724
| Nature | Vol 619 | 27 July 2023
Article
Diverse organic-mineral associations in
Jezero crater, Mars
Sunanda Sharma
1,26
✉
, Ryan D. Roppel
2,26
, Ashley E. Murphy
3
, Luther W. Beegle
4
,
Rohit Bhartia
5
, Andrew
Steele
6
, Joseph Razzell Hollis
7
, Sandra
Siljeström
8
,
Francis M. McCubbin
9
, Sanford A. Asher
2
, William J. Abbey
1
, Abigail C. Allwood
1
,
Eve L. Berger
9,1 0,1 1
, Benjamin L. Bleefeld
12
, Aaron S. Burton
9
, Sergei V. Bykov
2
,
Emily L. Cardarelli
1
, Pamela G. Conrad
6
, Andrea
Corpolongo
13
, Andrew D. Czaja
13
,
Lauren P. DeFlores
1
, Kenneth
Edgett
12
, Kenneth A. Farley
14
, Teresa
Fornaro
15
, Allison C. Fox
9,1 0,1 1
,
Marc D. Fries
9
, David
Harker
12
, Keyron
Hickman-Lewis
7
, Joshua
Huggett
12
, Samara
Imbeah
12
,
Ryan S. Jakubek
9,1 1
, Linda C. Kah
16
, Carina
Lee
9,1 0,1 1
, Yang
Liu
1
, Angela
Magee
12
, Michelle
Minitti
17
,
Kelsey R. Moore
14
, Alyssa
Pascuzzo
12
, Carolina
Rodriguez Sanchez-Vahamonde
12
,
Eva L. Scheller
18
, Svetlana
Shkolyar
19,20,21
, Kathryn M. Stack
1
, Kim
Steadman
1
, Michael
Tuite
1
,
Kyle Uckert
1
, Alyssa
Werynski
12
, Roger C. Wiens
22
, Amy J. Williams
23
, Katherine
Winchell
24
,
Megan R. Kennedy
12
& Anastasia
Yanchilina
25
The presence and distribution of preserved organic matter on the surface of Mars
can provide key information about the Martian carbon cycle and the potential of the
planet to host life throughout its history. Several types of organic molecules have
been previously detected in Martian meteorites
1
and at Gale crater, Mars
2
–
4
. Evaluating
the diversity and detectability of organic matter elsewhere on Mars is important for
understanding the extent and diversity of Martian surface processes and the potential
availability of carbon sources
1
,
5
,
6
. Here we report the detection of Raman and
fluorescence spectra consistent with several species of aromatic organic molecules in
the Máaz and Séítah formations within the Crater Floor sequences of Jezero crater,
Mars. We report specific fluorescence-mineral associations consistent with many
classes of organic molecules occurring in different spatial patterns within these
compositionally distinct formations, potentially indicating different fates of carbon
across environments. Our findings suggest there may be a diversity of aromatic
molecules prevalent on the Martian surface, and these materials persist despite
exposure to surface conditions. These potential organic molecules are largely found
within minerals linked to aqueous processes, indicating that these processes may
have had a key role in organic synthesis, transport or preservation.
There are multiple origin hypotheses for the presence of organic
matter on Mars from meteorite and mission studies. These include
in situ formation through water–rock interactions
5
or electrochemical
reduction of CO
2
(ref.
6
), or deposition from exogenous sources such
as interplanetary dust and meteoritic infall
1
, although a biotic origin
has not been excluded. Understanding the fine-scale spatial association
between minerals, textures and organic compounds has been crucial in
explaining the potential pools of organic carbon on Mars. The Scanning
Habitable Environments with Raman and Luminescence for Organics
and Chemicals (SHERLOC) instrument is a tool that enables this on
the Martian surface.
The Perseverance rover was designed for in situ science with the
ability to collect a suite of samples for eventual return to Earth
7
. The
rover’s landing site within Jezero crater combines a high potential for
past habitability as the site of an ancient lake basin
8
with diverse miner
-
als, including carbonates, clays and sulfates
9
that may preserve organic
materials and potential biosignatures
10
. The Jezero crater floor includes
three formations (fm)
11
; two of these, Máaz and Séítah, were explored
https://doi.org/10.1038/s41586-023-06143-z
Received: 7 June 2022
Accepted: 27 April 2023
Published online: 12 July 2023
Open access
Check for updates
1
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
2
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA, USA.
3
Planetary Science Institute,
Tucson, AZ, USA.
4
Melanie Sauer and Associates, LLC, Sierra Madre, CA, USA.
5
Photon Systems Incorporated, Covina, CA, USA.
