Earth and Planetary Science Letters 390 (2014) 157–164
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Laboratory spectroscopic detection of hydration in pristine
lunar regolith
Matthew R.M. Izawa a,∗ , Edward A. Cloutis a , Daniel M. Applin a , Michael A. Craig b ,
Paul Mann a , Matthew Cuddy a
a
Hyperspectral Optical Sensing for Extraterrestrial Reconnaissance Laboratory, Dept. Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg,
Manitoba R3B 2E9, Canada
b
Department of Earth Sciences/Centre for Planetary Science and Exploration, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B7,
Canada
a r t i c l e
i n f o
Article history:
Received 21 August 2013
Received in revised form 30 October 2013
Accepted 7 January 2014
Available online xxxx
Editor: C. Sotin
Keywords:
lunar regolith
lunar hydration
reflectance spectroscopy
Apollo samples
a b s t r a c t
Reflectance spectroscopy of Apollo lunar soil samples curated in an air- and water-free, sealed
environment since recovery and return to Earth has been carried out under water-, oxygen-, CO2 - and
organic-controlled conditions. Spectra of these pristine samples contain features near 3 μm wavelength
similar to those observed from the lunar surface by the Chandrayaan-1 Moon Mineralogy Mapper (M3 ),
Cassini Visual and Infrared Mapping Spectrometer (VIMS), and Deep Impact Extrasolar Planet Observation
and Deep Impact Extended Investigation (EPOXI) High-Resolution Instrument (HRI) instruments. Spectral
feature characteristics and inferred OH/H2 O concentrations are within the range of those observed by
spacecraft instruments. These findings confirm that the 3 μm feature from the lunar surface results from
the presence of hydration in the form of bound OH and H2 O. Implantation of solar wind H+ appears to
be the most plausible formation mechanism for most of the observed lunar OH and H2 O.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The prevailing view following analysis of lunar materials returned by the US Apollo and Soviet lunar programs was that the
moon is virtually anhydrous, with the possible exception of local
cold traps, e.g., near the poles (Urey, 1952; Watson et al., 1961;
Nozette et al., 1996; Feldman et al., 1998; Colaprete et al., 2010).
Recently, the anhydrous model has been upended by the detection of spectral features near 3 μm in wavelength, attributable to
hydroxyl and water by the M3 (Pieters et al., 2009), VIMS (Clark,
2009) and HRI (Sunshine et al., 2009) instruments. In parallel to
the remote detection of hydration in the lunar regolith, microanalytical studies of lunar materials using ion microprobe and infrared
spectromicroscopy showed unexpectedly high levels of H in lunar
minerals and volcanic glasses (Saal et al., 2008; McCubbin et al.,
2010; Hui et al., 2013; Saal et al., 2013) and soil agglutinate particles (Liu et al., 2012). Some of the microanalytically-detected H
in lunar materials has been ascribed to solar wind bombardment
(Liu et al., 2012), and the remainder is ascribed to internal water, though the total concentrations inferred for regolith materials,
ranging up to hundreds of ppm in H2 O equivalent (Liu et al., 2012)
*
Corresponding author.
E-mail address: [email protected] (M.R.M. Izawa).
0012-821X/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.epsl.2014.01.007
are much lower than the values of up to 0.3–0.5 wt.% derived
from remote spectral analysis (Clark, 2009; Pieters et al., 2009;
Sunshine et al., 2009; McCord et al., 2011). Therefore, some additional hydroxyl and water must be contained in lunar regolith,
probably as highly disordered hydroxyl groups at or near grain
surfaces, and as strongly hydrogen-bonded adsorbed water. The
objective of this study was to provide laboratory confirmation of
remote detections of hydrogen in the form of hydroxyl groups
and/or bound molecular water in lunar regolith. To accomplish
this goal, we have examined six Apollo regolith samples that have
remained unopened since collection and return to Earth, under
oxygen-, water-, CO2 , and organic-controlled conditions using infrared reflectance spectroscopy. Our results show that these regolith samples, recovered from just below the lunar surface, contain adsorbed water and hydroxyl. The band depths, shapes, and
feature positions observed here are similar to those observed for
the lunar surface by M3 , VIMS and HRI. Implantation of hydrogen
by the solar wind has been suggested as the most likely source
for lunar hydroxyl and water, a process first suggested by Zeller et
al. (1966). Following the M3 , VIMS and HRI observations, McCord
et al. (2011) presented systematic arguments for the solar wind
implantation model. Some contribution of internal (magmatic) hydration may also be remotely detectable in some areas of the lunar
surface (Klima et al., 2013).
