LETTERS
Hydrated silicate minerals on Mars observed by the
Mars Reconnaissance Orbiter CRISM instrument
John F. Mustard1, S. L. Murchie2, S. M. Pelkey1, B. L. Ehlmann1, R. E. Milliken3, J. A. Grant4, J.-P. Bibring5, F. Poulet5,
J. Bishop6, E. Noe Dobrea3, L. Roach1, F. Seelos2, R. E. Arvidson7, S. Wiseman7, R. Green3, C. Hash8, D. Humm2,
E. Malaret8, J. A. McGovern2, K. Seelos2, T. Clancy9, R. Clark10, D. Des Marais6, N. Izenberg2, A. Knudson7,
Y. Langevin5, T. Martin3, P. McGuire7, R. Morris11, M. Robinson12, T. Roush6, M. Smith13, G. Swayze9, H. Taylor2,
T. Titus14 & M. Wolff9
Phyllosilicates, a class of hydrous mineral first definitively iden-
tified on Mars by the OMEGA (Observatoire pour la Mineralogie,
L?Eau, les Glaces et l?Activitie?) instrument1,2, preserve a record of
the interaction of water with rocks on Mars. Global mapping
showed that phyllosilicates are widespread but are apparently
restricted to ancient terrains and a relatively narrow range of min-
eralogy (Fe/Mg and Al smectite clays). This was interpreted to
indicate that phyllosilicate formation occurred during the
Noachian (the earliest geological era of Mars), and that the condi-
tions necessary for phyllosilicate formation (moderate to high pH
and high water activity3) were specific to surface environments
during the earliest era of Mars?s history4. Here we report results
from the Compact Reconnaissance Imaging Spectrometer for
Mars (CRISM)5 of phyllosilicate-rich regions. We expand the
diversity of phyllosilicate mineralogy with the identification of
kaolinite, chlorite and illite or muscovite, and a new class of
hydrated silicate (hydrated silica). We observe diverse Fe/Mg-
OH phyllosilicates and find that smectites such as nontronite
and saponite are the most common, but chlorites are also present
in some locations. Stratigraphic relationships in the Nili Fossae
region show olivine-rich materials overlying phyllosilicate-bear-
ing units, indicating the cessation of aqueous alteration before
emplacement of the olivine-bearing unit. Hundreds of detections
of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters
in the southern highland Noachian cratered terrain indicate
excavation of altered crust from depth. We also find phyllosili-
cate in sedimentary deposits clearly laid by water. These results
point to a rich diversity of Noachian environments conducive to
habitability.
High-spatial-resolution, precision-pointing and nested observa-
tions of CRISM5, the Context Imager (CTX)6, and the High
Resolution Imaging Science Experiment (HiRISE)7 instruments on
theMars Reconnaissance Orbiter (MRO) resolvemineralogical, stra-
tigraphic and geological relationships for phyllosilicate deposits. This
combination of instruments permits mineralogical mapping at 18m
per pixel with CRISM linked with metre-scale geomorphology
from CTX and HiRISE. We focus here on the stratigraphic setting
of phyllosilicate-bearing rocks in three regions and report the detec-
tion of phyllosilicate in sedimentary settings.
