Click
Here
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E03003, doi:10.1029/2008JE003249, 2009
for
Full
Article
Irradiation of olivine by 4 keV He+: Simulation of space weathering
by the solar wind
M. J. Loeffler,1 C. A. Dukes,1 and R. A. Baragiola1
Received 8 August 2008; revised 22 December 2008; accepted 5 January 2009; published 10 March 2009.
[1] We have studied the effects of 4 keV He+ ion irradiation on olivine while
measuring, in situ, changes in the near-infrared (NIR) reflectance and in the chemical
composition of the surface. The observed changes in reflectance are reddening and the
attenuation of the Fe-3d absorption bands in the NIR. Spectral reddening of
irradiated olivine powder (<45 mm) correlates with the amount of metallic iron formed
by ion impact, consistent with the idea that space-weathering effects in the reflectance
of olivine-bearing S type asteroids are due to the formation of metallic iron. The
metallization rate of the powder is about half that of a sectioned rock of olivine, which
we propose is a consequence of redeposition of sputtered material. The NIR
spectral changes observed in ion irradiation experiments are similar to those observed
in our previous experiments on vapor redeposition, indicating that different space
weathering mechanisms can lead to similar final effects on reflectance. Finally, we
estimate that at 1 AU the spectral reddening caused by the solar wind saturates
approximately 2 orders of magnitude faster than comparable reddening caused by
micrometeorite impacts.
Citation: Loeffler, M. J., C. A. Dukes, and R. A. Baragiola (2009), Irradiation of olivine by 4 keV He+: Simulation of space
weathering by the solar wind, J. Geophys. Res., 114, E03003, doi:10.1029/2008JE003249.
1. Introduction
[2] Space weathering is the physical and chemical alteration of surfaces of airless bodies exposed to the space
environment [Hapke, 2001; Clark et al., 2002; Chapman,
2004]. There they are subject to bombardment by ions from
the solar wind, magnetosphere ions, energetic electrons,
cosmic rays, ultraviolet photons and fast interplanetary dust
(micrometeorites), with the relative flux of each of those
projectiles depending on the location of the surface in the
solar system. Any of these impacts can produce chemical
alterations by breaking molecular bonds, synthesizing new
molecules or by changing the elemental composition via ion
implantation and preferential sputtering. Micrometeorites
may, in addition, physically alter the particle size distribution through impacts that melt and reform surface particles,
sinter them, break them into smaller pieces, or bring larger
subsurface particles to the surface, mixing the soil. Changes
in grain size alter the path of the light in the solid leading to
changes in extinction and, therefore, reflectance. Many of
these alterations caused by space weathering can be inferred
from the spectral reflectance of the planetary body and,
in some cases, seen directly on returned samples, such as
lunar soil [Pillinger, 1979; Keller and McKay, 1993;
Christoffersen et al., 1996] and interplanetary dust [Bradley,
1994].
1
Laboratory for Atomic and Surface Physics, University of Virginia,
Charlottesville, Virginia, USA.
[3] Most laboratory simulations of space weathering
phenomena on the Moon, Mercury and asteroids have
focused on what are believed to be the two most important
types of weathering agents: solar wind ions and micrometeorites. The solar wind, consisting of mostly hydrogen
and helium ions (>99%), has a broad velocity distribution
with a mean value of 468 km/sec (1 keV/amu) [Gosling,
2007]. For this reason, most labratory studies have been
done with 1 keV H and 4 keV He.
[4] Several keV ion irradiation experiments on minerals
have been published on the simulation of solar wind impact
on mineral surfaces [Nash, 1967; Hapke, 1973; Dukes et al.,
1999; Davoisne et al., 2008]. Hapke [1973] showed that H+
irradiation of loose mineral powders, analogous to the
Moon regolith, visibly darkens the irradiated surface, which
he attributed to the formation of metallic iron in the
sputtered vapor and their redeposition on nearby grains.
Dukes et al. [1999] showed, by in situ surface analysis, that
ion irradiation of olivine can reduce its Fe2+ into metallic
iron, Fe0, near the surface and that the reduction is more
efficient for He+ than for H+. These experiments were done
on cleaved rock surfaces, showing that metallization did not
require redeposition of Fe atoms on nearby grain surfaces.
Other experiments with higher-energy ions (60 – 400 keV H,
He, Ar), more characteristic of high-velocity shocks found
in the interstellar media [Bringa et al., 2007], were shown to
darken and redden the reflectance spectra of mineral rocks
and powders between 0.25 and 2.5 mm [Brunetto and
Strazzulla, 2005; Strazzulla et al., 2005; Brunetto et al.,
2006b].
Copyright 2009 by the American Geophysical Union.
0148-0227/09/2008JE003249$09.00
E03003
1 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
[5] The correlation between optical effects and metallic
iron content that was observed in lunar soil has been
simulated in the laboratory. Adding metallic iron to silicate
gels having featureless spectra causes the reddening of
spectral slopes and darkening in the visible-NIR [Noble et
al., 2007], consistent with the effects seen on lunar rocks.
[6] The second source of space weathering, micrometeorite impact, has been simulated in the laboratory using
pulsed laser beams with instantaneous power densities
similar to those of typical impact by micron-sized meteorites [Moroz et al., 1996; Sasaki et al., 2001; Brunetto et al.,
2006a; Loeffler et al., 2008a, 2008b]. Those studies show
changes in the reflectance spectra of silicates that can be
larger than those seen in ion irradiation experiments. However, to compare these two effects, one needs to consider
relative fluxes of the impactors. The relative role of solar
wind versus micrometeorites has been a subject of controversy for several decades. Recently, it has been argued that
the inverse dependence of the degree of reddening of
asteroids with their distance from the Sun implies that the
solar wind is likely the primary mechanism for space
weathering [Marchi et al., 2006]. However, this inverse
dependence is tenuous because of the large scatter of the
data, and may be affected by the differences in composition
with distance to the Sun. Thus, even if this dependence is
related to space weathering, it does not preclude the
importance of micrometeorite impacts, as closeness to the
Sun not only implies larger ion fluxes but also micrometeorites at higher fluxes and velocities [Cintala, 1992].
[7] To further the understanding of the role of ion
irradiation on space weathering, we performed experiments
on olivine, a common mineral found on many asteroids
[Gaffey et al., 1993; Clark et al., 2002] that has been shown
to be susceptible to space weathering [Moroz et al., 1996;
Sasaki et al., 2001; Brunetto et al., 2006a; Loeffler et al.,
2008a, 2008b]. Specifically, in these experiments, we irradiated sectioned (‘‘flat’’) rock fragments and powders of
San Carlos olivine with 4 keV He+ ions. A unique aspect of
experiments is that during irradiation, we monitor the NIR
spectra and the surface chemical composition of olivine in
situ, e.g., without exposing the samples to the atmosphere.
This ability will enable us to directly determine whether
there is a correlation between irradiation-induced changes in
the reflectance spectra and chemical composition of olivine.
2. Experimental Methods
2.1. Sample Preparation and Analysis
[8] Olivine samples used in these experiments were
natural San Carlos olivine (Mg1.8Fe0.2SiO4) from Penn
Minerals (Scranton, PA, USA) and from Arizona Gems
and Crystals (Tucson, Az, USA). We also used synthetic
forsterite (99.4% Mg2SiO4) from Reade Advanced Materials (Riverside, RI, USA) with the percent concentration of
impurities listed as: Al2O3 (0.03), Fe2O3 (0.02), TiO2 (0.09),
Na2O (0.11), CaO (0.22), and K2O (0.12). To prepare
olivine powder samples for use, we separated the large
grains from rock inclusions, crushed them with agate mortar
and pestle and used a sieve to obtain a grain size <45 mm.
