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Icarus 195 (2008) 622–629
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Laboratory simulations of sulfur depletion at Eros
M.J. Loeffler a , C.A. Dukes a , W.Y. Chang a , L.A. McFadden b , R.A. Baragiola a,∗
a University of Virginia, Laboratory for Atomic and Surface Physics, Charlottesville, VA 22904, USA
b University of Maryland, Department of Astronomy, College Park, MD 20742, USA
Received 15 August 2007; revised 5 December 2007
Available online 29 February 2008
Abstract
The X-ray spectrometer of the Near-Earth Asteroid Rendezvous (NEAR) mission discovered a low abundance of sulfur on the surface of
asteroid Eros, which is seemingly inconsistent with the match of the overall surface composition to that of ordinary chondrites. Since troilite, FeS,
is the primary sulfur-bearing mineral in ordinary chondrites, we investigated the hypothesis that sulfur loss from surface FeS could result from
‘space weathering’ by impact of solar wind ions and micrometeorites. We performed laboratory studies on the chemical alteration of FeS by 4
keV ions simulating exposure to the solar wind and by nanosecond laser pulses simulating pulsed heating by micrometeorite impact. We found
that the combination of laser irradiation followed by ion impact lowers the S:Fe atomic ratio on the surface by a factor of up to 2.5, which is
consistent with the value of at least 1.5 deduced from the NEAR measurements. Thus, our results support the hypothesis that the low abundance
of sulfur at the surface of Eros is caused by space weathering.
© 2008 Elsevier Inc. All rights reserved.
Keywords: Near-Earth objects; Impact processes; Asteroids; Experimental techniques; Asteroid Eros
1. Introduction
The Near-Earth Asteroid Rendezvous (NEAR) mission’s
year-long orbital study of Asteroid 433 Eros revealed fascinating new information about the asteroid (Veverka et al., 2000;
Robinson et al., 2001; Thomas et al., 2001; Veverka et al.,
2001a, 2001b). Among the new findings are the presence of
a complex regolith with very low color contrast, a population
of craters depleted in small sizes, an abundance of boulders
and flat regions (“ponds”) without structure down to the 1.2 cm
resolution level and, when compared to the reflectance of the
constituent minerals assumed to be on the surface, Eros appears
darker, redder, and with attenuated mafic absorption bands in
the near infrared. The latter characteristics are typical effects
of exposure of minerals to “space weathering” by solar radiation and micrometeorite impact (Clark et al., 2001, 2002). The
NEAR data taken by the visible-near infrared spectrometer and
the X-ray/Gamma-ray Spectrometer (X/GRS) suggest that the
* Corresponding author.
E-mail address: [email protected] (R.A. Baragiola).
0019-1035/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2008.02.002
surface of Eros best matches an ordinary chondrite (McCoy et
al., 2001; McFadden et al., 2001), consistent with Earth-based,
near-infrared observations (Murchie and Pieters, 1996; Kelley
et al., 2001). However, the NEAR XRS data show a striking
mismatch between Eros and ordinary chondrites: the surface
appears to be strongly depleted in sulfur (McCoy et al., 2001;
Nittler et al., 2001; Trombka et al., 2001). This is evidenced by
an upper limit of 0.05 for the S:Si weight ratio, measured on the
surface of Eros, in contrast to ratios of 0.075–0.165 in ordinary
chondrites (Nittler et al., 2001).
The short attenuation length of the X-rays detected by XRS
implies that the elemental composition is obtained only from
a shallow depth. Values of ∼3.4, ∼5.5 and 95 µm for Si, S,
and Fe, respectively, are estimated from tabulated cross sections (Henke et al., 1993), assuming the asteroid regolith has
a composition similar to olivine but with a lower density of
1.7 g/cm3 . Since the range of grain size on Eros is 50–100 µm
(Li et al., 2004), the shallow XRS sampling depths mean that
all of the information obtained from XRS is likely to originate
from the top layer of grains on the surface. However, regolith
turnover by meteorite impact implies that sulfur depletion probably extends to grains located much deeper in the regolith.
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Sulfur depletion at Eros
623
Fig. 1. Experimental setup.
