Choosing Martian Samples for Fossil Biosignature Analysis of Kerogen:

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© NASA/JPL, Spirit rover

5.
Discussion, Conclusions, and
Future Steps

Choosing Martian Samples for Fossil
Biosignature

Analysis of
Kerogen
:



Fluorescence
microscopy and Raman spectroscopy applications to analog samples





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
.


1. Abstract

S
p
e
c
i
f
i
c
a
t
i
o
n

V
a
l
u
e

R
a
n
g
e

9
0



2
0
0
0

c
m
-
1

L
a
s
e
r

5
2
3

n
m

g
r
e
e
n

m
o
n
o
c
h
r
o
m
a
t
i
c

C
o
m
p
a
s
s

S
o
l
i
d

S
t
a
t
e
,

Y
A
G

C
o
n
f
i
g
u
r
a
t
i
o
n

m
i
c
r
o
p
r
o
b
e

(
p
o
i
n
t

a
n
a
l
y
s
i
s
)

M
a
x
i
m
u
m

O
u
t
p
u
t

P
o
w
e
r

1
0
0

m
W

L
a
s
e
r

S
p
o
t

S
i
z
e

1

m
i
c
r
o
n

S
p
e
c
t
r
a
l

R
e
s
o
l
u
t
i
o
n

2
.
3
c
m
-
1

a
t

t
h
e

h
i
g
h
e
s
t

r
e
s
o
l
u
t
i
o
n

O
b
j
e
c
t
i
v
e

5
0
x

u
l
t
r
a

l
o
n
g
,

w
o
r
k
i
n
g
-
d
i
s
t
a
n
c
e




Sample Type

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

Sulfate
(Gypsum)

Guerrero Negro, Baja Sur, Mexico

Holocene

(12,000 YA)

Gypsum, calcite,
kerogen



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)
.




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
.



Fig
.

4
.

Raw

spectra

of

fluorescing

samples
.

Raw

spectra

of

four

highly

fluorescent

samples

showing

challenges

of

ID
’i
ng

kerogen

and

mineral

phases
.

In

Figs
.

5
-
7
,

Crystal

Sleuth

[
9
]

was

used

to

subtract

this

fluorescence
.


Acknowledgements
:

The

NASA

ASU

Space

Grant

Consortium

is

acknowledged

for

supporting

a

portion

of

this

work

through

a

Fellowship

to

S
.
S
.

Also
,

NASA

A
s
trobiology

Institute

and

NASA

Mars

exploration

Program

are

also

acknowledged

for

support
.


References
:

[
1
]

Blacksberg
,

J
.

et

al
.

(
2010
)
.

Applied

Optics
,

49
(
26
),

4951
-
4962
.

[
2
]

Wang,

et

al
.

(
2006
)
.

Geochim

et

Cosmochim

Acta
,

70
,

6118

6135
.

[
3
]

Marshall,

C
.

P,

et

al
.
,

(
201

0
)
.

Astrobio

10
(
2
)
:

229
-
243
.

[
4
]

Senftle
,

J
.

T
.
,

et

al,

(
1987
)
.

Intern

J

Coal

Geo
,

7
,

105
-
117
.

[
5
]

Farmer,

J
.

D
.

and

Des

Marais,

D
.

J
.

(
1999
)
.

J

Geophys

R

104
:

26977
-
26995
.

[
6
]

Gendrin
,

A
.

et

al
.
,

(
2005
)
.

Science
,

307
,

pp
.

1587
-
91
.

[
7
]

Schopf
,

W
.

et

al
.

(
2012
)
.

Astrobio
,

(
7
),

619
-
33
.

[
8
]

Bertrand,

P
.
,

et

al
.

(
1986
)
.

Adv

in

Org

Geochem
,

10
,

641
-
7
.

[
9
]

Laetsch

T,

and

Downs,

R
.

(
2006
)
.

Abstract,

19
th

GM

of

the

IMA,

Kobe,

Japan
.
[
10
]

Blacksberg
,

J
.

et

al
.

(
2011
)
.

Abstract

#
1166
,

42
nd

LPSC,

Houston,

TX
.


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
]
.

3. Methods and Samples

2. Introduction/Motivations

4.2 Raman Spectroscopy
Results

4.3. Fluorescence Observations

Review general
use

of Raman
to ID minerals
and
kerogen

in
diverse analogs

Understand challenges
of background
fluorescence in
IDing

kerogen

in sulfates,
carbonates, and shale.

Assess potential of
gated Raman system
for overcoming
fluorescence
problems.

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),

p
roblems

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
.

Fig. 3.
Raman system setup
(top left
) with zoomed setup view
(top right) and
summary of Raman Instrument
specifications (bottom).

Fig. 2. Summary of three
-
phase procedure

Figure 1. Summary of aims of the present study

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
.



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
.






Table 1. Summary of sample characteristics

Fig.
8
. No fluorescence example



Fig. 5
. Green River Fm. Oil
Shale.
A
background
-
subtracted
spectrum for
amber, A, and dark,
B,
region types
imaged.


Fig.
6. Guerrero Negro,
Baja Sur
Sulfate.
Background
-
subtracted spectra
of microfossil imaged.
K
erogen

peaks are
not
seen, possibly
due to
phyllosilicate

(clay) or immature
kerogen

fluorescence.

Fig
.

7
.

Both

cherts
.

No

background

removal

needed

due

to

no

fluorescence
.

These

had

the

most

identifiable

kerogen
.


~308
cm
-
1
:Calcite

~1350 cm
-
1,
~1535
cm
-
1
:
Kerogen
?

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
.


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
.




This

study

highlights

many

challenges

of

miniaturized

Raman

systems

proposed

for

flight
.




Challenges
,

especially

those

not

discussed

in

the

relevant

literature,

are
:



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
?


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)


3. Lack
of
integrated testing using natural analog geological materials
to
inform engineering design
considerations






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.

Settings
:

6

-

10

exp
.

of

20
-
180

seconds

and

40
-
175

uW
.









200 400 600 800 1000 1200 1400

1600 1800




Raman Shift (cm
-
1
)


205 395 685 775 965 1155 1345

1535 1725 1915



Raman
Shift (cm
-
1
)

Image Courtesy E. Soignard

~463 cm
-
1
:

Quartz

~1350 cm
-
1
, ~1600
cm
-
1
:

Kerogen
?

Settings
:
13
60
-
120 sec
at

4.5
-
13
uW



200 400 600 800 1000 1200 1400

1600 1800


Raman Shift (cm
-
1
)

Intensity

Intensity

~480 cm
-
1
:

Gypsum

~790 cm
-
1
:

A
ndalusite
?


Ferrierite
-
Mg?

Kerogen
??


210 410 610 810 1010 1210 1410

1610 1810


Raman Shift (cm
-
1
)

Intensity

Intensity