Meteoritics & Pinnetno Science 36, 1617-1637 (2001)
Available online at http //www uark eddmeteor
Space weathering on Eros:
Constraints from albedo and spectral measurements of Psyche crater
B. E. CLARKIt*, P. LUCEY2, P. HELFENSTEINI, J. F. BELL, 1111, C. PETERSONI, J. VEVERKAI,
T. McCONNOCHIE1, M. S. ROBINSON3, B. BUSSEY3, S. L. MURCHIE4, N. I. IZENBERG4
AND c . R. CHAPMAN5
'Cornell University, Center for Radiophysics and Space Research, Ithaca, New York 14853, USA
2University of Hawai'i, HIGP, 2525 Correa Road, Honolulu, Hawai'i 96822, USA
3Northwestern University, Geological Sciences, 1847 Sheridan Road, Evanston, Illinois 60208, USA
4Johns Hopkins University, John Hopkins Road, Laurel, Maryland 20723, USA
5Southwest Research, 1050 Walnut Street, Suite 426, Boulder, Colorado 80302, USA
?Present address: Ithaca College, Physics Department, Ithaca, New York 14850, USA
*Correspondence author's e-mail address: [email protected]
(Received 2001 June 28; accepted in revised form 2001 September 26)
(Part of a series ofpapers on the NEAR-Shoemaker mission to 433 Eros)
Abstract-We present combined multi-spectral imager (MSI) (0.95 pm) and near-infrared spectrometer
(NIS) (0.8-2.4pm) observations of Psyche crater on S-type asteroid 433 Eros obtained by the NearEarth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft. At 5.3 km in diameter, Psyche is one of
the largest craters on Eros which exhibit distinctive brightness patterns consistent with downslope
motion of dark regolith material overlying a substrate of brighter material. At spatial scales of620 m/
spectrum, Psyche crater wall materials exhibit albedo contrasts of 3 2 4 0 % at 0.946 pm. Associated
spectral variations occur at a much lower level of 4-8% (+2-4%). We report results of scattering
model and lunar analogy investigations into several possible causes for these albedo and spectral
trends: grain size differences, olivine, pyroxene, and troilite variations, and optical surface maturation.
We find that the albedo contrasts in Psyche crater are not consistent with a cause due solely to
variations in grain size, olivine, pyroxene or lunar-like optical maturation. A grain size change
sufficient to explain the observed albedo contrasts would result in strong color variations that are not
observed. Olivine and pyroxene variations would produce strong band-correlated variations that are
not observed. A simple lunar-like optical maturation effect would produce strong reddening that is
not observed. The contrasts and associated spectral variation trends are most consistent with a
combination of enhanced troilite (a dark spectrally neutral component simulating optical effects of
shock) and lunar-like optical maturation. These results suggest that space weathering processes may
affect the spectral properties of Eros materials, causing surface exposures to differ optically from
subsurface bedrock. However, there are significant spectral differences between Eros' proposed
analog meteorites (ordinary chondrites and/or primitive achondrites), and Eros' freshest exposures
of subsurface bright materials. After accounting for all differences in the measurement units of our
reflectance comparisons, we have found that the bright materials on Eros have reflectance values at
0.946pm consistent with meteorites, but spectral continua that are much redder than meteorites from
1.5 to 2.4 pm. Most importantly, we calculate that average Eros surface materials are 3040%
darker than meteorites.
INTRODUCTION
The near-Earth asteroid rendezvous (NEAR)-Shoemaker
observations of asteroid 433 Eros reveal a large crater, called
Psyche, centered at 25" north latitude and 88" west longitude
(Fig. 1). At 5.3 km in diameter, Psyche is the largest example of
the craters on Eros which exhibit brightness contrasts on their walls
(Murchie et af.,2002; Thomas et al., 2002). Murchie et al. (2002)
PvelLcde preprint MS#4579
show a correlation between gravitational slope on Eros and the
brightness of the surface. Steep slopes tend to have higher albedo
materials than exist on the surrounding plains. Murchie et al. (2002)
suggest that downslope movement has exposed subsurface
material which is both brighter and somewhat less red than
average Eros surface material, characteristics consistent with a
space weathering process on Eros which has not had sufficient
time to weather the bright, presumably subsurface material.
1617
0 Meteoritical Society, 2001. Printed in USA.
1618
Clark et al.
FIG. 1. Centrally located on asteroid 433 Eros, Psyche crater is -5.3 km
in diameter. This image mosaic was obtained by the NEARShoemaker multi-spectral imager (MSI) shortly after orbital insertion
of the spacecraft in February 2000.
At spatial scales of up to 180 mipixel from NEAR'S multispectral imager (MSI) data, the subsurface materials exposed
on Psyche's steep slopes have albedo contrasts with the
surroundings of 83% at 0.95 p m (Murchie et al., 2002). In
color ratio images (0.76/0.95 pm) these albedo contrasts are
associated with minor color variations (10%). In near-infrared
spectrometer (NIS) global maps, the albedo contrasts are also
detectable (Bell et al., 2002). For the purposes of this
investigation, we assume that the suggestions of Thomas et al.
(2002) are correct: the regolith of Eros has experienced
downslope motions on steep slopes. This mass wasting
mechanism has caused the dark surface materials to slide down
steep crater walls, exposing brighter subsurface materials below
(Fig. 2). The brighter substrate is probably also particulate
material. Thus, although the albedo markings have been linked
with macroscopic processes, the microscopic causes of the
contrasts (light scattering behavior) have yet to be identified.
While the spectral relationships among bright and dark
terrains on Eros, and the spectral relationship between Eros
and proposed analogs are qualitatively consistent with
operation of a space weathering process, these relationships
have not been examined quantitatively. That is the subject
of this paper.
We present a study of Psyche crater which integrates images
and spectroscopic observations. We present measurements of
the albedo contrasts and their associated spectral variations.
Using mixing models, meteorite spectra, and lunar analogies
we discuss three possible explanations of Psyche's albedo
contrasts: (1) grain size differences, (2) olivine, pyroxene, and
troilite compositional variations, and (3) maturation of the
optical surface by space weathering processes.
We will show that the spectral relationships among terrains
of different albedos on Eros are quantitatively consistent with
FIG. 2. A glimpse inside of Psyche by the MSI reveals a boulder
strewn field at the bottom of the crater and evidence of downslope
processes on the gravitationallysteep crater walls. This image was
obtained at a spatial resolution of up to 3.5 dpixel, at mission elapsed
time I40150800 s, after the spectrometer failed. The image is looking
south-southeast centered at 3" north latitude and 87" west longitude,
and is 1.4 km wide. It was obtained from a range of 36 km.
the operation of a space weathering process which must include
both the accumulation of submicroscopic iron and some dark,
spectrally neutral weathering product. The latter product is
represented in the meteorite collection in the form of black
chondrites. We will also show that the brightest terrain on
Eros, consistent with the least weathered material, is not
consistent with the presence of ordinary chondrite material.
The Psyche bright material shares a similar albedo to that of
ordinary chondrites, but is much redder and exhibits less
contrast. In other words, even the bright material appears quite
weathered and requires a precursor material which is brighter
than ordinary chondrites.
PREVIOUS WORK
There is a variety of evidence that asteroids in general
possess regoliths (of unknown depth). Asteroid regoliths are
thought to be dominated by grains of a different average size
than lunar regolith due to the fact that asteroids have different
Space weathering on Eros
gravitational fields, different agglutination rates, and probably
retain less impact ejecta (Housen and Wilkening, 1982; Dollfus
et al.,1989). Polarization studies indicate a surface grain size
range on S-type asteroids of 30-300pm (Dollfus et al., 1989).
For Eros at least, Veverka et al. (2001) show convincing
evidence that the regolith could be quite deep. In addition,
observations of 951 Gaspra and 243 Ida by the Galileo
spacecraft revealed surfaces with degraded crater morphology
and evidence of retained crater ejecta, suggesting that regolith
formation and evolution processes were at work (Sullivan et
al., 1996; Lee et al.,1996; Carr et al., 1994; Geissler et al.,
1996).
Since the Galileo results at Ida and Gaspra, it has been
suspected that a surface modification process (or processes)
occurs on asteroids which changes the brightness and color of
the surface with time. It has not been determined whether that
process is simply comminution (reduction of grain size) or
chemical alteration due to exposure of the surface to the space
environment (i.e.,space weathering). In this paper, we use the
term "space weathering" to mean micrometeorite impact, solar
wind particle implantation, and any other processes (known
and unknown) that may tend to change-over time-the
apparent traits (optical properties, physical structure, chemical
or mineralogical properties) of the immediate, remotely-sensed
surface of an airless body from analogous traits of the body's
inherent bulk material as analysed in the laboratory.
