Choosing Martian Samples for Fossil Biosignature Analysis of Kerogen:
Fluorescence microscopy and Raman spectroscopy applications to analog samples
1. Abstract
4.2 Raman Spectroscopy Results
2. Introduction/Motivations
Raman spectroscopy can be a potentially useful tool in planetary exploration for
selecting samples for in situ analysis, or for return to terrestrial labs. The ability
to identify both mineral and organic materials in samples non-destructively
makes the technique appealing to search for fossils preserved on Mars.
Although Raman instruments have been developed and proposed for missions to
Mars (e.g. ESA ExoMars and Mars Exploration Rovers), problems with background
fluorescence frequently challenge the identification of minerals and organic
materials in complex mineral mixtures. As a result, no Raman has yet flown to
Mars. The goal of this study is to identify specific challenges in developing Raman
for flight missions and to explore pathways for future development. See Fig. 1.
Fluorescence is seen in immature kerogen. As it
matures, kerogen becomes less fluorescent [8]. Here,
microscopy was uninformative of the locations of
kerogen. Not all kerogens displayed the anticipated
properties.
Immature kerogen was seen as translucent or amber
and mature kerogen was seen as dark brown to black,
opaque in transmitted white light. Kerogen in young
and old samples fluoresced ambiguously (or not at all).
Image Courtesy E. Soignard
Specification
Range
Laser
Configuration
Maximum Output Power
Laser Spot Size
Spectral Resolution
Objective
Value
90 – 2000 cm-1
523 nm green monochromatic Compass Solid State, YAG
microprobe (point analysis)
100 mW
1 micron
2.3cm-1 at the highest resolution
50x ultra long, working-distance
Fig. 8. No fluorescence example
Fig. 3. Raman system setup (top left) with zoomed setup view (top right) and summary of Raman Instrument
specifications (bottom).
Settings: 6 - 10 exp. of 20-180 seconds and 40-175 uW.
Fig. 4. Raw spectra of fluorescing
samples. Raw spectra of four
highly
fluorescent
samples
showing challenges of ID’ing
kerogen and mineral phases. In
Figs. 5-7, Crystal Sleuth [9] was
used to subtract this fluorescence.
The stromatolite showed filamentous structures
suggestive of extracellular sheath materials, but these
did not fluoresce (Fig. 8). The Guerrero Negro sulfate
also showed similar features.
5. Discussion, Conclusions, and Future Steps
• Major aqueous mineral groups studied here showed high preservation potential of kerogen as a
biosignature.
• In a MSR context, they would be high on the list to be brought back to Earth for definitive
biosignature identification.
Intensity
Raman spectroscopy has been shown to be a useful method for non-destructive
in situ identification of kerogen within mineral matrices, making it an appealing
method for sample selection for in situ robotic analysis. Howeverm Laser
Raman does present challenges due to the common occurrence of background
flurescence from matrix minerals, or immature organic matter. The goal of this
study is to identify the challenges of laser Raman as a stand-alone method for
identifying kerogen in sedimentary rocks, and if possible, pose identify
technology solutions that might make the Raman technique more effective in
situations of high background fluorescence. The empirical work reported here
analyzed 7 thin sections of Martian analog materials representing a variety of
environments and ages. This was done by: (1) mapping and imaging suspected
kerogen using UV fluorescence and petrographic analyses to understand
microtextural context, and (2) using a green (532 nm) micro-Raman system to
perform localized analyses of areas suspected to contain kerogen. Challenges
and lessons learned for developing Raman spectroscopy for missions are
discussed. Recommended techniques for further exploration which might
resolve fluorescence and other challenges are discussed.
4.3. Fluorescence Observations
• This study highlights many challenges of miniaturized Raman systems proposed for flight.
200
400
600
800
1000
Raman Shift (cm-1)
1200
1400
1600
• Challenges, especially those not discussed in the relevant literature, are:
1800
1. Sample-dependence of Raman spectroscopy
 Most samples with matrices composed of aqueous minerals (sulfate, carbonate, clays-Fig. 6)
fluoresce: major challenge for kerogen detections, except for cherts)
 False negatives (Fig. 5)
o
Kerogen under translucent matrix? (Fig. 6)
o
Kerogen peaks obtained after much effort and background subtraction (Fig. 6)
 Simplest samples most readily showed kerogen (Fig. 7)
 Analaysis of natural (unprepared) surfaces?