Chemical analysis, using X-ray photoelectron spectroscopy
(XPS), showed no discernable difference between the
uncrushed grains and the powder. To obtain the sample size
E03003
needed for XPS measurements (6 – 8 mm), we pressed the
powder to 3000 psi inside an aluminum ring (inner
diameter 10 mm), placed between two stainless steel
disks. In addition, we also performed experiments on a
solid piece of olivine, since we were interested in learning
whether changes induced by irradiation depend on physical
properties of the sample, such as grain size, as has been
noted previously [Hapke, 2001, and references therein]. To
obtain flat specimen, we cut sections of olivine using a
diamond blade, and removed the cutting fluid from the
surface with successive acetone, isopropanol, and HPLC
water baths.
[9] The samples were mounted on a copper holder and
transferred into an oil free ion-pumped ultra-high vacuum
(UHV) chamber with a base pressure between 5 and 10 10 10 Torr (Figure 1). Surface analyses of the samples were
performed before, during and after irradiation using XPS
and near infrared reflectance (NIR) spectroscopy.
[10] XPS is a widely used surface analysis method, which
gives both elemental abundance and chemical information.
In this technique, X-rays eject photoelectrons from a sample
surface with a kinetic energy (Ek) given by Ek = Ex-ray
EB f, where the X-ray energy, Ex-ray , is 1486.6 eV (Al-K),
EB is the electron binding energy with respect to the Fermi
level, and 8 is the work function of the spectrometer. The
energy distribution (spectra) of photoelectrons is given as
number of counts versus binding energy. The spectra show a
continuous background on which photoelectron and Auger
peaks are superimposed. The escape depth of the electrons
analyzed by XPS depends on their kinetic energy; it is 5 nm
for the Fe-2p and O-1s, and 8 nm for Si-2p, and Mg-2p
electrons in olivine [Seah and Dench, 1979]. The relative
area of the photoelectron peaks is used together with sensitivity factors to obtain elemental composition, whereas the
precise value of the electron binding energy gives information on the chemical state of the element [Moulder et al.,
1992].
[11] XPS analysis was performed in a Physical Electronics 560 system, equipped with a double-pass, cylindrical
mirror electron energy analyzer (CMA) [Dukes et al., 1999;
Loeffler et al., 2008b]. The spectrometer was operated in
fixed transmission mode at 200 eV pass energy for survey
spectra and 40 eV for high-resolution spectra, providing
energy resolutions of 3.2 eV and 0.6 eV, respectively. The
ultimate energy resolution of the XPS spectra is limited by
the width of the Al-K X-ray line, 0.85 eV FWHM. Survey
spectra were measured in 5 min, while the more detailed
spectra took 60 min.
[12] After taking an XPS spectrum, we moved the sample
within the vacuum chamber to the focus of our ThermoNicolet Nexus 670 Fourier Transform Infrared Spectrometer
(FTIR). The NIR spectrum was measured in the 15000 –
4000 cm 1 region (0.66 – 2.5 mm) at a spectral resolution set
to 16 cm 1; each spectrum was an average of 800 scans,
taking 10 min. Light from a halogen lamp was incident
normal to the sample and the reflected light was collected at
17 degrees with mirrors and focused onto an InGas detector.
To obtain the absolute bidirectional reflectance of the
samples, the raw spectrum of the olivine was divided by
that of a PTFE (Teflon) powder (Avocado Research Chemicals), which has been calibrated against an absolute reflec-
2 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
E03003
Figure 1. Experimental setup.
tance standard: spectralon (Labsphere certified reflectance
standard).
2.2. Ion Irradiation
[13] Irradiations were performed with a differentially
pumped, low-energy ion gun. During irradiation the chamber pressure remained in the mid 10 8 Torr range, where the
mean free path of residual gas in the chamber is low enough
that it does not affect the trajectory of the projectiles or of
the sputtered particles. We used He gas of purity better than
99.99%; the lack of significant amounts of implanted
contaminants was verified in the XPS analyses. Four keV
He ions, which simulate the solar wind, were rastered
uniformly over the sample surface analyzed by XPS at
normal incidence. The ion current density was measured
before and after each experiment with a Faraday cup to be
1.4 1013 ions/cm/sec; precise to within 5%. During
irradiation, we used an electron flood gun, which emits lowenergy (<1 eV) electrons, to prevent electrostatic charging
of the sample, which could ultimately deflect the ion beam
away from the sample.
3. Results
3.1. Fluence Dependence of Near-Infrared Reflectance
[14] Figure 2 (top) shows the in situ NIR spectra of a
<45 mm powder of synthetic forsterite and olivine after
different fluences of ion irradiation. The spectrum of forsterite, which contains 10 times less iron than San Carlos
olivine, is featureless in this region and changes only
slightly during irradiation. The olivine spectrum has the
absorption features characteristic of polycrystalline olivine,
because of electronic excitation of the outer shell (3d)
electrons of Fe2+ [Burns, 1993]. The spectrum of the
unirradiated sample is similar to spectra reported in previous
studies on San Carlos olivine [Yamada et al., 1999; Sasaki
et al., 2001; Brunetto et al., 2006a].
[15] During irradiation, there are clear changes in the
spectral slope, termed ‘‘reddening,’’ generally consistent
with previous experiments that use comparable ion fluences
[Hapke, 2001]. We define reddening, r, from reflectance
values R outside the main absorptions as: r = (R2.0mm
R0.7mm)/1.3mm, where 1.3 mm is the difference in wavelength. We note that after subtracting a fitted continuum (see
below in Figure 8), we find that there is no shift in the band
center. Even after smoothing each spectrum and taking the
derivative, we find that the shift in the 1 mm absorption
band is at most 0.001 mm as a result of irradiation. These
spectral alterations can be seen more clearly in Figure 2
(bottom), where we have divided the spectrum of the
irradiated sample by the spectrum of the unirradiated
sample. Interestingly, this change in spectral shape is similar
to that reported previously for an olivine powder sample
coated by pulsed-laser deposition of olivine [Loeffler et al.,
2008a], laser ablation of olivine [Brunetto et al., 2007],
olivine impacted by higher-energy ions [Strazzulla et al.,
2005; Brunetto et al., 2006b]. Furthermore, the reddening is
also similar to many asteroid spectra, including, but not
limited to that of Eros [Clark et al., 2002].
[16] The similarity of the weathered and unweathered San
Carlos olivine reflectance curves in Figure 2 show the
difficulty that would be encountered when estimating the
degree of space weathering in an actual asteroid spectrum.
The small differences can be emphasized by differentiating
the spectrum. This is shown in Figure 3, where it is quite
clear that reddening occurs essentially below 1.07 mm, and
that the degree of weathering can be ascertained by taking
the ratios of the dips at 0.77 and 0.90 mm to the peaks at
1.11 and 1.4 mm. For this reason we propose to use
derivative spectra to analyze weathering from asteroids.