On Ida in particular, regolith processes such as crater
formation have been linked with albedo and color variations
by Sullivan et al. (1996). These color variations were attributed
to the exposure (by impact cratering) of fresh subsurface
materials which contrasted with the surrounding surface
(Chapman, 1996). The nature of color contrast exposures in
and around craters on Gaspra and Ida argues that the albedo
and color contrasts are caused by macroscopic ejecta
emplacement. On the Moon, albedo and color contrasts are
commonly observed around fresh craters and are due to
differences between the optically mature surface and the
optically less mature subsurface.
Some information on the spectral effects of space weathering
on asteroids has been inferred from measurements and
experiments with meteorites and minerals in the laboratory
(Moroz et al., 1996; Clark et al.,1992; Britt and Pieters, 1989;
etc.), and by analogy with the Moon (Pieters et al., 2000).
Recently, several studies have shown that energetic heating of
a regolith to simulate micrometeorite bombardment on asteroids
can cause optical alteration patterns similar to those observed
on Ida (Sasaki et al.,2001; Hiroi and Sasaki, 2001). These
optical alteration patterns are also similar to those required to
explain the discrepancies between certain meteorite classes and
their proposed asteroid parent bodies (Hapke, 2001).
In addition, progress in understanding space weathering on
the Moon has made it feasible to extrapolate studies to asteroids
(Pieters et al.,2000; Hapke, 2001). Currently the model for
space weathering on the Moon which is most consistent with
1619
all of the data is that micrometeorite bombardment, perhaps
aided by solar wind sputtering, produces a vapor from
particulate target materials which, upon redeposition at the
surface, is chemically reduced such that iron previously existing
in silicates condenses out as a coating of submicroscopic
particles on individual grains (Hapke, 2001). Because we do
not have samples of an asteroid surface for analysis in the
laboratory, we do not know to what extent the lunar weathering
process(es) operates on asteroids (McKay et al., 1989).
However, meteorite evidence and lunar soil studies indicate
that asteroid surfaces are probably characterized by complex
particle size distributions (Horz and Cintala, 1996), impact
shock effects (Keil et al., 1992), and mineral specific responses
to impacts, solar wind, and cosmic-ray bombardment (Sasaki
et al.,2001; Moroz et al., 1996; Clark and Johnson, 1996;
Hapke, 2001). The arguments presented by Pieters et al.(2000)
show that many aspects of lunar-type space weathering must
occur on asteroids; however, the rates and relative importance
of different effects (vapor deposition, comminution, glass
production, etc.) may be quite different yielding cumulative
optical effects which may differ in detail fiomthe lunar example.
The main components detectable in 0.8-2.4 p m spectra of
Eros and other S-type asteroids are the minerals olivine and
pyroxene, common constituents of ordinary chondrite and
primitive achondrite meteorites (Sears and Dodd, 1988; McCoy
et al.,2000). These minerals and meteorites exhibit absorption
features in reflectance spectra centered near 1.O and 2.0 pm,
the precise central wavelength depends on mineral chemistry
(Cloutis and Gaffey, 1991).
Albedo and color variations in imaging data of Eros are
compared with similar measurements of asteroids 243 Ida and
95 1 Gaspra by Murchie et al. (2002). Preliminary reports on
the geology of Eros have been presented by Veverka et al. (2000,
2001), Thomas et al. (2002) and Murchie et al. (2002). In
their paper, Thomas et al. (2002) compare Psyche crater with
other craters on Eros. Psyche is found to have a rim consistent
with impact crater morphology in gravitational profiles, although
it is somewhat degraded. It is not known to what extent the
formation of Psyche may have affected the rest of Eros, although
areas to the southwest show possible infilling of craters. Psyche
has a deptwdiameter ratio of -0.21, a high value compared
with typical craters on Gaspra and Ida (-0.15) (Thomas et al.,
2002).
OBSERVATIONS
Images and spectra obtained by the NEAR-Shoemaker
spacecraft are identified by mission elapsed time (MET)
(measured in seconds since launch on the spacecraft clock).
Imaging observations were conducted throughout the mission
by the MSI; however, the NIS only obtained data until MET
133588706, whereupon the instrument ceased to function due
to a failure in the electronics (Bell et al.,2002). Although the
coverage of asteroid terrain by both instruments is quite good
1620
Clark et a1
(100% in the case of MSI, and 90% in the case of NIS; Veverka
et al., 2000; Bell et al., 2002), there are comparatively few
occurrences of simultaneous MSI-NIS observations of Psyche
crater.
Listed in Table 1 are all MSI observations used in figures
for this paper. Corresponding NIS measurements are listed for
individual spectra. However it would not be feasible to list
each of the 250 spectra used in the global mapping. Instead,
selection criteria for NIS spectra are given below.
TABLE1. MSI-NIS Observations of Psyche crater.
Figure
1
2
4
8
13a
13b
MSI MET*
NIS MET*
125956839, 125957025,
125957087, 125957273
140150800
132393412
Basemap (mosaic)+
133148793
133147433
-
-
-
133148862-133148874
133147380-133147392
Images
Figure 3 is an oblique view across the top of Psyche crater.
This image indicates the steepness of the sides of the crater
walls, which have slope angles up to 35" (Thomas et al., 2002).
The contact between bright crater wall materials and the
surrounding terrain does not support the existence of bright
ejecta deposits around the rim, nor do the bright markings tend
to emerge from specific source ridges, blocks, or crater rims.
Downslope, the bright material margins commonly have very
sharp transitions to darker material, a characteristic very
different from the diffusive boundaries observed on Phobos,
Deimos, and Gaspra. The bright markings terminate against
the bottoms of craters, suggesting that whatever they are, they
do not run out over flat areas, and thus are not high velocity
failures (Thomas et al., 2001). As concluded by Thomas et al.
(200 l ) , downslope processes can apparently continue to
function long after crater formation on Eros. Figure 3 suggests
that since Psyche formed, the optical surface has equilibrated
to match the mean surface surrounding the crater (hereafter
called the plains). Any bright crater rays or ejecta deposits
that might have formed with the Psyche impact have since faded
or been covered with dark material. Apparently, bright materials
FIG. 3. This oblique view across the top of Psyche shows the steep
northern slopes of the crater. Steep slopes allow gravitational
processes to compete successfully with surface darkening processes.
*MET is mission elapsed time, in seconds since launch.
+SeeBussey et al. (2001) for basemap details.
are exposed on the crater walls because the slopes are (or have
been) locally unstable for the upper layers containing the dark
materials. These general relationships have also been described
by Murchie et al. (2002).
The association between dark and bright materials on Eros
is entirely different from associations observed on Gaspra and
Ida (Sullivan et al., 1996). On Eros, bright materials are
correlated with steep crater walls (Thomas et al., 2002; Murchie
et al., 2002), whereas on Gaspra and Ida we see only color
contrasts associated with craters and ejecta emplacement
(Geissler etal., 1996; Lee etal., 1996; Chapman, 1996). These
geomorphologic associations are important constraints on
models of the causes of the microscopic differences controlling
the albedo contrasts in Psyche crater.
Near-Infrared Spectroscopy
Spectroscopic data used for this analysis have been
calibrated (Izenberg et al., 2000) and photometrically corrected
(Clark et al., 2002). Calibration procedures correct instrumental
fluxes for the effects of dark current, detector crosstalk, and
bias. Photometric correction procedures correct each spectrum
for variations due to illumination and viewing geometry (Clark
et al., 2002). These corrections are further discussed in the
section "Models" where we review the importance of viewing
geometry to spectrum characterization. Figure 4 shows the
observation phase angle and spatial resolution of the data.
The spectra we used were constrained to phase angles <IOO",
incidence and emission angles <75", spatial resolution 400800 &spectrum, and areal distribution centered on Psyche
(-30 to 70" north latitude, and 35 to 175" west longitude). For
the purposes of comparing NEAR spectra with laboratory
spectra of analogue materials, we have photometrically
corrected all of the data to both 0 and 30" phase angle.
In reflectance spectrum analysis, it is customary to normalize
spectra at 0.55 pm, the wavelength where most asteroid
brightness and albedo information is measured. In this paper,
we are restricted to wavelengths obtained by the NIS
spectrometer (0.8-2.4 pm) and therefore we will normalize all
spectra to a wavelength near the continuum at 1.3 pm. When
Space weathering on Eros
1
-
Average Spectral Properties
Psyche High Resolution NIS Data
0
$ 500
p:
L
400
50
1621
60
70
80
90
Phase angle (degrees)
100
110
FIG. 4. This figure shows the phase angle and spatial resolution
coverage of the NIS spectra we used in our analysis of Psyche crater.