We identify two principal classes of mineral in the CRISM data on
the basis of observed absorptions: Al phyllosilicates and the more
common and spatially dominant Fe/Mg phyllosilicates. The
increased spatial and spectral resolutions of CRISM have revealed a
diversity of absorption band shapes, positions and combinations
indicating variations in phyllosilicate type and composition (Fig. 1;
see Methods for processing and identification details). Most spectra
show a band at ,1.4 mm from the overtone of the OH stretch, a
strong 1.9-mm H2O band and absorptions near 2.28?2.30 mm (for
example, spectrum 3 in Fig. 1b), which are consistent with Fe/Mg
phyllosilicates having interlayer water, such as the smectite clays
nontronite and saponite8. Other spectra in Nili Fossae have a dom-
inant absorption near or longward of 2.34 mm, a weak band or inflec-
tion near 2.25 mm, and a weak or absent 1.9 mm absorption band
(spectrum 4 in Fig. 1b). This combination of absorptions is consist-
ent with high-Fe chlorites9?11. Some spectra fromMawrth Vallis show
strong 1.9-mm and 2.2-mm features and a weak 1.4-mm band (spec-
trum 1 in Fig. 1b) diagnostic of montmorillonite, an Al-rich smectite
clay, confirming previous detections2. K,Al phyllosilicates have also
been recognized in a new location, the ejecta of a 50-km crater west of
Nili Fossae. These spectra show 1.4-mm, 2.2-mm and ,2.35-mm
absorptions but lack a 1.9-mm band (spectrum 6 in Fig. 1b). These
absorptions are consistent with the presence of micaminerals such as
illite and/or muscovite10. Additionally, in some small-scale outcrops
a distinct doublet absorption with resolved absorptions at 2.16 and
2.2 mm, and a strong 1.4-mmband is observed (spectrum 5 in Fig. 1b).
This combination of bands is diagnostic of kaolinite. Kaolinite was
first observed by OMEGA in one small region of the southern high-
lands12, whereas CRISM has observed kaolinite in Nili Fossae and
Mawrth Vallis.
A new class of hydrated silicate has been identified (spectrum 2 in
Fig. 1b), characterized by a 2.20?2.25-mm absorption that is dis-
tinct from that observed with Al-OH phyllosilicates such as mont-
morillonite in that the absorption is broader and centred at longer
wavelengths. These regions commonly show 1.4-mm and 1.9-mm
1Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. 2Johns Hopkins University/Applied Physics Laboratory, 11100 Johns Hopkins Road,
Laurel, Maryland 20723, USA. 3Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 183-301, 4800 Oak Grove Drive, Pasadena, California 91109, USA. 4Center for
Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Independence Avenue at 6th Street SW, Washington, DC 20560, USA. 5Institut
d?Astrophysique Spatiale, Universite? Paris Sud 11, 91405 Orsay, France. 6National Aeronautics and Space Administration, Ames Research Center, 515 N. Whisman Road, Mountain
View, California 94043, USA. 7Department of Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130, USA. 8Applied Coherent Technology, 112 Elden Street
Suite K, Herndon, Virginia 22070, USA. 9Space Science Institute, 4750Walnut Street, Suite 205, Boulder, Colorado 80301, USA. 10US Geological Survey, MS 964 Box 25046, Denver
Federal Center, Denver, Colorado 80225, USA. 11ARES Code KR, National Aeronautics and Space Administration, Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058,
USA. 12School of Earth and Space Exploration. Box 871404, Arizona State University, Tempe, Arizona 85287-1404, USA. 13National Aeronautics and Space Administration, Goddard
Space Flight Center, Code 693.0, Greenbelt, Maryland 20771, USA. 14US Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona, 86001, USA.
doi:10.1038/nature07097
305
 ?2008 Macmillan Publishers Limited. All rights reserved
bands, and this combination of spectral features is consistent with
hydrated silica glasses such as opal or volcanic glass.
In the Mawrth Vallis area (Fig. 2), smectites rich in Al and Fe have
been observed over a 300 km3 400 km region but are commonly
found along the flanks of the outflow channel and within the
surrounding plateau13,14. The units hosting these minerals are typ-
ically light-toned outcrops of early-Noachian to mid-Noachian
materials that have been exposed by erosion from beneath a dark
mantling unit that shows pyroxene absorptions. Some occurrences of
phyllosilicate are clearly associated with layered rocks. This leads to
0.20
0.16
0.12
a
b
c
1.0
0
0.2
1.2
1.0 1.4 1.8
s1
1
2
3
4
5
6
s2
Wavelength (?m)
Nontronite
Saponite
Chlorite
Illite
Muscovite
Montmorillonite
Hydrated silica
Kaolinite
La
bo
ra
to
ry
 re
fle
ct
an
ce
C
R
IS
M
 ra
tio
ed
 re
fle
ct
an
ce
C
R
IS
M
 L
am
be
rt
 a
lb
ed
o
2.2 2.6 1.0 1.4 1.8 2.2 2.6
Figure 1 | CRISM and laboratory
reflectance spectra showing
hydrated silicate mineral diversity.