3.2. Chemical Analysis of the Surface Using X-Ray
Photoelectron Spectroscopy
[17] In addition to measuring the NIR spectrum of olivine
during irradiation we also monitored chemical changes in
3 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
Figure 2. (top) Absolute reflectance spectra of <45 mm
forsterite powder before (a) and after (b) irradiation with
2.3 1018 ions/cm2. The lower curves are the absolute
reflectance spectra of <45 mm olivine powder as a function
of 4 keV He+ fluence of 0, 1.5, 5.3, and 25 (1017 ions/cm2)
from top to bottom at 700 nm. (bottom) Reflectance of
<45 mm olivine powder divided by that from a virgin
sample; the fluences are 3, 5.3, and 25 (1017 ions/cm2) from
top to bottom at 700 nm.
the surface with X-ray photoelectron spectroscopy. We
monitored O, C, Fe, Mg, and Si and list the measured
electron binding energies in Table 1. Electrostatic charging
of the surface by photoelectron emission and changes in the
work function of the spectrometer may cause the peak
position to shift between irradiations, and thus the spectra
were compared after the following procedure. First, the
spectra were all energy shifted so that the peak position of
Si-2p oxide coincided at a particular binding energy; this
small shift was always <0.5 eV. Next, the absolute binding
energy scale was obtained by using the standard energy of
284.8 eV [Powell et al., 1979; Dukes et al., 1999] for the 1s
peak of the adventitious carbon (present in the unirradiated
spectra because of exposure to atmospheric contaminants).
The binding energies measured for San Carlos olivine are in
good agreement with previously published values of olivine
[Dukes et al., 1999], forsterite [Hochella and Brown, 1988],
fayalite [Schott and Berner, 1983], and Mg1.2Fe0.8SiO4 [Yin
et al., 1971].
E03003
[18] The photoelectron peak due to adventitious carbon
quickly disappears to the noise level after a fluence of
1017 ions/cm2. However, there is a weak low-energy
shoulder at 283.8 eV, which remained throughout the
experiment indicating that it is intrinsic to the olivine
sample, as expected from terrestrial olivine.
[19] Besides the removal of atmospheric contaminants at
low fluences, the other major change observed with XPS is
in the Fe-2p photoelectron region. The spectra are shown as
a function of fluence in Figure 4. The initial Fe-2p3/2 peak
position at 711 eV indicates the presence of Fe3+ in addition
to the intrinsic Fe2+ in olivine (at 709.1 eV). These results
are in agreement with previous experiments [Schott and
Berner, 1983; Dukes et al., 1999] and are attributed to
oxidation of surface iron. During bombardment, the surface
ferric iron (Fe3+) was removed and reduced to Fe2+ and Fe0
(at 706.7 eV), because of the preferential loss of oxygen
from the surface, detected by XPS, via the breaking of Fe-O
bonds.
[20] No significant change in the binding energy of Si or
Mg core electrons was observed under ion bombardment
(Figure 5). The different behavior of Si and Mg compared to
Fe is expected on the basis of the higher strength of the Si-O
and Mg-O bonds. Finally, through the irradiation series, the
peak position of the O-1s transition remained at 531.2 ±
0.2 eV, indicative of the intact SiO4 silicate tetrahedra.
[21] Previous in situ XPS studies of irradiation of olivine
are in general agreement with our results: ion irradiation of
olivine forms metallic iron in the sample. The notable
exception is the result by Davoisne et al. [2008], which
shows the iron in olivine oxidized toward Fe3+ rather than
reduced to Fe0 when the flux of He ions was below 6 1014 ions/cm2/sec, but reduced to Fe0 at higher fluxes. We
speculate that this effect was caused by some minor
contamination, e.g., an oxygen containing species in the
background gas. In our experiments, where irradiations
were done at much lower pressures and with He+ flux 40
times lower than their threshold flux of Davoisne et al.
Figure 3. Derivative of the reflectance of a virgin sample
(v) and a sample irradiated with 4 keV He+ to 3.1 1018
ions/cm2 (i), together with their difference (D). Note that
the effect of irradiation appears below 1.07 mm.
4 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
Table 1. Electron Binding Energies in San Carlos Olivinea
Olivine Constituents
Binding Energy (eV)
Fe3+ (2p3/2)
Fe2+ (2p3/2)
Fe0 (2p3/2)
O (1s)
C (1s)
Si (2p)
Mg (2p)
711.4
709.1
706.7
531.4
283.8
101.8
50.1
a
The binding energies reported here are the experimental values shifted
as described in the text. The value listed for C (1s) is for the intrinsic carbon
left at high fluences. Adventitious carbon (Eb = 284.8 eV) is used to fix the
absolute binding energy scale.
[2008], we found that the iron reduced toward Fe0 and did
not form additional Fe3+. Furthermore, increasing the ion
flux by a factor of six yielded the same results.
3.3. Comparison of Reddening and Metallization
[22] To compare our XPS data with spectral reddening
data, we quantified the fraction of surface Fe that is metallic
by taking peak areas after deconvolving the Fe 2p3/2
photoelectron region (Figure 4) into oxide (Fe3+ and Fe2+)
and metallic (Fe0) components using the known positions of
each component and a Shirley baseline fit [Shirley, 1972]. In
Figure 6, we show the fluence dependence of metallization
of iron (based on the XPS data) and the fluence dependence
of spectral reddening. From this plot, it is clear that the
reddening is correlated with the increase of metallic iron in
the sample. This result agrees with previous observations
that lunar soils containing metallic iron nanoparticles had
strong red slopes in the visible (380– 850 nm) reflectance
[Keller et al., 1998b].
[23] We note that the rate of metallization of the <45 mm
powder olivine sample is 2 times slower than for a flat
Figure 4. XPS spectra of the Fe-2p iron region during
irradiation of <45 mm olivine powder with 4 keV He+ ions.
Fluence from bottom to top (1017 ions/cm2) are 0, 3, 5.3,
7.8, 11, 15, 20, 25, and 31. The spectra have been displaced
vertically for clarity.
E03003
olivine section (Figure 7). That the reduction of iron occurs
in the flat surface, where redeposition is unimportant, will
be discussed below.
3.4. Attenuation of the Fe2+ Absorption Band and
Darkening
[24] In addition to the reddening, we also quantified the
area of the 1 mm absorption band by converting the
spectrum to optical depth units, taking the natural logarithm
of the reflectance, and subtracting a polynomial baseline
(Figure 8). This method is similar to the one used in our
previous works on ices [Loeffler et al., 2006]. Figure 8
shows that, as the sample is irradiated, the area of the 1 mm
absorption band decreases by 11% after 3 1018 He+/
cm2. One can see from Figure 2 that darkening by ion
irradiation is at most 2% at 2.0 mm.
3.5. Reoxidation Upon Exposure to Atmosphere
[25] These studies are the first in which ion-induced
chemical and reflectance changes in olivine have been
measured simultaneously in situ. We also addressed whether
in situ measurements were necessary, by studying the effect
of exposing samples to air before analysis. After He+
irradiation, we removed a sample from vacuum, exposed
it to atmosphere and remeasured its reflectance and XPS
spectra. The NIR reflectance did not change, within 3%,
after exposure to atmosphere for 10 min. However, the
amount of metallic iron on the surface decreased signifi-
Figure 5. XPS spectra of the Mg-2p and Si-2p regions
before (0) and after (i) irradiation of a <45 mm olivine
powder with 4 keV He+ ions to a fluence of 3.1 1018 ions/
cm2. Notice the constancy of the peak positions, in contrast
with those of Fe-2p in Figure 4. The increase in intensity
after irradiation is due to the removal of an impurity
overlayer. In the top spectra one also notices the reduction
of iron in the position of the weak Fe-3p photoelectron line.
5 of 13
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
E03003
E03003
Figure 6. Comparison of reddening (circles) in the NIR and formation of metallic iron (crosses)
measured with XPS on <45 mm olivine powder as a function of 4 keV He+ fluence. See text for definition
of reddening.
cantly because of reoxidation, as first pointed out by Dukes
et al. [1999]. Figure 9 (top) shows the Fe-2p region after we
exposed it to atmosphere by placing it beside our vacuum
system in our air-conditioned laboratory for different time
periods for the powder and rock previously irradiated with
4 keV He+. Our shortest exposure time (10 min) causes the
percentage of surface Fe metal to drop by a factor of two.
