1. LETTERS
PUBLISHED ONLINE: 25 AUGUST 2013 | DOI: 10.1038/NGEO1909
Remote detection of magmatic water in Bullialdus
Crater on the Moon
R. Klima1
*, J. Cahill1
, J. Hagerty2
and D. Lawrence1
Once considered dry compared with Earth, laboratory analyses
of igneous components of lunar samples have suggested that
the Moon’s interior is not entirely anhydrous1,2
. Water and
hydroxyl have also been detected from orbit on the lunar
surface, but these have been attributed to nonindigenous
sources3–5
, such as interactions with the solar wind. Magmatic
lunar volatiles—evidence for water indigenous to the lunar
interior—have not previously been detected remotely. Here
we analyse spectroscopic data from the Moon Mineralogy
Mapper (M3
) and report that the central peak of Bullialdus
Crater is significantly enhanced in hydroxyl relative to its
surroundings. We suggest that the strong and localized
hydroxyl absorption features are inconsistent with a surficial
origin. Instead, they are consistent with hydroxyl bound to
magmatic minerals that were excavated from depth by the
impact that formed Bullialdus Crater. Furthermore, estimates
of thorium concentration in the central peak using data from
the Lunar Prospector orbiter indicate an enhancement in
incompatible elements, in contrast to the compositions of
water-bearing lunar samples2
. We suggest that the hydroxyl-
bearing material was excavated from a magmatic source that is
distinct from that of samples analysed thus far.
Hydrogen, hydroxyl (OH−
) and water have been detected on the
lunar surface from orbit using a range of different instruments3–7
.
Hypotheses for globally distributed surficial H2O/OH−
molecules
or larger deposits of water ice in the permanently shadowed regions
of the Moon include that OH−
and H2O molecules are produced
in situ through the interaction of solar-wind-derived protons with
lunar soils3,5,8
and/or that they are delivered by comets and other
impactors9
. Broadly distributed molecular OH−
or H2O formed
by solar-wind interaction with surface silicates is expected to be
loosely bound to the lunar surface8
and may dissociate from surface
soils during solar heating, migrating along ballistic trajectories
until ultimately becoming cold-trapped in permanently shadowed
regions and/or buried10,11
. New research suggests that hydroxyl
formed by solar-wind bombardment may also become embedded
in agglutinates during micrometeorite impacts as part of the space
weathering process12
. Until now, bound, magmatic lunar volatiles
have not been detected remotely anywhere on the Moon.
The 61-km-diameter Bullialdus Crater, centred at 20.7◦
S,
337.8◦
E in Mare Nubium (Fig. 1), lies along the southern edge
of the Procellarum KREEP (potassium, rare earth elements and
phosphorus) Terrane (PKT), a region on the nearside of the
Moon that is highly enriched in incompatible elements13
. Lunar
Prospector measurements suggest that Bullialdus Crater coincides
with a localized concentration of thorium14
. Bullialdus Crater
is mineralogically distinct and its central peak has long been
recognized as exhibiting strong spectral signatures typical of norite,
1Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA, 2US Geological Survey, Astrogeology Science Center, Flagstaff,
Arizona 86001, USA. *e-mail: Rachel.Klima@jhuapl.edu
an intrusively formed igneous rock dominated by orthopyroxene
(Ca-poor pyroxene) and anorthite (Ca-rich plagioclase feldspar)15
.
Stratigraphic interpretations of materials exposed in Bullialdus
Crater walls, coupled with the presence of exhumed noritic
materials in its central peak, led previous investigators to suggest
that the Bullialdus impact excavated a layered mafic intrusion15,16
.
Data from M3
provide an opportunity to examine Bullialdus Crater
both at high spatial (∼140 m per pixel) and spectral (20–40 nm
sampling) resolution. They also provide, for the first time, the
opportunity to characterize visible to near-infrared wavelength
reflectance spectra (that is, 0.6–3 µm). The 3 µm region in particular
is critical for near-infrared volatile assessment of the Moon or any
other airless body because both water and hydroxyl are strongly
absorbing in this portion of the electromagnetic spectrum.
