Icarus 211 (2011) 1082–1088
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Icarus
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Solar wind contribution to surficial lunar water: Laboratory investigations
D.J. Burke, C.A. Dukes, J.-H. Kim, J. Shi, M. Famá, R.A. Baragiola ⇑
Laboratory for Atomic and Space Physics, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904, United States
a r t i c l e
i n f o
Article history:
Received 8 July 2010
Revised 25 October 2010
Accepted 1 November 2010
Available online 17 November 2010
Keywords:
Solar wind
Moon, Surface
Spectroscopy
a b s t r a c t
Remote infrared spectroscopic measurements have recently re-opened the possibility that water is present on the surface of the Moon. Analyses of infrared absorption spectra obtained by three independent
space instruments have identified water and hydroxyl (–OH) absorption bands at 3 lm within the lunar
surface. These reports are surprising since there are many mechanisms that can remove water but no
clear mechanism for replenishment. One hypothesis, based on the spatial distribution of the –OH signal,
is that water is formed by the interaction of the solar wind with silicates and other oxides in the lunar
basalt. To test this hypothesis, we have performed a series of laboratory simulations that examine the
effect of proton irradiation on two minerals: anorthite and ilmenite. Bi-directional infrared reflection
absorption spectra do not show any discernable enhancement of infrared absorption in the 3 lm spectral
region following 1 or 100 keV proton irradiation at fluences between 1016 and 1018 ions cm2. In fact, the
post-irradiation spectra are characterized by a decrease in the residual O–H band within both minerals.
Similarly, secondary ion mass spectrometry shows a decrease rather than an increase of the water group
ions following proton bombardment of ilmenite. The absence of significant formation of either –OH or
H2O is ascribed to the preferential depletion of oxygen by sputtering during proton irradiation, which
is confirmed by post-irradiation surface analysis using X-ray photoelectron spectroscopy measurements.
Our results provide no evidence to support the formation of H2O in the lunar regolith via implantation of
solar wind protons as a mechanism responsible for the significant O–H absorption in recent spacecraft
data. We determine an upper limit for the production of surficial –OH on the lunar surface by solar wind
irradiation to be 0.5% (absorption depth).
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
The existence of H2O on the lunar surface has been a matter of
intense debate and speculation amongst astronomers for over
40 years. Based on analyses of returned samples from the Apollo
missions, the consensus was that the Moon is a nominally anhydrous body, devoid of any intrinsic water content. However, recent
analysis of infrared spectroscopic measurements from the lunar
surface obtained by three independent space instruments, the
Moon Mineralogy Mapper (M3) on Chandrayaan-1, the Visual and
Infrared Mapping Spectrometer (VIMS) on Cassini and the HighResolution Instrument infrared spectrometer onboard Deep Impact
(DI), indicate the existence of modest concentrations of H2O (and
OH groups) within the uppermost layers of the lunar surface sampled by mid-infrared spectroscopy. Upper limit estimates for –OH/
H2O bearing species vary between reports and range from 3% to
14% in absorption band depth (800 ppb) (Clark, 2009; Pieters
et al., 2009; Sunshine et al., 2009). Pieters et al. (2009) observed
⇑ Corresponding author. Fax: +1 434 924 1353.
E-mail address: [email protected] (R.A. Baragiola).
0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2010.11.007
a variation in –O–H band depth based on mineralogical and geographical differences across the lunar surface. Sunshine et al.
(2009) observed a diurnal dependence of the infrared absorption
band suggesting that the regolith undergoes a dynamic H2O loss
and rehydration cycle over the lunar day. However, an alternative
explanation has been proposed suggesting that variations in band
depth are due to surface topographic effects on reflectance (Clark,
2009). The three reports agree in that H2O or –OH are spatially distributed across the entire lunar landscape, with larger abundances
observed at higher latitudes and the poles.
