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Raman spectroscopy as a
screening tool for ancient life
detection on Mars
Craig P. Marshall and Alison Olcott Marshall
rsta.royalsocietypublishing.org
Research
Cite this article: Marshall CP, Olcott Marshall
A. 2014 Raman spectroscopy as a screening
tool for ancient life detection on Mars. Phil.
Trans. R. Soc. A 372: 20140195.
http://dx.doi.org/10.1098/rsta.2014.0195
One contribution of 14 to a Theme Issue
‘Raman spectroscopy meets extremophiles on
Earth and Mars: studies for successful search
of life’.
Subject Areas:
geochemistry
Keywords:
Raman spectroscopy, reduced carbon,
biomarkers, biosignatures, gas
chromatography–mass spectrometry,
sample screening
Author for correspondence:
Craig P. Marshall
e-mail: [email protected]
Department of Geology, The University of Kansas, Lawrence,
KS 66045-7613, USA
The search for sp2 -bonded carbonaceous material
is one of the major life detection strategies of the
astrobiological exploration programmes of National
Aeronautics and Space Administration and European
Space Agency (ESA). The ESA ExoMars rover
scheduled for launch in 2018 will include a Raman
spectrometer with the goal of detecting sp2 -bonded
carbonaceous material as potential evidence of ancient
life. However, sp2 -bonded carbonaceous material will
yield the same Raman spectra of well-developed
G and D bands whether they are synthesized
biologically or non-biologically. Therefore, the origin
and source of sp2 -bonded carbonaceous material
cannot be elucidated by Raman spectroscopy
alone. Here, we report the combined approach of
Raman spectroscopy and gas chromatography–mass
spectrometry biomarker analysis to Precambrian
sedimentary rocks, which taken together, provides
a promising new methodology for readily detecting
and rapidly screening samples for immature organic
material amenable to successful biomarker analysis.
1. Introduction
The search for reduced carbon is one of the major goals of
astrobiological exploration programmes of the National
Aeronautics and Space Administration (NASA) and the
European Space Agency (ESA) as part of life detection
strategies (e.g. [1]). The upcoming ExoMars mission,
slated to fly in 2018, will include a miniaturized Raman
spectrometer [2] that will be used to detect various
analytes, in particular reduced carbon [1]. Reduced
carbon, more properly referred to as carbonaceous
sp2 -bonded network solids, are substances of which
the major component is carbon. In addition, these
materials can contain varying quantities of substituents
2014 The Author(s) Published by the Royal Society. All rights reserved.
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(a) Samples
Samples were collected from Precambrian glacial sedimentary rocks from the Vazante Group of
Brazil. The Vazante Basin is located in the northwestern part of Minas Gerais, on the São Francisco
Craton. The craton is composed of Archean and Paleoproterozoic basement rocks covered by
Meso- and Neo-proterozoic sedimentary sequences. Sedimentary rocks of the Vazante Group
outcrop over ca 10 000 km2 , but are generally too highly metamorphosed for organic geochemical
studies [24]. However, some subsurface samples have been drilled, which exhibit only slight
metamorphism [25,26]. Seven separate formations are recognized within the Vazante Group (from
oldest to youngest): Santo Antônio do Bonito, Rocina, Lagamar, Serra do Garrote, Serra do Poço
Verde, Morro do Calcário and Lapa. The Vazante Group was first recognized as glaciogenic [27],
although its age has long been contentious. On the basis of lithology and chemostratigraphy,
.........................................................
2. Material and methods
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for example, hydrogen and heteroatoms such as oxygen, nitrogen, sulfur and phosphorous
contingent upon the source and grade of metamorphism that these materials have experienced.
From a molecular perspective, the degree of structural organization of carbonaceous sp2 -bonded
network solids can be variable, ranging from well crystalline, consisting of flat stacked layers
of sp2 -bonded carbon in sixfold rings, to poorly crystalline with short, curved or discontinuous
layers of sp2 -bonded carbon in sixfold rings that lack long-range continuity, to nanocrystalline
carbon, which consists of nanometre-sized crystallites of sp2 -bonded carbon.
