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Shock metamorphism of clay minerals
on Mars by meteor impact
Joseph R. Michalski
1
, Timothy D. Glotch
2
, Lonia R. Friedlander
3
, M. Darby Dyar
4
,
David L. Bish
5
, Thomas G. Sharp
6
, and John Carter
7
1
Department of Earth Sciences and Laboratory for Space Research, University of Hong Kong, Pokfulam, Hong Kong,
2
Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York, USA,
3
Blaustein Institutes
for Desert Research, Ben- Gurion University of the Negev, Beersheba, Israel,
4
Mount Holyoke College, South Hadley,
Massachusetts, USA,
5
Department of Earth and Atmospheric Sciences, University of Indiana, Bloomington, Bloomington,
Indiana, USA,
6
School of Earth and Space Exploration, Arizona State University, Tempe, Arizona,USA,
7
Institut d’Astrophysique
Spatiale, Université de Paris, Paris, France
Abstract A large fraction of clay minerals detected on Mars by infrared remote sensing represent
materials exhumed from the subsurface by meteor impact, begging the question of whether the infrared
features used to detect the clays are affected by shock associated with the impacts. We used X-ray diffraction
and infrared and Mössbauer spectroscopy to evaluate the mineralogy of five clay minerals after
experimentally shocking them to six shock pressures from ~10 to 40 GPa. The shocked clays exhibit three
main relevant shock effects: (1) an overall decrease in infrared spectral contrast in the impact-fragmented
materials, (2) oxidation of Fe in ferrous clays, and (3) loss of some spectral structure in relatively well-ordered
clays such as kaolinite. Other than the widespread oxidation of ferrous clays, shock metamorphism likely has
little effect on the accurate interpretation of clay mineralogy on Mars from remote sensing data. However,
we are able to identify rare cases of extreme shock in some Martian clay deposits.
Plain Language Summary One of the most significant achievements in planetary sciences in the
last 20 years has been the identification and mapping of Martian surface mineralogy by infrared remote
sensing. Of major interest is the characterization of clay minerals that formed in ancient habitable
environments on Mars >3.5 Ga ago. Because most of these deposits are extremely ancient, they exist within
crustal materials that have experienced numerous meteor impacts and therefore could have been affected
by heat and pressure associated with those events. The paper contains three main points: (1) the main
features that are used to interpret clay mineralogy from infrared data remain intact despite shock
metamorphic effects up to pressures of ~40 GPa, (2) shocked clays display some key features that can be used
to identify effects of shock remotely, and (3) we describe some of the global-scale spectral biases in
interpretation of clays on Mars that are likely to arise from shock effects and we show some concrete
examples of shocked Martian clays in certain cases. We believe that this would be the first publication of clear
evidence for shocked minerals detected by remote sensing on any planet.
1. Introduction
Over a decade of near-infrared remote sensing exploration of Mars has revealed thousands of deposits of
clay minerals, a large fraction of which occur in materials exhumed from the subsurface by impact craters
[Ehlmann et al., 2011]. Clay minerals detected by near-infrared spectroscopy (λ≈1–3μm) occur within
igneous rocks, sedimentary strata, and hydrothermal deposits in the central peaks [Sun and Milliken,
2015], uplifted rims, and ejecta of impact craters [Carter et al., 2013] (Figure 1). Given this context, it
stands to reason that clay minerals exhumed from subsurface environments could be affected by shock
metamorphism associated with meteor impacts [Boslough et al., 1986; Johnson et al., 2002]. While litho-
static pressures in the shallow Martian crust (<50 km) only are <1 GPa, meteor impacts result in large
parts of impacted crust experiencing shock pressures of 10–50 GPa, with heterogeneous, local peak
pressures reaching an order of magnitude higher[French, 1998]. The question is whether shock-induced
pressure [Kraus et al., 2013] or temperature effects [Gavin and Chevrier, 2010; Che et al., 2011] would
confound attempts to interpret clay mineralogy [Weldon et al., 1982; Boslough et al., 1986] of Mars from
remote sensing data.