The presence of water on the lunar surface is surprising considering that many potential removal mechanisms exist, such as photodesorption and photodissociation by ultraviolet radiation,
meteorite impact, radiation damage and sputtering. The mechanism for water production or delivery to the lunar surface is unknown. However, some possible sources include: cometary or
meteoritic delivery combined with surface diffusion, water/ice diffusing from the Moon’s interior, or chemical effects due to solar
wind impact on the lunar regolith (Arnold, 1979). An often-considered hypothesis is that –OH and H2O are formed by the interaction
of the solar wind with silicates and other oxygen-bearing minerals
within the lunar basalt. Ion irradiation over a range of energies
D.J. Burke et al. / Icarus 211 (2011) 1082–1088
(0.5–3 keV) can lead to the breaking of chemical bonds within various minerals. Implanted or trapped protons from the solar wind
may attach to those broken bonds and form –OH and subsequently
H2O. Such a mechanism is implied by Sunshine et al. (2009) to explain the apparent rehydration of the entire lunar surface that requires a daily renewal source to propagate –OH/H2O production.
To explore this hypothesis, we have performed a series of laboratory experiments designed to determine whether proton irradiation of minerals can form significant amounts of –OH/H2O.
Minerals that are abundant in the lunar regolith, ilmenite and
anorthite, were irradiated with 1 and 100 keV protons and the effects of irradiation probed with several infrared absorption techniques: bi-directional (BD-IRAS), hemispherical diffuse (DRIFTS)
and attenuated total reflectance (ATR), in addition to the more surface-specific X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS).
2. Experimental
In this initial study, we focus on terrestrial minerals found in lunar basalts, rather than lunar soil. This is because terrestrial analogues can be relatively well defined, without the complex
lithology of lunar materials, especially with regard to morphology
and phase content. The differences between our samples and lunar
material regarding the likelihood of O–H bond formation by proton
implantation will be addressed later. Ilmenite, FeTiO3, was chosen
because its reduction to metallic Fe and TiO2 in an H2 atmosphere
had been shown to produce water as a by-product in the laboratory (Allen et al., 1992). Anorthite, a Ca-rich plagioclase feldspar,
CaAl2Si2O8, is similar in composition to the anorthite that is ubiquitous in the lunar highlands, regions that show stronger hydroxyl
absorptions in the Cassini and M3 data, particularly in fresh crater
regions, as well as less thermal variation as observed by DI. Mineral
sections (Fig. 1) were rough cut with a diamond saw, lightly polished, ultrasonically cleaned in acetone, rinsed in denatured ethanol and then baked at 120 °C for 12–24 h in either air or high
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vacuum to remove intrinsic and surface water. Surfaces are rough
on a micron scale. All experiments were performed at room temperature (300 K) in ultra-high vacuum (UHV) to minimize the
adsorption of residual H2O onto the mineral surface.
The first series of experiments were performed at UHV
(109 Torr) in a chamber that allows ion irradiation using a low
energy ion source and surface characterization with XPS and BDIRAS. The experimental setup has been described previously (Loeffler et al., 2009). XPS is a standard surface analysis technique used
to quantitatively determine the surface composition and surface
chemistry of the outermost 30–50 Å. XPS cannot detect hydrogen,
but the chemical state of oxygen (as an oxide, in –OH, or in H2O)
can be inferred from the position of the photoelectron energies.
High resolution XPS spectra were taken with an energy resolution
of 0.8 eV.
Infrared spectroscopy was performed with a Thermo-Nicolet
670 FTIR spectrometer, using a liquid-nitrogen cooled MCT (mercury–cadmium–telluride) infrared detector operating at a resolution of 16 cm1. The infrared beam was transmitted out of the
spectrometer to the vacuum chamber at 8.5° from normal incidence to the sample. Outside the vacuum chamber, the infrared
beam path was purged with dry air. Infrared spectra were uncorrected for residual atmospheric water vapor and CO2 to avoid spurious spectral structure generated by an imperfect ratio or
subtraction of the background.
