The samples were ground by hand in an agate mortar to achieve particle size distributions approximately similar to terrestrial soils, as reported elsewhere (Mautner and Sinaj, 2001). For example, the particle size distribution of Murchison powder is equivalent to silty clay soils with 57% clay-size particles (<2μ), 41% silt-size particles, 2 - 20μ, and 2% sand-size particles > 20μ. Samples of 80 - 200 mg of Allende powder and 40 - 80 mg of Murchison powder were placed in polythene tubes washed in 10% acetic acid for 24 hours to remove electrolyte impurities. Deionized water was added at various solid/solution ratios and the powders were extracted for four days at 20 ^oC with vortex shaking for one minute twice daily at the natural pH of 7.0 - 8.0 established by the powders. Extractions of several minerals showed that constant equilibrium concentrations are obtained in 2 - 8 day extractions under similar conditions (Mautner, 2001).

            At solid/water ratios < 1, the suspended solids could be separated from the liquid by centrifuging and removing the liquid for analysis. However, at higher solid/water ratios the mixtures formed pastes that required special methods. The main method used for these samples was a rapid flush technique. These samples were extracted in 3 ml polythene syringes. After the extraction, twice 2 ml of deionized water was added to the syringes and pressed rapidly through a pre-washed filter. The flushing water was in contact with the extractant/solid paste for less than one minute, sufficient to dilute and remove the entrained extracts but not to dissolve significant amounts of further solutes. This assumption was tested by flushing non-extracted solid powders similarly. The small amounts dissolved from these non-extracted solids by the rapid flush were used as reference blanks.

            As a control some of the samples were also analyzed by a different method, where a portion of the extracts in the pastes obtained at solid/water ratios of 1.0 and 2.0 was adsorbed on dry filter paper. The paper was weighed to determine the amount of extract adsorbed, which was approximately 50% of the total entrained extract. The paper was subsequently extracted into 4 ml of deionized water to analyze the solutes.

Trace metals were extracted from Allende and Murchison at r_solid/water = 0.027 g/ml by 1M NH₄OAc solution, a standard soil extractant (Blakemore et al., 1987).

            Anions in the extracts were analyzed by ion exchange chromatography using a Waters Ion exchange Chromatograph and Waters Baseline 810 software. The method used was Waters Ion Chromatography Method A-102 "Anion Exchange Analysis Using IC-Pak A HC Column Borate/Gluconate Eluent", with the samples filtered through a 0.1 micron filter paper prior to analysis. Cations were analyzed by a Shimadzu AA-6200 Atomic Absorption Flame Emission Spectrophotometer. Phosphate was analyzed by colorimetry using malachite green solutions (van Veldhoven and Mannerts, 1987). The uncertainty in the reported concentrations is estimated as ±30% from the results of replicate measurements. Similarly, the standard deviation of values obtained by the three different extraction methods for Allende at r_solid/water = 1.0 was ±24%, and the average standard deviation of the constant c_solid values at r_solid/water = 1 - 10 in Figures 3 and 4 below was ±18%.  From these observations, the uncertainty in the data in Table 1 is estimated as ±30%.

2. Microbial Cultures

A main objective of the present studies was to observe the development of mixed microbial populations in meteorite microcosms simulating asteroid and cometary interiors.

The main limitation of meteorite-based microcosm studies is the small amount of available materials. In this work, typically 20 mg was required per microcosm, and typical experiment series required several times this amount for various treatments and replicates. The minimum sizes of usable microcosms are defined by the requirement that the amounts of the chemicals and microorganisms should be sufficient to detect. Consideration of the current analytical methods and the usual range of extractable materials shows that extractions need to use 0.001 - 1 g mineral samples for anion and cation analysis, 0.01 - 0.1 g for phosphate analysis, 0.001 - 1 g for algal bioassays and 10^­-6 - 10^­-2 g for microbial studies (Mautner, 2001).   Each of the present microbial cultures required 20 - 40 mg of materials.

Typically, our microcosm contains 0.2 - 1 ml extracts that allow repeated microbial population analysis. The small microcosms were prepared in 2 ml polythene microfuge tubes. In the present study two samples each of 20 mg of Allende and Murchison, and for reference acid-washed sand were extracted and sterilized in 1 ml of deionized water at 121 ^oC for 15 minutes, and inoculated with 20 microliters of the mixed microbial populations. These cultures will be denoted as "extracts". In another set of experiments, 100 mg of each solid was inoculated and wetted directly by 20 microliters of the inoculating solution. These cultures will be denoted as "wet solids". These extracts contained sufficient nutrients for microbial populations up to 10⁸ CFU/ml on the "wet solids", in the post-log steady-state populations from 4 to > 31 days, if all the limiting nutrients would be utilized.

            The inoculating solutions were chosen from a natural source with a mix of microorganisms that is expected to contain aerobes and anaerobes, autotrophs such as algae and heterotrophs adapted to humic or kerogen-like materials similar to those found in meteorites. For this purpose, samples of liquid and of wet soil from a local peat bog wetland reserve (Travis Swamp, Christchurch, New Zealand) was collected from the surface to a depth of one meter. The mixed sample from several layers was kept in a sealed jar terrarium allowing a slow diffusion of air for one year, creating a Winogradsky column (Winogradsky, 1949). During this time the soil differentiated into a 2 cm dark brown top layer, a 0.5 cm reddish brown middle layer and a 1.5 cm medium brown bottom layer. This layering may be due to microbial activity at various oxygen levels. The top level supported plant and algal growth indicating slowly exchanging aerobic conditions. The reddish brown layer may be due to sulphur or iron-oxidizing bacteria. In an oxygen restricted environment, the bottom layer is expected to be anaerobic or microoxic.