Data were constrained to latitudes -30 to 70" , and longitudes 35 to
175". A tiny spot is plotted for each of the 1246 spectra in this Psyche
crater subset of the NIS dataset.
spectra are normalized at 0.55 pm, any differences in the
visible wavelengths are minimized and differences in the
infrared are enhanced. When spectra are normalized at 1.3 prq
the similarities at wavelengths from 0.8 to 1.4 p m are
enhanced and the differences in both the short wavelength
visible (0.3-0.8 pm) and in the infrared (1.4-2.5 pm) are
emphasized.
To study the spectral effects of the albedo contrasts observed
in Psyche crater, we defined a study area which spatially
encloses Psyche and which also captures the surrounding plains.
Figure 5 is a simple cylindrical map showing the areal coverage
of the Psyche study area. Each spectrum covers an area
associated with a triangular plate number in the shape model
(for details on the shape model see Thomas et al., 2002). In
Fig. 5 the plate model is shaded according to the photometrically
corrected normal albedo values at 0.946pm and reprojected to
a simple cylindrical map. Thus, although the spectrometer
footprint is rectangular in shape (Izenberg et al., 2000),
resolution elements in our coverage maps are triangular in shape
because we use a plate model of Eros. Each plate is covered at
least 80% by the specified spectrum. The highest spatial
resolution of the NIS spectra is 0.55 km across (see Fig. 4).
This map can be compared with Fig. 6, which shows imaging
at the same scale, but at much higher spatial resolution (for
details on the basemap see Bussey et al., 2002). Figure 7 shows
a histogram of the normal albedo values at 0.946 p m mapped
in Fig. 5. The mean normal albedo value is 0.22 ? 0.04 and
the values are weakly enhanced in the bright tail of the
distribution. Figure 8 shows a histogram of the reflectance values
at 0.946pm for data photometrically corrected to 30" phase angle.
To compare the spectral properties of the darker ambient
materials with the brighter crater wall materials in Psyche, we
iteratively mapped the locations of bright and dark spectra until
the spatial locations of each concurred with the imaging data
FIG. 5. A simple cylindrical map representing the NIS observations of Psyche crater on Eros. Mapped are the photometrically corrected
normal albedo values at 0.946pm. The highest spatial resolution element is 550 m across. This can be compared with Fig. 6, which shows
imaging at the same scale, but at much higher spatial resolution ( 1 80 dpixel).
1622
Clark et al.
FIG.6. A simple cylindrical represention of a MSI basemap mosaic. For details on basemap construction, see Bussey et al. (2001). The area
around Psyche crater (centered at +20" latitude and 90" longitude) can be compared with Fig. 5 , which shows the NIS coverage at much lower
spatial resolution (550-800 dspectrum).
Histograms
u)
Q)
8
0
100
GI0
k
sE
k
iz
2
0
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
u)
$ 60
50
8
0
30
8
20
i$
10
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
u)
- - - - - Bri ht Materials
-Dar! Materials
P
z o . . . . ., . . .
8
El
Q)
58
40
%
100
0
-
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Normal Albedo (0.946p.m)
FIG.7. The distribution of reflectance values at 0.946 p m for data
photometrically corrected to 0" phase angle. The top panel shows a
histogram for all spectra used to make the map of Psyche crater in
Fig. 5 . The bottom panel shows histograms for the spectra used to
make the maps of Psiche bright and da;k materials in Figs. 9 and 10.
60
50
40
%
30
8
20
P
Bright Materials - - - - Dark Materials -
i$ 10
z o
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Reflectance (0.946p.m)
FIG.8. The distribution ofnormal albedo values at 0.946pm for data
photometrically corrected to 30" phase angle. The top panel shows a
histogram for all spectra used to make the map of Psyche crater in
Fig. 5 . The bottom panel shows histograms for the spectra of Psyche
bright and dark materials.
Space weathering on Eros
shown in Fig. 6. The MSI basemap image in Fig. 6 was used
only to aid in defining contacts between bright and dark units.
It is possible that a given location on Eros was classified in
both bright and dark bins because spectrum footprints may
overlap. In addition, uncertainties in normal albedo values due
to unresolved topographic slopes may be folded into the
bright/dark contact zones. These uncertainties could not be
avoided, and it is our hope that on average we have succeeded
in binning the bright and dark units sufficiently. We found a
total of 2 17 spectra in the dark areas and 76 spectra in the bright
areas, the remaining -1000 spectra in the Psyche NIS dataset
are probably mixtures of the two terrains. Results are shown in
Figs. 7, 9, and 10, and listed in Table 2. The resulting bright
and dark unit distribution maps nicely match the gravitational
slope map reproduced here from Thomas et al. (200 1) in Fig. 11.
1623
Note that the materials we call the "dark unit" are actually
average Eros surface materials, represented in both the plains
and in crater floors.
Figure 9 shows the spatial location of the 217 spectra we
have binned to represent the spectral properties of average dark
materials and Fig. 10 shows the spatial location of the 76 spectra
we have binned to represent the spectral properties of average
bright materials in Psyche. The mean dark value is 0.19 (0.08
at 30") and the mean bright value is 0.26 (0.11 at 30"), thus
there is not quite the 83% contrast in the NIS observations as
in the MSI observations (Murche et al., 2001). This is probably
due to the differing spatial resolutions of the two datasets. The
MSI data were obtained at 180 &pixel, whereas the NIS data
were obtained at 620 dspectrum. One would expect higher
resolution data to resolve more detail and contrasting surface
FIG. 9. The spatial locations of the 217 spectra binned for the average spectral properties of dark materials in and around Psyche crater.
TABLE2. Psyche NIS Spectra.*
Rim
Plains
Bright
Dark
Bright average
Dark average
Phase
angle
average
Incidence
angle
average
Emission
angle
average
Average
resolution
(dspectrum)
Reflectance
(0.946 pm)
68
68
66
67
72
24
38
37
41
62
60
46
30
41
591
600
600
60 1
624
616
0.10
0.08
0.12
0.08
0.11
0.08
80
40
96
100
*Note that all angles are given in degrees, and all reflectances are given at phase angle 30".
Reflectance
Reflectance
(0.946pm)
(0.946pm)
(minimum)
(maximum)
0.10
0.06
-
0.13
0.10
1624
Clark et al.
FIG.10. The spatial locations of the 76 spectra binned for the average spectral properties of bright materials in the walls of Psyche crater.
FIG.1 1. This simple cylindrical map shows gravitational slopes on Eros, reproduced from Thomas et al. (2001). Note the high correlation
between gravitational slope and albedo around Psyche crater (shown in Fig. 5 ) .
areas than lower resolution data because there is less averaging
within the field-of-view of the detector. If we use the equation
percentage contrast
= ((rhigh - rlow)/ravg) x
100
(l)
to describe the contrast, where rhigh is the average bright
reflectance value at 0.946 p m , 'low is the average dark value
and ravg is the average of the two, then we find a 32% albedo
contrast between bright and dark materials at spatial scales of
620 dspectrum. The maximum contrast level is difficult to
gauge with certainty; however, at 30" phase angle there are some
Space weathering on Eros
spectra with values higher than 0.13 and some with values as
low as 0.06, more than a factor of 2 difference, or 74%contrast.
This compares well with the Murchie et al. (2002) study in
which they find albedo contrasts of 83% at spatial scales of
180 d p i x e l .
Figure 12 compares the average dark materials spectrum
with the average bright materials spectrum. This figure shows
that darker materials on Eros have slightly broader 1 p m bands
and slightly redder spectral slopes than brighter materials. The
total variation is ~ 4 (%2 2 % ) .
This analysis indicates that on average the albedo contrasts
in Psyche crater are 32%. However, because we have binned
and averaged many spectra there is a chance that we are
including shadows or topographic facets not captured by the
shape model (Clark et al., 2002). On average, these problems
occur in both senses (there are just as many dark shadows as
there are bright facets) such that binning of a statistically
significant number of spectra will even things out. However,
for confirmation, in the following section we examine the level
of contrast observable at close range (i.e., with individual spectra
rather than with large areal averages).
Local Spectral Properties
We have searched through the imaging and spectroscopy
datasets for examples of both dark and bright material exposures
1.o
0.251
Q)
0
9
c,
.
.
' -
1.5
within Psyche crater. A constraint we imposed was that the
images and the spectra must have been obtained within 1 min
of each other, otherwise the motion of the spacecraft makes it
difficult to superpose the data such that the NIS field-of-view
is exactly placed on the MSI image. Figure 13 shows two
examples of local occurrences fitting our search criteria (spectra
are listed in Table 2). This figulle shows exposures of both
bright and dark materials close to each other, with minimal
shadows and minimal differences in viewing angles. Minimal
differences in viewing geometry helps to assure that
uncertainties in the photometric correction (Clark et al., 2001)
are not important to the spectral comparison.