Sulfur in chondrites is primarily bound in the form of
troilite (FeS). Due to a relatively high volatility, sulfur can be
preferentially removed from troilite by the two primary space
weathering mechanisms considered active on the Moon and
asteroids: sputtering by solar wind ions and impact vaporization by micrometeorites (Chapman, 2004). Sulfur depletion by
sputtering (Trombka et al., 2000; Killen, 2003; Kracher and
Sears, 2005) has been considered previously using a first order description of the phenomenon where the sputtering rate is
assumed to be constant. As we will show later, ion irradiation
indeed sputters sulfur preferentially from FeS, but its removal
rate decreases with time as sulfur is removed. The alternative
mechanism of sulfur depletion during impact vaporization by
micrometeorites has also been considered (Burbine et al., 2001;
Killen, 2003; Kracher and Sears, 2005), with widely different
estimates of the degree of chemical alteration.
Here we address experimentally whether the extent of sulfur depletion on Eros revealed by XRS can be explained by
ion irradiation and/or micrometeorite impact. We irradiated
troilite (FeS) with 4 keV He+ ions typical of the solar wind
and nanosecond laser pulses that mimic the fast-pulsed heating that accompanies micrometeorite impact. During irradiation, we monitored the atomic S:Fe ratio with X-ray photoelectron spectroscopy and ion scattering spectroscopy and, after the
experiment, examined the irradiated surface with electron microscopy.
the powder with ∼3000 psi into a disk inside a 10-mm aluminum ring placed in between two stainless steel disks, similar
to the technique used by Sasaki et al. (2001). Since leaving the
sample in air for a day or more produces detectable amounts of
sulfur oxides on the surface, for every experiment we prepared
a fresh sample using the method above and placed it in vacuum
within ∼15 min after crushing the powder.
The pressed samples were mounted on a copper holder and
transferred into an ion-pumped ultra-high vacuum chamber
with a base pressure of 5–10 × 10−9 Torr (Fig. 1). For surface
analysis, we used the sample manipulator to position the sample at the focus of the electron energy analyzer and to orient
it so that it could be irradiated with ions and illuminated with
X-rays at the same time. For laser irradiation we positioned the
sample normal to the incident laser beam. All irradiations and
chemical analysis were performed without removing the samples from vacuum.
Surface analysis of the troilite was done before, during
and after irradiation using two techniques: X-ray photoelectron
spectroscopy (XPS) and Ion Scattering Spectroscopy (ISS).
XPS is a widely used surface analysis method, which gives both
elemental abundance and chemical information. In this technique, irradiation of the surface with X-rays (in our case Al–K
X-rays of energy EX-ray = 1486.6 eV) ejects electrons into vacuum with a kinetic energy (Ek ) given by
2. Experimental procedures
where EB is the electron binding energy with respect to the
Fermi level, and φ is the work function of the spectrometer.
The energy distribution (spectra) of photoelectrons is measured
with double pass cylindrical-mirror electron energy spectrometer (Dukes et al., 1999) and is given as number of counts vs
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 2–3 nm for the Fe-2p and S-2p electrons in
2.1. Sample preparation and analysis
Synthetic iron sulfide (FeS) rods were obtained from the
Alfa Aesar company. To prepare powder samples, the rods were
cleaved, crushed with an agate mortar and pestle, and sieved to
obtain grain sizes between 45–125 µm, the range of the reported
average particle size on Eros (Li et al., 2004). Next, we pressed
Ek = EX-ray − EB − φ,
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troilite (Seah and Dench, 1979). The relative area of the photoelectron peaks is used together with sensitivity factors (see
below) 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).
For these XPS measurements, the spectrometer was operated
in fixed transmission mode at a 200 eV pass energy for survey
spectra and 40 eV for high resolution spectra, providing energy
resolutions of 3.2 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 ∼30 min.
To obtain absolute ratios of S:Fe in our XPS measurements,
we measured the relative sensitivity factors for pure iron and
sulfur, using an iron sheet and sulfur powder pressed into a flat
pellet. To prevent sulfur from outgassing in the chamber, we
cooled the sample holder down to 200 K before introducing
the sulfur. The surface of both samples was cleaned by sputtering with 4 keV Ar+ . To obtain the relative sensitivity factors for
iron and sulfur, we maintained the same incident X-ray intensity
on both samples and determined the areas of the Fe-2p (∼700–
740 eV) and the S-2p (150–175 eV) photoelectron peaks. Then
we obtained the sensitivity factors relative to O-1s (a standard
practice) by measuring the intensity of the O-1s photoelectrons
from a condensed ice film under the same conditions. The sensitivity factors relative to O-1s were Fe-2p = 2.54 and S-2p =
0.29, within 10%.