After long exposures, up to a month, this effect saturates,
leaving 15% metallic iron on the surface from an asirradiated level of 75%. This effect suggests that the lunar
soils which had been exposed to the air and had a weak
metallic peak in XPS, [e.g., Baron et al., 1977] may have
been completely metallic before handling in the laboratory.
[26] Interestingly, there are stronger effects of exposing
olivine to atmosphere when irradiation was done with 4 keV
Ar+ ions, as shown in Figure 9 (bottom). After 10 min, only
a weak shoulder of metallic iron remained in the Fe-2p XPS
spectrum, indicating nearly full reoxidation over the depth
of XPS; measurements a few days later show that the entire
surface had reoxidized. This difference in the reoxidation
times for different ions is attributed to easier oxidation of
the shallower and more damaged region created by Ar, as
compared to He.
absorption is typically due to electronic transitions between
levels split by the crystal field, color centers, or overtones of
fundamental molecular vibrations that occur at longer wavelengths [Hapke, 1993; Clark, 1999]. As mentioned earlier,
the olivine absorption in this wavelength range is due to
electronic excitation of the outer shell 3d electrons in the
Fe2+ ions.
[28] If the sample also contains particles of size d, smaller
than the wavelength of light, e.g., iron nanoparticles, then
the reflectance spectra will be altered by the wavelength
dependence of scattering and absorption. The efficiencies
for scattering and absorption decrease as (d/l)4 and d/l,
respectively and, for d/l < 0.3 for iron [Hapke, 1993], the
spectral reddening is dominated by absorption. We note that
4. Discussion
4.1. Overview of Sample Reflectance
[27] Electromagnetic radiation incident on the surface of a
grain can be absorbed, reflected, or transmitted. The degree
to which each occurs depends on the optical constants of the
material. For samples that contain grains much larger than
the wavelength of light, the reflected wave will leave in a
specular direction from each point on the surface. Because
of roughness, the surface normal varies locally causing light
to reflect in a variety of directions. The reflected and
transmitted light may enter another grain, where it can be
reflected or transmitted with some loss of intensity because
of absorption by the sample. In the range of 0.6– 2.5 mm,
Figure 7. Metallization of iron, as described by XPS, as a
function of fluence for the flat olivine rock and <45 mm
powder samples.
6 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
E03003
aggregate into bubbles of gas. Futagami et al. [1990] found
that irradiation of olivine and other minerals with 3.6 keV
He ions produced an accummulation of implanted He with a
maximum concentration of 0.5– 1 1017/cm3 that saturated
above a fluence of 1018/cm2, as expected from the
competition with sputtering. The overlap of damage regions
with increasing fluence in olivine irradiated by 4 keV He
leads to amorphization above a fluence of 1016 ions/cm2
[Bradley et al., 1996; Demyk et al., 2001; Carrez et al.,
2002], to swelling [Wilson et al., 1996] and surface roughness [Wilson et al., 1996; Davoisne et al., 2008].
[30] A direct manifestation of the atomic displacements is
sputtering, the ejection of atoms and molecules via collisions with the solid, a process known to be an important
surface alteration mechanism for both rocky and icy bodies
in space that are subject to radiation [Johnson, 1990; Hapke,
2001; Baragiola, 2003]. Sputtering results in erosion of the
surface, but because the probability of ejection depends on
the local binding energy of the atoms and on their mass, it is
often (though not always) the case that the chemical
composition is altered significantly by differential sputter-
Figure 8. (top) Decrease in the 1 mm band area (circles) as
a function of He ion fluence for a <45 mm olivine powder.
(bottom) Polynomial fit of the baseline in the olivine optical
depth spectra (in ln (R) units) used to measure the band
area, before (a) and after (b) irradiation with 4 keV He+ to a
fluence of 3.1 1018 ions/cm2.
only a small amount of metallic iron is needed to significantly alter the reflectance. As can be seen in Figure 10, Fe
has an extremely low absorption length, compared to
olivine (e.g., at a wavelength of 1 mm, 20 nm for Fe
[Johnson and Christy, 1974] and 500 microns for olivine
[Brunetto et al., 2007]).
4.2. Overview of Irradiation of Silicates
[29] KeV ions entering a solid sample lose energy mainly
as a result of binary collisions with target atoms. Some of
the collisions displace target atoms from their equilibrium
lattice positions and ultimately lead to damage. A small
fraction of the projectiles are reflected, the majority come to
rest in the solid. The cascade of displaced target atoms
produces atomic mixing in multicomponent samples. While
the cascade cools (times of the order of picoseconds)
diffusion allows the lattice to heal, but some point defects
(vacancies and interstitials) remain, which accumulate to
form line defects and voids. A large accummulation of
damage leads to amorphization. Voids assist in the formation of projectile aggregates, and He atoms are known to
Figure 9. XPS spectra of olivine rock and <45 mm powder
samples after different times of exposure to the atmosphere.
(top) Effects after He+ irradiation. From top to bottom,
samples after exposure to atmosphere for 0 min (rock), 0 min
(powder), 10 min (rock), 10 min (powder), 100 min
(powder), 3 days (powder), and 8 days (powder). (bottom)
Effects after Ar+ irradiation. From top to bottom, samples
after exposure to atmosphere for 0 min (rock), 0 min
(powder), 10 min (rock), 10 min (powder), and 2 days
(rock).
7 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
E03003
Figure 10. Absorption path length for olivine (data adapted from Brunetto et al. [2007]) and iron (data
taken from Johnson and Christy [1974]).
ing, i.e., the sputtering yield depends on the type of atom, in
addition to its surface concentration. The depth of the
altered layer is of the order of the penetration range of the
ions [Kelly, 1989]. In our in situ experiments, we only detect
slight oxygen loss correlated with reduction of iron. Previous reports of Mg loss [Bradley et al., 1996; Demyk et al.,
2001; Toppani et al., 2006] were likely affected by exposure
of the irradiated samples to the air, water or solvents prior to
analysis, as mentioned in our previous reports [Cantando et
al., 2009].
[31] On very rough surfaces, such as powders, the sputtering yield decreases because of the redeposition of some
of the sputtered flux. The amount of redeposition depends
on the shape of the surface. An experimental report indicated that a powder sample lost 10 times less mass than a
bulk sample [Hapke and Cassidy, 1978], but this was done
with a unidirectional ion beam at extremely high fluences
(8 1020 H/cm2). Even at fluences one tenth as large,
needle structures in the direction of the ion source develop
from the irregular grain surfaces [McDonnell and Flavill,
1974]. These extreme structures allow redeposition of a
large fraction of the sputtered ejecta, which strongly darken
the surface [Wu et al., 2001]. Thus, the darkening of
surfaces by ions reported by Hapke [2001] is likely complicated by trapping of light by structures generated via
large, unidirectional irradiation. These structures would not
exist on a surface in space, which is irradiated at different
angles of incidence as it rotates with respect to the solar
wind.
4.3. Test for the Effect of Redeposition
[32] Hapke and collaborators [Hapke, 2001, and references
therein] have clearly shown the importance of high-fluence
ion irradiation (hence the solar wind) in the darkening of
Fe-bearing minerals. Whereas they conclude that metallic
Fe only forms in redeposited coatings, we found that it is
created also in flat rocks, where redeposition is negligible.
[33] Thus, we need to examine in more detail the relative
role of direct alteration of the surface versus redeposition.
We find that, although the changes in chemical composition
of the olivine rock and the powder samples are very similar,
the formation rate of metallic iron is a factor of two slower
for the powder, which we postulate to be related to
redeposition.
