ASTROBIOLOGY AND THE ORIGINS OF LIFE
THE SEARCH FOR BIOMARKERS
AND THE ANALYSIS OF
TO THE MARTIAN METEORITE ALH84001
DR. TERRY BRIDGES
Meteorites are small pieces of rocks formed in the solar nebula at the same time planets formed, so they are composed by the same chemical elements and compounds; they are also the remnant of asteroid impacts and some are parts of the rocks expelled from large impacts on other planets.
If fundamental chemical compounds for the development of life were present in the solar nebula, they are also embedded within meteorites, if some kind of life developed on the surface of another planet, their traces must be preserved into meteorites.
The search for life in meteorites is difficult; we have to find fossilized microorganism or thei biological remnant. There are some constrains that make the search for life difficult:
Elimination of terrestrial contamination.
Development of representative experiments.
Unbiased analysis of results.
In this essay, we will first introduce the martian meteorite ALH84001. In the second part, we will give a description of several types of biomarkers and then a summary of representative analysis done on it.
INTRODUCTION TO METEORITE ALH84001
ALH84001 (figure 1) is classified as an Orthopyroxenite, it is a fragment expelled 14 My ago from the surface of Mars, and landed in Antarctica 13,000 yr ago [Jull et al. 1995]. We know that it is martian [Mittlefehldt 1994a, 1994b] due to:
young crystallization age,
composition of gases trapped in its interior that resembles those of the Martian atmosphere measured by Viking landers [Bogard, Johnson, 1983, Miura et al. 1995]
ratio of oxygen isotopes that are different from those in terrestrial rocks [Clayton, Mayeda 1996].
We also known that it suffered different shocks processes and that it was (originally) an igneous rock, slowly cooled and exposed to a wet environment when it was on Mars. The importance of this meteorite relies on the controversial finding of structures similar to fossilized bacteria.
THE SEARCH FOR BIOMARKERS
All living forms interact with the environment and are able to make changes in the surface, the hydrosphere and the atmosphere of the planet; this changes are preserved into sedimentary rocks, so they can be traced and measured in direct or indirect form [Rothschild 1994, Hoover 2001; Hoover 2003]. The problem is just to recognize those changes, called biomarkers, in an unambiguous way, because there are also similar changes produced by natural (abiotic) processes.
The only known life is on Earth, we have to find analogue or identical modifications on another planet if we want to find life elsewhere.
Meteorites are normally collected thousand of years after they fall, they suffer from contamination and weathering, that can produce misinterpretations of experimental results.
We will describe, now, several biomarkers and techniques developed to assess their presence.
Cellular and subcellular structures can be identified directly under the microscope as ovoids, rods or spheres of a few micron in size (figure 2), it is possible to identify multicellular or chained organism, normally they are found in small fractures of rocks or forming thin layers in sedimentary rocks. To distinguish them from their abiotic counterparts, they are covered by some substances that react with nucleic acid which emit fluorescent light when exposed to UV (figure 3). Several techniques like Fluorescent, confocal laser scanning, scanning electron and transmission microscopy are successfully used to identify microbial organism in rocks.
Gas chromatography can be used to find living cells analysing their biochemical properties. Cells are exposed to nutrients and the gases emitted during their life cycle are measured. This technique is useful to find terrestrial contamination, because it is very improbably that living cells can survive into meteorites [Oyama et al, 1977; Oyama, Berdhal 1977].
Carbon and other isotopes
Another useful biomarker is the isotopic ratio of 12C to 13C. Biogenetic 12C is observed in excess compared to the inorganic oxidised carbon derived by the atmospheric CO2 and marine ion bicarbonate dissolution [Schidlowski 1992]. The determination of 14N to 15N; 16O to 17O and 18O; and 34S to 32 S provide important information on biological activities [Farquhar et al. 2000].
Fourier transform Raman spectroscopy can be used to observe the presence of organic compounds like chlorophyll and calcium oxalate [Pasteris et al. 2003].
Minerals produced by degradation of organic material are distinguishable from their abiotic counterpart abiotic because of their distinctive crystallography, morphology and isotopic ratio. Magnetite is one of them (figure 4) [Schwartz 1992]. Mossbauer spectroscopy is used to identify iron derived minerals [Bishop et al. 1993; Bishop et al. 1995; Bradley 2000].