To produce inoculants for the aerobic cultures, 2 ml of wet soil was taken from the top layer and supplemented by 2 ml of algae cultures isolated from New Zealand soils and grown in algal nutrient cultures. The algal cultures included unicellular green algae Chlorella sp. and Chlorosarcinopsis sp., filamentuos blue-greens Leptolyngbya sp,.  and Phhormidium sp. and a gold-brown diatom Navicula sp., which were identified microscopically. For anaerobic inoculants, 2 ml of wet soil was extracted from the bottom layer of the terrarium. The inoculating soil and algal material can be assumed to contain many bacteria and fungi, some of which were identified in the microcosm cultures as discussed below. The solids in the samples were washed three times with 8 ml of deionized water deareated with nitrogen, and centrifuged to separate the suspended solids and microorganisms. However, traces of oxygen could not be excluded and the experiments were suitable tested for facultative anaerobes.

No buffers were used, to avoid carrying over the buffer materials into the microcosms. Microorganisms used in the previous meteorite microcosms survived similar washing procedures without lysing (Mautner et al., 1997). Considering that  <0.1 ml of water remained in the samples after each step, the procedure diluted all soluble components by a factor of 10⁶ to remove soluble nutrients. Samples of 20 microliters of the final suspension of the washed solids, containing about 1 mg of the soil solids, were used for inoculations.

Following inoculation, the meteorite cultures were developed at 20 ^oC under natural light and dark cycles to allow the development of mixed populations including both photosynthetic autotrophs and heterotrophs. The culture vials were contained in sealed glass jars kept at 100% humidity to prevent drying. The jars were filled with air for the aerobic cultures and with a mix of 90% N₂ and 10% CO₂ for anaerobic cultures.

For monitoring the microbial populations, the cultures were plated on nutrient agar under aerobic or anaerobic conditions as appropriate, and on algal nutrient agar. Samples of colonies obtained from the mixed cultures were grown as isolates on separate plates and analyzed by Gram staining, oxidase and catalase responses and 96 well carbon source test plates (Biolog, Hayward, California). 

Results: Meteorite Extracts

1. Aqueous Extractions

            As noted in the experimental section, the powders of Allende and Murchison were extracted in this work at solid/solution ratios < 1.0 by direct extraction and ratios > 2.0 by the flush or paper adsorption methods. For comparison, all three methods were applied to the Allende samples at solid/solution ratio of 1.0. For all the cations measured, the three methods gave values with standard deviation < ±30%, which shows that the consistency between the various methods is similar to the uncertainty of the replicate direct extractions themselves. The agreement amongst the methods is also supported by the absence of discontinuities in Figure 1 that include data points measured by all three methods.

            The results of the extractions may be expressed in several related ways that reflect different physical quantities. The actual measurements yield aqueous concentrations in the extracts, c_aq (mg/l), which can be converted into the extractable content in the solid c_solid (mg/g) using equation (1).

                                                 c_aq (mg/l) V_aq (ml)

            c_solid (mg/g)       =   ---------------------------

1,000 w_solid (g)

c_aq (mg/l)

            =   -----------------------------                               (1)    

                                               1,000 r_{solid/solution} (g/ml)

            If the total extractable content at infinite dilution is also known the results can be converted to desorption isotherms, which will be presented elsewhere (Mautner et al., 2001). In relation to the microbial experiments, the relevant results are reported in terms of solution concentrations in Figure 1, that shows the values of c_aq as a function of the solid/solution ratio used in the extractions of the Allende meteorite. The extractions of Murchison yielded similar trends although at much higher absolute solute concentrations, that will be reported elsewhere (Mautner et al., 2001).

            Figure 1 shows that the solute concentrations increase through the range of the solid/solution ratios applied. Table 1 reports the concentrations in the dilute solutions obtained at r_{solid/solution} of 0.02 used for the microbial cultures. The Table also reports concentrations obtained at r_{solid/solution} of 10.0 that simulate natural extractions by water in the meteorite and asteroid pores, at a porosity of 20% by volume. As r_{solid/solution} increases by a factor of 500 between these extractions, the concentrations of most elements increases by factors of 200 - 500 over this range The results therefore suggest that most of the extractable electrolytes are present in the meteorite as soluble salts that dissolve fully even in the minimum amount of water used (r_{solid/solution} = 10.0 g/ml). In the alternative case, if the concentrations had been controlled by adsorption/desorption equilibria on mineral surfaces, the extracted amounts c_solid would have increased significantly with increasing amounts of water. However, c_solid varied only moderately and became constant at r_{solid/solution} = 1 - 10. These c_solid values can be calculated from the data in Table 1 using equation (1) and the reported c_aq values.