Shown in Fig. 14a is a comparison of spectra of materials
on the rim of Psyche crater with materials in the plains
surrounding Psyche. Shown in Fig. 14b is a comparison of
spectra of the brightest and darkest materials. The top panels
show each spectrum in normal albedo units, the middle panels
show each spectrum in reflectance at 30" phase angle, and the
bottom panels show a ratio of the normalized spectra. This
ratio brings out the subtle spectral differences between bright
and dark materials. At 0.946pm, the bright crater wall materials
have a reflectance at 30" phase angle of 0.12 and the dark crater
wall materials have a reflectance of 0.08. Similarly, the bright
rim materials have a reflectance of 0.10 and the dark plains
materials have a reflectance of 0.08. Using Eq. (1) again, we
calculate an albedo contrast between these two areas to be 40 and
2.0
1.0
.
0.20:
1625
'*04
1
0.96
1
1.5
2.0
DarklBright Ratio
0.15
4 0.10 i
0
0.05;
0.00 l
1.o
1.5
2.0
Wavelength (microns)
1.0
1.5
2.0
Wavelength (microns)
FIG. 12. Average spectra for Psyche crater dark materials and bright materials. Error bars are the standard deviations of the mean. Panel (a)
IS i n units of reflectance at 0" phase angle, and panel (b) is a ratio of dark materials over bright materials and normalized to 1.3 ,urn to bring
out the subtle differences between the two avcrage spectra. A straight dotted line is plotted to aid in comparison. Points along the line are
wavelengths for which the normalized spectra show no spectral differences. Panel (c) shows the same spectra in units of reflectance at 30"
phase angle, and panel (d) shows their normalized ratio. This figure shows that the phase angle ofphotometric correction has no effect on the
trends observed in the spectrum ratios.
1626
Clark et al.
FIG.13. The spatial relationships of spectra obtained of Psyche bright and dark materials deposits. (left) A spectrum of bright rim materials
is outlined in red and a spectrum of dark plains materials is outlined in blue. (right) A spectrum of bright crater wall materials is outlined in
red, and a spectrum of dark crater wall materials is outlined in blue. Image and spectrum MET numbers are given in Table 1. Associated
viewing and illumination angles are given in Table 2.
1.06
-
1.04
:
- PlaindRim
0
-
B
1.02-
l.OO
3 #
--B
0.98
0.96 0.94 -
-
...%
0.0 0 0..
..oo0
a
Dark/Bright
- 1.04 -:.23 1.02 -:
-
-
1.06
*, **+ ++++
-
+ j
-
4*++++
b
0.96
0.94
+
-
-
FIG. 14. These figures compare the spectra of local dark material deposits with local bright material deposits. (a) A region on the bright rim
of Psyche crater is compared with a region in the darker surrounding plains. (b) A dark region on the wall of Psyche crater is compared with
a spectrum of a nearby area of bright materials (see Fig. 13). Spectra were obtained within 12 s of each other, minimizing instrumental drift
and viewing angle differences. In addition, as shown in Fig. 13, there are no large shadows in either spectrum footprint. The top panels show
each spectrum in normal albedo, the middle panels show each spectrum in reflectance at 30" phase, and the bottom panels show a ratio of dark
(plains) materials to bright (rim) materials. Viewing angles for all spectra are listed in Table 2.
Space weathering on Eros
22%, respectively. In total, the peak-to-peak spectral variations
are 8% (?4%) and are most significant in the shape of the 1 p m
band from 0.8 to 1.0 pm, and in the slope of the continuum
from 1.5 to 2.4 pm. Similar to Fig. 12, Fig. 14 shows that
individual spectra of dark materials on Eros have slightly
broader 1 p m bands and slightly redder spectral slopes than
brighter materials.
In sum, the global averages show contrasts of 32% and
spectral variations of 4%, and the local spectra show contrasts
of up to 40% with attendant spectral variations of up to 8%.
We do not distinguish which measure of the contrasts and
variations is most representative of the surface of Eros, but
instead use the range of values as a general guide.
1627
.
'
l
.
"
.
'
Spectral Photometric Model
In this section we begin by briefly explaining the photometric
model. Although the photometric correction procedure is
described in detail in Clark et al. (2001), we touch on some
aspects here in order to illustrate how photometric effects are
important to the comparison of asteroid and meteorite data.
The main sources of variation in the NIS spectral dataset
are photometric effects: variations in reflectance level due to
changes in illumination and viewing geometry (Veverka et al.,
2000; Clark et al., 2001; Bell et al., 2001). Because photometric
effects can cause large differences in spectral reflectance (Fig. 1 3 ,
it is necessary to analyze spectra of Eros at viewing angles
similar to those of analog minerals and meteorites measured in
the laboratory. For this study we use the model of Clark et al.
(2001) to photometrically correct our measured spectra of Eros
to the viewing geometry of common laboratory measurements.
The model consists of a set of Hapke parameter values for each
of 52 wavelengths. The parameters modeled are average single
scatter albedo (w),a single particle phase function HenyeyGreenstein asymmetry parameter (g),macroscopic roughness
mean slope angle (@,opposition surge amplitude (Bo),and the
.
"
l
.
l
"
l
a 0.40
i
c,
$
0.30;
p:
3 0.20 1
M
z
0.10
MODELS
In this section we use scattering models and lunar analogies
to explore three plausible explanations for the albedo and color
contrasts in Psyche: grain size, mineral chemistry and modal
abundance, and optical maturation processes. We then compare
predicted spectra1 variation trends with the measured variations
presented in the previous section. We begin with a brief
explanation of the photometric correction procedure, used to
correct all NIS spectra to a viewing geometry comparable to
laboratory viewing angles (Clark et al., 2002). We then use
mineral mixture models to simulate the photometrically
corrected spectra. Finally, we modify our mineral mixtures to
simulate the spectral consequences of variations in grain size,
composition, and optical maturity. We constrain our model
mixture variations to be consistent with the observed albedo
contrasts of 32-40%.
.
Eros Model Spectra
,
.
-
.
"
'
,
.
'
'
.
,
l
o Model Spectrum 37,29 29
m Measured Spectrum 33,29,29
0.05/..1 . . . .
1.0
I
1.5
. . . .
t
. . . .
2.0
2.5
Wavelength (microns)
FIG. 15. This figure shows models and measurements of Eros'
spectrum. The top panel shows model spectra calculated at various
viewing geometries using the Hapke parameters provided in Clark et
al. (2002). To the right side of each spectrum are the viewing angles
listed in order of incidence, emission,and phase angles, respectively.
The brightest spectrum is calculated at normal viewing geometry,
where incidence, emission, and phase angle are all exactly 0". Most
laboratory spectra of analog materials used to study Eros' composition
are measured at the 30,0,30 viewing geometry (last spectrum). The
bottom panel compares an average of 56 NIS spectra measured at
incidence = 37", emission = 29", and phase = 29" with a model
spectrum calculated at the same viewing geometry. The model agrees
quite well with the measurement, lending a degree of confidence in the
photometric model of Eros. Error bars have been supressed for this
comparison-for more detail on uncertainties see Clark et al. (2002).
opposition surge angular width parameter (A). Parameter values
are provided in Clark et al. (2001) in the form of spectra of
Hapke parameters. The model predicts the spectral reflectance
of Eros as a function of viewing geometry, and is accurate to
within 2 2 % on average.
In Fig. 15 we use the spectral photometric model of Eros to
show some effects of viewing geometry on Eros spectra. The
top panel shows how changing incidence, emission, and phase
angles affect the overall brightness of the Eros spectrum. For
1628
Clark et al.
example, the model spectrum calculated at normal reflectance
(incidence = emission = phase = 0) is 2 . 5 brighter
~
than any of
the spectra calculated for moderate laboratory-like reflectance
geometries (generally constrained to phase angles from 15 to
30"). As a check on the accuracy of the photometric model, the
bottom panel of Fig. 15 compares a measured spectrum with a
model spectrum. The measured spectrum is an average of 56
NIS spectra obtained at phase angles between 28 and 32". For
this average spectrum, the average incidence and emission
angles were 37 and 29", respectively. The model spectrum was
calculated at the same viewing geometry. There is good
agreement between the model and the measurement, lending a
high degree of confidence in the photometric model.
We can now compare Eros to laboratory spectra of analog
materials. Our suite of analog mineral samples were measured
at the Reflectance Laboratory Facility at Brown University
(RELAB) (Pieters, 1983; Sunshine et al., 1990; Britt et al.,
1992). At RELAB the standard viewing geometry is incidence =
30", emission = 0", and phase = 30".