6
Earth and Planets Laboratory, Carnegie Institution for Science,
Washington, DC, USA.
7
The Natural History Museum, London, UK.
8
Department of Methodology, Textiles and Medical Technology, RISE Research Institutes of Sweden, Stockholm, Sweden.
9
Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX, USA.
10
Texas State University, Houston, TX, USA.
11
Jacobs JETS II, Houston, TX, USA.
12
Malin Space Science Systems, Inc., San Diego, CA, USA.
13
Department of Geosciences, University of Cincinnati, Cincinnati, OH, USA.
14
Division of Geological and Planetary Sciences,
California Institute of Technology, Pasadena, CA, USA.
15
Astrophysical Observatory of Arcetri, INAF, Florence, Italy.
16
Department of Earth and Planetary Sciences, University of Tennessee,
Knoxville, TN, USA.
17
Framework, Silver Spring, MD, USA.
18
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA.
19
Department of Astronomy, University of Maryland, College Park, MD, USA.
20
Planetary Geology, Geophysics and Geochemistry Lab, NASA Goddard Space Flight Center, Greenbelt, MD, USA.
21
Blue Marble Space Institute of Science, Seattle, WA, USA.
22
Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, Lafayette, IN, USA.
23
Department of Geological
Sciences, University of Florida, Gainesville, FL, USA.
24
School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA.
25
Impossible Sensing, LLC, St. Louis, MO, USA.
26
These authors contributed equally: Sunanda Sharma, Ryan D. Roppel.
✉
e-mail:
sunanda.sharma@jpl.nasa.gov
Nature | Vol 619 | 27 July 2023 |
725
as part of the mission’s first campaign. Máaz, previously mapped as the
crater floor fractured rough unit, is highly cratered and broadly mafic
in composition; rover observations to date indicate a composition rich
in pyroxene and plagioclase
12
. Séítah, previously mapped as the crater
floor fractured 1 unit, is underlying and therefore presumed older than
Máaz and contains rocks that represent an ultramafic olivine-bearing
cumulate
13
. SHERLOC has observed three natural (as found) rock sur-
faces in Máaz and seven freshly abraded surfaces across Máaz and
Séítah (Fig.
1
and Extended Data Figs. 1 and 2). Abrasion consists of
removing the outer layer of the rock, which is weathered and covered
by Martian dust, using an abrading bit on the drill to create a 45 mm
diameter cylindrical hole of 8–10 mm deep. The gaseous dust removal
tool then removes residual fines with nitrogen gas
14
to reveal a flat,
dust-free surface for analysis. Four abrasion targets are associated
with rock cores that may be returned to Earth during the Mars Sample
Return campaign.
The SHERLOC instrument is a deep ultraviolet (DUV) Raman and
fluorescence spectrometer designed to map the distribution of organic
molecules and minerals on rock surfaces at a resolution of 100 μm
(ref.
15
). This approach enables spectral separation of weak Raman
scattering from stronger fluorescence emission, which can have
cross-sections that are 10
5
–10
8
times larger than Raman
15
, allowing
for measurement of both signals simultaneously. SHERLOC can detect
Raman scattering from roughly 700 to 4,000 cm
−1
and fluorescence
photons from 253 to 355 nm (see Methods for more detailed descrip
-
tions). SHERLOC includes a autofocus context imager (ACI) cob
-
oresighted with the spectrometer to collect high spatial resolution
(roughly 10.1 μm per pixel) grayscale images to place spectral maps
within the context of texture and grain sizes. The wide-angle topo
-
graphic sensor for operations and engineering (WATSON) imager pro
-
vides colour imaging and broader spatial context. Combined, these
enable spatial associations between organics and minerals to assess
formation, deposition and preservation mechanisms. SHERLOC has
previously observed fluorescence signatures consistent with small
aromatic compounds in three targets across the crater floor
16
that
align with previous findings on Mars and within Martian meteorites.
Fluorescence signals in the crater floor
Fluorescence signals were detected on all ten targets observed by
SHERLOC in the Jezero crater floor. They can be summarized by four
main feature groups (Fig.