The extractable amounts of trace elements were also examined, using extraction by 1M NH₄OAc­. The extractable amounts were (c_solid (microgram/g), Allende, Murchison): B, 0.1, 0.5; Fe, 0.4, 9.0; Mn, 0.7, 11.0; Al, 0.06, 0.03; Cd, 0.0, 3.8; Cr, 0.0, 0.2; Cu, 0.004, 0.04; Ni, 85, 101, respectively.

            Organic compounds constitute 18 mg/g of Murchison and 2.5 mg/g of Allende. Previous studies showed that 10% of the organic content is released under hydrothermal extraction at 121 ^oC, and about half as much may be extracted at 20^o C (Mautner et al., 1995; Mautner, 1997a). The calculated amounts of organics released by aqueous extraction are about 1 mg/g organic C from Murchison. Assuming the same relations for Allende, 0.1 mg/g organic C is released by aqueous extraction.

Results: Microbiology

2. Microbiology

The main culturable microorganisms that grew in the meteorite and sand extracts from the inoculates, and the steady-state populations that were established, are listed in Table 2. Two sets of cultures, in extracts and on wet solids, were grown in parallel. The development of the populations in the extracts is shown in Figures 2 - 4. Table 2 summarizes the steady-state populations obtained in both sets of experiments after 31 days, and the populations on the wet solids after 8 months of incubation. 

            Tentative identification of the observed species, obtained using Biolog carbon source plates, is listed in Table 2. Although the identifications are tentative, the tests provide useful information on the utilization of potential carbon sources in the meteorites, as discussed below. The similarity of our microorganisms to the species contained in the Biolog database is variable, with possibly reliable identification of Eureobacterium saperdae and Pseudomonas putida. The other identifications are tentative. However, all the species gave distinct colonies on nutrient agar which allowed counting the various microbial populations.

            Several algae were also included in the cultures. The initial populations of the unicellular, filamentous and diatom species described above were, in the extract cultures, 10,000, 200 and 80 CFU/ml, respectively. Plate counts of the populations were obtained 15 days after inoculation, during the post-log phase of the bacterial and fungal populations. The algal populations in the extracts and on the wet solids are listed in Table 3. The algal populations increased in all the cultures after inoculation but reached smaller populations than the bacteria, as is also the case in terrestrial soil populations.

            Microbial populations that were expected to contain anaerobes were grown in preliminary experiments under microoxic conditions. The inoculating cultures were obtained from the bottom layer of the wetland Winogradsky column described above. Samples of the cultures were plated on nutrient agar 27 days after inoculation and the plates were developed under microoxic conditions. Judging by colony morphology, the resulting populations resembled those in the aerobic samples. Under these conditions the main species remained C. michiganense, with smaller populations of the other original inoculating bacteria shown in Table 1. However, the late takeover by Corynebacterium sp. and the development of the yeasts and filamentuous fungi did not occur. The observations suggest that the bacterial species or strains isolated from the wetland are tolerant of microoxic conditions and can grow under such conditions on the meteorite extracts. We are investigating if true anaerobes can also grow on these materials.  

Discussion

1. Applications of planetary microcosms
 

Microcosms are often used to simulate ecosystems under controlled conditions (Odum and Hoskins, 1957; Beyers, 1969). Meteorites allow studies, based on actual planetary materials, to simulate some aspects of planetary ecosystems that are not accessible otherwise.

The design of planetary microcosms depends on the objectives of the simulation, such as models of early life or future terraforming. Early life on Earth or in asteroids and comets would probably be anaerobic, while the aim of terraforming is to create habitable oxygen-rich environments. The latter aerobic systems are addressed primarily in this work.

The minimum usable sizes of microcosms are defined by the requirement of measurable amounts of chemicals and microorganisms. Current common analytical methods and the usual range of extractable contents require 0.001 - 1 g meteorite or mineral samples for nutrient analysis and for algal essays, and 10^­-6 - 10^­-2 g samples for microbial studies. A more detailed analysis is given elsewhere (Mautner, 2002).   The present microbial cultures used 20 - 40 mg of materials per microcosm contained in solutions of 0.1 - 1 ml. Typical experimental series required several times this amount for various treatments and replicates.

2. Solution chemistry of asteroid and cometary interiors
 

Carbonaceous chondrite materials may be exposed to water in nature at widely varying solid/solution ratios. For example, meteorites that land in water are extracted at virtually infinite dilutions at very low solid/solution ratios. At the other extreme, water filling the pores of meteorites that fall on land are extracted by the penetrating water at a high excess of solid at solid/solution ratios of about 10 g/ml, given the porosity of about 20% by volume of CM2 meteorites (Corrigan et al., 1997).

The latter conditions also apply in asteroids during aqueous alteration when the internal pores are filled with water. A high concentration of soluble electrolytes in these fluids was implicit in our previous data (Mautner et al., 1995; Mautner, 1997a) and more recently in discussions by other authors (Bodnar and Zolensky, 2000; Cohen and Coker, 2000). The temperatures in these objects may range from 25 to 150 ^oC for thousands of years (Bunch and Chang, 1980; Tomeoka and Buseck, 1985; Brearley and Jones, 1998; Shearer et al., 1998).