a, CRISM reflectance spectra from
1.0 to 2.6 mm used in the spectral
ratio (s1/s2) for spectrum 4 in
b. b, Stacked spectral ratios: (1)
from Mawrth Vallis, green
(HRL0000285A); (2) from the
southern highlands, red
(FRT00035DB); (3) from Nili
Fossae, blue (FRT00003E12); (4)
from Nili Fossae, black
(FRT00050F2); (5) from Nili
Fossae, magenta (FRT00003FB9);
(6) from Nili Fossae, orange
(HRS0002FC5). Spectra were
selected on the basis of strong
BD2210 and D2300 parameter
values. Vertical lines are placed at
1.4, 2.2 and 2.3mm. c, Stacked
library reflectance spectra of
minerals with similar absorption
features. The hydrated silica is an
altered, hydrated volcanic ash.
20? 0? 0? Wa
c
b
20? 0? 0? W
Fe/Mg phyllosilicates
Al phyllosilicates
Fe/Mg phyllosilicates
Al phyllosilicate
Fe2+
km
20
? 
0?
 0
?
 N
25
? 
0?
 0
?
 N
Figure 2 | Mineral diversity and
stratigraphy of Mawrth Vallis.
a, MOC map of Mawrth Vallis with
OMEGA phyllosilicate mineral
indicators11 for Fe/Mg
phyllosilicates (blue) and Al
phyllosilicates (green). b, CTX
image of the red box from a, with a
superimposed mineral indicator
map from CRISM observation
HRL000043EC_07, in which green
areas are enriched in Al
phyllosilicate, blue areas are
enriched in Fe/Mg phyllosilicate,
and red areas indicate an Fe21
slope. c, Subset of the CTX image
from the white box in b, where the
Fe/Mg phyllosilicate (Fe/Mg)
occurs as a darker smooth surface.
Al phyllosilicates (Al) are within
brighter and rougher terrain
beneath a capping unit (cap).
LETTERS NATURE
306
 ?2008 Macmillan Publishers Limited. All rights reserved
the hypothesis that they may have formed by deposition in an aque-
ous environment, by aqueous alteration of a volcanic ash deposit, or
by impact-related alteration13.
CRISM observation HRL000043EC falls in the region west of the
mouth of Mawrth Vallis (Fig. 2a)13. In these light-toned outcrops we
find discrete layers of Al and Fe/Mg phyllosilicates. In this region Al
phyllosilicates are more prevalent (Fig. 2b); however, elsewhere in
Mawrth Vallis Fe/Mg phyllosilicates are typically more abundant. In
addition to CRISM spectra showing absorptions diagnostic of mon-
tmorillonite as observed previously with OMEGA, we also observe in
small outcrops absorptions characteristic of kaolinite and hydrated
silica or glass. Sandwiched between the Al and Fe/Mg phyllosilicate
layers in CRISM image HRL000043EC is an additional material con-
taining an Fe21 slope and a broadened 2.2?2.3-mm feature.
High-resolution Mars Orbiting Camera (MOC) and High
Resolution Stereo Camera (HRSC) images13,14, as well as new observa-
tions acquired by CTX and HiRISE, reveal that the light-toned out-
crops inMawrthVallis are layered and often show a distinct polygonal
texture. The Fe/Mg-phyllosilicate-bearing unit is the lowest stra-
tigraphic level and is overlain by Al-phyllosilicate-bearing strata; both
are exposed by erosion from beneath a dark-toned capping unit
(Fig. 2c). Despite the close proximity of these phyllosilicate-bearing
units, we see no evidence for mixing of the two phyllosilicate signa-
tures at spatial resolutions of,40m per pixel. This indicates that the
minerals occur in spatially distinct stratigraphic units.