Irradiations used 2 keV Hþ
2 at nearly normal incidence on each
section, and the ion beam flux was measured with a Faraday cup
before and after irradiation. Ion scattering spectrometry showed
that the H2+ fragments break on impact into two 1 keV protons.
An electron flood gun (<2 eV) minimized surface charging during
ion irradiation, as well as during XPS analysis. Each mineral was
introduced into a preparation chamber where it was baked in a
vacuum of 106 Torr. Afterward, the mineral section was inserted
directly into the analysis/irradiation chamber without exposure to
atmosphere. Samples were first characterized by XPS and BD-IRAS,
irradiated to fluences in the range 1017–1018 H+ cm2, which corre-
Fig. 1. (A) Rough polished anorthite shows grains sizes 62 mm for anorthite (white) and pyroxene inclusions (dark) under the optical microscope. (C) Magnified (10,000)
secondary electron images of anorthite show additional structure on the micron scale. (B) Rough polished ilmenite appears relatively homogenous. (D) However, at greater
magnification (10,000), secondary electron image suggests a rough microstructure on the ilmenite at the micron scale.
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D.J. Burke et al. / Icarus 211 (2011) 1082–1088
spond to exposure times of 30–300 years on the Moon (Loeffler
et al., 2009). The samples were then reanalyzed with XPS and IRAS.
After completion of the in situ experiments, ATR and DRIFTS infrared spectra were taken ex situ within the FTIR spectrometer and
compared with those of identical polished, unirradiated, minerals.
The maximum penetration depth of the implanted H+ is <50 nm
(Ziegler et al., 2008), much lower than the penetration depth of
incident infrared light (Clark and Roush, 1984).
In a second UHV chamber, SIMS measurements were performed
using 4 keV Ar+ and a Hiden EQS 300/HAL 4 quadrupole mass spectrometer. Positive and negative ion SIMS spectra were obtained at
a number of ion energies before and after irradiating unpolished
ilmenite with 1018 H+ cm2. In addition, negative ion spectra were
collected periodically during proton irradiation.
Finally, a third series of experiments were performed in another
UHV chamber connected to an ion implanter and equipped with a
BD-IRAS setup. Ilmenite and anorthite were irradiated with
100 keV protons; the higher proton energy was used to increase
the depth through which –OH/H2O would form, better matching
the infrared penetration depth. The higher energy also decreases
the probability of oxygen sputter removal during irradiation, as
the proton sputtering yield, which follows the nuclear stopping
power at low energies, is maximal around 1 keV and decreases as
the beam energy increases. The maximum implantation range for
100 keV H+ protons is 0.9 lm in anorthite and 0.7 lm in ilmenite,
calculated using TRIM (Ziegler et al., 2008). These depths are on the
same order as the hydrogen implantation–diffusion depths (0.1–
0.5 lm) for ‘‘suprathermal’’ solar protons measured in lunar soils
(Leich et al., 1974). For these experiments, reflectance spectra were
taken before, during and after proton bombardment. BD-IRAS spectra were recorded with a FTIR spectrometer (Thermo-Nicolet 670)
at 35° from normal incidence, using a liquid-nitrogen cooled MCT
detector with a resolution of 4 cm1 and 8 cm1 for anorthite
and ilmenite respectively.
3. Results
Fig. 2 shows single beam (Fig. 2A and B) infrared spectra of
anorthite and ilmenite samples taken before and after irradiation
with 1018 H+ cm2 at 1 keV. The spectral region shown focuses
on the position of the fundamental O–H stretch at 3 lm (H2O)
and 2.8–2.9 lm (OH groups) (Farmer, 1974). The spectra of the
unirradiated and irradiated anorthite and ilmenite samples clearly
exhibit an inherently weak H2O absorption band at 3 lm. This is
a result of residual H2O that was not removed by the 120 °C bake
under high vacuum. The sharper features located between 3 and
4 lm are attributed to C–H stretch vibrations of surface-bound
hydrocarbon contaminants. Fig. 2C shows the ratios of the postto pre-irradiation spectra, to aid in distinguishing the H2O signal
generated by the implanted protons from residual H2O. The ratio
spectra clearly show no discernable enhancement of either the –
OH or H2O spectral region following irradiation. Rather, the ratio
spectra show a minor decrease in the –OH/H2O spectral region.