MICHALSKI ET AL. SHOCKED CLAYS ON MARS 6562
PUBLICATION
S
Geophysical Research Letters
RESEARCH LETTER
10.1002/2017GL073423
Key Points:
•Five clay minerals known to exist on
Mars were experimentally shocked
from 10 to 40 GPa and products were
analyzed with infrared spectroscopy
•At 10 GPa, chlorite exhibits
impact-induced oxidation of Fe, and
at 20–30 GPa, kaolinite exhibits signs
of structural disorder
•Laboratory results are applied to
infrared remote sensing of clays
on Mars
Supporting Information:
•Supporting Information S1
Correspondence to:
J. R. Michalski,
jmichal@hku.hk
Citation:
Michalski, J. R., T. D. Glotch,
L. R. Friedlander, M. Darby Dyar,
D. L. Bish, T. G. Sharp, and J. Carter
(2017), Shock metamorphism of clay
minerals on Mars by meteor impact,
Geophys. Res. Lett.,44, 6562–6569,
doi:10.1002/2017GL073423.
Received 14 MAR 2017
Accepted 19 JUN 2017
Accepted article online 20 JUN 2017
Published online 12 JUL 2017
©2017. The Authors.
This is an open access article under the
terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and distri-
bution in any medium, provided the
original work is properly cited, the use is
non-commercial and no modifications
or adaptations are made.
Not all clay minerals associated with craters were necessarily uplifted/exposed by the impact, and clay miner-
als could be formed by impacts themselves [Tornabene et al., 2013]. Undoubtedly, impact-induced clay for-
mation must have happened on Mars [Marzo et al., 2010]. However, most clay minerals associated with
impact craters occur within units that appear to be uplifted from the subsurface in fractured but intact
clay-bearing terranes [e.g., Michalski and Niles, 2010], rather than postimpact hydrothermally altered rocks
(Figure 1). Impact craters are also the most common type of sedimentary basin on Mars. Therefore, it is impor-
tant to differentiate between rocks exhumed by the impact and rocks that formed later in the basin, many of
which could contain clays and other hydrated phases detectable by infrared remote sensing (Figure 1).
Understanding the mineralogy of clay minerals exhumed by meteor impact on Mars is important for several
reasons. The detailed mineralogy and crystal chemistry of the clays trace the chemical conditions of ancient
aqueous processes within the crust [Ehlmann et al., 2013]. These include diagenetic and metamorphic pro-
cesses relevant to understanding the nature of the planet’s crust and overall water budget. Also, life could have
formed and been sustained within deep crustal hydrothermal systems [Michalski et al., 2013]. But to truly under-
stand the 3-D composition of the crust and subsurface alteration, as indicated by clay minerals exhumed by
impact, it is important to determine how shock affects those interpretations of clay mineralogy. Some recent
studies have investigated the effects of shock on clay mineralogy at low shock pressures (<20 GPa) [Gavin et al.,
2013] and described some detailed relationships between shock and clay spectroscopy at higher pressures
[Friedlander et al., 2015, 2016]. Here we summarize the results of a series of shock experiments carried out on
clay minerals in the laboratory and relate those results to the spectroscopically observed mineralogy of Mars.
2. Experimental Setup and Analytical Methods
The <2μm size fraction of each of five well-characterized phyllosilicate samples was separated, dried, and
gently ground with mortar and pestle into loose powder. In an effort to decrease porosity, which can
Figure 1. Clay minerals are found in several different contexts associated with impact craters on Mars. (a) Some contexts
represent preimpact clays that have been exhumed by the impact event (e.g., central peaks, ejecta, and uplifted rims) and
might contain shocked materials. Other contexts include clays deposited postimpact into crater basins. A global map shows
alteration minerals associated with impact craters on Mars. (b) The green stars show all hydrated minerals occurring in
impact craters [Carter et al., 2013], and the white stars show only clay minerals associated with crater central peaks [Sun and
Milliken, 2015]. Both maps include many examples of exhumed clay minerals and some examples of postimpact clays.