The carbonaceous sp2 -bonded network solids can be formed from carbonaceous material
of biological or non-biological origin. It is worth noting that biological processes in and
of themselves cannot form this material, although biologically produced organic material in
sedimentary rocks can be thermally altered to form carbonaceous sp2 -bonded network solids. By
contrast, carbonaceous sp2 -bonded network solids can be formed by high-temperature heating
of inorganic compounds [3,4], inorganic precipitation from hydrothermal fluids [5–7] and redox
reactions during serpentinization [8,9], all of which are carbonaceous material of non-biological
origin. Despite this abiotic pathway of formation, the Raman signal acquired from carbonaceous
sp2 -bonded network solids has been erroneously postulated by some (e.g. [10–13]) as evidence of
the thermal alteration of biological remains, and thus evidence of the presence of life. However
others [3,14,15] have clearly demonstrated that Raman signals acquired from biologically and
non-biologically synthesized carbonaceous sp2 -bonded network solids are indistinguishable.
Similarly, it has been proposed that the presence of a shoulder on the low-frequency side of the
D band of carbonaceous sp2 -bonded network solids should be used as a definitive biosignature
[10]. However, carbonaceous sp2 -bonded network solids synthesized non-biologically such as
nanostructured carbons (e.g. multi-walled carbon nanotubes) and inorganic chars can also display
this low-frequency shoulder on the D band [16,17].
Despite these limitations, Raman spectroscopy can be useful to prospect for biosignatures,
but only if used in conjunction with other approaches, rather than as definitive proof [18–21].
For example, gas chromatography–mass spectrometry (GC–MS) is an analytical technique used
for identifying biomarker compounds in thermally immature rocks. These compounds are
derived exclusively from formerly living organisms. Biomarkers are complex compounds that
are composed of carbon, hydrogen and heteroatoms. Structurally, they show little or no change
from their parent organic molecules in living organisms (e.g. [22]). Biomarkers originate from
many biological sources that are discernible from their diagnostic structural characteristics.
Contributions from bacteria, algae, vascular plants and animals have been recognized among the
complex assemblage of molecular species present in sediments and sedimentary rocks (e.g. [23]).
Here, we illustrate the utility of Raman spectroscopy as a screening tool for the elucidation
of promising samples that may yield organic molecules of unequivocal biological origin upon
subsequent analysis with mass spectrometric techniques.
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Carbonaceous sp2 -bonded network solids were isolated by agitation in HCl and HF for 2 days and
ca 2 mg were weighed into a tin sample boat. The elemental H and C contents were determined
using a Perkin–Elmer 2400 Elemental Analyzer (the H/C ratio values were measured by the
commercial laboratory Baseline Resolution, Shenandoah, TX, USA).
(c) Raman spectroscopy
Standard petrographic thin sections were analysed using Raman spectroscopy. Spectra were
collected at several different locations in multiple thin sections with a Renishaw InVia Reflex
Raman Microprobe (Renishaw plc, Wotton-under-Edge, UK), equipped with a Peltier cooled
charge-coupled device (CCD) camera (1024 × 256 pixels). The collection optics are based on a
Leica DMLM microscope. A refractive glass ×100 (0.9 NA) objective lens was used to focus the
laser to a 1 μm spot to collect backscattered radiation. The Raman light was dispersed by a
diffraction grating with 2400 mm per line, and the signal was analysed with a Peltier cooled CCD
camera at room temperature (1024 × 256 pixels). The 514.5 nm line of a 5 W Ar+ laser (SpectraPhysics Stabilite 2017 laser) orientated normal to the sample and to the laminations in the shale
was used to excite the sample. Calibration of the Raman shift is achieved by recording the Raman
spectrum of the silicon F1g mode for one accumulation and 10 s. If necessary, an offset correction
is performed to ensure that the position of the F1g band is at 520.50 ± 0.10 cm−1 . Surface laser
power settings of 1.0–1.5 mW were used to minimize laser induced heating of the carbonaceous
sp2 -bonded network solids. Such heating can be readily detected as a downward shift in the G
band to 1565 cm−1 , which was not observed for any of the spectra acquired. Each spectrum was
acquired using 10 scans and an accumulation time of 30 s, which gave good signal-to-noise ratio.
Scan ranges were 1000–1800 cm−1 in the carbon first-order region, and whole spectral region scans
100–4000 cm−1 were also collected. Multiple analyses were performed on each thin section on
carbon clots a few millimetre away from one another.
(d) Biomarker analysis
Samples for biomarker analysis were processed one at a time to avoid cross contamination.