Moreover, the intensity of this absorption feature is so small that
the effects of proton irradiation can be considered negligible; that
is, the results provide no evidence, within the noise level, to support the formation of surface bound –OH or H2O via H+
implantation.
In agreement with the low energy proton experiments, Figs. 3
and 4 (bottom trace) show that irradiation with 100 keV protons
does not enhance infrared absorption in the O–H spectral region
in either sample. In the case of anorthite, irradiation at higher energies actually produces a decrease in the O–H absorption compared
to the unirradiated sample, shown by an increased positive infrared band in the ratio at 2.8 lm (Fig. 3). This increase is attributed
to ion-impact desorption of residual surface water. This effect is
not observed for ilmenite samples.
Fig. 4 illustrates the potential problem of detection of low levels
of H2O using IRAS. The top trace shows the ratio of post/pre-irradiated spectra of ilmenite for the case where the MCT detector was
cooled prior to the start of proton irradiation and maintained at liquid nitrogen temperatures throughout the experiment, rather
than cooling after irradiation was complete, as in all other cases
(Figs. 2 and 3). The spectrum taken after 7 h of proton irradiation
shows a clear 3.1 lm band increase due to H2O, which is observed
simultaneously during irradiation. However, a similar feature is
observed in the corresponding IRAS spectrum of the control experiment also taken after 7 h of cooling (Fig. 4 middle trace) with no
irradiation. Thus, it is clear that the 3.1 lm feature is not due to
Fig. 2. IRAS single beam spectra of unirradiated and proton irradiated anorthite and ilmenite (A and B). Irradiation was with 1 keV H+ with a fluence of 1 1018 ions cm2.
The single beam spectra in panel B do not exhibit darkening and have been offset for clarity. Panel C shows the corresponding ratio of irradiated/unirradiated spectra to
distinguish between residual and proton generated –OH/H2O infrared contributions.
D.J. Burke et al. / Icarus 211 (2011) 1082–1088
Fig. 3. IRAS ratio of irradiated/unirradiated spectra for anorthite following 100 keV
H+ irradiation with a fluence of 1.0 1017 ions cm2. The inversion of the
absorption feature shows that adsorbed water is lost, rather than produced, on
the surface of anorthite following proton irradiation.
Fig. 4. IRAS ratio of irradiated/unirradiated spectra for ilmenite. The upper trace
shows a small absorption feature at 3.1 lm which appears following irradiation
with 100 keV protons with a fluence of 1.4 1016 ions cm2. The center trace shows
the spectrum for an unirradiated ilmenite specimen under identical experimental
conditions. The absorption feature at 3.1 lm is due to condensed water on the
window of the liquid-nitrogen cooled MCT detector, and is not produced by
irradiation. The difference spectra (bottom trace) obtained by subtracting the
irradiated spectrum from the unirradiated spectrum confirms that no water is
formed by proton bombardment. The spectra are offset for clarity.
water formed by proton irradiation. Subtraction of the irradiated
and unirradiated spectra (Fig. 4 bottom trace) shows a decrease
in the O–H absorption region post-irradiation, consistent with
our other data. The appearance of the band is attributed to H2O
that has condensed onto the window of the liquid-nitrogen cooled
detector (Theocharous, 2005). This 3.1 lm feature is only observed
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during measurements where the detector is cooled at the beginning of the irradiation/control experiment and its depth grows
with time.