Geophysical Research Letters 10.1002/2017GL073423
MICHALSKI ET AL. SHOCKED CLAYS ON MARS 6563
result in increased shock melting during experimental impacts, ~150 mg samples of each powder were
pressed at ~70 MPa into disks ~2 mm thick using a hydraulic hand press. Each pellet was loaded into a sample
container precisely milled to fit the pellet, minimizing void space. These samples were then shocked experi-
mentally using the flat plate accelerator (FPA) at NASA’s Johnson Space Center.
At the FPA, stainless steel or fansteel flyer plate projectiles were launched horizontally at the secured sample
containers. Lasers mounted in the flight path were used to determine the projectile velocity and mounted
cameras were positioned to characterize projectile tilt (experiments with tilt >3° were rerun). Velocities of
0.872–1.349 km/s were converted to pressure using one-dimensional shock-stress relationships [Gault and
Heitowit, 1963].
The experimental setup was designed to produce six runs resulting in six peak shock pressures ofapproximately
10, 20, 25, 30, 35, and 40 GPa for each of five clay samples; achieved shock pressures differed by up to a few
percent. Recovery was nearly 100% for most runs, although the highest-pressure experiments resulted in some
sample loss because the sample holder was highly deformed. Samples chosen for shock experiments include
five important phyllosilicates known to exist on Mars from infrared remote sensing: two dioctahedral clay
minerals (kaolinite and nontronite) and three trioctahedral clay minerals (serpentine, chlorite, and saponite).
Of primary interest in this work were the near-infrared reflectance (NIR) (λ=1–3μm) spectra because these
data are directly relevant to measurements of clay minerals made on Mars by orbital remote sensing instru-
ments. In addition to NIR analyses, the unshocked and shocked samples were analyzed by Mössbauer spec-
troscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). We provide a brief summary of
analytical methods here. Additional details are available in Friedlander et al. [2016].
Bidirectional NIR data were collected under a nitrogen-purged environment from λ= 0.35–2.5 μm with a sam-
pling interval of 1.4–2 nm at the Center for Planetary Exploration at Stony Brook University. Mössbauer spec-
tra were acquired using a source of ~60 mCi
57
Co in Rh on a WEB Research Co. (now SEE Co.) model WT302
spectrometer (Mount Holyoke College) at 295 K. Spectra were calibrated against an Fe metal foil and fitted
using standard methods [Cuadros et al., 2013]. XRD analyses were carried out using a Bruker D8 diffract-
ometer at Indiana University, with Cu Kαradiation and a SolX solid-state point detector. All samples were ana-
lyzed before shock experiments using random-powder cavity mounts, and run products were mounted on
“zero-background”quartz plates. The clay particles were examined by TEM using a Philips CM200FEG
S/TEM instrument in the LeRoy-Eyring Center for Solid State Science at Arizona State University. Bright field
images were used to characterize particle size and morphology, and selected area electron diffraction (SAED)
patterns were collected to determine crystallinity and disorder.
3. Effects of Shock on Near-Infrared Spectroscopy of Clay Minerals
The most important result of this work is the demonstration that shock effects generally do not obfuscate
identification or interpretation of clay minerals from NIR spectra even if they are shocked to pressures up
to ~40 GPa. Major absorptions used to identify and characterize clay minerals with NIR remote sensing
remain intact despite shock effects [Gavin et al., 2013], although interesting and important modifications
to some of those spectral features occur. The conservation of most NIR spectral features upon exposure to
high shock pressures stands in contrast to the midinfrared spectral characteristics, which, depending on
the species, typically begin to resemble amorphous silicates between 20 and 40 GPa [Friedlander et al., 2016].
Essentially all clay minerals exhibit some similar spectral features related to basic phyllosilicate mineralogy
[Farmer, 1968] (Figure 2). Absorptions located near 1.4 μm result from OH stretching overtones. H
2
O
adsorbed onto surfaces and between layers within the phyllosilicate structure result in infrared absorption
at ~1.9 μm. Metal-OH deformations within the octahedral sheets produce diagnostic absorptions located
from 2.2 to 2.35 μm, where the position of this feature is an important indication of octahedral chemistry.