Figure 1 and Table 1 illustrate that the equilibrium concentrations of ions in these solutions can vary widely with solid/solution ratio. Table 1 compares the concentrations of the extracted electrolytes from Allende and Murchison with terrestrial soil solutions. The results show that the concentrations in the dilute Allende extracts obtained a r_{solid/solution} = 0.02 - 0.1 g/ml are generally below soil solutions or in the lower range of soil solutions. The concentrations of electrolytes in Murchison extracts at this solid/solution ratio are remarkably close to the median soil solution values except for higher levels of sulfate and phosphate.

For the solutions obtained at r_{solid/solution} = 10 g/ml in Allende, the concentrations of cations and Cl exceed the upper limits of soil solutions by about an order of magnitude, while NO₃-N, SO₄-S and PO₄-P are in the range of terrestrial soil solutions. However, the concentrations of cations in the Murchison extracts at r_{solid/solution} = 10 g/ml are higher than the upper range of soil solutions by over two orders of magnitude. Even the concentrations of NO₃-N and PO₄-P are comparable to the upper range of soil solutions. Sulfate is in large excess, with some possible implications discussed below. The high concentrations in these Murchison extracts may be in the toxic range for plants. The micronutrients and possibly toxic elements Mn and Fe are higher than the upper limit in typical surface soil solutions by factors of 55 and 360 respectively, and Ni is present at very high levels. However, some of these elements may also serve as oxidizable energy sources for microbial communities. 

In molar units, the total concentration of the ions at r_sold/water = 10.0 in Table 2 in the Allende extract is 0.097 mol/l with an ionic charge of 0.15eq/l, and in the Murchison extract 3.8 mol/l with an ionic charge of 6.6 eq/l. The latter values are much higher than the median electrolyte concentration of 0.031 mol/l and ionic charge of 0.034 meq/l in average surface soil solutions. The high electrolyte concentrations in the meteorite solutions imply that these solutions have high ionic strengths and osmotic pressures.

The amount of organic carbon that can be extracted from Murchison at 20 ^oC is about 1 mg/g (Mautner, 1997a). If this applies at  r_{solid/solution} = 10 g/ml, the concentration of organic carbon in the asteroid fluids can be estimated as 10 g/l or about 1 mol/l in the pores of carbonaceous chondrites. This represents only 10% of the total organic carbon in Murchison, the rest remaining as insoluble compounds and organic polymer. If this insoluble carbon was present in the parent bodies originally as unpolymerized soluble material, the concentration in solutions in the parent asteroid or cometary fluids may have been in the range of 10 mol/l. The next section will discuss the prebiotic implications of these results.

3. Biogenesis in carbonaceous chondrite asteroids and meteorites 
 

The preceding section suggests that the pores of meteorites and asteroids contain strong electrolytes with a total ionic concentration of >3mol/l, composed of the ions in Table 1; silicates not measured here; organic C of 1- 10 mol/l; pH 7 - 8 (Mautner, 1997a) and catalytic minerals including clay-like phyllosilicates with a large specific surface area of 3.7x10⁴ m²/kg (Mautner, 1999). The dissolved metals and the clays present and minerals such as FeS can serve as catalysts (Bernal, 1951; Cairns-Smith and Hartman, 1986). The high concentrations and catalysts are conducive to complex organic synthesis, and the trapped products can undergo further reactions leading to large molecules.

The concentrations and osmotic pressures of these solutions can be comparable to that of a cell interior. Chemistry similar to that in cells can therefore occur in the meteorite/asteroid fluids without the need for enclosure in membranes. With such solutions an entire solution-filled meteorite or asteroid, or a solution layer in a comet, may function as a giant cell. This is also assisted by the relative concentrations of the N, P, K and soluble C, and in another group, Ca, Mg, Na, Cl and S, in these solutions that are similar to those in bacterial or algal biomass. Alternatively or later, primitive cells bound by inefficient or weak membranes may form (Deamer, 1985; Deamer, 1992) and survive without osmotic rupture under these conditions, dividing the solution into cells.

The asteroid environments also included CO₂ and H₂ captured from gases in the solar nebula, significant amounts of sulfur and sulfides (3% in Murchison) and temperatures of 25 - 150 ^oC in the mesophilic, thermophilic and hyperthermophylic range. These conditions are suitable for archaebacteria, possibly methanogens and sulphur bacteria. In other words, the conditions in the asteroids, or in similar solutions on meteorites that landed on early aqueous Earth and Mars are therefore consistent with the possible origins of life in the interiors of carbonaceous chondrite objects, or early adaptation to such environments. These conditions were at least as suitable as for the primitive microorganisms as hydrothermal vents. In addition, the asteroid and meteorite interiors had the advantage of trapping chemicals and microorganisms for thousands of years, allowing for continued chemical and biological evolution. Although meteorites were only a small fraction of the organics imported by dust particles, the 10¹⁴ kg meteorites landed in the first 10⁸ years (Chyba and Sagan, 1992) could have accommodated 10¹⁰ kg biomass in 20²⁴ microorganisms (see below), more than enough for a first evolving biota. 

4. Indications of past microbial activity in asteroids
 

Several independent observations suggest that carbonaceous chondrites in our Solar System may have contained microbial life. For example, the Murchison organics contain amino acids and adenine and the polymer fraction resembles kerogen formed from biological materials. Microfossils in carbonaceous chondrites were reported by several observers (Claus and Nagy, 1961; Urey, 1962; Hoover, 2001). Life on Earth originated soon after the period of late heavy bombardment suggesting that microorganisms may have been delivered to Earth by meteorites, asteroids or comets.