Previous interpretations of this area suggest that the Bullialdus
impactor penetrated the basalt-flooded Nubium Basin, excavating
a range of intrusive crustal rocks. The central peaks of Bullialdus
are mineralogically diverse, with the westernmost peak exhibiting
a more clinopyroxene-rich spectral signature (Fig. 1e, cyan) than
the northern peaks. On the basis of radiative transfer models of
Clementine reflectance data, this western portion of the peak has
been classified as anorthositic gabbronorite, whereas the northern
peaks have been classified as anorthositic norite or norite17
. Norite
(Fig. 1e, yellow) dominates the bulk of the central peak, though
there is a region of anorthositic material (Fig. 1e, dark blue) exposed
towards the centre of the peak. Anorthositic material is also exposed
in the crater rim and proximal ejecta (Fig. 1d, black). A map of the
2.8 µm band depth (Fig. 1f,g) reveals that hydroxyl is detected in
only the central peak and is explicitly found in association with the
noritic and anorthositic peak material.
In laboratory measurements, bound OH−
can be distinguished
from adsorbed OH−
and H2O molecules by examining the spectral
shape. Unfortunately, the specific spectral shape of the hydroxyl
absorption band cannot be precisely characterized by M3
because
of the 40 nm spectral sampling in the 3 µm region. The surficial
OH−
and H2O molecules detected previously by M3
typically
exhibit a broad absorption beyond 2.8 µm (ref. 8). In contrast,
the hydroxyl absorption observed in the central peak of Bullialdus
Crater is significantly stronger and sharper than is observed at
similar latitudes in other nearby terrains (Fig. 2 and Supplementary
Fig. S1). The absorption observed in both the anorthositic and
noritic material exhibits a clear band minimum at 2.8 µm. The
band shape is significantly sharper than laboratory measurements
of agglutinates12
, but is consistent in both energy and band
shape with OH−
measured in transmission spectra of internally
bound hydroxyl in nominally anhydrous terrestrial minerals18
such
as anorthite and orthopyroxene, and minor hydrated terrestrial
minerals such as apatite19
(Supplementary Fig. S2).
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2. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1909
23° W 22° W 21° W
7
6
5
1 2
4
3
b
22° S
d
f
c
e
g
23° W 22° W 21° W
23° W 22° W 21° W
Bullialdus
Mare Nubium
a
8
20° S
21° S
22° S
20° S
21° S
20° S
21° S
22° S
Figure 1 | Context and spectral parameter maps of Bullialdus Crater. a, LROC wide-angle camera (WAC) view of Bullialdus Crater. Scale bar, 100 km.
Image credit: NASA/GSFC/ASU. b,c, M3 750 nm albedo map of Bullialdus Crater (80 km image width; b) and central peak (scale bar, 5 km; c). d,e, M3
false colour composite (Supplementary Table S1) highlighting mineralogy of the crater (d) and central peak (e). f,g, 2.8 µm OH− absorption strength in
Bullialdus Crater (f) and central peak (g). Absorption strength grades black (0%) to white (4%). Numbers on a–c indicate locations from which Fig. 2
spectra were extracted.
If the hydroxyl observed at Bullialdus Crater is solar-wind
implanted, it could either be locked stably within agglutinates12
or it could be unstable, adhered loosely to mineral grains but
continuously forming in the solar-wind flux10
. The former would
imply that the central peak of Bullialdus is more enriched in
agglutinates (or older) than any other material within at least 10◦
latitude in any direction, which is inconsistent with surface maturity
maps of the region20
. The strongest water/hydroxyl absorption
bands are found along and just below the peak ridge on both
sides and coincide with bands of large (tens to ∼150-m-diameter)
boulders and potentially bedrock (Fig. 3). The coarse texture and
high albedo of the OH−
-rich regions of the central peak suggest
that it has not experienced unusually high degrees of agglutination
relative to its surroundings.
If OH−
in Bullialdus Crater is loosely adhered to the surface,
the band depths of the OH−
absorption might weaken later in the
lunar day, as the surface becomes warmer, or when the Moon is
within the Earth’s magnetotail and shielded from the solar wind.
The central peak of Bullialdus was imaged by M3
at different times
of the lunar day. Hydroxyl absorptions are deepest near the apexes
of the peaks and weaken with increasing distance downslope. They
occur symmetrically about the central peak ridges, but because
of the illumination geometry in each of the optical periods, we
compare a slightly weaker absorption on a flatter slope that is
not in shadow during any of the three optical periods. After
photometric correction, there is little to no change in absorption
band depth observed to suggest hydroxyl mobility (Fig. 2 and
Supplementary Fig. S3).
