Icarus 196 (2008) 285–292
www.elsevier.com/locate/icarus
Laboratory simulations of redeposition of impact ejecta on mineral surfaces
M.J. Loeffler a,∗ , R.A. Baragiola a , M. Murayama b
a University of Virginia, Laboratory for Atomic and Surface Physics (LASP), Thornton Hall B113, Charlottesville, VA 22903, USA
b University of Virginia, Department of Materials Science & Engineering, Charlottesville, VA 22904, USA
Received 30 July 2007; revised 20 February 2008
Available online 21 March 2008
Abstract
We present quantitative laboratory studies that simulate the effect of redeposition of impact-ejecta on mineral surfaces. We produced deposits
of natural olivine (Fo90) and forsterite on olivine and forsterite powder samples by ns-pulsed laser ablation. The deposits produce changes in
the optical reflectance (0.66–2.5 µm). We show that significant darkening and reddening of the surface occurs when the deposit is olivine but
not if it is forsterite. This is attributed to the formation of metallic iron nanoparticles in the olivine deposits. We also characterized structural
and chemical changes using scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). In
situ XPS measurements show that the olivine deposits are reduced, with 50% of the iron becoming metallic. Transmission electron microscope
studies confirm the presence of 2–3 nm crystalline iron nanoparticles in the olivine deposits. The scanning electron microscope shows that both
olivine and forsterite deposits smoothen the topography of the powder surface, which could have effects on processes such as exosphere–surface
interactions and sputtering. We conclude that the effect of coatings produced by micrometeorite impacts will not be uniform on airless bodies but
will depend on the composition of the terrain.
© 2008 Elsevier Inc. All rights reserved.
Keywords: Near-Earth objects; Impact processes; Asteroids; Experimental techniques; Spectroscopy
1. Introduction
Space weathering is the alteration of the surface properties
of airless bodies due to bombardment by cosmic rays, ions from
the solar wind, magnetosphere ions, and fast interplanetary
dust (micrometeorites) (e.g., Hapke, 2001; Clark et al., 2002;
Chapman, 2004). The magnitude of the effect and the relative
flux of the impactors depend on the location of the surface
in the Solar System. Radiation and dust impacts can change
the surface chemical properties by destroying or synthesizing
molecules or by changing the composition by preferential sputtering of species. Micrometeorites may, in addition, physically
alter the grain size distribution of the surface through impacts
that melt and reform surface particles, break them into smaller
pieces, or bring larger subsurface particles to the surface, mixing the soil. Most laboratory simulations of space weathering
* Corresponding author.
E-mail address: [email protected] (M.J. Loeffler).
0019-1035/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2008.02.021
phenomena on the Moon and asteroids, have focused on simulating what is believed to be the two most important types of
weathering agents: solar wind radiation (mostly 1 keV H+ and
4 keV He+ ions) and micrometeorites.
Several low-energy ion irradiation experiments on minerals have been performed to simulate the solar wind impact
on surfaces (Nash, 1967; Hapke, 1973; Dukes et al., 1999).
Hapke first pointed out that different effects should occur for
powder-like regoliths than for compact rock surfaces. H+ irradiation of loose powders, analogous to Moon rocks, produces
darkening of the irradiated surface (Hapke, 1973), which was
attributed to the formation of metallic iron in the deposit sputtered onto nearby grains. On the other hand, Dukes et al. (1999)
showed that ion irradiation can also reduce the surface of compact olivine leading to significant amounts of metallic iron near
the surface. Higher energy ions (400 keV Ar2+ ), characteristic
of more violent stellar environments, were found to darken and
redden mineral samples as seen by optical reflectance between
0.25 and 2.5 µm (Brunetto and Strazzulla, 2005).
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Fig. 1. Top: schematic of chamber used in the ablation experiments. Bottom: photograph of target holder in ablation chamber. The targets are located inside the
aluminum rings.
Laser irradiation experiments have been used to simulate
impact vaporization processes (Hapke et al., 1975), impact
melting and recrystallization (Moroz et al., 1996), and micrometeorite impacts (Yamada et al., 1999; Sasaki et al., 2001;
Bentley et al., 2005; Brunetto et al., 2006; Loeffler et al., 2008).