[34] To test the magnitude of redeposition that occurred in
our experiments, we analyzed the relative rate of surface
depletion of adventitious carbon on the flat rock versus the
powder. After sputter-cleaning each surface we exposed it to
the atmosphere for 10 min to allow the deposition of
atmospheric hydrocarbons. Then, we reinserted the samples
into vacuum and measured with XPS the amount of
adventitious carbon on the surface and its decrease as a
result of He+ irradiation. Since the hydrocarbon content in
the air can change from day to day and with time, we
repeated each experiment on different days but found
variations of less than 10%. Our results for the carbon
depletion versus fluence are shown in Figure 11 for flat rock
and powder samples. We found that the carbon removal rate
for the powder is about a third lower than that for the flat
surface. Thus, we can conclude that about two thirds of the
ejecta is redeposited. This rough estimate is similar to the to
0.70 predicted by modeling of redeposition [Cassidy and
Johnson, 2005]. The difference with the value of 0.9
measured by Hapke and Cassidy [1978] is most likely
due to the needle topography that is expected in the experiments by these authors, as discussed above.
[35] To understand the different conclusions proposed by
us and Hapke [2001, and references therein] on the role of
direct metallization of a surface, we note that those authors’
8 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
Figure 11. Removal of of adventitious carbon from the
surface of an olivine rock and olivine powder by sputtering.
The solid lines are double exponentials fit to the data.
conclusions that only sputtered deposits will contain metallic iron were based mainly on experiments where irradiation
was done on a ball situated in an alumina crucible, both
made of impure alumina (clean alumina did not darken)
[Hapke, 2001]. In such geometry, only the top of the ball
and the crucible were irradiated and only the lower half of
the ball would have sputtered material deposited onto it.
The results showed that the lower half darkened much more
than the top half of the ball, and were not affected by ioninduced topography mentioned above. However, they were
interpreted to mean that redeposition of the aluminum (on
the bottom of the ball) was more important than direct
irradiation (of the top of the ball) in altering the optical
reflectance. We argue that this difference is caused simply
by the fact that the thickness of the altered region of the
top of the ball is on the order of the ion range and is
limited by sputtering, whereas the thickness of the deposit
in the bottom of the ball can increase indefinitely with
fluence, since it is being sputtered from another surface
(the crucible).
[36] The fact that metallization does not require redeposition, as evidenced in this and previous work on sectioned
surfaces [Dukes et al., 1999; Davoisne et al., 2008], does
not imply that irradiation of sectioned surfaces will cause
significant optical effects. The effect on the spectral reflectance will be larger on powders, only because scattering of
the grains causes light to pass through the altered regions
multiple times, irrespective of whether the main mechanism
for formation of metallic iron is vapor redeposition or direct
ion bombardment.
[37] In closing this discussion, we propose that the reason
why the powders metallize at a slower rate than a flat
surface is that there is more more redeposition in the powder
and the redeposited material is partially oxidized, as commonly happens in the sputter deposition of oxides in
vacuum [Mauvernay et al., 2007].
E03003
4.4. Mechanism for Alteration of Reflectance Spectra
of Silicates
[38] As mentioned above, Hapke [2001] has shown that
darkening of visible reflectance spectra in several types of
mineral oxides was correlated with the sputter redeposition
of coatings containing metallic iron.
[ 39 ] Recent experiments in Catania [Brunetto and
Strazzulla, 2005; Strazzulla et al., 2005; Brunetto et al.,
2006b] have shown that higher-energy (60 – 400 keV) heavy
ions can also cause significant alteration of spectral reflectance but at much lower ion fluences than used by Hapke
[2001]. Although heavy ions are unimportant in the solar
wind, these experiments can help in understanding weathering mechanisms. The Catania group proposed that the
alteration they observed at lower fluences was caused by
amorphization, which would be an additional mechanism
independent of the metallic iron in the sample. However, we
suggest that the lower fluences needed to change the
spectral reflectance in these higher-energy heavy ion experiments are due to the difference in density of energy
deposition and projectile range, not to a separate mechanism. The 60– 400 keV Ar+ ions used by the Catania group
have 100 times higher stopping power and penetrate 2.5–
14 times more than the 1 keV H ions used by Hapke’s group
(J. F. Ziegler and J. P. Biersack, Stopping and Range of Ions
in Matter (SRIM)-2006, 2006, available at http://srim.org)
and will cause more damage, create more metallic iron, and
cause more spectral alteration for the same ion fluence. As
an additional test, we irradiated a sectioned olivine rock
with 4 keV Ar, which has a stopping power 11 times
higher than 4 keV He used in our experiments and found
that the metallization rate is 26 times faster than it was for
He. Regarding the role of amorphization, we note the subtle
change of optical properties due to amorphization would
have a negligible effect in optical reflectance at near infrared
wavelengths. In fact, we detected no reflectance changes at
fluences of 1016 – 1017 ions/cm2 where olivine has already
been amorphized by 4 keV He [Carrez et al., 2002], and no
reddening after irradiating iron-free forsterite with 2 1018 He
ions/cm2 (Figure 2 (top)). It is only at the high fluences of
irradiation of olivine, where formation of metallic iron is
significant (Figure 4), that we observe spectral reflectance
changes. Therefore, we conclude that metallization, rather
than amorphization, is responsible for the changes in the
spectral reflectance.
4.5. Formation of Metallic Iron in Silicates by the Solar
Wind
[40] During irradiation of olivine, metallic iron is formed
in the sample, because oxygen is preferentially removed by
sputtering, and radiation enhanced diffusion assists in the
nucleation of the reduced iron. Metallic iron can also form
in a sputtered deposit because Fe atoms deposit preferentially compared to the lighter O atoms, as shown by Hapke
[2001].
[41] The effect of metallic iron particles on the reflectance
spectra will depend on their size. Lunar soil analysis has
shown spectrally darker soils contain large iron nanoparticles (>10 nm), while smaller nanoparticles (<5 nm) are
more prominent in soils that are spectrally reddened [Keller
et al., 1998a]. These results are consistent with more recent
laboratory measurements [Noble et al., 2007] on silicate
9 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
Figure 12. Comparison of our reflectance ratio (Figure 2
(bottom)) with that of spectra extrapolated from Sasaki et al.
[2001] and Brunetto et al. [2006a], all for San Carlos
olivine. We have normalized spectra at 2 mm before taking
the reflectance ratio. The data from top to bottom at 700 nm
after 4 keV He irradiation, a pulse of 12 J/cm2 [Sasaki et al.,
2001], a pulse of 24 J/cm2 [Sasaki et al., 2001], and 13–
14 pulses of 2 J/cm2 [Brunetto et al., 2006a].
gels, which show that addition of Fe nanoparticles smaller
than 15 nm produce reddening while addition of Fe particles
larger than 40 nm induce darkening of the silicate. Therefore we conclude that since the He bombardment reddens
our olivine samples but barely darkens their spectral reflectance, the metallic iron formed in our experiments exists in
very small precipitates.
4.6. Astronomical Implications
[42] We can scale our results to an effective exposure time
in space. The estimates are approximate because fluxes of
solar wind ions and micrometeorites have changed through
history. We consider the case of the Moon; similar estimates
can be done for Mercury and asteroids. For the Moon, we
use the mean number density (n/cm3) and mean velocity
(468 km/sec) given by Gosling [2007] at 1 AU and obtain a
proton flux of 4.1 108 protons/cm2/sec. The incident flux
(F0) of He is estimated to be 1.9 107 He/cm2/sec,
obtained from the product of the proton flux by the average
relative abundance of helium ions in the solar wind (0.047).