~1350 cm-1, ~1535 cm-1: Kerogen?
~308 cm-1 :Calcite
Intensity
Figure 1. Summary of aims of the present study
3. Methods and Samples
Chemical and optical analyses offer the best assessment of kerogens [4] and are
standard in analyses of this type [4]. Kerogen identification was performed in 2
phases on the samples according to Fig. 2.
205
395
685
775
965
1155
Raman Shift (cm-1)
1345
1535
1725
2. Approaches for removing fluorescence
 Optimize the excitation wavelength range
 Laser power setting and integration times difficult to optimize without visual imaging aid
 Background removal to reveal kerogen peaks?
 Targeting location of kerogen before analysis: Feasibility under mission scenario? (Fig. 5)
1915
Fig. 5. Green River Fm. Oil Shale. A background-subtracted spectrum for amber, A, and dark, B, region types imaged.
Samples (Table 1) were chosen as analogs for high priority geologic environments
previously identified as important astrobiological targets for Mars Exploration,
especially sulfates [5;6;7]. Petrographic thin sections (35 microns) were used.
~480 cm-1:
Gypsum
3. Lack of integrated testing using natural analog geological materials to inform engineering design
considerations
Kerogen??
• Potential solution for background fluorescence: time-resolved laser system using laser pulse for
fluorescence rejection. See Figs 9-10 for time-resolved (T-R) results for Mars‐relevant minerals.
Intensity
~790 cm-1:
Andalusite?
Ferrierite-Mg?
Fig. 2. Summary of three-phase procedure
Locality
Age
Mineralogy/Composition
Chert
Chinaman Creek, Pilbara, W.
Australia
Archaean
(3.8-2.5 BYA)
Chert (silica, kerogen)
Chert
Gunflint Formation, Chert dyke,
Apex Chert
Proterozoic (2.5 BYA 540 MYA)
Chert (quartz, kerogen)
Oil Shale
Green River Formation, WY
Eocene
(56 - 34 MYA)
Clays, kerogen
Stromatolite
Walker Lake, NV
Holocene
(12,000 YA)
Columnar Stromatolite
(calcite, kerogen)
Sulfate
(Gypsum)
Castille Formation, NM
Permian
(300 – 250 MYA)
Gypsum, calcite, kerogen
Sulfate
(Gypsum)
Castille Formation, NM
Permian
(300 – 250 MYA)
Gypsum, calcite, kerogen
Guerrero Negro, Baja Sur, Mexico
Holocene
(12,000 YA)
Sulfate
(Gypsum)
210
610
810
1010
Raman Shift (cm-1)
1210
1410
1610
1810
Fig. 6. Guerrero Negro, Baja Sur Sulfate. Background-subtracted spectra of microfossil imaged. Kerogen peaks are
not seen, possibly due to phyllosilicate (clay) or immature kerogen fluorescence.
Settings: 13 60-120 sec at 4.5-13 uW
~1350 cm-1, ~1600 cm-1:
Kerogen?
~463 cm-1:
Quartz
200
Gypsum, calcite, kerogen
410
Intensity
Sample Type
400
600
800
1000
1200
-1
Raman Shift (cm )
1400
1600
1800
Fig. 7. Both cherts. No background removal needed due to no fluorescence. These had the most identifiable kerogen.
Table 1. Summary of sample characteristics
Fig. 9. Mg sulfate, obscured by fluorescence when
measured using regular Raman (red). With T-R Raman
(green) the mineral was identified and compared to
others. From [10] and refs therein.
Fig. 10. T-R Raman spectra of spodumene (green) from
pulsed Raman and regular spectrum (red). RRUFF
spectrum is shown for reference. Image from [1].
Acknowledgements: The NASA ASU Space Grant Consortium is acknowledged for supporting a portion of this work through a Fellowship to S.S. Also, NASA Astrobiology Institute and NASA Mars exploration Program are also acknowledged for support.
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641-7. [9] Laetsch T, and Downs, R. (2006). Abstract, 19th GM of the IMA, Kobe, Japan.[10] Blacksberg, J. et al. (2011). Abstract #1166, 42nd LPSC, Houston, TX.
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