2
). Group 1 is a doublet at roughly 303 and
325 nm; group 2 is a single broad band at roughly 335–350 nm; group 3
is a single broad band between roughly 270 and 295 nm and group 4 is a
pair of bands centred at roughly 290 and 330 nm. The scan parameters
are given in Extended Data Table 1. A two-sample Kolmogorov–Smirnov
test was done on the observed fluorescence maxima for each group to
determine whether they were statistically distinct from one another
a
b
Sol 83 Nataani
Sol 98 Bi la sana
Sol 141 Foux
Sol 161/162 Guillaumes
Sol 186 Bellegar
de
Sol 207/208 Gar
de
Sol 257/269 Dourbes
Sol 293/304 Quartier
Sol 349 MontpezatS
ol 370 Alfalfa
Séítah
Máaz
c
Natural (Maaz)
Abraded (Maaz)
Abraded (Seitah
)
*
*
*
Nataani
Bi la sana
Alfalfa
Guillaumes
Bellegard
e
Dourbes
Quartier
Gar
de
Montpezat
Foux
Average number of uor
escence detections
500
400
300
Number of detections
Nataani
Bi la sana
Foux
Guillaumes
Be
llegar
de
Gar
de
Dourbes
Quartier
Mon
tpezat
Alfalfa
200
100
0
Fig. 1 | Overview of targets analysed by SHERLOC during the crater f loor
campaign.
a
, High Resolution Imaging ScienceExperiment (HiRISE) image
of the region studied with the rover’s traverse marked in white, the boundary
between the Séítah and Máaz fm delineated by the light blue line, and each rock
target labelled. Scale bar, 100 m.
b
, Average number of f luorescence detections
(out of 1,296 points) from survey scans for each target interrogated by SHERLOC,
arranged in order of observation. *The acquisition conditions were different
for dust-covered natural targets as compared to relatively dust-free abraded
targets, possibly resulting in reduced detections.
c
, WATSON images of natural
(red box) and abraded targets (Máaz is the blue box, Séítah is the green box)
analysed in this study, with SHERLOC survey scan footprints outlined in white.
Two survey scans were performed on Guillaumes, Dourbes and Quartier. Sol 141
imaging on Foux had an incomplete overlap of WATSON imaging and SHERLOC
spectroscopy mapping. Scale bars, 5 mm.
726
| Nature | Vol 619 | 27 July 2023
Article
and found that groups 1–3 had null probabilities (likelihood that two
groups are samples of the same distribution) of less than 10
−40
. Group 4
was too small for a statistical assessment, but is considered qualitatively
different from the others.
The four fluorescence feature categories observed in the ten targets
presented here are all consistent with emission in the spectral range
shown by single ring aromatics and polycyclic aromatic hydrocar
-
bons
15
,
17
(Extended Data Table 2 and Extended Data Fig. 3). The num
-
ber of rings in aromatic compounds can be estimated following the
reported trend of emission spectra under DUV excitation
18
, in which
increasing emission wavelength is positively correlated with number
of aromatic rings; this was used to define the four fluorescence feature
categories used in this study (Extended Data Table 3). However, the
potential for non-organic luminescence
19
must also be considered for
each group and is discussed herein.
Group 1: doublet at roughly 303 and 325 nm
Two targets, Bellegarde and Quartier, showed the distinctive group
1 fluorescence feature (Fig.
3
). These peaks appear together with
constant relative positions and intensities, probably indicating
a single emitter. The Bellegarde target, located on the Rochette
rock in the Máaz fm, yielded detections on white crystals that are
probably hydrated Ca-sulfate based on SHERLOC and PIXL observa-
tions
16
; the fluorescence doublet feature was associated with these
areas (Fig.
3a,c
). The Quartier target, located on the Issole rock in
the Séítah fm, similarly contained white crystals that showed Raman
peaks at 1,010–1,020 cm
−1
and a broad band at roughly 3,500 cm
−1
whose intensities were positively correlated (Fig.
3b,d
). Sometimes,
minor peaks at roughly 1,140 and at 1,215–1,225 cm
−1
were also pre
-
sent. These peaks are consistent with a mix of sulfates
20
, potentially
including both Ca- and Mg-sulfate at different hydration states.