However, we note that the cross section being struck by the
solar wind is pR2, while the surface area of a sphere is 4pR2,
E03003
so the average flux is actually 1/4 of F 0 or 4.8 106 ions/cm2/sec. Using this value, we find that it takes
13,000 years for an exposed grain to reach a fluence
where alteration induced by the solar wind saturates (2 1018 He ions/cm2). The additional effects of solar wind
protons can be assumed to scale roughly as the product of
their flux times the sputtering yield [Johnson and Baragiola,
1991], since sputtering is governed by the same type of
collisions that cause surface damage. Thus, we obtain a
characteristic time for weathering of 5000 years. This does
not mean that the appearance of all surface material older than
5000 years will be the same, because there is a complex
interaction between the effects caused by the solar wind and
micrometeorites.
[43] Calculating the timescales for effects caused by
micrometerorite impacts is non trivial. The impacts are
not uniform [Gault et al., 1974; Horz et al., 1974] and
can modify grains by vaporization and changing their
composition. In addition, impacts mix the regolith, break
large grains, form craters and eject other grains indirectly by
the shock wave induced in the impact. The effect of
chemical alterations on reflectance was studied by Sasaki
et al. [2001] using ns pulsed laser irradiation as an analog
for micrometeorite impacts. In comparison with our experiments, they reported somewhat more spectral reddening and
darkening after one laser pulse (24 J/cm2), which they
estimated to be equivalent to 108 years exposure at
1 A.U. In addition, they assumed that this exposure time
might be a lower limit, since laser irradiation may be more
efficient than dust impacts in altering the sample. This
assertion is supported by experiments comparing thermoionic plasma emitted from samples irradiated with pulsed
lasers and dust impacts show that similar secondary ion
mass spectra and absolute yields could be obtained if the
power density of the laser was 100 times lower than that
of the dust [Kissel and Krueger, 1987].
[44] On the other hand, Sasaki et al. [2001] obtained the
timescale of 108 years by estimating that the energy flux
could be obtained by applying the cumulative flux of
micrometeorites to 10 12 g particles. However, we note
that it has previously been estimated that >75– 80% of the
material vaporized by impacts at 1 A.U. comes from heavier
projectiles between 10 8 and 10 4 g (10 – 230 mm particles
assuming a density of 2 g/cm3) [Gault et al., 1972; Cintala,
1992], even though their flux is much lower. Thus to obtain
a more accurate estimate of timescales for vaporization
caused by micrometeorite impacts, we need to integrate
the differential energy flux over the range of micrometeorites responsible for vaporization. The differential flux (f(m))
of a micrometeorite is given by f(m) = dF(m)/dm [Grun et
al., 2001], where F is the cumulative flux of particles of
mass larger than m. An approximation for F has been given
by Gault et al. [1972] for 10 13 g < m < 103 g at 1 A.U. By
integrating the product of the impact energy and dF/dm
over masses between 10 13 g and 103 g, assuming an
average impact velocity of 13 km/sec [Cintala, 1992], we
obtain an energy flux of 1000 J/cm2/106 a.
[45] In Figure 12, we replot the ratio of the reflectance
spectra of irradiated samples to that of the virgin samples,
shown in Figure 2 together with previous laser ablation
studies [Sasaki et al., 2001; Brunetto et al., 2006a]. We have
normalized all spectra at 2 microns before taking the ratios,
10 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
to compare reddening and not darkening, which can depend
on microstructure caused by laser ablation. The NIR spectrum measured by Sasaki et al. [2001] after one laser pulse
(12 J/cm2) is quite similar to our ion irradiation experiment.
The energy fluence of 12 J/cm2 is equivalent to 106 years
of exposure to micrometeorite impacts, taking into account
the increased efficiency of the laser mentioned above. Thus,
we estimate that spectral reddening caused by the solar wind
saturates about 2 orders of magnitude faster than estimated
to occur by micrometeorite impacts. However, we note that
if the grain survives for more than a million years, as
is likely the case on the surface of the Moon, micrometeorite impacts may become more important as the spectral reddening effect is not saturated after one impact (see
Figure 12).
[46] The importance of the solar wind is substantiated
also by the fact that amorphous rims in regolith grains have
a thickness of the order of the range of solar wind ions
(considering slow and fast components) [Christoffersen et
al., 1996; Keller and McKay, 1997]. Furthermore, we note
that our experimental results also lead to the conclusion that
reduction of iron in the lunar regolith in the irradiated region
can explain the formation of submicron Fe particles and
their effect on optical reflectance, in contrast to the assertion
of Hapke [2001] that direct ion impact produces only
negligible effects. Our conclusion, of course, does not
preclude the contribution of redeposition of micrometeorite
impact ejecta and, perhaps, redeposition of sputtered
particles.
[47] With respect to space weathering effects on asteroids,
our experiments strengthen the idea that, solar wind irradiation and the consequent formation of metallic iron alter the
reflectance spectra of olivine-bearing S type asteroids. More
specifically we can apply our results, which show ion
irradiation of olivine causes reddening and attenuation of
the 1 mm absorption band but little darkening of its overall
spectrum, to the main belt asteroids 243 Ida (pyroxene rich)
and 951 Gaspra (olivine rich). Like in our experiments, the
spectra of each of these asteroids show variations in slope
and absorption band depth but minor albedo variations
across their respective surfaces [Chapman, 1996; Clark et
al., 2002]. We speculate that this is a consequence of their
location, because although both the solar wind and micrometeorite fluxes decline as one goes from the Sun to the
asteroid belt, the impact speed of the micrometeorites also
decreases. This will in turn decrease significantly the impact
energy and hence, the amount of material vaporized [Bloch
et al., 1971; Mandeville and Vedder, 1971; Cintala, 1992].
Thus, micrometeorite impacts in the main asteroid belt will
mainly act to bring fresh material to the surface, while the
solar wind will be the primary space weathering agent.
[48] Closer to the Sun, micrometeorites have higher
fluxes and velocities and thus weathering becomes more
complicated. As an example, Itokawa and Eros, near-Earth
S type asteroids located at similar heliocentric distances,
have spectra that exhibit different space weathering trends
and variations across their surfaces [Clark et al., 2002;
Murchie et al., 2002; Hiroi et al., 2006; Ishiguro et al.,
2007]. These differences likely indicate that other properties, besides ion and micrometeorite impact determine the
appearance of those objects. Still, the discovery of weathered large boulders on Itokawa [Ishiguro et al., 2007] can
E03003
be explained by our results that show metallic iron can be
formed by direct ion irradiation of a flat rock of olivine,
where redeposition does not occur.
5. Conclusions
[49] Four keV He ions, typical of the solar wind, alter the
reflectance spectrum of olivine powders by reddening the
spectral slope and attenuating the 1 mm absorption feature, a
manner consistent with observations of space weathering.
Furthermore, as judged by in situ chemical analysis, spectral
reddening corresponds directly to the formation of metallic
iron in the outer 50 – 80 Å of the mineral surface. In
addition, we find that exposing the irradiated surfaces to
the atmosphere causes the metallic Fe to reoxidize, showing
that in situ measurements are necessary to study chemical
changes occurring during ion bombardment of minerals and
that previous XPS measurements of lunar soils may have
underestimated the metallic content of the surface.
[50] Our results show that metallic iron forms more
slowly on a powder than on a flat rock of olivine, which
we suggest is due to partial oxidation of redeposited
sputtered material. Interestingly, the spectral changes observed in our ion irradiation experiments are similar to
previous weathering experiments using laser impact, indicating that both processes affect similarly the spectral
reflectance of olivine. In addition, we estimate that at
1 A.U. the spectral reddening of olivine induced by He
bombardment will saturate 2 orders of magnitude faster than
the time it takes for similar reddening effects to occur as a
result of micrometeorite impacts.