'
1.0
1 '
'
'
' '
' 1 .
Olivine
0.0'.
I . .
.
.
I
,
.
'
1 '
' .
,
.-./---
,
,
I
,
.--,
0.8-,
'
//-\//\TL
\-I
.
.,'
' 1 '
1.0
' '
'
'
' ' '
"
"
'
' ,
Orthopyroxene
'
0.8 .
0.0
. # . . . . # . . . . I . . . . _
Neutral Dark
\
0)
$
0.6 -
Y
0
Modeling the Spectral Relationship Between Psyche Bright
and Dark Material
While the differences between Psyche bright and dark
material are qualitatively consistent with a space weathering
relationship, it is plausible that grain size, mineral composition
or other reasonable mechanisms can account for these
differences. We will use an intimate mixing model based on
the work of Hapke to quantitatively explore the effects of several
parameters. We will present spectral effects of variations in
grain size, mineral chemistry, relative abundance of olivine and
pyroxene, abundance of a spectrally neutral dark component,
and submicroscopic iron. We will show that a model which
includes processes consistent with current understanding of
space weathering (Pieters et al., 2000; Hapke, 2001) can account
for the differences between Psyche bright and dark material,
though grain size differences are also at least spectrally
plausible. Our model relies largely upon the work of Hapke
(1993, 2001). Hapke (1993) presented a compilation and
extension of his work on radiative transfer in powders. Hapke
(2001) presents the methods necessary to compute the optical
effects of submicroscopic iron which serves an important role
in lunar space weathering optical effects (see also Hapke, 2000).
Our model allows the computation of the reflectance spectrum
of a meteorite-like assemblage consisting of intimately mixed
olivine, ortho- and clinopyroxene, plagioclase feldspar, troilite,
Fe-Ni metal, iron-bearing glass, and submicroscopic Fe metal.
Figure 16 shows the major mineral endmembers included in
our model. We can compute the spectrum of an arbitrary
mixture of these components at any grain size and, within limits,
mineral chemistry.
The model begins with a definition of the relative
abundances of the components and the mineral chemistries of
the components for which this is an important variable. For
Q,
9
w
Coarse
0.4
0.4.
---
0.2 .
:
0.0
Plagioclase
w
1.5 2.0 2.5
Wavelength (pm)
1.0
0.0
1.0
1.5
2.0
2.5
Wavelength (pn)
FIG. 16. Shown here are RELAB measurements of the reflectance
spectra of our four mineral endmembers used in the mixing model of
Eros' spectrum. Grain size separates are labeled in the legend at the
right.
the transparent minerals (olivine, pyroxene, plagioclase and
glass) we compute the optical constants from defined
chemistries. Chemistry-dependent optical constant coeficients
for olivine and pyroxene are from Lucey (1998). The
chemistries of olivine and orthopyroxene were linked to
conform to the relationship between olivine and pyroxene Mgnumber in ordinary chondrites from Keil and Fredrikkson
(1964). Optical constants for glass as a function of iron content
were computed by Lucey (1998). We used the Hapke-based
methods presented in Lucey (1998) to compute the irondependent optical constants of plagioclase from reflectance
spectra ofplagioclase in the U.S.G.S. Denver spectrum library.
Next, using the methods outlined in Hapke (1981, 1984,
1986, 1993) modified to include the effects of submicroscopic
iron presented by Hapke (2001) we compute the single
scattering albedos of the transparent components from the
optical constants, grain sizes and the defined abundance of
submicroscopic iron, assuming a coating of metal for the
minerals, and a uniform distribution of metal throughout the
grain for glass. We note that lunar and asteroidal
submicroscopic iron deposition may be quantitatively different
Space weathering on Eros
because the asteroidal submicroscopic iron may also contain
nickel. Our models assume that the submicroscopic coatings
are pure iron, which appears to be a good assumption on the
Moon (Hapke, 2001). The extent to which Ni will alter the
optical constants of iron is not clear. For troilite, the single
scattering albedo at a given grain size is computed from a fit to
derived single scattering albedos for three different sizes of
troilite. For metal, the single scattering albedo at a given grain
size is computed from Mie theory. In this implementation all
components share the same particle size (except for
submicroscopic iron).
The single scattering albedos are combined using Eq. (1 7)
of Hapke (1981). The single particle phase functions are
combined using the methods of Helfenstein et al. (1994, 1996).
The mixture single scattering albedo is converted to reflectance
using Eq. (37) of Hapke (1981).
Shown in Fig. 17 are our mixing model spectra of Eros
compared with the measured spectra for the bright and dark
materials in Psyche crater. Mixing ratios for each endmember
are listed in Table 3. The best model fits are achieved with a
grain size of 63 pm. Note that because we do not have
constraints on the actual composition of the minerals present at
the surface of Eros, these mixing models are not meant to be
construed as unique determinations of the composition of Eros,
but should instead be taken as reasonable interpretations of the
reflectance spectra. In reality, the surface is likely to be an
extremely complex mixture of particles of different sizes (Horz
and Cintala, 1996), different shock histories (Keil et al., 1992),
and mineral specific responses to impacts, solar wind, and
cosmic-ray bombardment (Sasaki et al., 2001; Moroz et al.,
1996). In this effort we were unable to mimic any spectra of
Eros without including the reddening effect of submicroscopic
iron. However, our approach to modeling the effect of
macroscopic Fe-Ni metal grains may not be an accurate
representation of the true effect. Note that between these two
fits there have been only minor changes in composition and
grain size, but large increases in the amount of troilite (which
in our case we use to represent both the effects of the mineral,
and any other dark neutral component), and submicroscopic
iron.
Our approach to using this model to explore the parameter
space is to begin with the inputs resulting in the fit to the Psyche
bright material, then modify these by varying individual
parameters to illustrate these parameters' effects on the bright
Psyche material. Our goal in each case is to attempt to transform
0.20
1629
1
1.0
1.5
2.0
Wavelength (microns)
FIG.17. Intimate mixture models for the spectra of Eros bright and
dark materials. The Eros spectra are shown in symbols and the models
are shown in solid lines.
the bright material spectrum into the spectrum of the dark
material using model variations.
Grain Size
The first plausible explanation for the albedo contrasts
observed in Psyche crater is grain size. As shown by A d a m
and Filice (1976), Johnson and Fanale (1973), Clark et al. (1992)
and Clark (1999, the reflectance ofmeteorite and mineral samples
increases with decrease in the average grain size.
We began with our nominal Eros bright material mixture
model spectrum (grain size 63 p m ) and produced three
comparison spectra at 88,107, and 126pmgrain size. Figure 18
shows the spectral consequences of this variation. For the
purposes of comparing grain size variations to our observed
variations in Psyche crater, we ratio the dark spectra (coarser
grain sizes) to the nominal bright materials mixture model
spectrum. In the normalized spectral ratios, the amplitude of
expected spectral variation is as high as 18% (+5-13%), and
there are minor variations correlated with the absorption band
centers near 1 and 2 pm. In contrast, the observed spectral
ratio variation amplitude on Eros is at the level of 4-8%, and
there are no variations correlated with band centers.
TABLE3. Mixing model parameters.*
Olivine
Bright
Dark
54
49
Orthopyroxene Plagioclase Neutral dark
17
16
15
14
2.5
Grain size
SMFe
Mg-number (01)
14
63
0.03
22
77
0.07
70
70
*Note that all endmembers are given in weight percentages, and grain sizes are given in microns.
Abbreviations: SMFe = submicroscopic iron.
1630
Clark et al.
I
.
.
.
.
I
'
.
.
.
I
.
.
.
1.20 i
.
j
- Variations
GrainSize
0-20;
88 microns - - - - 107 microns - - 126 microns -.---.-.
0.05
.
....
I
1.0
I
.
.
.
.
#
.
.
.
0.80
.
1.5
2.0
Wavelength (pm)
2.5
1.0
1.5
2.0
Wavelength (pm)
2.5
FIG. 18. Variations in grain size of our mineral mixture are shown: the left panel shows the nominal mixture model (grain size 63 pm)
compared with mixture models made with grain sizes of 88, 107, and 126pm. Also plotted are the dark material mixture model (solid line),
and the observed darkhright material spectrum ratio (solid dots) to aid in comparison. The right panel shows the coarser grain size mixtures
ratioed to the nominal grain size.
0.20
. Forsterite
0
.H
4
d
1.10
i
0.15
0.10
0.05
Mg58 - - - - -
I
.
1.0
.
.
.
I
.
.
.
.
I
.
.
1.5
2.0
Wavelength (pm)
.
