SWINBURNE UNIVERSITY
OF TECHNOLOGY
HET618
ASTROBIOLOGY AND THE ORIGINS OF LIFE
ESSAY
THE SEARCH FOR BIOMARKERS
AND THE ANALYSIS OF
TO THE MARTIAN METEORITE ALH84001
Roberto Bartali
COURSE INSTRUCTOR
DR. TERRY BRIDGES
ABSTRACT
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:
- Defining biomarkers.
- 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.
Direct observation
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
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].
Organic compounds
Fourier transform Raman spectroscopy can be used to observe the presence of organic
compounds like chlorophyll and calcium oxalate [Pasteris et al. 2003].
Specific minerals
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].
Kerogen
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].
Nanobacteria
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
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
Low temperature
 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].
High Temperature
 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].
Shock event
 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.].
Inorganic
 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.].
Biologic
 Similar structures found on other martian meteorites and on the Earth, are
validating the hypothesis of biological origin [Gibson et al.].
Magnetite
Biologic
 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.].
Non biologic
 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 hightemperature (>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 ].
Terrestrial contamination
 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.].
CONCLUSIONS
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.
IMAGE CREDITS
Figure 1
ALH84001
http://www.nasa.gov/audience/forstudents/postsecondary/features/mars_life_feature_1015_
prt.htm
Figure 2
Microbes: Exobiology in the Solar System and the Search for life on Mars, ESA, 1999.
Figure 3
Fluorescent microbes (adapted from):
Exobiology in the Solar System and the Search for life on Mars, ESA, 1999.
Figure 4 left
Magnetite (micro-crystals):
http://www.astrobio.nau.edu/~koerner/ast180/lectures/lec15.html
Figure 4 right
Magnetite: http://gwydir.demon.co.uk/jo/minerals/pix/magnetite2.jpg
Figure 5 left
Columbia River microfossil:
http://zebu.uoregon.edu/~imamura/121/images/lower_columbia_bacteria.gif
Figure 5 centre
Apex chert microfossil:
http://astronomy.com/asy/objects/images/micro_rb_line_0205_500.jpg
Figure 5 right
Stromatolite:
http://www2.aclyon.fr/enseigne/biologie/photossql/photos.php?RollID=images&FrameID=stromatolite
Figure 6
ALH84001 Microfossils: http://hubble.uhh.hawaii.edu/UHH/tubules.gif
Figure 7
ALH84001 Carbonate globules:
http://www.lpi.usra.edu/publications/newsletters/lpib/lpib82/alh84001.html
Figure 8
ALH84001 Magnetite crystals:
http://science.nasa.gov/headlines/images/mars_life/crystal_compare.jpg
Figure 9
Magnetoactic bacteria: http://science.nasa.gov/headlines/y2000/ast20dec_1.htm
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