[51] Acknowledgments. This work was supported by the NASA
Cosmochemistry and Lunar Advanced Science and Exploration Research
programs. We would like to thank W. Y. Chang for his technical assistance
in the beginning of this project. We would like to thank B. E. Clark and an
anonymous referee with their very helpful reviews.
References
Baragiola, R. A. (2003), Water ice on outer solar system surfaces: Basic
properties and radiation effects, Planet. Space Sci., 51, 953 – 961,
doi:10.1016/j.pss.2003.05.007.
Baron, R. L., et al. (1977), The surface composition of lunar soil grains: A
comparison of the results of auger and X-ray photoelectron (ESCA)
spectroscopy, Earth Planet. Sci. Lett., 37, 263 – 272, doi:10.1016/0012821X(77)90172-8.
Bloch, M. R., et al. (1971), Meteorite impact craters, crater simulations, and
the meteoroid flux in the early solar system, paper presented at Second
Lunar Science Conference, Lunar and Planet. Sci. Inst., Houston, Tex.
Bradley, J. P. (1994), Chemically anomalous, preaccretionally irradiated
grains in interplanetary dust from comets, Science, 265, 925 – 929,
doi:10.1126/science.265.5174.925.
Bradley, J. P., et al. (1996), Radiation processing and the origins of interplanetary dust, Lunar Planet. Sci., XXVII, 149 – 150.
Bringa, E. M., et al. (2007), Energetic processing of interstellar silicate
grains by cosmic rays, Astrophys. J., 662, 372 – 378, doi:10.1086/517865.
Brunetto, R., and G. Strazzulla (2005), Elastic collisions in ion irradiation
experiments: A mechanism for space weathering of silicates, Icarus, 179,
265 – 273.
Brunetto, R., et al. (2006a), Space weathering of silicates simulated by
nanosecond pulsed UV excimer laser, Icarus, 180, 546 – 554,
doi:10.1016/j.icarus.2005.10.016.
Brunetto, R., et al. (2006b), Modeling asteroid surfaces from observations
and irradiation experiments: The case of 832 Karin, Icarus, 184, 327 –
337, doi:10.1016/j.icarus.2006.05.019.
Brunetto, R., et al. (2007), Optical characterization of laser ablated silicates,
Icarus, 191, 381 – 393, doi:10.1016/j.icarus.2007.04.023.
Burns, R. G. (1993), Mineralogical Applications of Crystal Field Theory,
551 pp., Cambridge Univ. Press, New York.
11 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
Cantando, E. D., C. A. Dukes, and R. A. Baragiola (2009), Aqueous dissolution of olivine enhanced by ion irradiation, J. Geophys. Res., 113,
E09011, doi:10.1029/2008JE003119.
Carrez, P., et al. (2002), Low-energy helium ion irradiation-induced amorphization and chemical changes in olivine: Insights for silicate dust evolution in the interstellar medium, Meteorit. Planet. Sci., 37, 1599 – 1614.
Cassidy, T. A., and R. E. Johnson (2005), Monte Carlo model of sputtering
and other ejection processes with a regolith, Icarus, 176, 499 – 507,
doi:10.1016/j.icarus.2005.02.013.
Chapman, C. R. (1996), S-type asteroids, ordinary chondrites, and space
weathering: The evidence from Galileo’s fly bys of Gaspra and Ida,
Meteorit. Planet. Sci., 31, 699 – 725.
Chapman, C. R. (2004), Space weathering of asteroid surfaces, Annu. Rev.
Earth Planet. Sci., 32, 539 – 567, doi:10.1146/annurev.earth.
32.101802.120453.
Christoffersen, R., et al. (1996), Microstructure, chemistry, and origin of
grain rims on ilmenite from the lunar soil finest fraction, Meteorit. Planet.
Sci., 31, 835 – 848.
Cintala, M. J. (1992), Impact-induced thermal effects in the lunar and
Mercurian regoliths, J. Geophys. Res., 97, 947 – 973.
Clark, B. E., et al. (2002), Asteroid space weathering and regolith evolution, in Asteroids III, edited by W. F. Bottke Jr. et al., pp. 585 – 599, Univ.
of Ariz. Press, Tucson, Ariz.
Clark, R. N. (1999), Spectroscopy of rocks and minerals, and principles of
spectroscopy, in Manual of Remote Sensing, edited by A. N. Rencz,
pp. 3 – 58, Wiley, New York.
Davoisne, C., et al. (2008), Chemical and morphological evoultion of a
silicate surface under low-energy ion irradiation, Astron. Astrophys., 482,
541 – 548, doi:10.1051/0004-6361:20078964.
Demyk, K., et al. (2001), Structural and chemical alteration of crystalline
olivine under low energy He+ irradiation, Astron. Astrophys., 368, L38 –
L41, doi:10.1051/0004-6361:20010208.
Dukes, C. A, et al. (1999), Surface modification of olivine by H+ and He+
bombardment, J. Geophys. Res., 104, 1865 – 1872, doi:10.1029/98JE02820.
Futagami, T., et al. (1990), Helium ion implantation into minerals, Earth
Planet. Sci. Lett., 101, 63 – 67, doi:10.1016/0012-821X(90)90124-G.
Gaffey, M. J., et al. (1993), Mineralogical variations within the S-type
asteroid class, Icarus, 106, 573 – 602, doi:10.1006/icar.1993.1194.
Gault, D. E., et al. (1972), Effects of microcratering on the lunar surface,
Geochim. Cosmochim. Acta, 3, 2713 – 2734.
Gault, D. E., et al. (1974), Mixing of the lunar regolith, paper presented at
Fifth Lunar Conference, Lunar and Planet. Sci. Inst., Houston, Tex.
Gosling, J. T. (2007), The solar wind, in Encyclopedia of the Solar System,
2nd ed., edited by L.-A. McFadden, P. R. Weissman, and T. V. Johnson,
Academic, Amsterdam.
Grun, E., et al. (2001), Situ measurements of cosmic dust, in Interplanetary
Dust, edited by E. Grun et al., pp. 295 – 346, Springer, New York.
Hapke, B. W. (1973), Darkening of silicate rocks by solar wind sputtering,
Moon, 7, 342 – 355, doi:10.1007/BF00564639.
Hapke, B. W. (1993), Theory of Reflectance and Emittance Spectroscopy,
455 pp., Univ. of Cambridge, Cambridge, U. K.
Hapke, B. W. (2001), Space weathering from Mercury to the asteroid belt,
J. Geophys. Res., 106, 10,039 – 10,074, doi:10.1029/2000JE001338.
Hapke, B. W., and W. Cassidy (1978), Is the Moon really as smooth as a
billiard ball? Remarks concerning recent models of sputter-fractionation
on the lunar surface, Geophys. Res. Lett., 5, 297 – 300, doi:10.1029/
GL005i004p00297.
Hiroi, T., et al. (2006), Developing space weathering on the asteroid 25143
Itokawa, Nature, 443, 56 – 58, doi:10.1038/nature05073.
Hochella, M. F., Jr., and G. E. Brown Jr. (1988), Aspects of silicate surface
and bulk structure analysis using X-ray photoelectron spectroscopy, Geochim. Cosmochim. Acta, 52, 1641 – 1648, doi:10.1016/00167037(88)90232-3.
Horz, F., et al. (1974), Micrometeoroid abrasion of lunar rocks: A Monte
Carlo simulation, paper presented at Fifth Lunar Conference, Lunar and
Planet. Sci. Inst., Houston, Tex.
Ishiguro, M., et al. (2007), Global mapping of the degree of space weathering on asteroid 25143 Itokawa by Hayabusa/AMICA observations,
Meteorit. Planet. Sci., 42, 1791 – 1800.