A previous study10
showed that the likelihood of OH−
adsorption may depend on lithology. The central peak of Aristillus
Crater (33.9◦
N, 1.2◦
E) has a similar lithology to Bullialdus and
is also the focus of a thorium anomaly. Given that Aristillus is
more than 10◦
farther from the equator, it should be cooler and
more hospitable to OH−
adsorption than Bullialdus, resulting in
a central peak that is coated with OH−
. However, it does not
exhibit a prominent regional 3 µm anomaly (Fig. 2). The geologic
context coupled with the stability of the 2.8 µm absorption band
at Bullialdus Crater leads us to conclude that the water/hydroxyl
detected in the central peak is most likely bound within primary
magmatic materials rather than implanted by the solar wind or
within glassy agglutinates.
We can place limits on the amount of water/hydroxyl present
by making simplifying assumptions about the nature of the surface
and/or mineralogy. On the basis of radiative transfer modelling,
another study21
suggested that Bullialdus Crater central peak as a
whole contains about 57% mafic minerals and 43% plagioclase.
Using this bulk mineralogy, coupled with an orthopyroxene
composition of Mg75 (ref. 22), we can derive the single-scattering
albedo of the surface, from which the absorbance can be
calculated23–25
(Supplementary Fig. S4). If we assume that the local
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3. NATURE GEOSCIENCE DOI: 10.1038/NGEO1909 LETTERS
1 Central peak
2 Central peak
5 Crater floor
6 Crater wall
7 Mare Nubium soil
6 Highland soil south
3 Central peak
4 Central peak
0.00
1,000 1,500 2,000
Wavelength (nm)
2,500 3,000 1,000 1,500 2,000
Wavelength (nm)
2,500 3,000
1,000 1,500 2,000
Wavelength (nm)
2,500 3,000 1,000 1,500 2,000
Wavelength (nm)
2,500 3,000
0.05
0.10
0.15
0.20
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Scaledreflectance
ReflectanceScaledreflectance
Scaledreflectance
0.00
0.05
0.25
0.15
0.20
0.10
Optical period 1b
Optical period 2a
Optical period 2c1
Bullialdus central peak, 20.8° S, 22.3° W
Aristillus central peak, 33.9° N, 1.2° E
Highlands, 59.0° S, 43.5° W
a b
c d
Figure 2 | Spectra of Bullialdus Crater and the surrounding region. a, Strong absorption bands at 1,000 and 2,000 nm are indicative of pyroxene, whereas
flatter, brighter spectra are probably dominated by anorthosite. b, Bullialdus spectra scaled to unity at 2.736 µm. Only the spectra extracted from the
central peak exhibit a hydroxyl absorption. c, Spectra from Bullialdus Crater central peak compared with the higher latitude, noritic central peak of Aristillus
Crater and a higher-latitude region of highlands exhibiting a typical surficial water signature. d, Repeat spectral imaging during three optical periods. c,d are
scaled at 2.736 µm.
regolith exhibits a grain size between 15 and 150 µm, we estimate the
maximum abundance in the central peak to be 80±40 ppm as water.
In principle, one might expect that the bulk hydrogen signature
could be detected using neutron spectroscopy in a manner
similar to how enhanced hydrogen signatures were detected at the
lunar poles6
. However, the small spatial scale of the Bullialdus
central peak (∼5 km width) and the low hydrogen concentrations
([H] = (2/18)(80 ± 40) = 9 ± 4 ppm H) preclude the detection
of a hydrogen signature from either the Lunar Prospector or
Lunar Exploration Neutron Detector data because the hydrogen
sensitivity of these data sets is tens of ppm H and >100 ppm
H, respectively26,27
.
A general Th enhancement (5–6 ppm) focused on Bullialdus
Crater suggests that the excavated rocks also contain increased
amounts of KREEP. The broad spatial resolution (>30 km) of the
Th data14
precludes a unique quantification of the Th content of
the central peak. However, using forward modelling techniques
that have been validated28
elsewhere on the Moon, the Th
concentrations of the central peak region can be non-uniquely
estimated. Given the distinct mineralogical character of the central
peak, we have carried out forward modelling of the entire Bullialdus
region to infer the Th concentration of the central peak region
(Supplementary Information). Our modelling (Fig. 4) indicates
that the central peak could have a Th concentration as high as
16.5±2.5 ppm. Based on a Clementine-derived FeO estimate, the
central peak has FeO abundances ranging from 8 to 10 wt%. When
compared with values in the lunar sample suite these Th values
are consistent with alkali-suite lithologies such as alkali norites29
,
though the composition of the pyroxenes22
is more magnesian than
typical for alkali-suite samples.