In addition to XPS, the composition of the topmost layer
of the sample was studied with Ion Scattering Spectroscopy
(Brongersma et al., 2007). In this technique, low energy ions
(∼1 keV) are incident on the surface, and the spectrometer is
used to measure the energy spectra of ions backscattered at a
specific angle. In this low energy regime, the backscattered ions
result from elastic binary collisions with atoms in the outermost
surface layer, and thus their energy is given by energy and momentum conservation:
2
Eion = E0 cos θ ± A2 − sin2 θ /(1 + A)2 ,
where θ is the backscattering angle, E0 is the incident ion energy, A = m2 /m1 , and where m1 , m2 are the masses of the
projectile and target atoms, respectively. The scattered ions are
detected with our CMA by switching the polarity of the spectrometer with respect to that used to analyze electrons. The
backscattering angle of interest, θ is selected by a slit in a rotating drum (Fig. 1). In our experiments, we used 1 keV He+ and
θ = 150◦ . The ISS energy spectra were taken in <5 min using
the same parameters as in the survey spectra for XPS. The He+
flux was low enough that, during this time, it did not produce
detectable compositional changes in the surface, as determined
by ISS and by XPS. In this work we used ISS mainly to identify
the elements that are in the outermost surface layer.
In addition to surface analysis, during irradiation we also
monitored the molecules ejected from the surface with a
quadrupole mass spectrometer aimed at the sample. After completing all irradiation and analyses, we removed the sample and
imaged its surface ex situ using a JEOL 6700 field-emission
scanning electron microscope.
2.2. Ion irradiation
A low energy (0–5 kV) ion gun was used to irradiate the
sample surface. It is differentially pumped and thus during experiments 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. Ion irradiation was done
with 4 keV He+ and Ar+ rastered uniformly over the sample surface analyzed by XPS. The average ion incidence angle
was ∼45 degrees to the average sample surface, and varied locally at each grain. We measured the ion current density before
(∼4 × 1013 ions/cm/s) and after each experiment with a Faraday cup and found it to be stable within ∼5%.
The use of He+ ions should be adequate to simulate He2+
solar wind ions. It has been argued (Kracher and Sears, 2005)
that a higher charge state can enhance sputtering, based on
measurements on highly charged heavy ions that carry keV potential energy. However, there is neither experimental evidence
nor a theoretical justification for a charge state dependence of
sputtering yields for ions of such low charge state.
2.3. Laser irradiation
The traditional method to simulate micrometeorite or hypervelocity dust impacts in the laboratory is to accelerate silicate
grains, which mimic interplanetary dust, with an electrostatic
accelerator and collide them with the desired surface. Another
technique to simulate micrometeorite impacts, more suitable for
uniform irradiation over macroscopic areas, is the use of highenergy pulses from a nanosecond laser (Yamada et al., 1999;
Sasaki et al., 2001; Brunetto et al., 2006). This method has
been successful in producing the rapid melting and vaporization that is thought to occur in micrometeorite impacts (Sasaki
et al., 2001). Furthermore, it has been shown that ns-pulsed
laser irradiation alters reflectance spectra of silicates and meteorites in a manner that is consistent with the hypothesis of
space weathering (Sasaki et al., 2001). The justification of using ns-pulsed lasers to mimic dust impacts has been made
by comparing thermoionic plasma emitted from samples that
were impacted by accelerated dust and laser beams (Kissel
and Krueger, 1987), normalized to the deposited power density
(W/cm2 ). These authors found that, above the laser ablation
threshold, similar ion mass spectra and absolute yields are obtained when the power density of the laser beam is ∼100 times
lower than that for dust impact, as a result of the different efficiency of conversion of impact energy to temperature (Sugita
et al., 2003). With this in mind, we estimate that the UV laser
used in our experiments (∼108 W/cm2 ) is roughly equivalent
to a 1–20 µm particle (2 g/cm3 ) traveling ∼10–15 km/s (appropriate for Eros; Killen, 2003). On impact, this is equivalent
to a power density of 1010 W/cm2 assuming the energy is
deposited over an area given by the cross section of the dust
particle, and a depth of 3 particle diameters. We note that even
micrometeorite impacts as slow as 0.05 km/s can produce a
thermionic plasma and, hence, vaporization (Smith and Adams,
1973).