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Table 1
Sample data for Apollo lunar regolith samples investigated in this study.
Sample
Generic
Specific
Parent
Container
Weight
(g)
Apollo
Apollo
Apollo
Apollo
Apollo
Apollo
10 084
12 030
15 041
61 221
64 801
71 061
2008
185
71
176
176
14
162
7
20
25
4
5
99425
99440
PV
925444
925454
98990
0.536
0.511
0.500
0.517
0.510
0.520
11
12
15
16
16
17
2. Samples and analytical methods
Contamination of the Apollo samples with terrestrial water
would compromise the goal of this study; therefore special procedures were implemented to preserve their integrity. Spectra were
collected in a Plas-Labs 818 GBB glovebox with interior dimensions
97 × 152 × 79 cm; (Plas-labs Inc., Lansing, MI) under a dry N2 atmosphere, where dry N2 was passed through a series of Drierite
filters. The glovebox also contained Drierite and Chemisorb to remove any remaining water and CO2 prior to opening of the sample
containers. Humidity and O2 levels were monitored continuously
with a BW Technologies Gas Alert O2 Extreme oxygen meter, and
an Extech Instruments RH20 humidity/temperature datalogger, and
were 1% throughout the course of the measurements. Samples
were only opened within the glovebox. Additional checks on sample integrity were provided by the inclusion of nanophase iron
(50 nm nominal particle size) and calcium sulfide in the glove box,
which are extremely sensitive to oxidation, hydration, or adsorption of water. Nanophase iron and CaS samples were monitored
spectroscopically throughout the course of the experiment and by
X-ray diffractometry (XRD) before and after and no evidence of
oxidation, hydration or water adsorption within the glovebox environment was observed. For the XRD analysis, aliquots of nanophase
iron and calcium sulfide were removed (through an independently
purged airlock) from the glovebox at the beginning of the experiment and immediately analyzed using a Bruker D8 Discover
X-ray diffractometer with Co Kα1,2 radiation (λ = 1.78897 Å) operating at 40 kV and 40 mA. After the entire experiment, aliquots
of nanophase iron and calcium sulfide that had remained exposed
within the glovebox for the duration were again removed and analyzed by XRD.
An important issue which is not within the control of the experimenters is that of pre-analysis contamination of the samples,
either during their return to Earth or subsequently, either during
the 40–43 years of curation or during shipment to the laboratory.
The samples investigated in this study are splits of material that
(to the best of our knowledge) were not exposed to air or water
during either return or curation. The specific sample identities and
parentage of the materials investigated here are summarized in Table 1. The samples were delivered to HOSERLab in sealed sample
vials (packed under high-purity N2 atmosphere at NASA Johnson
Space Center) held in heavy plastic, within airtight steel containers
further wrapped in heavy plastic, all held within a steel briefcase.
There was no sign of damage to any of the packaging materials
when they were removed from the case within the purged glovebox. We also opened the successive layers of packaging material
with the O2 and RH meters nearby, to observe any spike in either
O2 concentration or relative humidity at the moment of opening (which could indicate the presence of contamination), no such
spike was observed.
Reflectance spectra from 2.0 to 5.2 μm were collected with
a Designs & Prototypes Model 102F spectrophotometer (Designs
& Prototypes, Simsbury, CT). Samples were illuminated with an
HOSERlab in-house 100 W quartz–tungsten–halogen light source
directed through an open-air aluminum pipe at 30° from normal.
The spectrometer is equipped with a Designs & Prototypes customized optical assembly that allows a spot size of ∼7 mm to
be viewed by the instrument. The spectrometer uses a Michelson
interferometer which contains the infrared optics, beam splitter,
and the scanning mirror assembly. The internal mirrors are servo
driven at a constant speed to produce the interferograms. Spectral resolution for all measurements in this study was 4 cm−1 .