Approximately 30 g of rock were washed in distilled water. Samples were air-dried at room
temperature and crushed to less than 5 cm pieces with a jaw-type rock crusher whose exposed
surfaces had been cleaned (4×) with acetone and then dichloromethane. The pieces were
sonicated in 9 : 1 dichloromethane and methanol for 2 min and crushed again to less than 1 cm
with a smaller rock crusher cleaned as above. The ultrasonic extraction was repeated, and the
.........................................................
(b) Elemental H/C analysis
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many researchers identify the sediments as Neoproterozoic [24,28], although some have proposed
an older age, ca 1000 Ma, on the basis of stromatolite biostratigraphy [29] and Re-Os chronometry
[30]. Regardless of age, the Lapa Formation is recognized as a post-glacial, cap carbonate, while
the Serra do Garrote, Serra do Poço Verde and Morro do Calcário formations are all synglacial
and contain glaciogenic lithologies such as diamictite and lonestones.
In 2002, the authors sampled the Serra do Poço Verde Formation from core MAF 42–88,
originally recovered in 2000 at 17◦ 29 49 S, 46◦ 49 49 W by the Brazilian mining company
Votorantim. Samples were collected from a black shale unit, as well as the flanking diamictic
and carbonate marl units. Thin sections of samples from the black shale unit reveal finely
disseminated solid crystalline carbonaceous material, as well as abundant pyrite and glendonite,
a pseudomorph of the hydrated low-temperature carbonate ikaite. Glendonite is viewed as an
indication that fresh organic material was buried, just as the formation of ikaite is thought
to require both cold temperatures and the rapid remineralization of organic matter to elevate
dissolved inorganic carbon [30]. By contrast, thin sections from the carbonate units flanking the
black shale contain little carbonaceous material or pyrite.
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(a) H/C values
The atomic hydrogen to carbon (H/C) ratios of the samples, 805, 812 and 816 (not determined)
m from the black shale, and 783 m from the carbonate marl varies from 0.48, 0.68 and 0.29,
respectively.
(b) Raman spectroscopic analyses
A collection of representative Raman spectra measured in the range from 100 to 4000 cm−1
acquired from the samples is displayed in figure 1. Samples from the black shale unit at 805,
812 and 816 m, and one from the carbonate marl unit (783 m) exhibit Raman spectra that are quite
different from one another (figure 1).
The Raman spectrum acquired from the carbonate marl sample at 783 m shows five broad
bands at ca 1350, 1600, 2720, 2935 and 3240 cm−1 . These bands are due to first- and secondorder phonon modes of carbonaceous sp2 -bonded network solids. The first-order bands at ca
1350 cm−1 is assigned to the D1 band which is a totally symmetric radial breathing mode of a
sp2 -bonded carbon sixfold ring mode (A1g symmetry) that becomes Raman-allowed when defects
are introduced into the lattice or decreasing dimension of crystallites, and the band at ca 1600 cm−1
is assigned to the G band which is due to a doubly degenerate in-plane C–C stretching mode
(E2g2 symmetry). Second-order bands result from overtone scattering (2 × 1360 = 2720 cm−1 , the
most intense, and 2 × 1620 = 3240 cm−1 , a weak band) and combination scattering (1620 + 830 =
2450 cm−1 , 1580 + 1355 = 2935 cm−1 ). The appearance of carbon second-order bands indicates
three-dimensional (triperiodic) ordering in carbonaceous sp2 -bonded network solids. On the
molecular level, the spectra indicate that the carbonaceous sp2 -bonded network solid is composed
of isolated polyaromatic layers that are becoming progressively stacked to form coherent
nanometric domains.
.........................................................
3. Results
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sample was powdered in a shatterbox that had been cleaned using grinding quartz sand followed
by acetone and dichloromethane rinses (4×). The powdered rock was extracted in a microwave
accelerated reaction system (MARS Express, CEM Corp.) as follows: 20 g rock powder were split
equally between five clean Teflon vessels; 25 ml of 9 : 1 dichloromethane and methanol was added
to each, and the samples were extracted at 100◦ C for 15 min with stirring. Extracts were filtered
through combusted glass fibre filters to remove particulates and the solvent was evaporated to
ca 30 ml under N2 at 35◦ C, taking care not to allow them to completely dry out. Elemental S
was removed by filtration through activated Cu and the S0 -free extract was evaporated to near
dryness under N2 . Silica gel column chromatography was used to separate the extracts into
aliphatic and aromatic fractions. The aliphatic fraction was transferred to a vial with hexane;
the solvent volume was reduced under N2 to 50 μl and the fraction analysed using GC–MS.