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Fig. 3. ISS spectra of FeS: (a) before, and after irradiation with 4 keV He+ to
a fluence of (b) 1.8 × 1017 ions/cm2 and (c) 1.8 × 1018 ions/cm2 .
Fig. 2. Top: survey XPS spectra of FeS. Bottom: high-resolution XPS spectra
of S-2p and Fe-2p photoelectrons in S, Fe, and FeS samples. (a) FeS before,
(b) FeS after 70 laser pulses, (c) FeS after 2.3 × 1018 He+ /cm2 , (d) FeS after
2 × 1018 Ar+ /cm2 , (e) FeS after 100 laser pulses and 1.1 × 1018 Ar+ /cm2 ,
and (f) metallic iron (left) and sulfur (right).
Laser irradiations were made with a GAM excimer laser
that emits 10 ns pulses at 193 nm (ArF) at 1 Hz. The laser
beam entered the analysis chamber through a quartz-fused silica window and was imaged to a 0.5 mm2 spot on the sample;
each pulse had an energy fluence of 0.7 J/cm2 . The beam was
rastered uniformly across the sample with mirrors actuated by
a motion control system synchronized to the laser.
3. Results
The comparison of XPS spectra of troilite before (virgin)
and after irradiation with 4 keV He+ and Ar+ , as well as laser
pulses, show notable differences (Fig. 2): sulfur increases with
laser irradiation and decreases with ion impact, while C and O
impurities decrease with both types of irradiation. Analysis of
the energies of the S-2p and Fe-2p photoelectron peaks shows
that the oxygen impurity is mainly bound to iron and not to
sulfur [the peak of S bonded to oxygen at ∼169 eV is very
weak (Fig. 2, bottom)]. This is consistent with the studies of
Jones et al. (1992) and Pratt et al. (1994) on pyrrhotite (Fe1−x S,
x < 0.25).
ISS spectra, giving information only of the outermost atomic
surface layer, is shown in Fig. 3 for samples before and after irradiation with 4 keV He+ . Iron and sulfur, which has not
been found as an impurity in other studies in our laboratory,
are present in the initial spectra indicating that the common air-
Fig. 4. (a) XPS and ISS data for virgin FeS vs fluence of 4 keV He+ , (b) XPS
data for virgin FeS and for laser irradiated FeS (total fluence 70 J/cm2 ), as
a function of fluence of 4 keV Ar+ . A fluence of He of 17 × 1017 /cm2 corresponds to an exposure of 4 × 104 years to the solar wind at Eros. The bars
represent the uncertainties in the data analysis.
borne surface contaminants (C and O impurities) do not cover
the entire surface.
Fig. 4a gives the S:Fe atomic ratio vs irradiation fluence from
the XPS and ISS measurements, where the ISS data have been
scaled so that the ratio at zero fluence matches the XPS data.
The similarity of the two sets of fluence dependencies, in spite
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Fig. 5. Comparison between unirradiated FeS (left) and laser irradiated (total fluence 70 J/cm2 ) FeS powder (right) at different magnifications [bar size is 10 µm
(top) and 200 nm (bottom)].
of the different depth information in ISS and XPS, indicates that
the alteration by ion impact is relatively uniform for depths less
than ∼3 nm. The same conclusion results from the constancy
of the ratio of Fe-2p (∼700 eV) and Fe-3p (55 eV) peak intensities, which, because of their different energies, also sample
different depths: 2.1 nm vs 3.6 nm (Seah and Dench, 1979).
Fig. 4a shows that the S:Fe ratio increases at low He+ fluences as the initial surface contaminants are removed, suggesting that those contaminants are not uniformly distributed but are
preferentially bound to the sulfur. We note that in addition to the
XPS and ISS measurements, we also observed that the flux of
sputtered sulfur, measured by the mass spectrometer, initially
increased at low ion fluences and subsequently decreased. The
initial increase in the S:Fe ratio was not seen in the Ar sputtering experiments, because at the lowest irradiation fluence
shown in Fig. 4b, Ar already removed the contaminants due to
its ∼15 fold larger sputtering yield. In both cases, the final S:Fe
ratio, obtained by extrapolating the fit of a single-exponential
decay to high fluences, is 0.55 ± 0.05.