Hapke’s results showed that laser ablation of Moon rocks created a deposit on a SiO2 substrate that appeared brownish,
which was speculated to be from the formation of sub-micron
metallic grains. Looking at the surface of laser ablated silicate powders, Sasaki et al. (2001, 2003) showed darkening and
reddening by reflectance spectra in the 0.5–2.5 µm range. The
authors used the transmission electron microscope (TEM) and
electron spin resonance (ESR) measurements to show that, in
addition to reflectance changes, the laser irradiated surface of
olivine and enstatite also contained metallic iron, which they
also suggested formed from redeposition of material evaporated from the laser. Darkening and reddening in olivine due
to laser irradiation have been recently attributed to structural
changes and not to redeposition of ablated material (Brunetto et
al., 2006).
To examine in more detail space weathering by deposits of
ejecta, we have laser ablated olivine and forsterite onto powders
of olivine and forsterite. We studied the deposits with infrared
spectroscopy, X-ray photoelectron spectroscopy, and electron
microscopy. The experiments enable us to separate the effects
due to the deposits from those of direct impact.
2. Experimental setup
Laser ablation experiments were performed in an ionpumped ultra-high vacuum (UHV) chamber with a base pressure of 5–10 × 10−9 Torr. A mass spectrometer shows that, at
these pressures, the background gas in the system is composed
of H2 , H2 O, CO, and CO2 with masses greater than 44 amu (indicative of hydrocarbons) being 400–3000 times less abundant
(<0.01% total concentration). In this study, we refer to the surface ablated by the laser as the target, and the surface catching
the deposit as the sample (Fig. 1).
The targets were irradiated with 10-ns pulses of 193 nm light
from a GAM ArF excimer laser at a repetition rate chosen to be
Redeposition of non-icy impact ejecta
1 Hz. The laser beam enters the chamber horizontally through
a fused silica window (measured to have 74% transmittance at
193 nm) and was imaged on the target mounted on a vertical
plane. The laser beam was focused to an area of 0.25 mm2
striking the target at 56◦ from the normal, yielding a fluence
per pulse of 1.0 ± 0.3 J/cm2 . A moving mirror, synchronized
with the 1 Hz repetition rate, rastered the laser uniformly across
the sample (8 × 8 mm) so the same spot was not hit successively. The ejecta produced from the target were collected on a
catcher plate (our sample) 1.3 cm away from the target. Due to
the geometry of the experiment, the reflectance of the sample
could not be analyzed in situ, and thus at successive fluence intervals, the chamber was opened and the reflectance spectrum
of the sample was measured using a Thermo-Nicolet Nexus
670 infrared spectrometer, where the probe light was focused
to a diameter of <2 mm. To measure the reflectance, we used a
Pike Easy Diff reflectance attachment where the light is incident
at 35◦ and an integrating sphere catches the reflected light. The
intensity reflected from the sample was divided by the intensity
reflected from a Teflon powder (<45 µm) standard to obtain a
reflectance spectrum. We will also show spectra in optical depth
units, defined as − ln(reflectance). Unless otherwise noted, the
resolution of the spectrometer was 32 cm−1 , and each spectrum
was an average of 400 scans, which took approximately 5 min.
After measuring the optical reflectance we placed the sample
back in the vacuum chamber, which was pumped down to obtain the next data point.
The minerals used in these experiments were natural San
Carlos olivine (Fo90 or Mg1.8 Fe0.2 SiO4 ) and iron-free synthetic forsterite (Fo100 or Mg2 SiO4 ). The natural olivine was
purchased from Penn Minerals in a lump about 6 inches in diameter. A large number of iron-rich inclusions were mixed with
the light green pieces of olivine, and thus we cleaved the lump
to remove the inclusions. To obtain a silicate similar to olivine
with no iron in it, we purchased synthetic forsterite granules
(+30 mesh by 1/2 ) from Reade Advanced Materials. The purity of this sample was given as 99.4 wt% Mg2 SiO4 , and the
impurities were listed as Al2 O3 (0.03%), Fe2 O3 (0.02%), TiO2
(0.09%), Na2 O (0.11%), CaO (0.22%), and K2 O (0.12%).