Johnson, P. B., and R. W. Christy (1974), Optical constants of transition
metals: Ti, V, Cr, Mn, Fe, Co, Ni, and Pd, Phys. Rev. B, 9, 5056 – 5070,
doi:10.1103/PhysRevB.9.5056.
Johnson, R. E. (1990), Energetic Charged-Particle Interactions With Atmospheres and Surfaces, Springer, Berlin.
Johnson, R. E., and R. Baragiola (1991), Lunar surface - sputtering and
secondary ion mass spectrometry, Geophys. Res. Lett., 18, 2169 – 2172,
doi:10.1029/91GL02095.
Keller, L. P., and D. S. McKay (1993), Discovery of vapor deposits in the
lunar regolith, Science, 261, 1305 – 1307, doi:10.1126/science.
261.5126.1305.
E03003
Keller, L. P., and D. S. McKay (1997), The nature and origin of rims on
lunar soil grains, Geochim. Cosmochim. Acta, 61, 2331 – 2341,
doi:10.1016/S0016-7037(97)00085-9.
Keller, L. P., et al. (1998a), Space weathering: Reflectance spectroscopy
and TEM analysis of individual lunar soil grains, paper presented at
Lunar and Planetary Institute Conference, Lunar and Planet. Sci. Inst.,
Houston, Tex., 1 Mar.
Keller, L. P., et al. (1998b), Surface-correlated nanophase iron metal in
lunar soils: Petrography and space weathering effects, paper presented
at Workshop on New Views of the Moon: Integrated Remotely Sensed,
Geophysical, and Sample Datasets, Lunar and Planet. Sci. Inst., Houston,
Tex.
Kelly, R. (1989), Bombardment-induced compositional change with alloys,
oxides, oxysalts, and halides: Part II. The role of segregation, Nucl.
Instrum. Methods Phys. Res. Sect. B, 39, 43 – 56, doi:10.1016/0168583X(89)90739-8.
Kissel, J., and F. R. Krueger (1987), Ion formation by impact of fast dust
particles and comparison with related techniques, Appl. Phys. A, 42, 69 –
85, doi:10.1007/BF00618161.
Loeffler, M. J., et al. (2006), Synthesis of hydrogen peroxide in water
ice by ion irradiation, Icarus, 180, 265 – 273, doi:10.1016/
j.icarus.2005.08.001.
Loeffler, M. J., et al. (2008a), Laboratory simulations of redeposition of
impact ejecta on mineral surfaces, Icarus, 196, 285 – 292, doi:10.1016/
j.icarus.2008.02.021.
Loeffler, M. J., et al. (2008b), Laboratory simulations of sulfur depletion at
Eros, Icarus, 195, 622 – 629, doi:10.1016/j.icarus.2008.02.002.
Mandeville, J. C., and J. F. Vedder (1971), Microcraters formed in glass by
low density projectiles, Earth Planet. Sci. Lett., 11, 297 – 306,
doi:10.1016/0012-821X(71)90183-X.
Marchi, S., et al. (2006), A general spectral slope-exposure relation for Stype Main belt and near-Earth asteroids, Astron. J., 131, 1138 – 1141,
doi:10.1086/498721.
Mauvernay, B., et al. (2007), Elaboration and characterization of Fe1 – xO
thin films sputter deposited from magnetite target, Thin Solid Films, 515,
6532 – 6536, doi:10.1016/j.tsf.2006.11.131.
McDonnell, J. A. M., and R. P. Flavill (1974), Solar wind sputtering on the
lunar surface: Equilibrium crater densities related to past and present
microparticle influx rates, Lunar Sci., III, 2441 – 2449.
Moroz, L. V., et al. (1996), Optical effects of regolith processes on Sasteroids as simulated by laser shots on ordinary chondrite and other
mafic materials, Icarus, 122, 366 – 382.
Moulder, J. F., et al. (1992), Handbook of X-Ray Photoelectron Spectroscopy, 261 pp., Phys. Electron., Eden Praire, Minn.
Murchie, S., et al. (2002), Color variations on Eros from NEAR multispectral imaging, Icarus, 155, 145 – 168, doi:10.1006/icar.2001.6756.
Nash, D. (1967), Proton irradiation darkening of rock powders: Contamination and temperature effects, and applications to solar wind darkening
of the Moon, J. Geophys. Res., 72, 3089 – 3104, doi:10.1029/
JZ072i012p03089.
Noble, S. K., et al. (2007), An experimental approach to understanding the
optical effects of space weathering, Icarus, 192, 629 – 642, doi:10.1016/
j.icarus.2007.07.021.
Pillinger, C. T. (1979), Solar-wind exposure effects in lunar soil, Rep. Prog.
Phys., 42, 897 – 961, doi:10.1088/0034-4885/42/5/003.
Powell, C. J., et al. (1979), Results of a joint Auger-ESCA round robin
sponsored by ASTM committee E-42 on surface analysis, I, ESCA results, J. Electr. Spectrosc. Related Phenomena, 17, 361 – 403,
doi:10.1016/0368-2048(79)80001-8.
Sasaki, S., et al. (2001), Production of iron nanoparticles by laser irradiation
in a simulation of lunar-like space weathering, Nature, 410, 555 – 557,
doi:10.1038/35069013.
Schott, J., and R. A. Berner (1983), X-ray photoelectron studies of the
mechanism of iron silicate dissolution during weathering, Geochim. Cosmochim. Acta, 47, 2233 – 2240, doi:10.1016/0016-7037(83)90046-7.
Seah, M. P., and W. A. Dench (1979), Quantitative electron spectroscopy of
surfaces: A standard data base for electron inelastic mean free path in
solids, Surf. Interface Anal., 1, 2 – 11, doi:10.1002/sia.740010103.
Shirley, D. A. (1972), High-resolution X-ray photoemission spectrum of
the valence bands of gold, Phys. Rev. B, 5, 4709 – 4714, doi:10.1103/
PhysRevB.5.4709.
Strazzulla, G., et al. (2005), Spectral alteration of the Meteorite Epinal (H5)
induced by heavy ion irradiation: A simulation of space weathering effects on near-Earth asteroids, Icarus, 174, 31 – 35, doi:10.1016/
j.icarus.2004.09.013.
Toppani, A., et al. (2006), Segregation of Mg, Ca, Al, and Ti in silicates
during ion irradiation, Lunar Planet. Sci., XXXVII, abstract 2056.
Wilson, C., et al. (1996), Atomic force microscopy of olivine, in Atomic
Force Microscopy/Scanning Tunneling Microscopy 3, edited by S. H.
Cohen and M. L. Lightbody, pp. 125 – 134, Springer, New York.
12 of 13
E03003
LOEFFLER ET AL.: IRRADIATION OF OLIVINE BY 4 KEV HE+
Wu, C., et al. (2001), Near-unity below-band-gap absorption by microstructured silicon, Appl. Phys. Lett., 78, 1850 – 1853, doi:10.1063/1.1358846.
Yamada, M., et al. (1999), Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation
on olivine and pyroxene samples, Earth Planets Space, 51, 1255 – 1265.
E03003
Yin, L. I., et al. (1971), Core binding energy difference between bridging
and nonbridging oxygen atoms in a silicate chain, Science, 173, 633 –
635, doi:10.1126/science.173.3997.633.
R. A. Baragiola, C. A. Dukes, and M. J. Loeffler, Laboratory for Atomic
and Surface Physics, University of Virginia, 351 McCormick Road, P. O.
Box 400238, Charlottesville, VA 22904, USA. ([email protected])
13 of 13