.
>
2.5
0.80
1.0
1.5
2.0
Wavelength (pm)
2.5
FIG. 19. Variations in forsterite composition (as measured by the irodmagnesium ratio-Mg number) of our mineral mixture. (left) The
nominal bright material model is compared with mixture models made using increasing proportions of the magnesium content of the olivine
and pyroxene endmembers. (right) The enhanced magnesium models ratioed to the nominal model spectrum. This figure shows that the
trends in spectral ratios associated with olivine magnesium content have amplitudes of -lo%, and result in wave-like spectral ratios. Also
plotted are the dark material mixture model (solid line), and the observed dark/bright material spectrum ratio (solid dots) to aid in comparison.
Composition
The second plausible explanation for the albedo contrasts
observed in Psyche is a compositional heterogeneity. We consider
variations in either the relative abundance of components, or the
chemistry of the components to be compositional variations, and
we simulate both effects in thls section.
We begin four comparison variations: one with a steadily
decreasing forsterite number (increasing proportions of iron in
the olivine and pyroxene), one with increasing olivine weight
percent in the mixture, one with increasing orthopyroxene
weight percent in the mixture, and finally one with increasing
troilite.
Figure 19 shows the spectral consequences of variations in
forsterite composition (as measured by the irodmagnesium
content). In this case, the total amplitude of expected spectral
variations is -10%. Note that forsterite (Mg number) variations
change the shape of the reflectance spectrum as band centers
shift in wavelength, causing the spectral ratios to exhibit a
distinct wave-like appearance. These trends are not observed
Space weathering on Eros
1631
not result in albedo contrasts at 0.946 p m comparable to the
observed albedo contrasts in Psyche.
Figure 2 1 shows the spectral consequences of the increase
in orthopyroxene proportion of the mixture composition. In
this case, the total amplitude of expected spectral variations is
13%. Again, however, increasing the pyroxene proportions
did not result in albedo contrasts comparable to the observed
contrasts in Psyche. In addition, pyroxene variations cause
in spectral ratios of Eros' materials. Also, the magnitude of the
change in forsterite content necessary to account for the albedo
contrast is extremely large, and probably unreasonable for a
single object.
Figure 20 shows the spectral consequences of the increase
in olivine proportion of the mixture composition. In this case,
the total amplitude of expected spectral variations is -15%.
Note, however, that increasing the proportion of olivine did
-
0
.
3
c,
a
p:
'.'
.- -
1.5
2.0
4
0.8
1.o
2.0
1.5
1.o
2.5
Wavelength (urn)
2.5
Wavelength (urn)
FIG.20. Variations in olivine proportions of our mineral mixture. (left) The nominal bright material model is compared with models made
with increasing weight percentages of the olivine endmember. (right) The enhanced olivine models are ratioed to the nominal model spectrum.
Also plotted are the dark material mixture model (solid line), and the observed darklbright material spectrum ratio (solid dots) to aid in
comparison. This figure illustrates that the trends in spectral ratios associated with olivine variations have amplitudes of-I 5%; however the
40% albedo contrasts observed in Psyche cannot be simulated with these increases in olivine.
1
.
2
q
q
30%-. -. -. -.
0.05.1 . . . .
1.o
#
1.5
.
.
.
. . . .
2.0
2.5
.
I
Wavelength (urn)
0.8
j
dark Model 1.o
1.5
2.0
Wavelength (urn)
,
2.5
FIG. 21. Variations in pyroxene proportions of our mineral mixture. (left) The nominal bright material model is compared with models made
by increasing the proportions ofthe pyroxene endmember. (right) The enhanced pyroxene models are ratioed to the nominal model spectrum.
Also plotted are the dark material mixture model (solid line), and the observed darwbright material spectrum ratio (solid dots) to aid in
comparison. This figure illustrates that the trends in spectral ratios associated with pyroxene compositional variations have amplitudes of
- 1 3%; however, the albedo contrasts O f 40% observed in Psyche crater cannot be simulated with increases in pyroxene proportions.
Clark et al.
1632
distinct band-correlated variations which are not observed in
spectral ratios of Eros' materials.
Figure 22 shows the spectral consequences of.& increase
in dark component (troilite) proportions of the' mixture
. . .T.'
composition.
....
Addition of troilite (a dark spectrally neutral component)
can match the albedo contrast observed within Psyche, but
model spectra with albedos similar to Psyche dark material are
much less red than observed within Psyche (and otherwise on
Eros).
"!
,
Optical Maturity
The only surface for which optical maturation is understood
is the Moon. There is no proof that optical maturation occurs
on asteroids, however it is reasonable to suppose that it might.
Pieters et al. (2000) showed that aspects of lunar-type space
weathering should occur on asteroids, including the accumulation
of submicroscopic iron (SMFe) from micrometeorite impact
vaporization.
Our model allows variations in the amount of lunar-like
optical maturation. We find that increasing the amount of SMFe
with the Eros bright material model as a starting point
substantially darkens the spectrum. However, the spectra redden
too rapidly to match Eros dark material (see Fig. 23). Model
spectra with the same composition as the Eros bright material
fits, but with sufficient SMFe to match the albedo of Eros dark
material, are substantially redder than Eros dark material.
Pieters et al. (2000) and Hapke (2001) noted that increases
both in SMFe and dark material were necessary to match the
spectra of lunar soil using a powder lunar rock starting material.
On the Moon, space weathering includes the effects of impact-
0.201
8El
2
+
shocked Fe-Ni and troilite components that have been dispersed
into grains larger than the observation wavelength. Indeed, in
meteorite regolith breccias shock-darkened materials (e.g.,
black chondrites) are common (Britt and Pieters, 1989). A
dark neutral component is thus a reasonable addition to an
asteroidal weathering model. We find that making the
abundance of our dark component (represented by troilite)
directly proportional to the abundance of SMFe gives rise to a
good match to Eros dark using the Eros bright model as a starting
composition. The excessive redness of the SMFe is
counteracted by the spectrally neutral effect of the dark
component (see Fig. 24).
DISCUSSION
Albedos of Eros and Meteorites
In our study we have taken pains to exploit the measurements
obtained by the NEAR NIS at many viewing geometries to
ensure that we can directly compare reflectances of Eros to the
reflectance of meteorites measured in the laboratory.
After accounting for all differences in brightness units of
our reflectance comparisons, we have found that the bright
materials on Eros have reflectance values at 0.946pm consistent
with meteorites (see Fig. 25), and that average Eros surface
materials are 3 0 4 0 % darker than meteorites.
Fanale et al. (1992) compared S-type asteroid albedos and
ordinary chondrite (OC) meteorite reflectances and showed that
there was significant overlap in values, indicating a possible
genetic relationship between the two populations. However,
in that study Fanale et al. did not take into account the fact that
OCs are measured in the laboratory at bidirectional viewing
Neutral Dark
Variations
/
0.15
0
EG 0.10
0.05
0.801. I . . . .
. .
. .:
1.0
1.5
2.0
2.5
Wavelength (pm)
I
1.0
1.5
2.0
Wavelength (pn)
2.5
.
.
FIG.22. Variations in the dark (troilite) proportions of our mineral mixture models. (left) The nominal bright materials model spectrum is
compared with spectra made by increasing the proportions of the dark component. (right) The dark component enhanced spectra are each
ratioed to the nominal bright spectrum and normalized at 1.3 pm. Also plotted are the dark material mixture model (solid line), and the
observed darkhright material spectrum ratio (solid dots) to aid in comparison. This figure illustrates that the trends in spectral ratios
associated with troilite variations consist of decrease in spectral contrast in the 1 p m band and flattening (or blueing) of the spectral continuum.
1633
Space weathering on Eros
I
0.20
I:
.
.
.
.
.
l
'
.
.
'
I
'
.
.
'
SMFe
Variations
0
.w
Y
4
Q)
0
9
9)
0.15
Y
0
9
0
4E
Y
0
4
0.10
0.05
E
I
.
1.0
.
.
.
I
.
.
.
1.5
.
I
.
.
.
.
2.0
2.5
1.0
Wavelength (pm)
1.5
2.0
2.5
Wavelength (pm)
FIG.23. The left panel shows our nominal bright material spectrum together with three different models of silicates and submicroscopic iron
metal (SMFe). The right panel shows the three SMFe-enhanced spectra ratioed to the nominal Eros spectrum. Also plotted are the dark
material mixture model (solid line), and the observed dark/bright material spectrum ratio (solid dots) to aid in comparison. This figure
illustrates that the trends in spectral ratios associated with SMFe variations consist of broadening of the 1 pm band and reddening of the
spectral continuum from 1.5 to 2.4 pm. These trends match the trends observed in Psyche crater; however, their amplitude is too high.