Hexadecanoic acid isobutyl ester was added to all samples as internal standard. Extracts from the
two intermediate stages of crushing were prepared and analysed following the same procedures.
All samples were analysed with a ThermoFinnigan Trace GC-DSQ quadrupole MS equipped with
a DB-5MS column (30 m × 0.25 mm, 0.25 μm film thickness). Aliquots (1 μl) were injected using a
PTV injector (35◦ C for 3 min, 14.5◦ C s−1 ramp to 200◦ C, then 12◦ C s−1 ramp to 350◦ C (held 3 min).
The column oven was programmed at 20◦ C min−1 ramp to 130◦ C, then 5◦ C min−1 ramp to 320◦ C
(held 20 min).
Laboratory blanks for all materials used in the laboratory procedures, including the solvents,
Cu, silica gel and MARS vessels, were analysed using GC–MS. No hydrocarbons, oil residues
or unresolved complex mixtures were observed. In addition, a block of basalt was baked
overnight at 450◦ C and spiked with a standard hydrocarbon solution and subjected to the entire
analytical procedure, no contaminants again being detected. Biomarker yields were confirmed
with replicate extractions of duplicate samples.
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5
core meterage
high-temperature
carbon band
carbonate band
812
C O band
C–H band
816
4000
3000
2000
Raman shift (cm–1)
1000
Figure 1. Representative Raman spectra measured in the range from 100 to 4000 cm−1 acquired from samples from the black
shale unit at 805, 812 and 816 m, and one from the carbonate marl unit (783 m).
Generally, the characterization of the molecular structure of carbonaceous sp2 -bonded network
solids are based on the dimensions and organization of these domains, that is, layer size (La ) and
stacking extension (Lc ). Raman spectroscopy can be used to elucidate La which is derived from the
intensity ratio of D and G bands given by: La = 44[ID /IG ]−1 (nm) [31]. The ID /IG ratio obtained
from the spectrum acquired from the carbonaceous sp2 -bonded network solid in the carbonate
marl sample is 1.68 which substituted into the above equation calculates a polyaromatic domain
layer size of 26 nm. Given that the diameter of a benzene ring is ca 0.28 nm, a polyaromatic layer
size of 26 nm consists of ca 93 fused aromatic rings.
By contrast, the Raman spectra acquired from samples 805, 812 and 816 m from the black shale
unit show vibration modes associated with organic functional groups. These spectra contain a
moderate broad band at 2700–3000 cm−1 , assigned to aliphatic C–Hx stretching modes, a band at
1650 cm−1 , assigned to ν (C=O) stretching mode, and a band between 1470 and 1435 cm−1 due to
the aliphatic deformational mode δ (C–H). Interestingly, the sample from a depth of 816 m from
the black shale unit contains minor G and D bands in addition to the functional group modes,
indicating that this functionalized organic material is undergoing carbonization and leading
to an enriched carbon composition with various degrees of condensation, aromatization and
two-dimensional (biperiodic) structural ordering.
(c) Gas chromatography–mass spectrometry analyses
Total ion chromatograms (TICs) for all the samples are presented in figure 2. The most abundant
compounds in all the extracts are long chain n-alkanes, based on their retention time, distribution
and fragmentograms (m/z 85). However, the distribution of n-alkanes varies for the samples
analysed (figure 2). While the extraction procedure does not recover n-alkanes lighter than ca C17 ,
samples from within black shale units, 805, 812 and 816 m are dominated by low molecular weight
(LMW) compounds, with the most abundant n-alkane less than n-C26 . By contrast, samples
extracted from carbonate marls (783 m) are dominated by higher molecular weight (HMW)
n-alkanes, with the most abundant at ca n-C30 .
Significantly, figure 3 shows that hopanes from C29 to C32 , dominated by the C29 αβ and C30 αβ
hopanes, were detected in all the black shale samples (805, 812 and 816 m), but not in samples from
the from carbonate marls (783 m). Similarly, figure 4 shows that C27 –C29 steranes, dominated by
the C27 20R ααα sterane, were detected in all the black shale samples but not in the carbonate
marl sample.
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805
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783
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meterage
783
805
812
816
retention time
Figure 2. TICs for all the samples from the black shale unit at 805, 812 and 816 m, and one from the carbonate marl unit (783 m).