PIXL established that two different sulfate minerals were present,
namely Mg-rich sulfate (66 wt% SO
3
, 27 wt% MgO, 3 wt% CaO, 4 wt%
FeO) and CaMg sulfate (61 wt% SO
3
, 18 wt% MgO, 19 wt% CaO, 2 wt%
FeO). A Raman peak at roughly 1,649 cm
−1
was detected at one point
within the hydrated sulfate crystal where doublet fluorescence was
also present. This peak was accompanied by a small peak at roughly
1,050 cm
−1
and a broader feature that seemed to contain several
Group 2
335–350
Group 1
303 and 325
Group 4
290 and 330
Group 3
270–295
Group 3
12
11
10
Group 2
Group 2
Group 4
Group 1
Group 3
b
a
Relative intensity
Relative intensity
Relative intensity
Relative intensity
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
1.0
0.8
0.6
0.4
0.2
0
0.4
0.2
0
260
280
300
320
340
260
280
300
320
340
260
280
300
320
340
260
280
300
320
340
Wavelength (nm)
Natural tar
gets (Máaz)
Abraded tar
gets (Máaz)
Abraded tar
gets (Séítah)
Group 1 r
oughly 303 and 325 nm
Group 2 r
oughly 335–350 nm
Group 3 r
oughly 270–295 nm
Group 4 r
oughly 290 and 330 nm
Nataani
Bi la sana
Foux
Guillaumes
Bellegar
de
Alfalfa
Gard
e
Dourbes
Quartier
Montpezat
Máaz (Natural)
Máaz (Abraded)
Séítah (Abraded)
9
3
5
4
1
350
330
310
290
270
350
330
310
290
270
350
330
310
290
270
0
140
20
14
12
10
8
6
4
2
0
0
40
60
80
100
120
2
6
8
7
Fig. 2 | Summary of f luorescence features across targets.
a
, Histograms of
the
λ
max
(measured from raw data) of four f luorescence features that were
observed in survey scans in natural targets in Máaz (top, n = 84), abraded targets
in Séítah (bottom, n = 82) and abraded targets in Máaz (middle, n = 1070). Bins
of 1 nm show variation in band centres,
y
axes scaled to each dataset.
b
, Filtered
mean spectra from each target representing each f luorescence feature
category demonstrate characteristic band positions, normalized relative
intensities and colocated features between targets. The range of the SHERLOC
CCD is 250–354 nm. The rise in baseline below 270 nm is a boundary artefact
introduced by the filter and not representative of the sample data
45
.
Nature | Vol 619 | 27 July 2023 |
727
peaks between 1,330 and 1,410 cm
−1
(Fig.
3d
). Eleven sols later, several
high-resolution scans subsequently performed on the same area of
Quartier showed a nearly identical roughly 1,649 cm
−1
peak at three
points within hydrated sulfate crystals. In each case, the distinc
-
tive doublet fluorescence was detected as well as a broader feature
at 1,330–1,410 cm
−1
.
The group 1 fluorescence observations in Quartier (Séítah) are
consistent with the presence of a one or two-ring aromatic organic
molecule(s) within a hydrated sulfate crystal. It is also possible that
the observed emission comes from Ce
3+
concentrated within the
sulfate, given the close match in emission wavelengths in laboratory
data. Three Raman peaks at 1,060, 1,330–1,410 and roughly 1,649 cm
−1
are colocated with the three most intense doublet fluorescence and
strong hydrated sulfate signals. They were detected even after 11 sols
of surface exposure, although the hydration feature (OH stretch at
roughly 3,300–3,500 cm
−1
) decreased in intensity, indicating a change
in the hydration state after exposure to the Martian atmosphere. On
the basis of the relative positions and intensities of these peaks, they
represent at least two possibilities: vibrational modes of an organic
molecule that include a preresonant C=C stretch
21
, or asymmetric
stretching and bending modes from nitrate within the sample
22
.
The possibility of organics occurring within sulfates is supported
by evidence from studies of Martian meteorites
5
and in Gale crater
21
,
which show that sulfates may have a key role in forming, preserving
or transporting organic molecules in the Martian environment. The
combination of Raman and fluorescence data reported here could
constitute two lines of evidence that support the detection of organic
molecules within hydrated sulfate crystals, which is the simplest expla
-
nation for these observations. If both Raman and fluorescence sig
-
nals are inorganic in origin, nitrate and Ce
3+
in sulfate would need to
be colocated.
Group 2: single band at roughly 335–350 nm
The most common fluorescence feature detected was a single broad
(roughly 30–40 nm full-width at half-maximum (FWHM)) band cen
-
tred at roughly 335–350 nm. Group 2 fluorescence was observed on
all targets across both formations and showed the highest intensities
among the four fluorescence feature categories (Fig.