To probe the effects of proton irradiation further, ATR and
DRIFTS measurements were performed on both polished anorthite
and ilmenite samples. These measurements were performed ex situ
following proton irradiation in UHV and compared with the corresponding spectra for the unirradiated samples. Residual internal
and surface-bound H2O (3 lm) in addition to hydrocarbon contamination (3.4 lm) were again evident in both unirradiated and irradiated samples. To distinguish the potential enhancement of
possible O–H absorption produced by proton bombardment, the
corresponding irradiated/unirradiated spectra were divided. In
agreement with the BD-IRAS data, the ATR and DRIFTS spectral ratios showed no evidence for the formation of –OH or H2O within
the mineral due to proton irradiation. However, ATR spectra obtained for anorthite irradiation gave rise to a clear reduction of
absorptions due to –OH and hydrocarbon contamination, attributed to sputter cleaning of the mineral surface by the incident protons. ATR data for ilmenite was also consistent with the
corresponding IRAS data, providing no evidence of –OH or H2O formation following irradiation but, in contrast to anorthite, showed
no discernable loss of –OH.
The comparison of SIMS data obtained before and after proton
bombardment of ilmenite (Fig. 5) shows no significant enhancement in any of the water group ions: OH+, H2O+, H3O+, or OH,
H2O after irradiation. Neither is there an increase in H+, O+ or
O to indicate water formation, with subsequent bond breakage
during SIMS measurement with 4 keV Ar+. In fact, a decrease in
the height of the oxygen peak is evident, as well as the removal
of surface hydrocarbons.
The XPS data (Fig. 6) showed a marked decrease in surface oxygen (photoelectron peak area) relative to iron after proton bombardment of ilmenite, consistent with the oxygen loss typical of
ion irradiation of oxides. The loss of oxygen relative to Fe and Ti
is greater than 90%. This loss is confirmed by the chemical reduction of Fe observed in the high resolution XPS spectra, where the
2p3/2 peak shifts in energy from Fe3+ to Fe2+ and splits, with the
lower binding energy (BE) component indicating the presence of
metallic Fe (BE = 707.0 eV). In addition, changes in surface chemistry are reflected in the electron-atom binding energies of the O-1s
peak. Before irradiation, a shoulder on the high binding energy side
of the O-1s primary photoelectron peak is apparent. The entire
oxygen peak was fit to three separate Gaussian–Lorentzian curves
of FWHM 2.0 eV with the binding energy scale fixed to adventitious carbon (BE = 284.8 eV). The primary O-1s component has a
binding energy of 530.1 eV, which is consistent with bulk ilmenite
(Fujii et al., 2008; Schulze et al., 1985). The major portion of the
shoulder at 532.1 eV can be attributed to hydroxyl groups within
the lattice or adsorbed on the mineral surface (Kloprogge et al.,
2006). Finally, the remaining portion of the shoulder, at 533.7 eV,
suggests molecular water adsorbed on the ilmenite surface
(Andersson et al., 2005). These binding energies are consistent with
previous XPS measurements for ilmenite and for water on corundum (Al2O3) (Kloprogge et al., 2006).
After irradiation, concurrent with the chemical reduction of Fe,
a large decrease in the O-1s shoulder is observed, indicating loss of
adsorbed H2O and –OH, consistent with our IRAS measurements
that show a similar loss (Fig. 2). Thus, XPS shows no indication
for the formation of either –OH or H2O within ilmenite due to proton bombardment.