Some of our observations are consistent with possible biological activity in the carbonaceous chondrite parent bodies. We found that the composition of the Murchison materials resembles biologically developed terrestrial soils in its overall organic content, C/N ratio, cation exchange capacity and concentrations of the available macronutrients (Mautner 1997b; Mautner, 1999). The effects of the Murchison meteorite on microorganisms are also similar to the effects of biologically developed soils (Mautner, 1997b; Mautner et al., 1997). As noted above, the ratios of soluble N, P, K and C, and those of soluble Ca, Mg, Na, Cl and S in the meteorite are also remarkably similar to those in biological materials. This similarity may suggest that the soluble elements were deposited from a microbial biomass. Alternatively, it may suggest that the present biological elemental ratios reflect the conditions of early life in carbonaceous chondrite asteroids or meteorites.

A further indication of possible microbial origin may be provided by the large concentration of soluble sulfate, 9.4 mg/g, observed in Murchison (Table 1 and Mautner, 1997a). This is much higher than in any other type of meteorite, including stony and Martian meteorites, igneous terrestrial analogues and serpentine (Mautner et al., 2001), where the soluble sulfate concentration is only 0.01 - 0.1 mg/g. However, the Murchison parent body was formed under reducing Solar Nebula conditions in the absence of oxygen, and it is uncertain if oxidised sulfur can be formed chemically under these conditions. It may have required sulfide-oxidising bacteria to oxidise sulfur under these conditions. We are testing this hypothesis by examining the SO₄-S isotopic composition in carbonaceous chondrites for biological signatures.  

In total, the observed chemistry and microbiology are consistent with the possible origin and past biology in carbonaceous chondrite objects. Implications for future life in space will be discussed in the next sections.

5. Microbial populations in meteorite solutions

In extension of the previous studies of pure microbial populations (Mautner, 1997a; Mautner et al., 1997), the present experiments used complex microbial inoculants. The results in Tables 2 and 3 show that complex microbial communities including bacteria, fungi and algae can grow on the meteorites. Table 2 shows that comparable population densities were observed in the dilute extracts and in the liquid on the wet solids.

Figures 2 and 3 and Table 2 show that the extracts of the two meteorites developed comparable microbial populations, with the Allende populations being slightly lower possibly because the lower concentration of nutrients in these extracts. On the other hand, Allende yielded the most diverse microbial populations, and in the long term its populations reached comparable levels to those in the Murchison solutions. Several additional microorganism species, not shown in Table 2, were also observed in Allende in small numbers. However, microorganisms in the extracts of both meteorites exhibited overall similar growth profiles, including the replacement of Clavibacter michiganense by Corynebacterium sp. as the dominant species after about 15 days.

The long-term survival of microorganisms on these materials was tested by measuring the populations after 8 months of incubation. Table 2 shows that the populations survived and in fact increased during this long period. The main species C. michiganense, M. imperiale and Corynebacterium sp. reached practically identical populations in both the Allende and Murchison cultures. Both also showed large populations of an additional unidentified species that form yellow globular colonies, and some additional new species in smaller numbers. The total population counted after 8 months in both cultures was also practically identical, 4.7 - 4.8 x10⁶ CFU/ml.

The general similarity of microbiology in the two meteorites is notable considering the much higher concentration of electrolytes in the Murchison extracts. Given that the microorganisms grew and survived also on nutrient-free acid-washed sand, these species may be oligotrophs possibly living on organics from the laboratory air. We demonstrated in the past that Murchison organics can provide the sole carbon source for microorganism (Mautner et al., 1997). The long-term survival of microorganisms on the wet meteorite solids after eight months of incubation shows at least that the meteorite solutions do not contain toxic components that would prevent microbial growth.

The total algal populations in the Allende extracts were also comparable to that in the Murchison extracts as seen in Table 3, while in the concentrated solutions on the wet solids Allende gave lower algal populations than Murchison, and even than wet sand. This suggests that the concentrated Allende solutions inhibited algal growth. Similar inhibitory effects of Allende extracts were observed on potato tissue cultures (Mautner, 1997a).

Both meteorites supported more diverse microbial populations than inert sand. The meteorite extracts also showed the development of Corynebacterium sp. as the dominant species after about 15 days. The large populations of C. michiganense and P. putida in the extracts of inert sand suggests that these microorganisms are oligotrophs. The fungi in later stages of the cultures may utilize the biomass produced by autotrophs and the detritus from the bacterial and algal populations.

The biomass that can be supported by any given nutrient x in the solid soil or meteorite can be calculated by equation (2).

m_biomass,xl (g) = 

1,000 c(x)_solid (mg/g) m_solid (g) / c(x)_bioamass (μg/g)                       (2)

Here m_bioamss,xl is the amount of biomass that can be obtained if limited by nutrient x; c(x)_solid is the concentration of extractable x in the solid, m_solid is the mass of the solid extracted, and c(x)_biomass is the concentration of bioavalable x in the dry biomass.