The association of OH−
with KREEP is enigmatic. Though it
has been suggested that any native lunar OH−
, such as KREEP,
would be concentrated in late-stage magma ocean fluids8
, studies of
hydroxyl in lunar apatites with varying KREEP abundance reveal an
anticorrelation between KREEP and OH−
, suggesting complexity
and heterogeneity in the volatile distribution in the lunar interior2
.
Bullialdus may have exposed further source-region heterogeneity by
excavating material that is enriched in both volatiles and KREEP.
The Bullialdus-forming impact is likely to have excavated
about 6–9 km deep into the lunar crust21
. However, the impact
occurred near the edge of Mare Nubium, near the inner ring
of the basin. This fortuitous placement of Bullialdus Crater may
explain its unusual composition. One possible scenario is that
material from a pluton, originally residing within the lower crust,
was uplifted to 6–9 km beneath the surface during the Nubium
basin-forming impact and subsequently excavated by the Bullialdus
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4. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1909
Noritic Anorthositic
a b c
Figure 3 | Geology of Bullialdus Crater central peak. a, Perspective view of central peak, looking towards the south. LROC narrow-angle camera (NAC)
images (Supplementary Fig. S5) overlain by 2.8 µm OH− band depth map from Fig. 1 and draped over LROC WAC 100 m digital elevation model30
(vertically exaggerated ×6). The strongest absorptions occur where large boulders are observed on either side of the ridges. Arrows indicate where the
images in b,c are located. b,c, Subset from LROC NAC image showing the surface geology in noritic (b) and anorthositic (c) spectral regions. Scale bars,
300 m.
6.24
6.09
5.95
5.80
5.65
5.51
5.36
5.21
5.07
4.92
4.77
Thabundance(ppm)
16.66
15.00
13.33
11.67
10.00
8.33
6.67
5.00
3.33
1.67
0.00
Thabundance(ppm)
Th abundance (ppm)
18.18
17.13
16.09
15.04
14.00
12.95
11.91
10.86
9.81
8.77
7.72
FeOabundance(wt%)
¬20
¬20
¬20
¬20 ¬20
¬20
0.4
12 14 16 18 20
0.6
0.8
1.0
1.2
1.4
1.6
χ2
a
c
b
d
Figure 4 | Thorium content of Bullialdus Crater central peak. a, LP-GRS Th map (0.5◦ ×0.5◦) of Bullialdus Crater. Scale bar, 50 km. b, Clementine FeO
map of the crater. c, Forward modelling results for the region. Central peak is outlined in black in a–c. d, χ2 error analysis of the central peak, indicating that
the optimum Th value for the peak is 16.5±2.5 ppm Th.
Crater-forming impact. Alternatively, deep fracturing during and
following the Nubium-Basin impact may have facilitated intrusion
of hydroxyl-bearing magma into the upper (∼10-km-deep) crust.
Our derived water/hydroxyl abundance is within the range of
OH−
concentrations for nominally anhydrous crustal pyroxenes on
Earth18
. High concentrations (>1,000 ppm weight) are strongly a
function of orthopyroxene aluminium content and typically form
at pressures greater than 1 GPa. A pressure of 1 GPa would imply
that any OH−
-bearing pyroxenes crystallized at a depth of near
to 200 km, well below the excavation depth of Bullialdus or even
the Nubium impact. A shallower origin is possible if the OH−
is
hosted within a minor hydrous mineral, as fluid inclusions, or in
a quenched silicate melt. If the minor phase makes up 2% of the
bulk rock, it would require ∼0.2–0.6 wt% OH−
to account for
the observed signature. This concentration is within the range of
OH−
concentrations measured in apatites from mare basalt samples
(some >1.5 wt% OH−
).
Methods
All M3
and Lunar Reconnaissance Orbiter camera (LROC) data used in this project
are available in the NASA planetary data system at http://pds.nasa.gov. Bullialdus
Crater was fully imaged by M3
during three optical periods: 1b, which was
illuminated from the east (average phase angle 60◦
) and has a spatial resolution of
140 m per pixel; 2a, which was illuminated from the west (average phase angle 68◦
)
and has a spatial resolution of 140 m per pixel; and 2c1, which was illuminated from
the west (average phase angle 40◦
). Aristillus Crater was imaged in optical periods 1b
and 2c1. Data were mosaicked for each of the optical periods and then parameters
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5. NATURE GEOSCIENCE DOI: 10.1038/NGEO1909 LETTERS
were selected to isolate and characterize key absorption features for more easy
visualization of spectral variability in its spatial context (Supplementary Table S1).