The Nili Fossae region shows expansive outcrops of phyllosili-
cates2,12 but also extensive spectral signatures of olivine in close prox-
imity15?19. Several high-resolution observations across this region
(Fig. 3a) show that much of the olivine is present within aeolian
bedforms. However, several observations provide definitive evidence
for the presence of phyllosilicate-bearing units in direct contact with
unaltered olivine-bearing units. In CRISM observation
FRT00003E12, stratigraphic relationships are resolved between the
lowest, phyllosilicate-bearing bedrock unit and the overlying olivine-
bearing bedrock unit (Fig. 3b). The phyllosilicate-rich unit shows
polygonal fractures and is recessed on slope-forming units, suggest-
ing that it is more easily eroded than the cap unit. The phyllosilicate
unit is capped by a regionally extensive, spectrally neutral, mesa-
forming unit. Although most of the cap unit is massive, HiRISE data
show the base is tens of metres thick and contains banding and/or
layering (Fig. 3c). Coordinated CRISM data show this banded unit is
olivine-rich and that it rests directly on top of the phyllosilicate-rich
material (Fig. 3c). This sequence of a spectrally neutral cap rock
overlying an unaltered olivine-rich unit resting on phyllosilicate-rich
rocks is observed in CRISM observations across a distance of
2,000 km from the Nili Fossae region to the southern edge of the
Isidis Basin20.
CRISM has targeted contacts between phyllosilicate-bearing rocks
and Hesperian-aged lavas from Syrtis Major. Some of the observa-
tions are of high-standing Noachian-aged massifs embayed by lava
(FRT000050F2 and FRT00005A3E) and other observations show
phyllosilicate-bearing outcrops beneath volcanic plains
(FRT000048B2, and Fig. 3d). In all these observations of volcanic
rocks in contact with phyllosilicate-bearing terrains, the volcanic
rocks are unaltered and provide no evidence for hydrated silicates.
A section of the Nili Fossae trough scarp that is,600m high exposes
rocks that are strongly enriched in phyllosilicates, whereas the rocks
capping the plateau are enriched in pyroxene (Fig. 3d).
In the ancient southern highlands, localized phyllosilicate deposits
were identified withOMEGA data19,21.With the use of a combination
of multispectral mapping and targeted observations, CRISM has sig-
nificantly expanded the number of locations to the thousands of
phyllosilicate occurrences (see, for example, Fig. 4a). These data show
phyllosilicates associated with craters of many sizes and within ejecta,
walls and central peaks. Full-resolution CRISM observation
FRT000049BB shows a highlands crater in which Fe/Mg smectite
clays are can be seen in outcrops at or near the rim of the crater along
its inward facing wall (Fig. 4b). Weaker phyllosilicate detections are
visible downslope of these outcrops, indicating physical breakdown
and mass wasting. In addition we observe phyllosilicate associated
with ejecta exterior to the crater and in mounds and knobs on the
crater floor. CRISM observation FRT00003E92 shows Fe/Mg phyllo-
silicates associated with the central peak of a crater 45 km in diameter
(Fig. 4c). This central peakmaterial probably originated from a depth
of 4?5 km, indicating alteration of the crust to at least this depth.
Elsewhere in the southern highlands we observe phyllosilicates in
central peaks that show a distinct 2.2-mm absorption indicative of
either Al phyllosilicate or hydrated glass (Fig. 1).
20
? 
N
70? E
2 km
Phyllosilicate
Phyllosilicate Low-Ca pyroxeneOlivine
Olivine
a
b
c
d
75? E
15
? 
N
Figure 3 | Stratigraphy of phyllosilicate-bearing strata in the Nili Fossae
region. a, OMEGA phyllosilicate detections (green) based on the D2300
parameter11 overlaid on a Viking digital image mosaic. Targeted CRISM
images are outlined; their full identifications are preceded by FRT0000.
b, CRISM scene FRT00003E12 overlaid with CRISM mineral indicators of
olivine (red) and phyllosilicate (blue). The black box indicates the location of
c. c, A portion of HiRISE image PSP_002176_2025. Phyllosilicates are found
in the bright layered material, overlain by a thin (one or two CRISM pixels)
layer of olivine capped by a spectrally neutral unit. d, Mineral indicators
from CRISM observations FRT000064D9 and FRT00007BC8 overlaid on
CTX observation P05_003986_20_11_XI_21N285W_070324. Low-Ca
pyroxene is shown in green, phyllosilicate in blue and magenta, and olivine
in red. The boundary between Syrtis Major lava and phyllosilicate-bearing
material within Nili Fossae trough material is shown by the white arrows.