Absorption at 2.2 μm is indicative of AlOH in kaolinite (the nontronite has a kaolinite contaminant in it).
FeOH in nontronite absorbs at ~2.28–2.29 μm. MgOH in saponite, serpentine, and chlorite absorbs at approxi-
mately 2.32, 2.33, and 2.34 μm, respectively. Absorptions at 2.35–2.5 μm are less diagnostic, resulting from
complex combination absorptions in the octahedral and tetrahedral sheets. Lastly, a broad absorption
located between 1 and 2 μm results from electronic crystal field effects in Fe within Fe
2+
-bearing samples.
This is most pronounced in the chlorite sample, which contains significant amounts of Fe
2+
.
Geophysical Research Letters 10.1002/2017GL073423
MICHALSKI ET AL. SHOCKED CLAYS ON MARS 6564
NIR spectra of shocked clays are affected by both physical and mineralogical changes. Physical changes result
from intense fragmentation of the sample during shock decompression. Such changes are evident in the XRD
analyses, which show peak broadening of the 001, 02 ℓ, and 06 ℓpeaks in all shocked samples, consistent with
a decrease in crystallite size in the shocked samples relative to the unshocked samples.
Changes in crystallite sizes and physical particle sizes are known to affect NIR spectral shape and contrast.
Here we quantify the effects of impact-induced changes on spectral contrast for the most diagnostic of
the NIR absorptions for clays, the
metal-OH feature located from (λ=)
2.17 to 2.35 μm (see supporting infor-
mation for details). For each mineral,
we created a simple spectral index
defined as
BD ¼1Rλ12ðÞ=Rλ2þRλ3
ðÞ;
where BD is the band depth, R
λ1
is the
reflectance at the metal-OH absorp-
tion center, and R
λ2
and R
λ3
are the
continuum reflectance values on
either side of the absorption. Using
kaolinite as an example, R
λ1
corre-
sponds to 2.210 μm, R
λ2
corresponds
to 2.127 μm, and R
λ3
corresponds to
2.231 μm. The result of this analysis
shows that the band depth of the
metal-OH feature decreased by an
average of ~40% due to shock
effects for all samples (Figure 3).
However, the effects are more pro-
nounced for dioctahedral clays (aver-
age R
2
= 0.74) than for trioctahedral
clays (average R
2
= 0.40). The band
depth for saponite (trioctahedral) is
essentially unaffected.
Figure 3. Shock metamorphism and fragmentation result in decreased
spectral contrast in the diagnostic metal-OH features used to identify clay
minerals on Mars with NIR data. The effect is more pronounced for diocta-
hedral clays (e.g., kaolinite and nontronite) (dashed lines) than for triocta-
hedral clays (e.g., chlorite, serpentine, and saponite).
Figure 2. NIR spectra are presented for shocked clay minerals, as well as for the corresponding unshocked samples. “MOH”
corresponds to metal-OH features for each mineral located from 2.2 to 2.35 μm. “HOH”corresponds to vibrational over-
tones in adsorbed H
2
O. “Fe
2+
”corresponds to a spectral slope located from 1 to 2 μm related to electronic transitions in Fe.
“OH”corresponds to hydroxyl deformation overtones located near 1.4 μm. “GPa”corresponds to the shock pressure
calculated for each experimentally altered mineral.
Geophysical Research Letters 10.1002/2017GL073423
MICHALSKI ET AL. SHOCKED CLAYS ON MARS 6565
Changes in the spectral slope of
chlorite between λ= 1 and 2 μm sug-
gest that Fe-oxidation is related to
shock, as has been observed in
igneous minerals [McCanta and
Dyar, 2017]. Mössbauer spectroscopy
results relating the amount of oxi-
dized Fe to total Fe show that even
relatively low amounts of shock pres-
sure (10–20 GPa) have a profound
effect on Fe oxidation for chlorite
(Figure 4). The trend is nonlinear
and incremental increases in shock
pressure above 20 GPa do not pro-
duce a proportional increase in Fe
oxidation (Figure 4). We interpret this
to result from heterogeneous distri-
bution of peak heating within the
samples likely due to collapse of resi-
dual pore space.