We used TRIM to calculate sputtering yields (Ziegler and
Biersack, 2006) to convert the fluence dependence into a depth
scale: 7 (100) nm removed by 1017 He (Ar)/cm2 . However,
Fig. 4 does not represent a depth distribution of the S:Fe ratio
for the unirradiated (or laser irradiated) solid. This is because
the instantaneous surface composition is affected by preferential sputtering (for 4 keV He impact, the sputtering yield of S
is ∼15% higher than that of Fe) and surface segregation. This
segregation is a result of defects and radiation enhanced diffusion over the maximum ion penetration depth (Kelly, 1989). For
our experiments at 45◦ incidence, the maximum depth is ∼80
(∼10) nm for 4 keV He (Ar) (Ziegler and Biersack, 2006).
A different behavior is observed under laser irradiation.
Fig. 4b shows the Ar+ fluence dependence of the S:Fe ratio for
FeS samples with and without irradiation by 100 laser pulses
(total 70 J/cm2 ). The initial (zero ion fluence) S:Fe ratio increases from ∼1.0 ± 0.2 to about 1.8 ± 0.2 after one laser pulse
and does not increase with further laser irradiation. Ion irradiation of laser impacted samples decreases the S:Fe ratio by a
factor of 2 after 1016 ion/cm2 , at which ∼4 nm of the surface
is removed. This dramatic drop indicates that sulfur enrichment
is limited to this shallow depth.
Further ion irradiation after laser processing lowered the
S:Fe ratio to ∼0.40 ± 0.04, indicating that even though repeated laser pulses do not alter the surface composition, they
deplete sulfur below the surface, causing lower S:Fe ratios at
high ion fluences. After irradiation we removed the samples
and examined them with the scanning electron microscope.
Fig. 5 shows micrographs of an unirradiated sample and one
that has been ablated with 100 laser pulses to a total fluence
of 70 J/cm2 . The unirradiated sample presents small grains of
FeS, which have adhered to larger grains during the sieving
process. The high-resolution images show that the surface of
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Sulfur depletion at Eros
the grains is fairly smooth. The laser irradiated sample shows
grains with smooth edges, and the absence of small particles.
High-resolution images show surface ripples typical of laser ablated materials, which indicate surface melting and rapid cooling (Foltyn, 1994).
4. Discussion
4.1. Irradiation effects
The laser experiments, that simulate pulse heating during
micrometeorite impact, show that sulfur segregates to the surface of troilite. This is consistent with the thermal surface segregation of sulfur observed in ordinary chondrites subject to
heating above 500 ◦ C (Lauretta et al., 1997); high temperatures
are needed to achieve sufficiently high rates of diffusion in iron
sulfide (Fryt et al., 1979). As shown in Fig. 4, the excess sulfur is in a thin surface layer that can be easily removed by a
subsequent impact or by sputtering.
The decrease of the S:Fe ratio at the outermost surface layer
upon ion irradiation, as revealed by ISS, demonstrates the preferential sputtering of the surface sulfur. The XPS data show
that the sulfur depletion region extends at least 3 nm deep (the
XPS sampling depth) but most likely continues through the
whole ion penetration depth (Kelly, 1989). This depletion below the layer that contributes to sputtering can be explained
by radiation driven diffusion, where defects generated by each
ion enhance the segregation of sulfur to the surface, even in
the absence of macroscopic heating. Our results for changes
of surface composition by ion irradiation are consistent with a
body of evidence on surface alteration of other types of multielement targets; for example, preferential sputtering of sulfur
is also seen in ion irradiation of FeS2 (Nikzad et al., 1992;
Chaturvedi et al., 1996). The similar equilibrium (high fluence)
S:Fe ratio we obtain for different types of ions, is consistent
with previous findings that equilibrium concentrations achieved
through ion irradiation are generally independent of the mass of
incident projectile (Liau et al., 1978).