2
). The relative
Wavelength (nm)
340
330
320
310
300
290
280
270
260
250
Intensity
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Raman shift (cm
–1
)
2,000
1,900
1,800
1,700
1,600
1,500
1,400
1,300
1,200
1,100
1,000
900
800
Intensity
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0
ab
cd
Quartier
Bellegarde
303
325
1,060
1,300–1,500
1,649
550
500
450
400
350
1,850
1,800
1,750
1,700
1,650
1,600
1,550
1,500
Fig. 3 | Group 1 (roughly 303 and 325 nm) doublet f luorescence feature
mineral associations in Bellegarde and Quartier.
a
, Colourized ACI image
of a region where a survey scan (36 × 36 points over 5 × 5 mm
2
) was performed
on the Bellegarde target from sol 186. Green rings (rough laser beam diameter)
represent locations where the roughly 303 and 325 nm f luorescence doublet
was detected.
b
, Colourized ACI image of a region where a detailed scan
(10 × 10 points over 1 × 1 mm
2
) was performed on the Quartier target from
sol 304. Green rings represent locations where the roughly 303 and 325 nm
f luorescence doublet was detected. Scale bars, 1 mm.
c
, Median f luorescence
spectra (unfiltered) from the green points indicated in Bellegarde (red, n = 33)
and Quartier (black, n = 26) normalized to 303 nm band and offset for clarity.
d
, Median Raman spectra of four points with highest f luorescence band
intensities from Quartier scans on sols 293 and 304. Roughly 1,010 cm
−1
sulfate
band is off scale; inset shows roughly 1,649 cm
−1
band with Voigt fit (FWHM
53.737, area 12, 559, height 192.79). In the inset, the unfitted spectrum (red),
fitted spectrum (blue) and baseline (green) are shown;
y
axis is intensity.
728
| Nature | Vol 619 | 27 July 2023
Article
occurrence of this feature observed in survey scans of abraded targets
was markedly higher in Máaz (189 ± 96 counts) versus Séítah (26 ± 6
counts). However, the average intensity of this feature was comparable
between survey scans performed in the two formations (Máaz 342 ± 76
counts; Séítah 361 ± 80 counts). The measured intensity can vary on
the basis of several factors, including the concentration of the emit
-
ter, the focus of the spectrometer and the presence of an absorbing
material; therefore, large standard deviations are expected. Scans
from all abraded targets show group 2 fluorescence detections that
seem to be at or near grain boundaries in most cases (Extended Data
Table 2 and Extended Data Fig. 1). In Máaz, the group 2 feature had an
average band centre position of 344.1 ± 1.5 nm in survey scans and was
observed to have band centres varying from roughly 338 to 349 nm,
whereas in Séítah, the average band centre position was 343.1 ± 0.5 nm
and the variance of the band centre had a narrower range, from roughly
340 to 345 nm (Fig.
2a
). In abraded targets in both formations, the
group 2 feature was associated with a common set of minerals detected
with Raman spectroscopy, including carbonate, phosphate, sulfate,
silicate and occasionally, potential perchlorate (Fig.
5
and Extended
Data Fig. 4)
16
,
20
. The key difference in mineral associations was that
in three Máaz fm targets (Montpezat, Bellegarde and Alfalfa), this
feature was also associated with possible detections of pyroxene. By
contrast, in the Séítah fm, this feature was associated with a possi
-
ble detection of olivine in at least one point on each target (Extended
Data Table 2).
One point in the high dynamic range (HDR) scan on Montpezat
showed a Raman peak at 1,597 cm
−1
as well as weak fluorescence at
roughly 340 nm (Fig.
4a,b
), and was colocated with a detection at
roughly 1,080 cm
−1
. The roughly 1,080 cm
−1
signal shows a broad-shaped
Raman band consistent with laboratory studies of carbonate and sili-
cate minerals
19
. Raman spectroscopy cannot resolve silicate phases
well because of the small degree of polarizability of the silicon-oxygen
tetrahedron
23
; therefore, it is provisionally assigned here as simply
silicate or carbonate. The roughly 1,597 cm
−1
peak closely matched
a
Montpezat (Máaz)
Raman shift (cm
–1
)
1,597.2 cm
–1
2,000
1,800
1,400
1,600
200
250
150
100
40
20
60
80
100
50
338 nm
300
200
100
0
260
280
300
320
340
Wa
velength (nm)
b
c
1,599.8 cm
–1
Raman shift (cm
–1
)
Meteorite (calibration tar
get)
100
20
40
60
80
100
120
80
120
140
160
180
200
220
1,700
1,800
1,500
1,400
1,600
d
260
280
300
320
340
400
300
200
100
0
340 nm
Wa
velength (nm)
Raman shift (cm
–1
)
1,400
1,600
1,800
e
20
10
30
50
240
260
280
300
320
40
Gar
de (Séítah)
1,403 cm
–1
340 nm
Wa
velength (nm)
10,000
8,000
6,000
4,000
2,000
260
280
300
320
340
f
Fig. 4 | Raman features of possible organic compounds.
a
, Raman spectrum
from point 40 of an HDR scan on Montpezat (sol 349) with a Lorentzian fit
(FWHM 49.873, area 8,069.1, height 103).