Fossil and microfossils
Fossil and microfossil direct research (figure 5) is difficult as we go back in time because of the loss of organic molecules. All these selection criteria must be taken simultaneously: samples must not belonging to metamorphosed sediments, must be authentic constituent of the rocks, several specimen presents, minimum size of cells, central cavity and structural details in excess of the inorganic surround material, and associated with carbonaceous matter [Barghoorn, Schopf, 1966; Schopf, Packer 1987].
The bulk of the body material degrades after the death of the Organism but a miniscule quantity escapes decomposition and it is transformed into an insoluble compound called Kerogen [Binet et al., 2003; Skrzypczak et al. 2005].
Comparison with Earth’s nanobacteria and similar formations on meteorites under the microscope [Folk] can give a first clue of fossilized life forms, but further chemical analysis must be done.
Polycyclic Aromatic Hydrocarbons
Polycyclic Aromatic Hydrocarbons (PAH) are organic molecule composed of Hydrogen and Carbon that consist of multiple loops of carbons with strong chemical bonds; they are building blocks or containers, metabolic units and genetic information carriers. They can be preserved even when exposed to heat and radiation [Ehrenfreund 2006].
Carbonate globules may have formed by biologic and inorganic processes; they may be also associated with PAH. If organic, they contains fossilized remnant of bacteria.
ANALYSIS OF ALH84001
McKey et al [McKay et al 1996] conclusion of the analysis of ALH84001 is that even if most features can be explained by abiotic processes when taken separately, it is probable that, if taken all together, they represent the remnant of biologic activity occurred in the past on Mars. Their (but not conclusive) evidences are: PAH associated with carbonate globules (figure 7) and formation (younger than the rock) (figure 6) that resembles fossilized nanobacteria, crystals of magnetite and siderite minerals similar to those produced by magnetoactic terrestrial bacteria (figure 8 and 9) and mineralization of an igneous rock occurred under wet conditions.
A large series of alternative experiments followed in order to assess an alternative scenario or a confirmation for the observed features.
Here we will get a representative (briefly summarized) series of analysiss, classified according to some particular feature observed in ALH84001.
Carbonate globule formation
Oxygen isotopes ratio (16O/17O) is consistent to low temperature formation of carbonates [Holland et al ]
Precipitation at low temperature from the water of a saline, evaporating lake. [McSween et al. ]
Measuring the rates of cation diffusion and its thermochemistry in carbonate minerals: Mg in calcite and Ca in magnesite suggest that they formed at temperatures below 250°C [Kent et al. ]
Grown of ellipsoidal globules, 10-50 of micrometers in the laboratory under abiotic conditions, at 150°C, from water solutions rich in Ca, Mg and Fe [Golden et al].
At low pressure, siderite and hematite should be stable to ~475°C, and siderite alone up to 600°C. [Koziol ].
Layers of mica rich in alumina and magnesium probably formed with the carbonates in a high-temperature reaction between from clay minerals [Bearley].
From Rb-Sr and U-Pb dating, globules could be formed during large impacts 3.9 Ga ago [Borg et al.].
Multiple and wide peaks in the Raman spectra, amorphous silica structure suggest structural disorder caused by shock. [Conney et al].
Organic Carbon is a terrestrial contamination; Isotope composition of the Carbon associated with the pyroxene minerals (including PAH and Kerogen) represent carbonaceous meteorite material that fell onto Mars [Becker et al.].
Relatively large amount of clorine can be explained by an evaporitic origin [Holland et al.].
PAH may be produced by a suffused gas rich in H, CO and CO2 at 1100°C, when the rock formed or after a large impact event [Zolotov et al.].
Similar structures found on other martian meteorites and on the Earth, are validating the hypothesis of biological origin [Gibson et al.].
Crystal composition and morphology of submicron grains and the intracellular magnetite grains in the Earth bacterium MV-1, are almost identical, suggesting a biogenic origin [Tomas-Keprta et al., 2000].
Biogenic, low-temperature magnetites can form within or outside cells. Intracellular magnetites are free of element substituents, size range 20 to 150 nm, and parallelepipeds. [Thomas-Kerpta et al.].
Oxygen isotope composition of small grains of olivine suggest that magnetite formed in a high-temperature event (900ºC.) [Shearer 1999].