To prepare our targets, the minerals were first cleaved and
then crushed with agate mortar and pestle. Next, the particles
were sieved to obtain the desired grain size and then pressed
with 3000 psi into pellets inside a 9-mm-dia. aluminum ring
placed in between two stainless steel disks. Since we were interested in studying the effect of the evaporated coatings, we chose
the grain size of the target being ablated to be fairly large: 500–
600 µm. This is because we found that small grains could be
ejected directly and form a crystalline deposit, as was evidenced
by the appearance of crystalline silicate absorption bands in the
infrared spectrum of the deposit. This effect was more marked
for the smaller grain sizes and disappeared if we increased grain
size to 500 µm. The sample that caught ejected material was either an optically flat Si wafer or a pressed mineral (olivine or
forsterite) powder (<45 µm), prepared in the same way as the
targets.
The cleanliness of the ion pumped, ultrahigh vacuum target system ensured that any darkening of our samples was
287
Fig. 2. Infrared absorbance in the Si–O stretching region for a 460 nm deposit
of forsterite (F) and a 600 nm deposit of olivine (O) ablated onto a Si wafer. For
comparison of band shape, we have also included a spectrum of polycrystalline
olivine (cr-O) that we obtained by ablating loosely packed powder onto a gold
substrate.
not induced by residual hydrocarbons; this was confirmed by
X-ray Photoelectron Spectroscopy (XPS) (Dukes et al., 1999;
Loeffler et al., 2008). The electron energy analyzer was operated in fixed analyzer transmission mode at a pass energy of
200 eV for survey spectra and 40 eV for high resolution spectra, which gave an instrumental resolution of 3.2 eV and 0.6 eV,
respectively.
In addition to reflectance and chemical analysis of the deposit, we also performed structural measurements using a JEOL
6700 Scanning Electron Microscope (SEM) and a 200 kV
JEOL JEM-2010F Transmission Electron Microscope (TEM)
equipped with an Oxford LINK energy dispersive spectrometer
(EDS) and a Gatan GIF200 Electron Energy Loss Spectrometer.
To prepare samples for TEM analysis, we scraped the surface
of the deposit onto lacy-carbon copper TEM grid.
3. Results and discussion
3.1. Calibration of the deposit thickness
We laser ablated olivine and forsterite targets, depositing
the evaporated material onto an optically flat Si wafer sample, using an accumulated fluence of 134 J/cm2 for olivine and
150 J/cm2 for forsterite (1 J/cm2 per pulse). We characterize
these deposits by transmission infrared spectroscopy in the Si–
O stretch region. The results, shown in Fig. 2, are compared
with a spectrum of polycrystalline olivine dust, measured in reflectance. The shape and position (10.60 ± 0.03 µm) of the band
shows that our deposit is amorphous, based on previous studies
(Scott and Duley, 1996; Brucato et al., 2004). Using the optical
constants of amorphous forsterite (Scott and Duley, 1996), we
derive a band strength,
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Fig. 3. NIR spectrum of unirradiated Fo90 olivine (O) and Fo100 forsterite (F)
powder (<45 µm) samples normalized to that of a Teflon powder (<45 µm).
4πkν
dν = 1.3 × 10−16 cm/molecule,
N
where k is the extinction, ν is frequency, and N is the number
density. With this number, we calculate that the thickness of
the deposits in Fig. 2 is 600 nm for olivine and 460 nm for
forsterite. These values, normalized to the laser fluence, were
used as calibration for the other experiments.
A=
3.2. Fluence dependence of NIR reflectance of a <45 µm
powder
After calibrating the laser ablation deposition rate, we replaced the silicon with pressed powder (<45 µm) of olivine or
iron-free forsterite. In Fig. 3, we show the initial NIR spectra
of each pressed powder sample referenced to Teflon. We note
that the forsterite appears brighter than the olivine sample and
lacks the Fe–O absorption band centered near 1.0 µm, which is
present in olivine, as expected.
Fig. 4 (top) shows the reflectance of an olivine powder as a
function of increasing olivine deposit thickness. We note that
the sample reddens even at the low thickness points and also
darkens with fluence. In addition, we note that the olivine darkens greatly in the visible as well, which can be seen by the
photographs in the inset. To see this effect on a non-absorbing
material, we deposited olivine onto forsterite (Fig. 4, bottom).