b
, Corresponding average fluorescence
spectrum to
a
(lambda max roughly 338 nm).
c
, Median Raman spectrum
(n = 100) from an HDR scan on the SaU008 meteorite calibration target (sol 181),
which contains the known graphitic (G) band, with a Lorentzian fit (FWHM
61.784, area 11,646, height 120).
d
, Corresponding average fluorescence
spectrum to
c
(lambda max roughly 338 nm).
e
, Average Raman spectrum of
points with the highest group 2 f luorescence (n = 28) on Garde (sol 207–208)
with a Lorentzian fit (FWHM 47, area 4, 500, height 60.953).
f
, Corresponding
average f luorescence spectrum to
c
(lambda max roughly 340 nm). In all
graphs, the unfitted spectrum (red), fitted spectrum (blue) and baseline
(green) are shown; the
y
axis is intensity.
Nature | Vol 619 | 27 July 2023 |
729
the known graphitic (G) band observed on a sample from the Martian
meteorite Sayh al Uhaymir (SaU008) calibration target in position and
shape (Fig.
4c,d
and Extended Data Fig. 5). In the calibration target, the
Raman peak at roughly 1,599 cm
−1
is known to be from macromolecular
carbon
15
,
24
; thus, the 1,597 cm
−1
peak is consistent with a carbon–carbon
bond. This point on SaU008 similarly shows weak group 2 fluores
-
cence at roughly 340 nm, although it seems to have lower intensity
and longer emission wavelength than the point on Montpezat. Higher
confidence in a specific Raman assignment would have been possible
if the peak was detected at a greater signal-to-noise and seen at more
than one point. Several nearby points showed possible peaks below
the detection threshold.
The mean spectrum of points where the highest intensity group 2
features were detected on Garde (Séítah fm) yielded a Raman peak at
roughly 1,403 cm
−1
(Fig.
4e,f
). These points were correlated to Raman
detections consistent with olivine (823 cm
−1
), phosphate (960 cm
−1
) and
carbonate (1,086 cm
−1
). Another possible peak in the mean spectrum
was visible at roughly 1,540 cm
−1
, but was at the lower limit of detect
-
able width (less than 3 pixels FWHM) so is unassigned. The roughly
1,403 cm
−1
peak could be due to an organic compound, such as a C=O
stretching vibration of an organic salt
25
. Organic salts are possible
oxidation and radiolysis products of organic matter and have been
indirectly detected on Mars previously
26
. Carbonyl groups and aromatic
or olefinic carbon have been correlated with carbonate in a Martian
meteorite
5
. Further work is continuing to rule out secondary modes
of matrix minerals.
The group 2 fluorescence (roughly 335–350 nm) feature is consistent
with a two-ring aromatic molecule, such as naphthalene. Alternatively,
the emission spectra are also consistent with Ce
3+
in phosphates, on
the basis of laboratory data
27
. Both aromatic organics
6
and Ce
3+
have
been associated with phosphate minerals in Martian meteorites
28
,
29
.
With the data collected from Perseverance and our laboratory analyses,
we cannot rule out a contribution from both inorganic and organic
sources. The aromatic compounds would probably exist with some
degree of chemical substitution or in specific steric configurations
with respect to surrounding minerals, that would result in blue- or
red-shifting from the expected fluorescence wavelengths for benzene
and naphthalene. Red-shifting of fluorescence due to the formation
of carboxylic acids on or near the aromatic ring is highly probable as
these compounds are exposed to high energy radiation in an oxidative
environment
30
,
31
, and previous studies of refractory organic carbon in
Martian meteorites have shown carboxyl functionality
5
,
6
. It is highly
probable that the detected fluorescence features, if organic, repre-
sent mixes of organic moieties rather than single emitters, and their
overlapping spectra could cause variability in the apparent position
and the FWHM of observed bands. This would align with the colocated
detections of many fluorescence features on the same points. If the
fluorescence is inorganic, the emissions could also be varied as Ce
3+
luminescence is highly matrix dependent and affected by changes in
mineralogy and mineral composition
19
.
Group 3: single band at roughly 270–295 nm
Fluorescence bands between roughly 270 and 295 nm (FWHM of
roughly 20 nm) were observed at many points in survey scans of three
abraded (Guillaumes, Bellegarde, Alfalfa) and one natural target (Foux)
in Máaz, and at few or no points in all other scans (Extended Data Figs. 1
and 2). On the targets with a substantial number of detections, points
where group 3 fluorescence was detected often appeared clustered
together on brown-toned, possibly iron-stained material (Extended
Data Figs. 2 and 5). In many cases, this feature was colocated with the
group 2 feature and was comparatively weaker in intensity (Fig.