I
1.10
0.05.1 . . . .
1 .o
1.5
I
.
.
.
.
I
. . . .
2.0
Wavelength (urn)
2.5
0.8d.
5%S 18%D----6%S 21% D- - 8%S 25%
Dark Model -I
I
I
.
.
. .
I
.
.
. .
I . .
1.o
1.5
2.0
Wavelength (urn)
. .1
2.5
FIG. 24. (left) The nominal mixture model spectrum of bright materials in Psyche is compared with mixture models made by increasing the
proportions of both the neutral dark and the SMFe endmembers. (right) Each comparison spectrum is ratioed to the nominal spectrum and
normalized to 1 .O at 1.3 p m . Also plotted are the dark material mixture model (solid line), and the observed darmright material spectrum
ratio (solid dots) to aid in comparison. This figure shows that the albedo contrasts and overall amplitude ofspectral variation trends observed
in Psyche crater can be simulated with a mixture model incorporating enhanced neutral dark and SMFe endmember components.
and illumination geometry which differs from that used for the
calculation of asteroid albedos. In fact, it is actually very
dificult to compare the two quantities. Asteroid albedo values
are measurements of the geometric albedc-defined as the ratio
of the brightness of a body at 0" phase angle to the brightness
of a perfect Lambert disk of the same radius and at the same
distance as the body, b u t illuminated and observed
perpendicularly (Hapke, 1993). Geometric albedo can thus be
thought of as the weighted average of the normal albedo over
the illuminated area of the body. The largest database of
geometric albedos of asteroids is from the infrared astronomical
satellite (IRAS) survey published by Tedesco (1992).
Laboratory reflectance quantities at RELAB a r e
bidirectional reflectance factors, defined as the ratio of the
brightness of a sample at 30" incidence and 0" emission, to the
brightness of halon (an almost perfect Lambertian surface)
illuminated identically (Pieters, 1983). At any given
wavelength, the value of the geometric albedo (0" phase) is
Clark et al.
1634
Eros Compared with Meteorites
Eros and Meteorite Analogs
CI
8
9
8
Y
BE
0.20
O0
3
0.15
0.10
1.0
1.5
I
-
.
2.0
2.5
Wavelength (microns)
.
-
-
-
-
.
-
-
-
-
b
8'
433ErosNEAFt o
433 Eros Telescopic Aca ulcoite
LL Ordinary C%ondrite
.
--
FIG. 26. This figure compares spectra of Eros obtained by NEAR
0.6t . . . . . . . . . . . . . . . .
1.0
1.5
2.0
1
2.5
Wavelength (microns)
FIG. 25. Both Psyche crater units, bright and dark materials, are
(global average) and by telescope (disk-integrated)(Murchie and
Pieters, 1996)with spectra of Eros' potential meteorite analog types,
the LL ordinary chondrites and the Acapulcoite primitive achondrites.
Note the considerable spectral differences between Eros and its
proposed meteorites beyond 0.8 pm. Meteorite spectra are from T.
Hiroi (pers. comm.), measured at the Brown University Reflectance
Laboratory (RELAB).
ratio vs. band center position analysis in the manner of Cloutis
et al. (1986) to estimate a relative abundance of olivine to
pyroxene of -2: 1, which is similar to LL ordinary chondrites.
Another class of viable analogs, the Acapulcoite and Lodranite
meteorites, known as primitive achondrites, have not been
completely ruled out (McCoy et al., 2000; Burbine et al., 200 1).
However, as shown in Fig. 26 neither proposed meteorite analog
therefore systematically higher than the value of the type for asteroid Eros is a match to the spectrum of Eros. Eros
bidirectional reflectance of a lab sample (30" phase). Using is both redder and exhibits less contrast. Meteorite spectra are
the photometric model of Clark et al. (2001), we estimate that from T. Hiroi, and were measured at the Brown University
the difference can be up to a factor of 2.5 (this is also shown in Reflectance Laboratory Facility (T. Hiroi, pers. comm.). Both
Fig. 15). Taking this photometric factor into consideration, the meteorite types appear to have reflectance spectra which are
results of Fanale et al. (1992) indicate that meteorites are 2 . 5 ~ too flat in the infrared, and which have absorption features too
brighter than asteroids on average. In other words, S-type deep to match the asteroid. The spectral differences observed
asteroids are generally much darker than OC meteorites. It is between Eros and its proposed meteorite analogs are typical of
the differences between S-type asteroids and meteorites (Fanale
therefore not surprising that Eros is darker than its proposed
et al., 1992; Pieters and McFadden, 1994; Meibom and Clark,
meteorite analogs.
1999).
An important question which must be answered is "Do the
Eros Materials Compared to Meteorites
Psyche crater bright wall materials spectrally resemble
Detailed analysis of the absorption features present in the meteorites more closely than does average Eros?" Shown in
spectrum of Eros suggest a surface composition similar to OC Fig. 25 is a comparison of Psyche crater materials with both
meteorites ( e . g . , Murchie and Pieters, 1996, and references proposed meteorite analogs for Eros. In this figure we compare
therein) (see Fig. 26). Preliminary NEAR results from the the local individual spectra (not the global averages) of both of
x-ray spectrometer (Trombka et al., 2000) and from the NIS the Psyche crater wall units, bright and dark materials, with the
(Veverka et al., 2000; Bell et al., 2001) corroborate this proposed Eros meteorite analog types. The top panel shows
compositional similarity. Bell et al. (2001) use a band-area- all spectra in reflectance at RELAB geometry (incidence = 30,
compared with proposed Eros meteorite analog types. The top panel
shows all spectra in reflectanceat RELAB geometry(incidence= 30,
emission = 0, and phase = 30"). The bottom panel shows all spectra
normalized to 1.0 at 1.4 pm. The bright materials are similar to
meteorites in brightness and spectral properties from 0.8 to 1.5 pm,
however neither analog spectrally matches either unit of Psyche crater
at wavelengths beyond 1.5 pm.
Space weathering on Eros
1635
models predict that grain size differences should result in band
correlated variations that are not observed.
It is interesting that the NEAR x-ray spectrometer team
reports marked depletions in sulfur elemental abundances at
Eros relative to ordinary chondrites. Other elements, Mg, Al,
Ca, and Fe, all seem to be consistent with OCs, suggesting that
if indeed Eros is similar to OCs then partial melting or impact
volatilization processes may have been active, resulting in
fractionation of sulfur at the surface (Trombka et al., 2000).
This evidence, together with our current understanding that the
lunar space weathering mechanism is partly a process involving
volatilization of target materials, implies that surface alteration
processes may be required to explain the optical differences
between bright and dark areas on Eros.
However, while the elementalresults allow (but do not require)
an OC composition (excepting sulfur) and mineralogical analysis
suggests that Eros bears an o1ivine:pyroxene ratio consistent
with LL chondrites, our results are difficult to reconcile with
an OC composition in detail. The brightest material on Eros
does not look like a powdered meteorite: it is red and exhibits
lower contrast. But the brightest material on Eros has a similar
albedo to that of analog meteorites. Our work suggests that if
space weathering is operating on Eros, unaltered Eros material
must be inherently brighter than previously proposed analogs.
For example, it is possible that Eros bright materials are of
much finer grain sizes than previously supposed.
Shown in Fig. 27a is a comparison between lunar rocks and
soils. When the spectra are normalized and ratioed in the
manner we have been utilizing in this paper, it is evident that
soils are redder than rocks. Similarly, if we use an asteroid
emission = 0, and phase = 30"). The bottom panel shows all
spectra normalized to 1.O at 1.4 pm. Neither analog is a good
spectral match to either unit of Psyche crater. The bright
materials are similar to the Acapulcoite meteorite in terms of
reflectance at 0.946 pm, however the spectra diverge beyond
1.5pm. The dark materials, which probably represent exposures
of Eros' average surface, are darker and redder than either
meteorite. The shapes of the absorption features at 1.0 p m
also differ, with the Eros units being broader and smoother than
the meteorites (which have well-defined minima and U-shaped
profiles). The meteorites shown here are typical and
representative of their classes, however this comparison is not
exhaustive. It is possible that better matches to Eros could be
found among the meteorite spectrum collection, however we
were not able to find them. We conclude that bright materials
in Psyche are optically fresher than dark materials, but do not
spectrally match meteorites.
Space Weathering on Eros
Our modeling suggests that the albedo and color contrasts
in Psyche crater are best explained by a space weathering
process which includes accumulation of both SMFe and a dark
neutral component. Geologic relationships suggest that the
bright material is being exposed from the subsurface and is
therefore younger. Both this study and that of Murchie et al.