4.2. Application to Eros
Our experiments test the effect of space weathering on the
sulfur abundance on Eros’ surface, under the assumption that
S is only in the form of troilite and that the concentration
of troilite is in the range found in ordinary chondrites. This
concentration can be inferred from the S:Si ratio, which is in
the range 0.075–0.165 in weight (Nittler et al., 2001). There
are two reports of S:Si weight ratios on Eros from XRS data:
0.014 ± 0.017 (McCoy et al., 2001) and <0.05 (Nittler et al.,
2001). This implies that the abundance of S on Eros is reduced
by a factor of at least 0.075/0.05 = 1.5 or 0.075/0.031 = 2.4,
from the two interpretations of the XRS data. Our measurements show a maximum depletion of 1.82 ± 0.17 for ion irradiation alone and 2.5 ± 0.25 for laser impact followed by
ion irradiation. However, the sulfur depletion by the solar wind
will be limited to the penetration depth of the ions, ∼0.2 µm
(Bradley, 1994), which is only a few percent of a typical grains
627
size and of the sampling depths of the NEAR XRS instrument.
Therefore, the solar wind by itself cannot be the source of the
sulfur depletion in the regolith observed by NEAR.
On the other hand, micrometeorite impact will produce transient heating that will enhance diffusion and bring sulfur from
depths greater than the ion range to the surface of the grain
where it will segregate, as seen in our laser simulations and
other heating experiments (Lauretta et al., 1997). In addition,
regolith turnover by meteorite impact brings grains to the surface where they are impacted by ions and micrometeorites; the
time for this turnover depends on depth. From lunar studies, a
layer as thick as the XRS information depth of a few microns
is turned over a few times in 104 years (Horz et al., 1991), during which impact heating produces sulfur sublimation and the
surface segregation mentioned above.
The time needed to sputter the segregated surface layer can
be estimated from the measurements of Fig. 4a. There we see
that a He+ fluence of ∼1.5 × 1018 ions/cm2 suffices to reduce
the surface concentration of S to an equilibrium level. Since
the flux of solar wind He at 1.34 AU—the current location of
Eros—is ∼4 × 106 ions/cm2 s, the depletion time by sputtering
will be about 104 years, or of the order of the turn over time for
the few micron depth sampled by the XRS on NEAR. Hence,
sulfur depletion on Eros to the upper limits of concentration
reported by XRS can occur on times scales on the order of 104 –
105 years by combined micrometeorite and solar wind impact.
These times are much smaller than the 3–50 million years estimate for the formation time of Eros regolith (Korycansky and
Asphaug, 2004).
Based on these arguments we conclude that space weathering is a plausible mechanism for sulfur depletion on Eros. For
completeness, we must consider that the XRS data does not exclude a much smaller sulfur concentration that would require
other explanations. A number of physical processes have been
proposed including partial melting and physical sorting. Partial
melting during the formation of the asteroid could have caused
the sulfur to evaporate or move from the surface into the interior (Nittler et al., 2001; McFadden et al., 2005), but it should
also have affected the relative concentration of Cr, Mn, and Ni,
which was not observed (Foley et al., 2006). The idea of physical sorting is based on the assumption that grains containing
troilite are differentiated in size or weight with the other grains
in the regolith. A variety of processes have been postulated
that could remove these grains with different physical properties from the surface (Asphaug et al., 2001; Nittler et al., 2001;
Robinson et al., 2001; Kracher and Sears, 2005). However,
there is not enough information on the regolith that would allow
evaluating the hypothesis that grains containing troilite have
different properties from other grains in the regolith.
We note that the processes described here will occur also
on other small airless bodies. In fact, sulfur depletion has been
detected on the Itokawa asteroid (T. Okada, private communication). In addition, our results may help explain the low S:Fe
ratios observed in some of the cometary dust captured by the
STARDUST mission (Zolensky et al., 2006).
Finally, while the space weathering effects will also occur
on the Moon and Mercury, it is important to note that the sig-
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nificant gravity of those bodies will cause most of the ejected
sulfur to return and coat the surface. We note that a large S enhancement has been observed on the rim of lunar grains (Keller
and McKay, 1993).
Acknowledgment
This work was funded by NASA Discovery Data Analysis
and Cosmochemistry Programs.
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