NATURE LETTERS
307
 ?2008 Macmillan Publishers Limited. All rights reserved
Phyllosilicates have also been identified by CRISM within sedi-
mentary basins, specifically in fans and deltas within Holden,
Eberswalde and Jezero craters, and represent the detection on Mars
of hydrated silicates within sediments clearly deposited by water. The
mineralogy of phyllosilicates within the sedimentary deposits does
not differ from that in nearby potential sediment source regions,
whether interior to the basin, as in Holden22,23, or from Jezero crater?s
exterior catchment24. The sedimentary phyllosilicates may have been
eroded and transported rather than being formed in situ, although
formation in situ cannot be ruled out; it is likely that some alteration
of materials within the crater occurred during the various periods of
aqueous activity. However, there is a relative paucity of phyllosilicate
spectral signatures in the uppermost units of the sedimentary depos-
its, implying that conditions favouring phyllosilicate formation,
deposition or preservation did not persist into the late stages of delta
formation in the Noachian.
The CRISM observations and coordinated CTX and HiRISE
imaging showed detailed geological relationships and compositional
stratigraphy. In the Nili Fossae and Isidis regions we observe region-
ally extensive phyllosilicate-bearing rocks overlain by both unaltered
rocks susceptible to alteration (olivine, pyroxene) and/or a spectrally
neutral mesa-forming unit and embayed by Hesperian Syrtis Major
lavas that show no evidence of alteration. Some of the rock units in
this region are cut by fractures associated with the Isidis impact basin,
which is dated to the late Noachian. Because the olivine-bearing units
were emplaced before or concurrently with the event that formed the
Isidis basin17,19, the lack of any aqueous alteration associated with the
olivine lithologies, and the lack of alteration of Hesperian lavas,
definitively document Noachian phyllosilicate formation.
The Mawrth Vallis and Nili Fossae regions show that thick, region-
ally extensive sections of Noachian crust are altered. Throughout the
southernhighlands,CRISMdata document phyllosilicates in thewalls,
ejecta and central peaks of craters in the southern highlands. This
striking association of phyllosilicates with impact craters may be the
result of impacts excavating pre-existing deposits or impact-associated
processes, leading to the formation of phyllosilicate and hydrated sil-
icate minerals. The possibility cannot be excluded that the alteration
occurred as a result of the impact hydrothermal processes25,26.
However, these systems are usually restricted to the subsurface and/
or interior of craters (for example melt sheets). The uniform distri-
bution of phyllosilicate-rich material in crater ejecta, and the presence
of phyllosilicates in the walls, floor deposits and within central peaks,
argue for the presence of hydrated silicates in the target rocks before
impact. With hydrated silicates observed in central peaks of impact
8? S
12? S
8? S
12? S
98? E94? E
10 km
Fe/Mg phyllosilicate
Pyroxene
StrongWeak
98? E
Weak
Phyllosilicate detection
Strong
94? Ea
b c
Phyllosilicate detection
Figure 4 | Phyllosilicate occurrences in the Noachian-aged southern
highlands. a, Map of phyllosilicatemineral indicator11 (green, weak; orange,
strong) derived from CRISM multispectral mapping merged with a MOC
wide-angle camera image mosaic. White arrows show distinct regions
enriched in phyllosilicate. b, High-resolution CRISM observation
FRT000049BB at 13.25u S, 105.25uE, showing a phyllosilicate mineral
indicator map (blue, low; green, high). c, High-resolution CRISM
observation FRT00003E92 at 13.5u S, 119.5uE, showing phyllosilicate (cyan)
and pyroxene (red) mineral indicator maps.