We note that as compared to the olivine in Fig. 4 (top) the
sample darkens more at lower wavelengths and less at longer
wavelengths, showing that the degree of reddening and darkening in the NIR is affected by both the deposit and the sample. We note that the spectral shape of this deposit is similar to that reported by Hapke when he evaporated material
analogous to Moon rocks onto a SiO2 window (Hapke, 1973;
Hapke et al., 1975), and that the spectral shape of the deposit
is smoother here than when the deposit is put on top of olivine
(Fig. 5).
Fig. 4. Top: NIR spectrum of San Carlos olivine powder (<45 µm) vs deposit
thickness of ablated olivine. From top to bottom at 700 nm the thickness of
deposit is: 0, 175, 351, 527, 700, 965, 1338, 1689, and 2040 nm. Inset: optical
image of the olivine powder sample before irradiation (left) and after receiving
a 2 µm deposit on the surface of the olivine (right). Bottom: NIR spectrum of
forsterite powder (<45 µm) vs deposit thickness of ablated olivine. From top to
bottom at 700 nm the thickness of the olivine deposit is: 0, 153, 306, 459, 613,
826, 1139, 1452, and 1752 nm. Inset: optical image of forsterite before (left)
and after receiving a 1.75 µm deposit of olivine on the surface (right).
Fig. 5. Ratio of an olivine powder’s (<45 µm) NIR after receiving the deposit to
that of the olivine powder before receiving any deposit. The ratios are derived
from Fig. 4 (top). From top to bottom at 700 nm the thickness of deposit is:
175, 351, 527, 700, 965, 1338, 1689, and 2040 nm.
Redeposition of non-icy impact ejecta
289
Fig. 7. Reddening of each powdered sample as a function of deposit thickness,
where forsterite is denoted F and olivine is denoted O.
3.3. X-ray photoelectron spectroscopy
Fig. 6. Top: NIR spectrum of forsterite powder (<45 µm) for selected deposit
thicknesses of ablated forsterite. From bottom to top at 2 µm the thickness of the
deposit is: 0, 693, and 1929 nm. Inset: optical image of forsterite before (left)
and after receiving a 1.9 µm deposit of forsterite on the surface (right). Bottom:
NIR spectrum of olivine powder (<45 µm) for selected deposit thicknesses of
ablated forsterite. From bottom to top at 2 µm the thickness of the deposit is:
817, 1886, and 0 nm. Inset: optical image of the olivine before (left) and after
receiving a 1.88 µm deposit of forsterite on the surface (right). We note that the
white spot on the top of the right picture allows one to see clearly the contrast
between the pure olivine (white spot) and surrounding olivine with forsterite
deposited on top.
In Fig. 6 (top), we plot the reflectance of the forsterite powder as a function of increasing forsterite deposit thickness. One
can see that the initial spectrum is, within error, flat between
0.66–2.5 µm, and that as we deposit forsterite there is little
darkening or reddening in this region. In the inset we show the
forsterite before and after depositing a 1.9 µm film of forsterite
on top, when it appears to be slightly yellow. This color may
result from partial oxygen deficiency in the deposit, since the
pale yellow color in Si–O is a result of a strong UV absorption
bands of Si–Si bonds (Philipp, 1971). Fig. 6 (bottom) shows the
reflectance of olivine as a function of increasing the forsterite
deposit thickness. The results are similar to the forsterite ablated onto forsterite data: there is little change in the reflectance
over this region and the sample appears to have a pale yellow
coloration change at high thickness.
In Fig. 7 we plot the reddening, defined here as the linear
slope (R2 µm − R0.7 µm )/(2 µm − 0.7 µm), versus deposit thickness, where Rλ denotes reflectance at wavelength λ. We note
that reddening occurs at a faster rate when we deposit olivine
onto forsterite and very little reddening occurs when forsterite
is deposited onto forsterite or olivine.