For calculating the limiting microbial population in the aqueous extract of the solid, the amount of microbial biomass in the solution can be expressed by equation (3).

m_biomass,aq (g) = c_{microorganisms} (CFU/ml) V_aq (ml) m_{microorganism} (g)   (3)

Here m_biomass,aq is the amount of biomass in V_aq volume of solution, contained in organisms each of m_{microorganism} dry biomass. Combining equations (2) and (3) yields the limiting microbial population density in solution.

c_{microorgansims(aq),xl} (CFU/ml)   = 

1,000 c(x)_solid (mg/g) r_s/w (g/ml)

--------------------------------------------                 (4)

                  c(x)_biomass (μg/g)  m_{microorganism} (g)

Here c_{microrganism(aq),xl} is the concentration of microorganisms allowed if x is the limiting nutrient, r_s/w is the solid/solution ratio in the extraction and c(x)_solid is the concentration of nutrient x in the solid that is extractable at this solid/solution ratio.

The last two rows of Table 1 show the sustainable populations of typical bacteria with a radius of 1 micron and dry mass of 2x10^-12g calculated using equation (4). The results show for example that the Ca content in the concentrated Allende solutions (at r_{solid/solution} = 10 g/ml) can sustain 4.1x10¹⁰ bacteria/ml, while the NO₃-N content in the extract is sufficient only for 3.5x10⁸ bacteria/ml. These calculations show that NO₃-N, PO₄-P, K, and even soluble C limit the bacterial populations to quite comparable levels of 3.5-10x10⁸ bacteria/ml in the concentrated Allende extracts. The nutrients K and P lead to similar limiting levels also in the Murchison extracts, consistent with the similarity of the populations observed in Table 2 in the two extracts. The limiting nutrients in the meteorite extracts are nitrate, phosphate and potassium, that are also typical limiting nutrients in many terrestrial ecosystems.

The other essential nutrients Ca, Mg, Na and SO₄-S are all sufficient to provide larger populations over 10¹⁰ bacteria/ml in the Allende and over10¹² bacteria/ml in the Murchison solutions.

Note that soluble elements in the group N, P, K and C and elements in the Ca, Mg, Na, Cl and S in the meteorite extracts can support mutually similar populations of bacterial biomass. This is a consequence of equation (4) as c(x)_solid and c(x)_biomass are similar in these groups of elements. In other words, the relative concentrations of the soluble forms of these elements in Murchison are similar to their relative amounts in bacterial biomass.

The high concentrations of sulfate and Ni, and possibly Fe and Mn, may be toxic to some organisms. However, we did not observe toxic effects, as the microbial populations in the concentrated solutions on the wet solids were comparable to or larger than in the more dilute solutions (Table 2).

The Biolog tests provide some useful information on the possible meteorite components utilized by the microorganisms. All of the microorganisms can use glycerol, a polyalcohol that may be present in Murchison considering the presence of various alcohols and other hydroxylated compounds such as carboxylic and amino acids. One of the microorganisms, Pseudomonas putida, also utilizes other Murchison components such as acetic acid and alanine. Murchison also contains a large number of other likely nutrients for heterotrophs that were not included in the Biolog tests.

6. Sustainable populations on asteroid resources
 

The measured amounts of bioavailable nutrients allows and experiment-based estimate of the biomass that could have existed in the early Solar System or that could be established in the future, based on carbonaceous chondrite resources. The biomass that could be obtained as limited by nutrient x is obtained from equation (2), using m_solid = 10²² kg as the total mass of the carbonaceous asteroids (Lewis, 1997) and the allowed microbial population is obtained from m_biomass,xl/m_{microorganism}, where m_microrganism = 2x10^-12 g is used as an estimated average. Further, the allowed human population can be calculated by assuming, for example, 10⁴ kg biomass supporting a human on a terraformed planet or space colony.

  n_population = 10^-4 c(x) _solid (mg/g) m_solid (g) / c(x)_biomass (μg/g)     (5)

Given the similarity of equations (4) and (5), the allowed biomass (kg) and human population, based on carbonaceous chondrite Murchison CM2 type asteroids, is obtained by multiplying the last row in Table 2 by 2x10⁹ and 2x10⁵, respectively. This of course yields again NO₃-N and PO₄-P as the liming nutrients. Both lead to similar values, with an NO₃-N limited biomass of 1.3x10¹⁸ kg in 6.5x10³² microorganisms, supporting a human population of 1.3x10¹⁴, or about 10^-4 kg biomass per kg asteroid material. The values based on PO₄-P are similar, being larger by a factor of 1.4. Note that the actual populations in Table 2 are lower by about a factor of 100 than the calculated nutrient-limiting populations. The asteroid-based populations would be lower by a factor of 100 using these actual observed populations.

6. Natural and directed panspermia
 

Comets were proposed as vehicles for natural panspermia (Hoyle and Wikramashinge, 1978). As noted above, comets may contain pockets of water at 20 - 100 ^oC during perihelion passes, containing fluids similar to the concentrated Murchison extracts, that support microbial growth. Comets may be also used deliberately for directed panspermia, by seeding them with microorganisms that can grow to a substantial biomass during perihelion passes. The microorganism bearing comets can then be fragmented and propelled or allowed to eject naturally to interstellar space (Mautner, 1997b).