Estimation of hydroxyl abundance in the central peak materials requires
conversion of spectra into an absorption coefficient, knowledge of the molar
absorption coefficient for the local lithology and an estimate of the path length
through the minerals. These calculations further require knowledge of surface
temperature, particle size and other scattering properties. We can place limits on
the amount of hydroxyl present by making several simplifying assumptions about
the nature of the surface and mineralogy. One advantage to using an absorption
coefficient as derived from the single-scattering albedo of a particulate surface is that
we are not subject to orientation effects, because the observed signal is inherently an
integrated absorption average combining all crystallographic directions.
Level 2c1 data were specifically selected for modelling hydroxyl abundance
because they were obtained at the lowest phase angle, which minimizes shadowing
and multiple scattering effects4
on the steep slopes of the central peak. We convert
the spectra to single-scattering albedo using the methods of ref. 23 and then solve for
the absorption coefficient using a particle size of 45 µm. To derive single-scattering
albedo, we calculate the albedo factor23
using the incidence and emission angles
and the reflectance values at each wavelength. Albedo factor is then converted
to single-scattering albedo23
. If we approximate the mean path ray length (in
micrometres) through a single particle as 45 µm, we can approximate absorbance23
.
This requires the use of a refractive index (n), which we calculate24
using pyroxene
with an Mg# of 0.75 as an estimate for the composition of the central peak of
Bullialdus22
. Single-scattering albedo and calculated absorbance spectra for the
central peak spectra are shown in Supplementary Fig. S3.
The baseline-removed hydroxyl absorption intensity of CP1 was fit by a
Gaussian curve and then the centre frequency (in wavenumbers) of the OH−
stretching absorption was used to calculate the integrated molar absorption
coefficient (εi; ref. 25). As this calibration for εi uses units of water per mol rather
than H or OH−
, we used the formulation of the Beer–Lambert law shown in
equation (1) to determine the concentration of water. The area of the Gaussian was
used as the integrated absorption intensity (Ai), the path length of 45 µm was used
for thickness (t) and density of 3.0 g cm−3
was used for the bulk rock density (ρ).
This results in a concentration (c) of 80 ppm for the chosen particle size of 45 µm.
c =
Ai
t ×p×εi
×1.8 (1)
To ensure that this estimate was not highly dependent on the composition of
the mineral hosting the hydroxyl, we calculated the absorption coefficient using the
refractive index of anorthite and quartz as well. In those cases, the baseline-removed
absorption coefficients were indistinguishable from the orthopyroxene and resulted
in the same concentration at a fixed grain size. As the scattering properties of the
rocks are controlled by the dominant mineral phase(s), we did not carry out an
analysis using apatite. For a given concentration of water, the strength of an OH−
absorption in apatite is comparable to the OH−
strength in a nominally anhydrous
mineral18,19
. As any minor phase would comprise only a few per cent of the total
rock, the amount of hydroxyl in that phase would be significantly higher (that is, we
are calculating the abundance in the bulk path length, not in the host phase per se).
In the laboratory, water content calculations based on infrared spectra are
subject to typically 10–30% relative uncertainty depending on the strength of the
absorption band and the approximation of the molar absorption coefficient12,25
.
As the calculation of single-scattering albedo is heavily dependent on grain size,
additional uncertainty is introduced. If the average grain size of the surface is closer
to 15 µm, the water abundance would be approximately 20% higher, whereas if it
is closer to 150 µm the water content would be approximately 20% lower, resulting
in a range of ∼45–125 ppm.
Received 19 April 2013; accepted 5 July 2013; published online
25 August 2013
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Acknowledgements
We thank the NASA Lunar Advanced Science and Engineering Program (NNX10AH62G
to RK/JHUAPL), the NASA National Lunar Science Institute Polar Exploration Node
(NNA09DB31A to JHUAPL) and the NASA Planetary Mission Data Analysis Program
(NNH09AL42I to JH/USGS) for supporting this research. We are also grateful to the
NASA Discovery Program, Indian Space Research Organization and M3
team.
Author contributions
All authors contributed extensively to this work. R.K. wrote the main manuscript with
comments and feedback from the whole team. R.K. led analysis and modelling of M3
data, J.C. led processing and analysis of LROC data and J.H. and D.L. contributed the
thorium analyses and forward modelling.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Correspondence
and requests for materials should be addressed to R.K.
Competing financial interests
The authors declare no competing financial interests.
NATURE GEOSCIENCE | VOL 6 | SEPTEMBER 2013 | www.nature.com/naturegeoscience 741