The thermo-electrically cooled detector (77 K) consists of InSb
over mercury cadmium telluride (MCT). The servo, electronics, and
wavelength calibration are referenced to a temperature controlled
laser diode. Samples were placed on an adjustable stage and were
placed into focus, aided by a removable Modicam 10 video camera
(Camea, Brno, Czech Republic) directed into the optical assembly.
Reflectance spectra were acquired relative to a 35 mm diameter
Infragold® diffuse gold-coated standard (Labsphere, North Sutton,
NH). Reference spectra were acquired with the same viewing geometry and illumination as the samples and with the same number (100) of co-adds.
3. Results and discussion
Spectra of pristine lunar regolith have spectral features due
to water and hydroxyl that are similar to those detected by M3 ,
VIMS and HRI, displaying a broad asymmetrical feature centered
near ∼3 μm and extending from ∼2.70 μm to ∼4.45 μm (Fig. 1).
A weak feature between ∼4.5 μm and ∼4.8 μm is ascribable to
the first overtones of the silicate Si–O fundamental vibrations. The
rise in reflectance towards longer wavelengths is due primarily to
rising reflectance approaching the principle Christiansen feature,
which occurs near ∼7.5–9.0 μm for many silicates (e.g., Conel,
1969; Prost, 1973; Logan et al., 1978; Cooper et al., 2002). Because the Infragold® standard and the regolith powders were held
within the same environment, and a standard spectrum was measured prior to each sample run, it is expected that temperature
differences between the sample and standard were small, of the
order of a few °C. Nevertheless, there may be a minute contribution of thermal excess affecting these spectra, the primary
effect of which would be to reduce the band depths. The precise location of the band centers of the hydration features, near
2.957 μm are indicative of a high degree of hydrogen bonding,
as would be expected for adsorbed water remaining in material
exposed to the hostile lunar surface environment (Clark, 2009).
Continuum-removed spectra show that in detail, the 3-μm band
in these samples consists of a sharp inflection near ∼2.80 μm, attributable to structural hydroxyl, and a pair of features with local
minima near ∼2.95 μm and ∼3.14 μm attributable to bound water and hydroxyl (Fig. 2). These feature locations are very close
to the values of ∼2.81 μm, ∼2.95 μm and ∼3.14 μm observed
by HRI (Sunshine et al., 2009). The feature near 3.14 μm is similar to that due to structurally bound water in silicate minerals
(Farmer, 1974). The apparent band depths (calculated as depth below a linear continuum anchored on the short-wavelength side
of the band, following the method described by McCord et al.,
2011) at 2.8 μm and 3.0 μm in our spectra of Apollo samples
are very similar to those observed by M3 near the lunar equator
(Fig. 3A).
Quantification of the water content of the lunar regolith samples in this study is subject to many uncertainties due to the
mineralogical and textural heterogeneity of the samples. Using
the Normalized Optical Path Length (NOPL) method of (Milliken
and Mustard, 2005) we calculate between ∼0.08 and ∼0.16 wt.%
M.R.M. Izawa et al. / Earth and Planetary Science Letters 390 (2014) 157–164
159
Fig. 1. Reflectance spectra of pristine lunar regolith samples from 2.0 to 5.2 μm showing absorption features due to OH and H2 O. The “3-μm” feature is a broad envelope
ranging from ∼2.70 μm to ∼4.45 μm wavelength (cf. Fig. 1C of Sunshine et al., 2009 and Fig. 2B of Clark, 2009). This feature is a mixture of numerous O–H and H–O–H
vibrational overtones and combinations, which are poorly resolved because of the vast diversity of possible O–H co-ordination environments in surficial and structural OH and
adsorbed H2 O. The weak feature between ∼4.5 μm and ∼4.8 μm is due to the first overtones of the silicate Si–O fundamental vibrations. (A) Spectra in absolute reflectance,
shifted vertically by +0.25 each for clarity. (B) Spectra normalized to the reflectance averaged over all bands between 2617 nm and 2697 nm (2.617–2.697 μm; the range
over which Pieters et al. (2009) calculated their approximate continuum).