XPS measurements on proton irradiated anorthite also show
oxygen loss but to a lesser degree, with a 70% and 60% decrease
in oxygen compared to aluminum and silicon, respectively. Because of the pyroxene inclusions in our anorthite, Mg and Fe are
observed in addition to Ca, Na, Al, Si, and O. However, with the
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D.J. Burke et al. / Icarus 211 (2011) 1082–1088
Fig. 5. Positive SIMS for ilmenite before (A) and after (B) irradiation. Fluences were greater than 1018 H+ cm2 at 1 keV. Both panels show the constituents of ilmenite, as
expected, are sputtered from the surface during irradiation: O+, Fe+, Ti+, Fe++, Ti++. Implanted Ar+ from the incident 4 keV Ar+ beam is also observed, along with a molecular
constituent: TiO+. Panel B shows no increase in H+, nor increases in any water group ion. Oxygen loss is observed as a result of proton bombardment.
exception of Fe, which chemically reduces from Fe3+ to Fe2+ and
Fe0, no clear binding energy shift occurs for any of the elements
after irradiation. None of these chemical changes are unexpected,
based on previous studies (Dukes et al., 1999).
4. Discussion
Our experiments provide an upper limit of 0.1% band depth in
the O–H spectral region attributed to 1 keV proton irradiation at
1018 ions cm2 or due to 100 keV irradiation at 1017 ions cm2.
We use the 1 keV proton measurements to calculate the band
depth limit, as the more deeply penetrating 100 keV proton data
were taken at 10 less fluence. Previously, several groups had reported an increased band depth after higher energy proton bombardment. Zeller et al. (1966) and Mattern et al. (1976) showed
an increased absorption between 2.75 and 3.8 lm in a silicate glass
after irradiation with 0.15–1 MeV protons of fluences between
1017 and 1019 ions cm2. However, the lack of experimental details
regarding vacuum conditions, sample preparation and the unusual
width of the absorption band(s), makes these measurements difficult to assess or compare to our data. A subsequent study determined an upper limit of 0.5% band depth at 2.8 lm for 15 keV
protons on sapphire (Gruen et al., 1976). This depth, measured outside vacuum, is slightly greater than our limit of 0.1%. The difference between results can be ascribed to an increased penetration
depth due to 15 higher energy and possible channeling effects
in the single crystal samples of Gruen et al. (1976), hence allowing
more hydrogen to be implanted before reaching saturation concentrations. Furthermore, the increased band depth could also be a result of atmospheric water adsorption after irradiation.
The O–H band depth is associated with the retention of hydrogen in the mineral, with the maximum hydrogen concentration
occurring at saturation fluence, where there is an equilibrium between implanted and desorbed hydrogen. The saturation fluence
is a function of implantation depth and sputtering rate, both of
which depend strongly on ion species, energy, incidence angle
and mineral type. A systematic measurement of the saturation
concentration for 2 keV H+ in forsterite was performed by Lord
(1968), who determined a maximum concentration of 5 1017
H+ cm2. A similar saturation concentration (2 1017 H2 cm2)
was found for 200 keV H2+ implanted into vitreous silica (Mattern et al., 1976). Our fluence of 1 keV protons exceeds
1018 H+ cm2, well beyond the fluence necessary for reaching the
saturation concentration, at least for the anorthite/pyroxene targets which better resemble forsterite or silica. Therefore, these
irradiations simulate the maximum effect in the 2.8 lm spectral
region that can be caused by 1 keV protons. We note that the saturation concentrations reported by Lord (1968) are more than 10
larger than those measured in the lunar soil (DesMarais et al.,
1974), implying that saturation occurs at even lower concentrations in lunar material.
Clearly, terrestrial analogs are not identical to lunar material,
which have a more complex morphology and heterogeneous composition. In our irradiated terrestrial minerals, the H-implanted
layer is confined to the surface, whereas in lunar soil, due to communition, H may also appear on the surface of deeper grains. The
attenuation length of 3 lm light in minerals is not fully understood, and depends strongly on impurities and Fe content. From
the analysis of Shkuratov et al. (1999), who give an average absorption coefficient k = 0.001 for silicate minerals at 3 lm, we derive an
attenuation length of 240 lm. The mean regolith particle size is
60 lm (McKay et al., 1991), but the average number of grains traversed by the photons is not given by the mean size, but by the inverse, which is strongly affected by the smallest grains. Using our
upper limit of 0.1% band depth and a mean number of five grains
traversed, we find an upper limit of 0.5% for the equivalent 3 lm
absorption by lunar soil. This 0.5% band depth is still much smaller
than reported values (3–14%) from the recent lunar observations.