In fact, the TEM data of kaolinite sam-
ples show strong evidence for het-
erogeneous distribution of shocked
Figure 4. The amount of oxidized iron (Fe
3+
) versus total Fe is plotted
against experimental shock pressure for chlorite. Even minor amounts of
shock (10 GPa) dramatically affect oxidation of Fe. The effect is nonlinear at
high shock pressures (>20 GPa). Estimated error bars of ±3% are shown for
Fe
3+
data derived from Mössbauer spectroscopy.
Figure 5. Bright-field TEM images and inset selected area electron diffraction (SAED) patterns of (a) unshocked kaolinite
and kaolinite shocked to (b) 10, (c) 20, (d) 29, (e) 36, and (f) 40 GPa. These are examples of the most deformed materials
that show a progression from ordered pseudo hexagonal plates (Figure 5a) through nearly amorphous material at 40 GPa.
At 10 GPa (Figure 5b), the crystals show minor deformation and rotational disorder in the SAED pattern. At 20 GPa
(Figure 5c), the plate-like morphology is reduced and the SAED shows a prominent and sharp ring pattern indicative of
rotational disorder. At 29, 36, and 40 GPa, the fragments have lost their plate-like morphology and the SAED patterns show
progressively more diffuse and weakening diffraction intensity.
Geophysical Research Letters 10.1002/2017GL073423
MICHALSKI ET AL. SHOCKED CLAYS ON MARS 6566
material. The unshocked kaolinite crystals are relatively coarsely crystalline, with hexagonal plates ~1–2μmin
diameter and tens of nanometer thickness (Figure 5). Like the unshocked sample, the 10 GPa shock material
was generally composed of large crystals and displays SAED patterns indicative of well-ordered material.
Significant changes are observed at 20 GPa where the material contains both amorphous domains and rela-
tively undeformed domains. This trend continues up to 25, 30, and 36 GPa, where the proportion of highly
deformed and disordered material steadily increases. The 40 GPa material that is nearly completely amor-
phous, but even in this highly shocked sample, domains of crystalline kaolinite exist (Figure 5).
These progressive shock effects in kaolinite result in discernible changes to NIR spectral structure of the sam-
ples (Figure 2). The NIR spectra of unshocked and low-shock (10 GPa) samples show spectral structure in the
2.17–2.21 μm region diagnostic of the AlAlOH vibrations in relatively well-ordered kaolinite-group minerals
[Crowley and Vergo, 1988]. With increasing shock, these overlapping features are broadened and merged into
a single spectral absorption centered at ~2.2–2.21 μm. Similar impact-induced disorder is observed in the loss
of the doublet spectral structure of the OH overtone absorptions located at 1.41 μm in shock pressures
above 20 GPa.
Figure 6. CRISM spectra show evidence for shocked kaolinite on Mars. (a and b) The blue-green colors in the CRISM images
correspond to kaolinite-bearing surfaces. (c) The central peak of Leighton crater (Figure 6a) and ejecta of an unnamed
crater located at 158.9°E, 25.83°S (Figure 6b) show systematic changes in their spectral features that could be related to
shock. Specifically, variations in the spectral structure of AlOH overtones at 2.17 and 2.21 μm are consistent with the
observed trends in experimentally shocked kaolinite. Some spectra within the kaolinite unit in Leighton crater show
extreme spectral changes (gray-black lines in Figure 6c). (d) The surface bearing these spectra appears brecciated and
possibly melted in HiRISE data, and therefore, the features observed with CRISM in these surfaces could correspondto melt
or altered melt rocks. CRISM false color images display I/F at 1.08 μm as blue, 1.51 μm as green, and 2.53 μm as red.