H2 O (∼800–1600 ppm) in our samples (Fig. 4), somewhat less
than the value of 0.3 wt.% calculated for high-latitude regions
(Sunshine et al., 2009). Although we report water concentrations
here, it must be acknowledged that the methods used both here
and by Sunshine et al. (2009) do not explicitly include hydroxyl,
which is likely to be the predominant source of the spectral
features near 3 μm. The water concentration values reported here
are best regarded as upper bounds.
The observations reported in this study are consistent with solar wind hydrogen implantation as the predominant mechanism
for the formation of lunar hydration, with initial implantation
of H+ , forming hydroxyl, some of which may form molecular water, if hydroxyl generation approaches the saturation rate (McCord
et al., 2011). The presence of similar hydration features in the
diverse lunar regolith lithologies studied here suggests that the
mechanisms of hydroxyl and water formation on the lunar surface
are similar in different surface materials. Nevertheless, there are
some minor but significant differences. There is a correlation between the depth of the ∼3 μm feature (where ∼3 μm refers to
the continuum-removed band centers of the absorption feature at
the center locations listed in Table 2) and the surface maturity parameter Is /FeO, the ratio of the value of the intensity in arbitrary
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Fig. 2. Continuum-removed spectra of lunar regolith samples reveal details of band structure. These spectra are similar to those measured by Cassini VIMS (cf. Fig 2B of Clark,
2009) and Deep Impact/EPOXI (cf. Figs. 1D and 4A–D of Sunshine et al., 2009). All spectra show an inflection near 2.80 μm and local minima near 2.96 μm and 3.14 μm
(marked by thin vertical lines), which are similar values to those in continuum-removed spectra of the lunar surface measured by Deep Impact/EPOXI (2.81 μm, 2.95 μm
and 3.14 μm).
units of the ferromagnetic resonance at g ∼ 2.1 to the FeO concentration (Morris, 1978) (Fig. 5A), consistent with hydroxyl and
water generation at or near the lunar surface. There is also a
weaker correlation between ∼3 μm feature depth and carbon content (Fig. 5B), as well as a correlation between carbon content and
the soil maturity parameter (Fig. 5C). Carbon is delivered to the
lunar surface by the solar wind and by a variety of exogenous
matter including cometary material, carbonaceous chondrites and
interplanetary dust particles, therefore the observed correlation
between C concentration and ∼3 μm feature depth may reflect input from a variety of sources. Apollo 12 sample 12 030 appears
anomalously rich in C compared to the depth of the ∼3 μm feature (Fig. 5B), and is also anomalously immature (Fig. 5C) indicating that processes that incorporate carbon into the lunar regolith do not always lead to enhanced hydration or increased
soil maturity. There is no obvious correlation between Al2 O3 content and ∼3 μm feature depth (Fig. 5D). To the extent that Al2 O3
M.R.M. Izawa et al. / Earth and Planetary Science Letters 390 (2014) 157–164
161
Fig. 3. Comparison of spectral properties derived from lunar regolith spectra reported in this study with observations from the Chandrayaan-1 M3 spectrometer (Pieters et
al., 2009; McCord et al., 2011). (A) Comparison of the apparent band depths at ∼2.8 μm and ∼3.0 μm (in percent reflectance). The depths of the features were calculated
from our observations to be most comparable with those shown by McCord et al. (2011), that is, as the depth below a straight-line continuum extrapolated from the
short-wavelength edge of the band (cf. Fig. 9 and Section 6.2 of McCord et al., 2011). Light-toned shaded regions represent the range of 2.8 and 3.0 μm feature depths for
M3 observations within 30° of the lunar equator (McCord et al., 2011), the dark-toned area is the region of overlap for both band depth parameters. Sample 61 121-176
(Apollo 16) falls slightly above the range of 3.0 μm band depths for M3 observations within 30° latitude of the lunar equator, but remains well within the range over all
latitudes (up to ∼20%). (B) Comparison of relative 3-μm feature strength, calculated from our spectra following the method of Pieters et al. (2009), that is, relative band
depth = 1 − ( R b / R c ), where R b is the average of channels at 2896 nm and 2936 nm, and R c is the approximate continuum given by the average of channels at 2617 nm,
2657 nm, and 2697 nm (cf., Fig. 2C of Pieters et al., 2009). We have averaged over our channels in the ranges cited to produce numbers directly comparable to those of
Pieters et al. (2009).