The internal standard is marked by a black dot and the most abundant compounds in all the extracts are long chain n-alkanes,
assignment is based on retention time, distribution and fragmentograms (m/z 85).
meterage
783
C29
Tm
C30
Ts
C31
C32
gammacerane
SR
SR
805
812
816
retention time
Figure 3. Selected-ion GC–MS chromatograms from black shale units and carbonate marl unit. Partial m/z 191 (hopanes) trace
from samples from the black shale unit at 805, 812 and 816 m, and one from the carbonate marl unit (783 m), shown at the
same scale. The S and R peak assignments define the stereochemistry at C-22, Ts denotes the C27 18α(H),22,29,30-trisnorhopane,
Tm denotes the C27 17α(H),22,29,30-trisnorhopane.
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meterage
7
783
C27 ba
S
R
C2720R aa + C2920R ba
C29 20S aaa
C29 20R + S abb
C28 20R aaa
C28 ba
S R
C29 20R aaa
805
812
816
retention time
Figure 4. Selected-ion GC–MS chromatograms from black shale units and carbonate marl unit. Partial m/z 217 (steranes) trace
from samples from the black shale unit at 805, 812 and 816 m, and one from the carbonate marl unit (783 m), shown at the
same scale. The S and R peak assignments define the stereochemistry at C-20, while βα, ααα, αββ denote 13β(H), 17α(H)diasteranes, 5α(H),14α(H),17α(H)-steranes and 5α(H),14β(H),17β(H)-steranes, respectively.
4. Discussion
(a) Elemental H/C
The H/C ratio of carbonaceous sp2 -bonded network solids is often used to ascertain the thermal
maturity of a sample, as increasing thermal alteration leads to dehydrogenation, condensation
and aromatization reactions which removes H and thus leads to lower H/C values (e.g. [32]).
This elemental ratio can be used as a potential screening for samples likely to contain biomarkers
however, this technique is laborious and moreover, affords no information on molecular structure.
Here, we show that Raman spectroscopy can be a rapid, successful screening methodology that
can delineate which samples are useful for subsequent biomarker analyses and additionally, this
technique provides information on molecular structure.
(b) Raman spectra
The Raman spectra acquired from the carbonate marl at 783 m indicate thermally over-mature
carbonaceous material. Both the presence of evolved D and G bands, indicating carbonaceous
sp2 -bonded network solids, and additionally, the presence of overtone bands is consistent with
this conclusion. The lineshape of the carbon first-order region are similar to those of other
over-mature samples that have undergone amphibolite facies metamorphic alteration in the
temperature range of 500–700◦ C (e.g. [33,34]). By contrast, the spectra acquired from the black
shale unit contain aliphatic hydrocarbon and carbonyl functional group vibrational modes
indicating this material is a solid crystalline aliphatic macromolecular hydrocarbon. While the
samples at 816 m also contains D and G bands in the spectra, they also have a band at 1650 cm−1 ,
indicative of C=O functional groups. This functional group chemistry is suggestive that the
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C27 20S abb + C29 20S ba
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C27 20S aaa
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The distribution of n-alkanes is quite different between the two black shale samples and the
carbonate marl sample (figure 2). The black shale samples are dominated by LMW n-alkanes,
whereas the carbonate marl sample is dominated by HMW n-alkanes. However, the high maturity
indicated for this sample by the low H/C value of 0.29 and Raman spectra of well-developed D
and G bands is inconsistent with the preservation of even HMW n-alkanes. Therefore, it is possible
that the HMW n-alkane envelope in the carbonate marl sample represents trace contamination of
geologically younger material, a common problem for Precambrian samples [36]. The n-alkanes
are not considered to be biomarkers as they can be synthesized biologically or non-biologically.
For example, n-alkanes can be synthesized non-biologically via a process known as Fischer–
Tropsch synthesis. The n-alkanes are formed during this abiotic process by the catalyzed addition
of single carbons, with no obvious patterns in distribution, chain lengths and carbon number [37].