The changes in reflectance suggest a compositional/chemical
change related to the iron, which is the main difference between
olivine and forsterite. To verify the state of iron, we measured
the XPS spectra of the deposits in situ, since we have previously
determined that exposure to air would oxidize any reduced
species. Thus we made olivine deposits inside the XPS analysis chamber (Loeffler et al., 2008). Fig. 8 shows high-resolution
spectra of uncoated olivine and olivine coated with a laser ablated olivine deposit for the main photoelectron peaks: Fe 2p,
Mg 2p, Si 2p, O 1s, and C 1s. The deposits have less carbon
than the powder samples. This is expected since the deposit is
formed in an ultra clean vacuum environment, while most of
the carbon on the powder surface originates from atmospheric
contaminants. This result in itself shows that the visible/NIR
changes of the sample with the deposit are not due to a buildup
of decomposed hydrocarbons. Besides this, the composition of
the olivine sample and the deposit are similar, except for iron,
where XPS shows that ∼50% iron in the deposit is in the metallic form instead of an oxide. This iron reduction is observed
already for the thinnest deposit in Fig. 4, that is, the percentage
of metallic iron in the first few monolayers does not increase
with further deposition. From this saturation of iron reduction
and the stoichiometry of San Carlos olivine, we infer that there
are ∼1.3 × 1014 Fe metal atoms/cm2 for every 1 nm deposited
onto the surface.
3.4. Electron microscopy
To characterize whether the deposit changes the surface
structure, we examined the powder samples before and after
laser deposition. Fig. 9 shows an SEM micrograph of synthetic
forsterite before and after depositing 1.9 µm of forsterite. It
is interesting to note that even in this sample where there is
no apparent weathering effect in near infrared reflectance, we
can observe surface alteration due to the deposit. The deposit
smoothens the surface by covering up small grains and blending
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Fig. 8. High-resolution XPS spectra in regions of interest for uncoated olivine (—) pressed powder (<45 µm) and a deposit of olivine from laser ablation.
separated grains together. This smoothing appears to decrease
the surface area and roughness of the sample surface. Furthermore, the gaps between smaller grains are reduced, indicating
that the macroscopic surface porosity has decreased.
For the more interesting olivine deposits, we attempted to
observe the metallic iron particles but they were beyond the resolving power of the SEM. We therefore scraped material from
the surface of the olivine deposit onto a TEM grid. Fig. 10
shows a high resolution TEM image of three nanoparticles
clearly visible in the deposit. We note that there appear to be
more nanoparticles in other regions of this photograph, but
since the grain is thicker as we move towards its center, the
particles are most evident on the edge of the grain. A magnified
image of one particle on the edge of the grain (Fig. 10, bottom
left) shows that the particles are ∼2–3 nm in size and X-ray
microanalysis show that they are iron. Selected area electron
diffraction (SAED) of the deposit shows a typical halo pattern,
indicating that the matrix is amorphous, in agreement with the
infrared measurements. On the other hand, the spotty SAED
pattern of the iron particle and corresponding parallel fringes
demonstrate that it is crystalline. The spacing of the parallel
fringes is ∼0.2 nm, which is in reasonable agreement with the
d-spacing of {110}Fe . The iron nanoparticles are smaller than
those reported by Sasaki et al. (2001), who were observing the
area impacted by the laser rather than the deposits. We attribute
this difference in particle size to aggregation of iron particles
on the target by repeated laser pulses.
4. Conclusions
Surfaces of interplanetary objects, such as asteroids, are constantly bombarded by the solar wind and occasionally are hit
with micrometeorite impacts. Ejecta are redeposited onto the
regoliths and they form reduced layers that have been identified by Keller and McKay (1997) on lunar samples. We find
that the effect of these deposits on the surface depends on
the nature of the surface and the deposit. We observe no reflectance changes for forsterite deposited onto forsterite or
olivine, whereas olivine deposits produce reddening and darkening due to the production of nanoparticles of metallic iron.
Therefore the effect of micrometeorite impacts rather than being uniform will be dependent on the composition of the terrain.
On the other hand, all deposits smoothen the surfaces by
covering up small grains, blending them together, and decreasing the void space in between. These structural effects could
affect processes that depend on surface structure. For instance,
a smoother surface would adsorb less gas from the exosphere
and would have a higher sputtering yield due to a reduced probability of redeposition of sputtered atoms.
Acknowledgments
This work was supported through the NASA Discovery Data
Analysis and Cosmochemistry Programs. We thank J.M. Fitz-
Redeposition of non-icy impact ejecta
291
Fig. 9. SEM micrographs of forsterite as prepared (top) and after depositing
1.9 µm of forsterite by laser ablation (bottom).
Gerald for assistance with the initial tests in his laser ablation
laboratory.
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