However, comets may have limited potential as liquid water may not form, or may be lost rapidly by evaporation. This also limits the surface temperatures to <180 K, too low for microorganisms  (Komle et al., 1991; Lewis, 1997). Liquid water may exist at best for short periods, in small pockets shielded by carbonaceous deposits. These conditions are not favorable for complex chemistry, and do not allow sustained evolving microbial populations.

In contrast, asteroid interiors during aqueous alteration can contain large volumes of water for thousands of years. For example, a 10 km radius asteroid or cometary nucleus of about 10¹⁶ kg with 10% water content can contain over 10¹⁵ liters of nutrient electrolyte solutions similar in composition to those in Table 1. Following the preceding section, this asteroid may accommodate a biomass of 10¹² kg in 5x10²⁶ bacteria and about 10²⁴ algae. As noted, the total asteroidal mass of 10²² kg can accommodate over 10³² bacteria and 10³⁰ algae.

In fact, large numbers of asteroids undergo aqueous alteration simultaneously. Collision amongst these objects are frequent (Lewis, 1997), and impact collisions can distribute microorganisms (Mileikowsky et al., 2000). Even small meteorites ejected by these collisions can provide enough shielding to protect microorganisms in space for months (Horneck et al., 1994), until recapture by another object. These considerations, combined with the present results, suggest that significant microbial populations can grow in aqueous asteroids. Collisions can distribute the microorganisms efficiently in the early Solar System.

Part of this population will be preserved at low temperatures in the interiors of asteroids and comets, and a fraction subsequently delivered to planets. Other asteroids and comets are ejected to interstellar space and some of these can spread the microorganisms to other protoplanetary nebulae, where they can multiply similarly and propel to yet further nebulae. This mechanism therefore proposes asteroids, protoplanetary nebulae and early solar systems for the growth and dispersion of microbial life, instead or in addition to the cometary proposals (Hoyle and Wickramasinghe, 1978).

A similar mechanism can be applied in directed panspermia (Crick and Orgel, 1973; Mautner and Matloff, 1979; Mautner, 1995; Mautner, 1997b). For example, comets may be seeded with microorganisms. Natural melting during perihelion passes may allow the microorganism to multiply. Microbial inoculants may be also inserted deeper into the cometary nuclei together with artificial heat sources to melt the ices and create sub-surface pools. Given the possible relations between carbonaceous asteroids and comets, concentrated solutions in these pools may be similar to those observed in the Murchison extracts, and can allow similarly the growth of large microbial populations. Eventually, the comet may be fragmented and ejected into interstellar space toward new Solar Systems in star-forming clouds, carrying the microbial payload. If non-periodic comets in parabolic orbits are seeded, this ejection will occur naturally.

The cometary interiors can shield the microbial content from prolonged space radiation, although the effects over transit times of millions of years are not known. If long-lived radioactive heat sources are used, a self-recycling microbial community can renew itself genetically during the long interstellar flights. The microorganisms can multiply and disperse collisionally in the target protoplanetary nebulae and asteroids, seed local planets and eventually disperse by comets to further Solar Systems. Using the above relations between nutrients and biomass, the 10²⁵ kg of comets in the Oort cloud can yield 10²¹ kg biomass comprised of over 10³⁵ microorganisms, sufficient to seed new Solar Systems throughout the entire Milky Way galaxy (Mautner and Matloff, 1979; Mautner, 1995; Mautner, 1997b). It is also possible that intelligent civilizations evolving in the seeded habitats will propagate life further in the galaxy deliberately.

7. Terraforming and space agriculture
 

Carbonaceous chondrites are likely to be the main sources of carbon and water in space-based agriculture. In space colonies and in terraformed asteroids, they may be used as soils and fertilizers. The carbonaceous moons Phobos and Deimos may be mined for soils and fertilizers in Martian terraforming. 

Algae are likely to be used as colonizing microorganisms (Friedmann and Ocampo-Friedmann, 1995). A viable soil ecology will subsequently require a diverse microbial population and the recycling of nutrients by bacteria and fungi. The microbial results above demonstrate that carbonceous chondrite soils can sustain diverse microbial ecosystems.

If ground into particles similarly to the present studies, the Murchison soil will be similar in particle size distribution to silty clay. In this form the agriculturally useable moisture content will be between the wilting point at 20% w/w and field capacity at 40% w/w. These moisture contents are in the range used in the extractions above and will yield nutrient ion concentrations similar to those of the concentrated solutions in Table 1. The dilute solutions in Table 1 may be also used for hydroponics. Using the figures in the preceding section, the total asteroid materials used as synthetic soils allow a biomass of 10¹⁸ kg and a human population of 10¹⁴ in these terraformed colonies.

Summary and Conclusions

The previously reported studies on carbonaceous chondrites (Mautner, 1997a; Mautner et al, 1997) have been extended here to natural aqueous conditions of high solid/solution ratios. The main experiment-based conclusions are:

1.       Planetary microcosms, based on actual extraterrestrial materials in meteorites, are useful tools in experimental astroecology.

2.       Based on microcosm studies, the interiors of carbonaceous chondrite meteorites, asteroids and comets can contain highly concentrated solutions of electrolytes, nutrients and organics. (Observation: >3 mol/l electrolytes, 1 - 10 mol/l organics)

3.       In the presence of mineral catalysts, these trapped concentrated solutions are suitable for prolonged, stepwise synthesis of complex organics.