2
, and
Extended Data Fig. 4). The average band centre position in natural
targets (276.1 ± 0.8 nm) and abraded targets (276.1 ± 1.4 nm) in Máaz
were similar. Given the few overall detections in Séítah, no quantitative
comparison was possible. No clear mineral associations were detected
with group 3 fluorescence in Máaz abraded targets, except Alfalfa.
Here, fluorescence was associated with a broad Raman peak at roughly
1,040–1,080 cm
−1
, assigned to possible silicate
2
,
19
, and peaks at roughly
1,085–1,100 cm
−1
, assigned to carbonate
2
,
19
, at or near boundaries of
black and grey grains. As with the group 2 feature, no clear textural asso
-
ciations were observed in natural targets, and no mineral signatures
could be identified in the spectra. The group 3 (roughly 270–295 nm)
feature is consistent with a single ring aromatic compound, such as
benzene
16
; possible non-organic sources, such as silica defects, are
discussed in the Methods.
Group 4: roughly 290 and 330 nm features
The feature with bands centred at roughly 290 and 330 nm was observed
on two targets, Guillaumes (Máaz) and Garde (Séítah). In both, it was
observed on several points in intergranular spaces; this was particularly
apparent on Garde as previously reported
16
. On Guillaumes, group 4
fluorescence was not clearly associated with specific minerals. On
Garde, it was colocated with Raman peaks at roughly 1,087–1,096 cm
−1
and a broad peak at roughly 1,080 cm
−1
, assigned to carbonate and
silicate, respectively
2
,
19
(Extended Data Fig. 6). The relative intensities
of the two peaks were not constant between points, indicating that they
could be from several emitters. It is also possible that it is not a distinct
category but simply a combination of group 2 and 3 species. The spectra
are consistent with a one or two-ringed aromatic compound(s), though
the possible inorganic sources of groups 2 and 3 may also apply to
group 4.
Relative abundance of organic compounds
The observed fluorescence response, if solely from organic molecules,
can be used to provide a conservative estimate of concentration using a
single ring aromatic (benzene) with a weak fluorescence cross-section
and an assumed depth of penetration of 75 μm (refs.
16
,
32
). This depth
is a conservative estimate based on DUV transmission of more than
150 μm through Mars simulants
32
. Comparing the survey scans of
the abraded surfaces, the localized concentrations are varied and
range from 20 to 400 pg of organics where Alfalfa (Máaz) has some of
the highest number of occurrences and localized concentrations.
Furthermore, the bulk concentration in Máaz is an order of magnitude
higher than in Séítah (roughly 20 versus 2 ppm).
Diverse fluorescence across formations
The Máaz and Séítah fm are two geologically and compositionally dis
-
tinct formations that also show two different patterns of fluorescence.
Following the hypothesis of the fluorescence being entirely organic in
origin, these findings would indicate different bulk quantities of organic
material, with Máaz having an order of magnitude more than Séítah.
While colocations between organic features and minerals associated
with aqueous processes were found in both formations, the coloca
-
tion with primary igneous minerals was different. The group 2 feature
was associated with olivine at many points in all Séítah targets and to
pyroxene in two Máaz targets (Fig.
5
). This suggests several mechanisms
of synthesis or preservation, which may be at least partially unique to
each formation. A similar pattern of organics associated with pyrox
-
ene and olivine has been shown in studies on meteorites ALH84001,
Nakhla and Tissint. In these cases, the organic material has been shown
to be synthesized in situ
5
. Further observation of the cored samples
is needed to confirm the provenance and formation mechanism of
this material.
Previous findings indicate that the two formations underwent
different alteration processes. Máaz seems to be aqueously altered
basaltic rock that contains Fe
3+
bearing alteration minerals
33
. Séítah is
proposed to be an olivine cumulate
13
altered by fluids at low water to
rock ratios
32
, and contains mafic minerals that have higher abundances
730
| Nature | Vol 619 | 27 July 2023
Article
of total FeO than Máaz rocks
34
. Owing to the presence in Máaz of Fe
3+
bearing minerals, which can attenuate the fluorescence response
35
,
we would expect fewer and lower intensity fluorescence detections
than in Séítah. However, our observations demonstrate the opposite,
with Máaz targets having more fluorescence detections and highest
localized fluorescence intensities. If the fluorescence is organic, this
demonstrates a correlation of organics occurrence and abundance
with the degree of water-driven alteration and suggests that these
signatures are driven by synthesis and/or transport mechanisms rather
than meteoritic deposition, which would probably affect both for
-
mations in a similar manner. The concentrations of organics associ
-
ated with more aqueously altered surfaces are consistent with known
bulk concentrations of organics observed in Martian meteorites at
roughly 11 ppm
1
and in situ analysis performed by Curiosity rover in
Gale crater that indicated organics concentrations from roughly 7 ppb
to 11 ppm
2
.