All the known shapes of magnetite grains have been reported in deposits from high-temperature (>500°C) fluids. Magnetite crystals in terrestrial bacteria are all single magnetic domain, but magnetites in ALH84001 are too small and aligned with respect to the carbonate grains. No known bacteria to do this [Scott 1999].
Magnetism of each small grains of pyrrhotite points in a different direction and could not have been hotter than 40ºC. A high temperature event follows the magnetization [Tremain ].
Magnetites could have formed by thermal decomposition of siderite or if oxygen pressure increased.[Koziol ].
Lead and PAH are correlated and due to the isotopic similarity of terrestrial lead, authors infer that lead and PAH are contaminants [Stephan et al.]..
Nanometer scale rods and spheres on small grains of calcite are contamination from the environment (inorganic or biologic), based on analysis of other meteorites. [Barrat et al.1999; Steele et al].
Portions of ALH84001 is contaminated by Actinomycetes bacteria. The organisms are present in dense colonies of Y-branched fibers, suggesting that they grew in the meteorite on Earth [Steele et al. 2000]..
Bacteria and bacteria like organic material
Micrometre size spherules of vaterite were grown from a solution of calcium chloride that was saturated in carbonate by bubbling carbon dioxide gas through it at 25°C [Vecht et al.2000].
ALH84001 contains terrestrial fossilized bacteria. [Gillet et al. 2000].
Sulfur eating bacteria are not responsible of changes in Sulfur isotopic ratios in meteorites. Sulfur may be affected by photochemical reactions [Farquhar et al.2000].
Inorganic spherules and ovoid shaped objects of 25 to 300 nm diameter, can be grown in laboratory from minerals in sterile environment or in the presence of living bacteria. [Kirkland et al 1999].
Bacteria Desulfovibrio desulfuricans does dissolve and alter maskelynite glass and other silicates, [VanCleve et al. 2006], when it was inoculated into terrestrial orthopyroxenite it grew and produced biofilms. Resulting surface of the orthopyroxene grains are similar to those of ALH84001 [Robbins et al.].
The wide distribution and the close association of PAH with aliphatic hydrocarbons suggest that it formed during a rapid cooling , after a large impact, in a metastable Fischer-Tropsch-like processes. [Zolotov et al.].
Microbes and nanobacteria can be found on carbonate hot-springs. Organic matter degrade, but biofilms are readily mineralized by sphere of silica of 50-300 nm diameter. [Allen et al.].
High I/Cl ratio suggest that carbonates are biogenic, but it can be also explained by non biologic processes. [Gilmour et al.].
Octahedral crystal of magnetite (300 nm side), are produced by thermophile bacteria (45 to 65ºC) by ferric oxide hydroxide process. Siderite globules of 3 to 5 micron diameter are formed by mesophilic bacteria (20 to 35ºC) as an extracellular product of their iron-reducing metabolisms. [Zhang et al.].
ALH84001 is the most controversial meteorite found. It contain evidences of extraterrestrial life, but maybe it contain only inorganic matter. Each new analysis give a full range of possibility, we think that we never get, from ALH84001, a definitive answer, we need to find many more samples of martian meteorites and almost one as soon as it fall on Earth.
If we take each evidence separately, we can find much different explanation for the observed features, but if we take all of them collectively, the most reliable conclusion is that ALH84001 contains fossilized martian microbes, but only a set of samples collected on the surface of Mars can prove or disprove this hypothesis.
We do not know yet, for sure, how life developed on Earth or from where it came from, so we have, first, to select unambiguous biomarkers and try to find them in meteorites, but this is a circular argument, because we are trying to find something that we do not know.
Finding extraterrestrial life evidence without any doubt is a very delicate issue and can imply severe political, social and religious consequences, and maybe, most people, and scientists too, are not yet ready.
Microbes: Exobiology in the Solar System and the Search for life on Mars, ESA, 1999.
Fluorescent microbes (adapted from):
Exobiology in the Solar System and the Search for life on Mars, ESA, 1999.
Figure 4 left
Figure 4 right
Figure 5 left
Columbia River microfossil:
Figure 5 centre
Apex chert microfossil:
Figure 5 right
ALH84001 Microfossils: http://hubble.uhh.hawaii.edu/UHH/tubules.gif
ALH84001 Carbonate globules:
ALH84001 Magnetite crystals:
Magnetoactic bacteria: http://science.nasa.gov/headlines/y2000/ast20dec_1.htm
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