Geophysical Research Letters 10.1002/2017GL073423
MICHALSKI ET AL. SHOCKED CLAYS ON MARS 6567
Nontronite and chlorite exhibit slightly more complex behavior in the 2.2–2.35 μm spectral region. Just as in
the pure kaolinite sample, the admixed kaolinite contaminant in the nontronite sample loses its spectral
structure at ~20 GPa [Friedlander et al., 2015], but a strong 2.2 μm AlOH feature persists to high pressures.
Over the same pressure range, the FeOH absorption at 2.28 μm becomes weaker and broader to the point
that it is unrecognizable against the dominant, stronger AlOH absorption profile. Similarly, chlorite contains
two metal-OH absorptions relating to AlMgOH at lower wavelength (~2.26 μm) and FeMgOH at higher wave-
length (2.35 μm). With increasing shock, the 2.35 μm feature is lost, potentially due in part to oxidation of Fe,
which destabilizes MgFeOH bonds in the octahedral sheet. The AlMgOH bonds are less affected, and there-
fore, the 2.26 μm absorption persists to high shock pressures.
4. Detection of Shocked Clays on Mars
Impact-driven fragmentation of the Martian crust would have resulted in the decrease in lithic particle size
and clay crystallite size in ancient, phyllosilicate-bearing materials. Some of the record of ancient Martian
clays is likely, as a result, either spectrally undetectable or spectroscopically similar to amorphous materials
[Friedlander et al., 2016]. However, some particular features of shocked clays allow for remote infrared detec-
tion of shock effects in some cases on Mars.
Spectra from the Compact Reconnaissance Imaging Spectrometer for Mars, of the ejecta of an unnamed cra-
ter located at 158.9°E, 25.83°S, and the uplifted central peak of Leighton crater (57.76°E, 3.16°N) both show
spectral trends consistent with shock metamorphism of kaolinite (Figure 6) (see supporting information for
data processing details). NIR spectra of kaolinite in crater ejecta (Figure 6c) show a decrease in spectral struc-
ture and increasing breadth. Spectra of kaolinite in the central peak of Leighton display systematic changes
to the AlOH spectral structure, as is observed in lab data of shocked kaolinite (Figure 6). Breccia observed
within the kaolinite unit in high-resolution visible images exhibits interesting spectral features that might
represent melted bedrock or altered melted bedrock. While neither of these examples demonstrates the
detection of shocked clays beyond a shadow of a doubt, both show trends consistent with shock processes
in appropriate geologic contexts. Shock effects might be more widespread on Mars, but the effects are more
easily detectable as they pertain to kaolinite, which is why these two examples were chosen.
5. Conclusions
Most of the Martian surface has been modified by impact cratering. Clay minerals that formed on early Mars
at the surface and in the subsurface have been affected by impact processes, potentially including shock
metamorphism. Given the importance of Martian clay mineralogy for understanding ancient climate and
early habitability of the planet [Bibring et al., 2006], the questions of if and how shock processes influence
our interpretation of the mineralogy of clay minerals on Mars are both interesting and necessary.
Importantly, our results show that most of the essential aspects of clay mineral identification by infrared
remote sensing of Mars are not obfuscated by shock effects. This result is important not only for remote sen-
sing of Mars but also for other impacted bodies with clay minerals such as asteroids and Ceres [Ammannito
et al., 2016].
Effects of shock up to 40 GPa do not dominate the NIR spectral features of clay minerals, but some effects are
useful, interesting, and important for Mars. On a global scale, impact fragmentation has reduced the spectral
contrast of impacted clays, and therefore, it is important to recognize that the current assessment of clay dis-
tribution and abundance on Mars is an underestimate. Second, impact-induced shock could affect the detec-
tion of Fe
2+
-rich clays and our analysis of the oxidation state under which early clays formed on Mars.
Therefore, it is likely that our interpretation clay mineralogy in the ancient, heavily impacted Martian crust
is biased against ferrous clays, which might have been abundant on early Mars.
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Acknowledgments
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