The two formations also showed different types of fluorescence
features. Whereas the group 1, 2 and 4 features were detected in both
formations, Séítah showed a near-complete absence of group 3 fea
-
tures. This could indicate selective synthesis or preservation mecha-
nisms that favour the organics associated with the longer wavelength
fluorescence or a degradation process that only affected the group 3
associated organic molecules. The group 2 feature was most frequently
detected in both formations, but showed differences in the abraded
targets in Máaz and Séítah. Although the average band centre posi
-
tions of the group 2 detections in both units were similar (Máaz
344.1 ± 1.5 nm; Séítah 343.1 ± 0.5 nm), the range of band centres in
abraded Séítah targets was narrower (roughly 340–345 nm), whereas
the band centres in abraded Máaz targets were more broadly distributed
(roughly 338–349 nm).
Potential mechanisms affecting organic matter
The four fluorescence features observed on the ten targets interrogated
by SHERLOC each show varying degrees of mineral association and
spatial patterning, suggesting that these features may originate from
more than one mechanism of formation, deposition or preservation.
Two of the features, groups 1 and 4, were highly localized to specific
minerals, whereas the other two features were associated with sev
-
eral minerals and more broadly distributed. Continuing the organic
hypothesis, the clearest association between a specific organic detec
-
tion, mineral detection and texture was the group 1 feature found on
Bellegarde and Quartier associated with white sulfate grains (Fig.
3
). One
possible mechanism consistent with this association is abiotic aqueous
organic synthesis. Aromatic molecules, including sulfur-containing
species, associated with sulfate have been found in Tissint, Nakhla
and NWA 1950 (ref.
5
) and were proposed in these cases to be the result
of electrochemical reduction of aqueous CO
2
to organic molecules
due to interactions of spinel-group materials, sulfides and a brine.
Organics in ALH84001 have been shown to be produced during car
-
bonation and serpentinization reactions, indicating that several abiotic
organic synthesis mechanisms can occur on Mars. Alternatively, this
organic-mineral association could be the result of mineral-mediated
selective preservation of transported organic compounds in sulfate.
Previous work has shown that sulfates, including gypsum and magne
-
sium sulfate, can protect organic molecules within their crystal lattices
355
345
315
275
700
975
1,000
1,100
1,075
1,050
1,025
950
925
900
875
850
825
800
775
750
725
265
285
295
305
325
335
Formation
Raman shift (cm
–1
)
Fluor
escence (nm)
Séítah
Máaz
Group 1
Fluore
scence
feature
gr
oups
Group 2
Group 3
Group 4
Olivine
Perchlorate
Phosphate
Sulfate
Pyroxene
Possible silicate
Carbonate
Fig. 5 | Summary of SHERLOC f luorescence-mineral associations across
features and formations.
Select mineral detections (Raman shift, cm
−1
) and
their fluorescence features (
λ
max
, nm) for abraded targets analysed using
unsmoothed data from HDR and detail scans; both Raman and f luorescence
data are measured on the same point. Máaz scans (blue) used between 250 and
500 ppp, yielding low signal-to-noise ratio (less than 2) in some cases that were
not included; Séítah scans (green) all used 500 ppp, allowing for comparatively
more Raman detections. Mineral classifications based on high confidence
Raman detections of major peaks are indicated by boxed regions: olivine
(roughly 825–847 cm
−1
)
2
,
19
,
26
, range of hydrated and dehydrated perchlorate
(roughly 925–980 cm
−1
)
26
,
46
, phosphate (roughly 961–975 cm
−1
)
19
,
26
,
46
, pyroxene
(roughly 1,000–1,026 cm
−1
)
19
, sulfate (roughly 990–1,041 cm
−1
)
2
,
19
,
26
, amorphous
silicate (broad peak at roughly 1,020–1,080 cm
−1
)
2
,
26
and carbonate (roughly
1,085–1,102 cm
−1
)
19
,
46
. Markers outside a boxed region do not have a mineral
assignment. Disambiguation of overlapping regions can generally be resolved
by consideration of minor Raman peaks (not marked here) and corroboration
by other instrument(s) (for example, PIXL/SuperCam)
47
.