4.       The interiors of asteroids during aqueous alteration, or meteorites landed on aqueous planets, are therefore suitable for potential biogenesis.

5.       The resulting indigenous, or introduced, microorganisms can grow in the interior solutions in meteorites and asteroids.

6.       Possible microfossils, the biomass-like ratios of macronutrients and high sulfate content may suggest past biological activity in carboanceous chondrite asteroids. (Indicative observations: Soil fertility properties of Murchison are similar to biologically developed soils; organic polymer similar to coal; ratios of soluble  N, P, K and C, and Ca, Mg, Na, Cl and S are comparable to those in bacterial biomass; high sulfate content in Murchison).

7.       Complex recycling communities of algae, bacteria and fungi develop and survive for substantial periods in these solutions. (Observation: Algal populations of >10⁵ and microbial populations >10⁶ CFU/ml of six species surviving over 8 months on wet Allende and Murchison).

8.       Carbonaceous asteroids containing nutrient solutions can distribute micoorganisms during a period of collision-mediated panspermia in the Solar Nebula and in the early Solar System. (Available nutrients allow a biomass of 10¹⁸ kg in a population over 10³² microorganisms in the asteroid belt.)

9.      Similarly, comets can be used as vehicles, and protoplanetary nebulae can be used as targets and incubators in directed panspermia missions for seeding new planetary systems with microbial life (The nutrients in the Oort belt comets allow a biomass of 10²¹ kg containing 10³⁵ microorganisms, sufficient to seed all new solar systems in the galaxy).

10.   Carbonaceous chondrites are suitable soil resources for planetary terraforming and space colonization. (Based on the limiting nutrients NO₃-N and PO₄-P, the total asteroid material can support a population of 10¹⁴ humans).   

The present microcosms examined separately the nutrient contents and the microbial populations in the microcosms. In real ecosystems, microbial activity and available nutrients are interdependent, and should be monitored simultaneously.

We are extending our planetary microcosm studies to further types of meteorites representing other asteroids, also to Martian meteorites and simulants (Mautner, 2002), plausible planetary conditions such as CO₂ atmospheres, and anaerobic microbial populations. These microcosm simulations will model the complex interactions between nutrients, pH, temperature, light flux, and biological populations in planetary ecosystems. For example, the first ranking of carbonaceous and Martian meteorites, based on extractable nutrients, and algal and plant yields, resulted in a ranking of fertilities as Martian basalts > terrestrial basalt > carbonaceous chondrites, lava ash > cumulate igneous rock. (Mautner, 2002). Eventually, these microcosm studies can help in targeting astrobiology exploration; identifying bioresources in the Solar System; and modeling ecosystems for terraforming, space colonization and directed panspermia.

Acknowledgements

I thank Ms. Catherine Trought for assistance with the aqueous extractions and analysis, Dr. Paul Broady for the algal samples, Dr. Mark Braithwaite for microbial identification and Dr. Eric Forbes for a review of the manuscript, and the Smithsonian Institution for a gift of Allende and Murchison samples. This work was supported by grant 99-LIU-014 ESA from the Marsden Fund administered by the Royal Society of New Zealand.

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Table 1. Solute concentrations (mg/l) in extracts of the Allende and Murchison meteorites, obtained by extractions at low and high solid/solution ratios.

Table 1 (continued)

 Footnotes to Table 1

a.       Concentrations (mg/l or ppm) in extracts obtained at 20 ^oC for 4 days at low (average data from 0.02-0.1 g/ml) and high (1-10 g/ml) solid/solution ratios. The corresponding concentrations of extractable elements in the solids (c_solid mg/g) may be obtained by dividing the listed values by 10³r_{solid/solution} (g/ml). The average values obtained at solid/solution ratios of 1, 2, 4 and 10 g/ml are given for the 10 g/ml extractions. Estimated uncertainty ±30%, except Cl and NO₃-N measured at low concentrations in extracts obtained at r_{solid/solution} = 0.1 - 1.0, where an uncertainty by of a factor of 2 may apply.

b.       Based on c_solid measured by extraction by 1M NH₄OAc for 24 hours at a solid/solution ratio of  0.028 g/ml. Solution concentrations calculated using this c_solid value and r_{solid/solution} = 0.02 or 10 in equation (1).

c.       Based on c_solid measured by extraction in 4 days at a solid/solution ratio of 1.0 g/ml (Mautner and Sinai, 2001), and using this c_solid value and r_{solid/solution} = 10 and in equation (1).

d.       Estimated as half of the yield of organic carbon obtained at 121 ^oC for 15 minutes at solid/solution ratios of 0.01 - 0.04 (Mautner et al., 1995).

e.       Elemental concentrations in soil solution. Concentrations in ppm (Bowen, 1966).

f.        Elemental concentrations in bacterial dry biomass in ppm (Bowen, 1966).

g.       Calculated maximum bacterial populations (CFU/ml) allowed by the concentration of a given nutrient in the extracts of Allende and Murchison obtained by extractions at r_{solid/solution} = 10 g/ml as given in rows 2 and 4. Calculated using equation (4) for bacteria with radius of 1 μm, dry mass of 2x10^-12 g, and elemental content per gram bacterial dry mass as in row 6 (Bowen, 1966).