Bacteriohopanepolyols are pentacyclic organic compounds, whereas sterols are tetracyclic
organic compounds (figure 5). Exclusively, biological processes form both these classes of
compounds and thus these molecules can be used as target biosignatures for unequivocal extant
life detection. Figure 5 shows that bacteriohopanepolyols and sterol molecules generally contain
one or more oxygen atoms and often have carbon–carbon double bonds. During diagenesis, a
low-temperature relatively shallow burial process occurring after the death of an organism, the
oxygen atoms are cleaved from the molecule and the doubly bonded carbons are hydrogenated
to produce the saturated-hydrocarbon geolipid form of the biomarkers (figure 5). In most cases,
the reactions outlined above that occur during diagenesis have little or no effect on the skeletal
part of the molecule. Therefore, the geologically derived hopanes and steranes can also be used
as target biosignatures for unequivocal ancient fossil life detection.
Hopanes and steranes were detected above the background signal only in samples from black
shale units (i.e. those containing LMW n-alkanes) and producing Raman spectra indicative of
the thermally immature macromolecular hydrocarbon. Unlike n-alkanes, which are ubiquitous
and not indicative of a specific biologic input, hopanes and steranes are indicative of bacterial
and algal inputs, respectively [38,39]. Like the n-alkanes, these compounds are generally not
preserved above ca 200◦ C and have never been reliably detected in rocks reaching greenschist
facies metamorphism [35].
(d) Significance for screening techniques and sample return missions
This work demonstrates that carbonaceous sp2 -bonded network solids and thermally immature
macromolecular hydrocarbon solids are readily and rapidly detected by Raman spectroscopy.
We show that Raman spectra acquired from samples displaying well-developed G and D bands
and additionally second-order bands are indicative of samples too thermally mature to contain
any evidence of biologically derived organic material. Therefore, the source or origin of this
material is impossible to elucidate by Raman spectroscopy alone. However, we demonstrate
that Raman spectra acquired from samples showing bands due to vibrational modes from
skeletal hydrocarbon backbones, and carbon–oxygen functional groups might be promising
for subsequent analysis by GC–MS that could potentially reveal organic compounds of bona
fide biological origin. Furthermore, these biomarker results obtained by GC–MS on ancient
sedimentary material can afford source information such as bacterial and micro-algae inputs by
the presence of hopanes and steranes, respectively.
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(c) Extractable biomarkers
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sample has not undergone a high degree of thermal alteration, as C=O moieties are not preserved
at high temperature (e.g. [35]). Alternatively, the presence of D and G Raman bands in spectra
acquired from these samples could arise from an immature carbonaceous material with an
unusual aromatic nature containing C=O and C–H functional groups, rather than from a lack
of thermal alteration.
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OH
OH
R H or OH
R
R H, CH3 to n-C5H11
R
R
R H, CH3, C2H5
R H, CH3, C2H5
HO
Figure 5. Molecular structure of bacteriohopanepolyols and sterols and their geologically derived saturated-hydrocarbon
biomarkers.
The major goal of the ESA 2018 mission to Mars, ExoMars, is the search for traces of
past or present life [40]. The instrument payload has been purported to contain a drill for
obtaining samples from depths of up to 2 m, PanCam a tool for observing rock outcrops, HR
camera and microscope to investigate rock/core surfaces and variety of chemical analytical
instruments for analysing mineralogy and organics (IR and Raman spectroscopies, GC– and
laser desorption–MS) [40]. Raman spectroscopic analyses of thermally over-mature carbonaceous
sp2 -bonded network solids and thermally immature macromolecular hydrocarbon solids is
not sufficient to determine the biogenecity of these organic materials. However, we show
that Raman spectroscopy provides a promising new approach in astrobiological prospecting
for rapidly screening samples containing thermally immature macromolecular hydrocarbon
materials amenable to potentially successful biomarker analysis by GC–MS.
5. Conclusion
Here, we have demonstrated that Raman spectroscopy can be a valuable approach for delineating
thermally immature geological organic matter as an effective methodology for rapid screening
of samples prior to laborious biomarker analysis by GC–MS. This is of great significance for
astrobiological prospecting on Mars during the ExoMars 2018 mission in the search for traces of
past or present life. Promising samples that have a Raman spectrum indicative of low thermally
mature functionalized hydrocarbon that are acquired in a relatively short period of time can then
be selected for more intensive GC–MS analysis that is amenable for the detection of organic matter
of an unequivocal biological origin.
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OH
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rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20140195
R
OH
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Acknowledgements. A. J. Kaufman for field assistance and A. L. Sessions and F. A. Corsetti for useful discussion.
Funding statement. We thank the Votorantim Metais company and T.F. de Oliveira for samples and a NSF grant
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