content corresponds to the presence of anorthosite-rich highlands material, this confirms previous suggestions (Clark, 2009;
Pieters et al., 2009; Sunshine et al., 2009; McCord et al., 2011)
that composition (in terms of maria vs. highland material) does
not strongly affect the equilibrium OH/H2 O content of lunar regolith. The fact that the anorthosite-rich Apollo 16 samples may
plot on a different linear array in the space of calculated H2 O content vs. ∼3 μm band depth (Fig. 4) suggests the possibility that the
higher-albedo anorthositic materials may be suppressing the measured ∼3 μm band depths, leading to a systematic underestimation
of lunar water content in highland areas.
The surface and near-subsurface lunar regolith is certainly hydrated, although the contributions of various mechanisms of hydration remain unquantified. The similarity of spectral features due
to hydration in diverse lithologies and the correlation between surface maturity and band depth are most consistent with solar wind
hydrogen implantation as the predominant source of lunar hydration. Observations of similar hydration features over much of the
lunar surface (Pieters et al., 2009; McCord et al., 2011) also support
the solar wind model. Traces of indigenous lunar hydrogen trapped
in minerals and glasses may also make a minor contribution to
hydration spectral features in some regions (Saal et al., 2008;
Hui et al., 2013; Klima et al., 2013), but the lack of strong
compositional correlation between hydration features in laboratory data (this study) or remote sensing argues against a major
spectral signature of lunar magmatic or metasomatic hydrogen
(Pieters et al., 2009; Sunshine et al., 2009; Clark et al., 2011;
McCord et al., 2011).
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Fig. 4. Concentrations of H2 O derived using the Normalized Optical Path Length (NOPL) (Milliken and Mustard, 2005) vs. continuum-removed band depths for the 3 μm
feature for lunar regolith samples in this study. The H2 O concentrations derived here are close to the modeled abundance of up to ∼770 ppm reported by Pieters et al.
(2009) based on M3 data, and overlap the upper range of the estimate of 10–1000 ppm given by Clark (2009) based on VIMS spectra. It is possible that the anorthosite-rich
Apollo 16 samples (diamonds) plot on a different linear array than the other samples (circles) suggesting the spectrally bright components may be suppressing the measured
∼3 μm band depths leading to a systematic underestimation of lunar water content in highland areas. Water concentrations obtained from these laboratory spectra are
below the estimate of ∼0.3 wt.% for high-latitude regions reported by Sunshine et al. (2009), possibly reflecting lower concentrations of H2 O and OH at the lower-latitude
Apollo sampling sites. Sunshine et al. (2009) used the Effective Single-Particle Absorption Thickness (ESPAT) parameter of Milliken and Mustard (2005). Water concentrations
derived by the ESPAT and NOPL techniques are comparable within the uncertainties of each method (Milliken and Mustard, 2005, 2007).
Table 2
Spectral characteristics and derived quantities.
Sample
Band center (3rd order polynomial fit, μm)
Band depth (non-continuum removed)
Band depth (continuum removed)
1 − Rb /Rc b
Apparent deptha at 2.8 μm
Apparent deptha at 3.0 μm
Water from NOPL (wt.%)
10 084-2008
Apollo 11
12 030-185
Apollo 12
15 041-71
Apollo 15
61 221-176
Apollo 16
64 801-176
Apollo 16
71 061-14
Apollo 17
2.960
0.027
0.128
0.176
0.017
0.026
0.16
2.949
0.031
0.087
0.114
0.021
0.031
0.10
2.965
0.030
0.120
0.059
0.023
0.032
0.14
2.955
0.052
0.101
0.134
0.033
0.051
0.09
2.965
0.050
0.120
0.155
0.034
0.050
0.12
2.955
0.025
0.076
0.100
0.017
0.024
0.08
a
Values are derived following the methods in McCord et al. (2011), that is, depth below a straight-line continuum anchored to the short-wavelength side of the band.