We now return to the question of whether the peculiar lithology of lunar soil can have a significant effect on O–H synthesis
by energetic protons. The first difference is the microstructure
(topography) of lunar regolith. Examination of our samples with
the optical microscope (Fig. 1A and B) shows that they have a
roughness on a scale comparable to the grain sizes in the regolith.
In addition, the secondary electron micrographs (Fig. 1C and D)
illustrate microstructure of the mineral surfaces that varies from
less than 0.05 lm to 2 lm. Roughness on this scale provides a large
D.J. Burke et al. / Icarus 211 (2011) 1082–1088
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Fig. 6. XPS spectra of the (A) Fe-2p, (B) Ti-2p, and (C and D) O-1s regions of ilmenite, before and after irradiation with 2 1017 H cm2. The oxygen peak area is reduced as the
area of Ti and Fe increases, thus indicating preferential oxygen loss under proton bombardment. Iron is chemically reduced from Fe3+ to Fe2+ and Fe0 by the ion beam as it
removes oxygen preferentially.
intrinsic surface area for oxygen or hydrogen adsorption, after their
sputter removal/backscattering, where they may react to form OH.
Therefore, processes enabled on the lunar regolith are also possible
in our samples.
Chemical differences between our samples and lunar material
are also not expected to be significant. Already at a fluence of
2 1017 H cm2, the surface Fe3+ fraction (from the 6% hematite in terrestrial samples) has disappeared entirely in favor of
Fe2+ as well as metallic Fe0. This is quite similar to what is seen
on the surface of mature and submature lunar regolith soils
(Baron et al., 1977; Housley and Grant, 1977; Yin et al., 1975).
Furthermore, the surface activation observed on lunar soil as
an enhanced reactivity to water (Gammage et al. 1974;
Cadenhead and Stetter, 1974), is also shown in samples irradiated in the laboratory (Holmes et al., 1975; Cantando et al.,
2008).
That solar wind hydrogen implantation, by far, is unable to produce the significant 3 lm absorption observed on the lunar surface, can be explained with thermodynamic arguments. During
the cooling of the collision cascade initiated by each proton, recoil
atoms will tend to associate with sites where thermal dissociation
is minimized. This favors bonds with higher free energy of dissociation (Kelly, 1989), in the order: Si–O (800), Ti–O (667), O–O (498),
H–H (436). O–H (430), O–Fe (407), where the numbers in parenthesis are the bond dissociation energies in kJ/mol (CRC, 2010).
In addition, irradiation hinders O–H synthesis by removing oxygen
preferentially; XPS shows a 90% and 65% oxygen loss in ilmenite
and anorthite respectively—a finding that is further substantiated
by SIMS measurements on ilmenite. This observation is consistent
with numerous reports of preferential oxygen loss from minerals
and metal oxides under ion bombardment (Betz and Wehner,
1984).
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D.J. Burke et al. / Icarus 211 (2011) 1082–1088
5. Conclusion
Laboratory simulations of keV proton irradiation of ilmenite and
anorthite give an upper limit of 0.5% in band depth at 2.8–3 lm for
lunar material. This implies that there is no evidence to support the
hypothesis that solar wind protons impacting the lunar surface
combine with oxygen in the regolith to form significant amounts
of –OH/H2O. In particular, we find proton bombardment alone cannot form enough O–H bonds to explain the 3–14% depth of the
3 lm absorption in the recent spacecraft observations.
Acknowledgments
This work was supported by NASA-Cosmochemistry (Grant
NNX08AG72G) and Planetary Geology and Geophysics (Grant
NNX08AMB6G) programs.
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