LETTERS NATURE
308
 ?2008 Macmillan Publishers Limited. All rights reserved
craters on the order of 45 km, a significant thickness of the ancient
martian crust has experienced some form of alteration. In contrast,
despitemanyobservations, no craters in terrains ofHesperian age or in
the northern plains where subsurface ice is known to be present show
evidence for phyllosilicates.
Identifying the types of phyllosilicate minerals on Mars also con-
strains their environment of formation. The majority of phyllosili-
cate spectra are most consistent with smectite clay minerals such as
montmorillonite, nontronite and saponite. Smectite clays require
significant water reservoirs and moderate to alkaline pH (ref. 3).
However, the higher spatial resolution of CRISM has augmented
the phyllosilicate mineral diversity beyond this class alone. CRISM
has expanded the geographic range of Al-OH-bearing phyllosilicates
with a new detection in theNili Fossae region, where spectra aremore
consistent with illite or muscovite and kaolinite rather than with
montmorillonite. In addition, several regions show absorption fea-
tures consistent with chlorite. Overall, the weathering products of Fe/
Mg mafic minerals are over-represented relative to alkaline minerals
such as plagioclase, given the basaltic mineralogy of the martian
crust. Kaolinite, which would be expected from an active hydrologi-
cal system operating in regions with good drainage and high levels of
flushing27, has been identified in only a few locations, and exposures
are less than 200m3 200m in spatial scale. The restriction of kaoli-
nite to such rare, small deposits argues against a widespread and
vigorous hydrological system in the Noachian or the possibility that
the lifetime of such a system was sufficient for the formation of
smectite clay but not kaolinite. However, the existence of localities
with kaolinite and other diverse phases such as chlorite and hydrated
silica suggests that microenvironments with a greater throughput of
liquid water or enhanced hydrothermal activity were also a feature of
early Mars.
METHODS SUMMARY
Standard processing approaches were used to convert CRISM data from instru-
ment units to apparent I/F (the ratio of reflected intensity to incident intensity of
sunlight) (ref. 5).We then assume a lambertian surface and divide the data by the
cosine of the incidence angle. We assume that surface and atmospheric contri-
butions are multiplicative and that the atmospheric contribution follows an
exponential variation with altitude28. The data are divided by a scaled, atmo-
spheric transmission spectrum obtained from an observation across Olympus
Mons, as used by the OMEGA instrument team1,15. This method removes only
the effects of gas absorption. However, it is efficient for the analysis of large
volumes of data and compares favourably with amore rigorous but time-intens-
ive retrieval of surface Lambert albedos using DISORT-based simulations and
regressions for each wavelength band between modelled CRISM radiances and a
suite of input surface albedos29 that addresses both gas absorptions and aerosols.
Spectral summary parameters are then derived for each observation, to highlight
areas with interesting mineralogy30.
Narrow absorptions in the 1.9?2.5-mm wavelength region are due to combina-
tions of molecular vibrations in phyllosilicate minerals. Residual calibration arte-
facts and errors in atmospheric removal cause systematic spikes (Fig. 1a) that are
suppressed in spectral ratios in which the spectrum for a region of interest is
divided by the spectrum of a nearby region of low spectral contrast. If water is
present in a mineral (for example interlayer water in smectite clays), then an
absorption centred near 1.9mm is observed as a result of the combination of the
H?O?H bending and stretching vibrations29. Absorptions near 2.2mm are indi-
cative of Al?OH vibrations, whereas those between 2.28 and 2.35mm are com-
monly indicative of Fe/Mg?OH vibrations8?10. Quantitative abundances can be
estimated from computationally intensive, nonlinear spectral deconvolution31.
Thus, our analyses focus on the detection of phyllosilicate mineralogy, and a
qualitativemeasure of relative abundance is provided by the spectral parameters30.
Received 24 April; accepted 8 May 2008.
Published online 16 July 2008.
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AcknowledgementsWe thank theMars Reconnaissance Orbiter team for building
and operating the spacecraft and the numerous people who have contributed to
the CRISM investigation. Support from NASA to the CRISM science team is
gratefully acknowledged.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. Correspondence and requests for materials should be
addressed to J.F.M. (john_mustard@brown.edu).
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