The quantity 1 − R b / R c is calculated following Pieters et al. (2009), that is, averaging channels over the interval from 2.617–2.697 μm to approximate the continuum
(R c ) and over the interval 2.896–2.936 μm for the band intensity (R b ).
b
4. Summary and conclusions
Authors contributions
Laboratory spectra of pristine lunar regolith reported here
confirm remote sensing observations of hydroxyl and water in
the lunar near-surface. Our results cannot conclusively exclude
small contributions from terrestrial contamination or other lunar H reservoirs such as volatile-rich minor phases, as hydrous
meteorites, comets and interplanetary dust particles also deliver
hydrogen to the lunar surface, some of which may become incorporated into the regolith (Anders et al., 1973). Traces of indigenous (magmatic) lunar hydrogen trapped in minerals and
glasses may also contribute to spectral features (Saal et al., 2008;
Hui et al., 2013). Nevertheless, based on the similarity between
laboratory and remote spectra and the correlations between spectral and soil maturity parameters (Is /FeO and C concentration,
both of which are proxies for near-surface exposure), we conclude
that the predominant source of the observed 3-μm region spectral features are hydroxyl and water native to the lunar surface,
most of which were created by solar wind hydrogen implantation.
E.A.C. designed the experiment. M.R.M.I., D.M.A., P.M. & M.C.
conducted the measurements. M.R.M.I., M.A.C., D.M.A. & E.A.C. analyzed the data. M.R.M.I. wrote the paper with contributions from
all other authors.
Acknowledgements
M.R.M.I. gratefully acknowledges funding from the NSERC CREATE Canadian Astrobiology Training Program and the Mineralogical Association of Canada. The University of Winnipeg’s HOSERLab
was established with funding from the Canada Foundation for Innovation, the Manitoba Research Innovations Fund and the Canadian Space Agency, whose support is gratefully acknowledged. This
study was supported by research grants from NSERC, the Canadian
Space Agency and the University of Winnipeg. Thanks to Thomas
McCord and Paul Lucey for highly constructive reviews, and to
Christophe Sotin for editorial handling. This research has made use
of the NASA Astrophysical Data System.
M.R.M. Izawa et al. / Earth and Planetary Science Letters 390 (2014) 157–164
163
Fig. 5. Correlation plots of spectral parameters and sample characteristics previously reported in the literature for different splits of the same parentage as the samples
studied here (Frondel et al., 1971; LSPET, 1973, 1974; Morris, 1978; Korotev, 1987; Korotev and Kremser, 1992; Korotev and Gillis, 2001). (A) Continuum-removed band
depth of the ∼3 μm feature in Apollo lunar samples is correlated with lunar soil maturity parameter Is /FeO. The soil maturity parameter is the ratio between ferromagnetic
resonance intensity at g ∼ 2.1 (g is the dimensionless magnetic moment) to the FeO concentration (Morris, 1976, 1978). The relationship between increased hydration
feature depth and maturity parameter supports solar wind H implantation as the predominant source of lunar surface OH and H2 O. (B) The general correlation between C
concentration and continuum-removed ∼3 μm feature depth may reflect input from solar wind implantation and a variety of infalling matter including cometary material,
carbonaceous chondrites and interplanetary dust particles, therefore, Apollo 12 sample 12 030 appears anomalously rich in C compared to the continuum-removed band
depth, indicating that the addition of carbon to the lunar regolith is not always accompanied by hydration. (C) There is a general correlation between soil maturity parameter
and C concentration for these samples, but the relationship between C concentration, maturity and hydration is not simple, possibly due to overlapping or competing
processes involved (solar wind implantation, infall of exogenous matter, and both creation and destruction/removal of OH/H2 O). Sample 12 030 is again anomalous, with a
very high C concentration for its maturity. (D) Although some correspondence between highland material rich in Al (primarily due to anorthitic plagioclase) and hydration
spectral features has been noted (Sunshine et al., 2009; McCord et al., 2011), the lack of a strong correlation between continuum-removed ∼3 μm feature band depth and
Al2 O3 content in these soil samples may indicate that a compositional or mineralogical control on OH/H2 O generation is not the predominant mechanism responsible for the
observed 3 μm feature enhancement in lunar spectra.
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