(2001) suggest that materials that are consistent with recent
exposure are brighter and less red than material consistent with
longer exposure to space. A combination of effects which
includes a grain size difference is also possible; however, our
--___----
Optical Maturity Variations
1.0
1.5
2.0
2.5
1.0
1.5
2.5
2.0
1.2
1.0
a
a
Y
0.5 !
0.51
0.01
0.0 -
-
Highland
7
1.0
4
L
.
1.5
-___
2.0
Wavelength (microns)
2.5
c,
0
o-."[
0.6
B 0.4
&
b
433-s
ACS ul o!b
LL ordinary CfonLte
I
1.0
1.5
2.0
--I
0
2.5
Wavelength (microns)
FIG.27. In this figure, (a) lunar surface soils are compared with lunar basalt rocks (reproduced from Pieters et al., 2000 by permission) and
(b) the photometric model spectrum of average Eros is compared with both of its meteorite analog types. If Eros' subsurface is similar to
ordinary chondrite or Acapulcoite meteorites, then the differences between Eros' surface soils (shown in diamonds) and the subsurface
(represented by meteorites shown in solid and dashed lines) are similar to the differences between lunar surface soils and subsurface rocks.
Clark et al.
1636
spectrum to represent asteroid soils, and meteorite spectra to
represent asteroid rocks, a spectral ratio shows that asteroids
are redder than meteorites in a sense that is very similar to the
lunar comparison (Fig. 27b).
If, as we favor, space weathering has been an important
process on Eros, then it must be pervasive because all portions
of Eros, even the brightest, appear weathered. It is possible
that Eros does not have color contrasts around the rims of craters
as Gaspra and Ida have because it is in a different weathering
environment in the solar system. Eros is in near-Earth space,
whereas Gaspra and Ida are in the main belt. This argues for
different rates of the competing processes of surface maturation
and impact cratering between near-Earth orbits and the asteroid
main belt.
indicating that since the formation of the crater the surface of
Eros has been weathered.
(9) These findings suggest that Eros's surface may be
completely weathered due to its exposure to the space
environment. Our results suggest that space weathering
processes affect Eros' spectral properties, but do not fully
explain the differences between Eros and its proposed analog
meteorites, the ordinary chondrites and/or the primitive
achondrites.
(10) Differences in optical maturation trends between Eros
and other S-type asteroids studied by spacecraft may indicate
different rates of the competing processes of surface maturation
and impact cratering between objects in near-Earth orbits and
objects in the main asteroid belt.
SUMMARY
Acknowledgnzents-We gratefully acknowledge the help of J. Joseph,
B. Carcich, A. Harch, M. Ockert-Bell, and B. Owen. Helpful reviews
were provided by B. Hapke and A. W. Harris (DLR). B. E. C.
especially thanks the NEAR project and the NEAR Science teams
for a wonderful mission. This research is supported by the NEAR
In summary, we find:
(1) The background surface of Eros in and around Psyche
crater is 3 2 4 0 % darker than the bright materials exposed on
steep slopes in the crater walls at spatial scales of -620 m/
spectrum.
(2) The dark material is 2 4 % redder than the bright material
at 1.5-2.4 pm, and has a 1 p m band which is 2 4 % broader at
0.8-1 .Opm. The total spectral variation is 4-8% peak-to-peak
from the normalized mean at 1.3 pm.
(3) The albedo and contrasts in Psyche crater are not
consistent with a cause due solely to either grain size or
compositional variations.
(4) The contrasts and associated spectral variation trends
observed in Psyche are consistent with a lunar-like optical
maturation which combines accumulation of SMFe with an
enhancement of the dark spectrally neutral component of Eros'
surface materials (presumably shocked silicates).
(5) Bright materials in Psyche appear optically less mature
than dark materials, but do not spectrally match meteorites.
The bright materials are approximately the same brightness as
meteorites, however their spectra are 33% too red from 1.5 to
2.4 pm.
( 6 ) We calculate that average Eros surface materials are
30-40% darker than meteorites. We also find that when
differences in reflectance measurement units and photometric
geometry are accounted for, most meteorites are 2-3x brighter
than asteroids.
(7) If Eros is compositionally similar to ordinary chondrites,
and if the darker materials on Eros are due to space weathering
(optical maturation of a fresher brighter substrate), then the
weathering process must be a strange two-step mechanism
whereby materials are reddened before darkened (bright
materials are redder than meteorites), and darkened more than
reddened (albedo contrasts are stronger than color contrasts).
(8) At the scale of our observations Psyche crater did not
excavate materials which are spectrally similar to meteorites.
Nor does Psyche have any bright crater ejecta deposits,
Project under APL contract number 779809.
Editorial handling: W. F. Huebner
REFERENCES
ADAMSJ. B. AND FILICEA. L. (1976) Spectral reflectance from 0.4
to 2.0 microns of silicate rock powders. J. Geophys. Res. 72,
5705-57 1 5.
BELLJ. F., 111 E T A L . (2002) Near-IR reflectance spectroscopy of433
Eros from the NIS instrument on the NEAR mission. 1. Low
phase angle observations. /carus (in press).
BRITTD. AND PIETERS
C. (1 989) The physical, chemical, and spectral
properties of opaque phases in optically altered ordinary
chondrites (abstract). Bull. Am. Astron. SOC.21, 967.
BRITTD., BELLJ. F., HAACK H. AND SCOTT E. R. D. (1992) The
reflectance spectrum of troilite (abstract). Lunar Planet. Sci. 23,
167-1 68.
BUmINE T., McCoy T., NITTLERL. AND BELLJ. F., 111 (2001) Could
433 Eros have a primitive achondritic composition? (abstract).
Lunar Planet. Sci. 32, #1860, Lunar and Planetary Institute,
Houston, Texas, USA (CD-ROM).
D. B. J., ROBINSONM. S., EDWARDS
K., THOMAS
P. C. AND
BUSSEY
JOSEPHJ. (2001) Digital cartographyof433 Eros using the NEAR
Shoemaker multispectral imager. Icarus (in press):
P., HEAD
CARR M. H., KIRK R., MCEWENA,, VEVERKA J., THOMAS
J. AND MURCHIEs. (1994) The geology of Gaspra. /carus 107,
61-71.
CHAPMAN
C. R. (1 996) 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.
CLARK
B. E. (1 995) Spectral mixing models of S-type asteroids. J.
Geophys. Res. 100, 14 443-14 456.
CLARK
B. E. AND JOHNSONR. E. (1 996) Interplanetary weathering:
Surface erosion in outer space. EOS 77, 141-145.
CLARKB. E., FANALE
F. P. AND SALISBURY
J. (1992) Meteoriteasteroid spectral comparison: The effects of comminution,
melting, and recrystallization. Icarus 97,288-297.
P., BELLJ. F., 111, PETERSONC., VEVERKA
CLARK
B. E., HELFENSTEIN
J., IZENBERG N., DOMINGUE
D., WELLNITZ
D. AND MCFADDEN
L. (2002) NEAR infrared spectrometer photometry of asteroid
433 Eros. Icarus (in press).
Space weathering on Eros
CLOUTIS E. AND GAFFEY M. J. (1991) Pyroxene spectroscopy
revisited: Spectral-compositional correlations and relationship
to geothermometry. J. Geophys. Res. 96, 22 809-22 826.
CLOUTIS E., GAFFEY M., TACKOWSKI
T. AND REED K. (1986)
Calibration of phase abundance, composition, and particle size
distribution for olivine-orthopyroxene mixtures from reflectance
spectra. J. Geophys. Res. 91, 11 641-1 1 653.
D. AND DOUGHERTY
DOLLFUSA., WOLFF M., GEAKE J., LUPISHKO
L. (1989) Photopolarimetry of asteroids. In Asteroids II (eds.
R. P. Binzel, T. Gehrels and M. S. Matthews), pp. 594-616. Univ.
Arizona Press, Tucson, Arizona, USA.
FANALEF. P., CLARKB. E. AND BELLJ. F. (1 992) A spectral analysis
ofordinary chondrites, S-type asteroids and their component minerals:
Genetic implications. J. Geophys. Res. 97,20 863-20 874.
R., BOTTKEW.,
GEISSLERP., PETITJ. M., DURDAD., GREENBERG
NOLANM. AND MOORE J. (1 996) Erosion and ejecta reaccretion
on 243 Ida and its moon. lcarus 120, 140-157.
HAPKEB. W. (I98 1) Bidirectional reflectance spectroscopy 1. Theory.
J. Geophys. Res. 86, 3039-3054.
HAPKEB. W. (1 984) Bidirectional reflectance spectroscopy 111.
Correction for macroscopic roughness. lcarus 5 9 , 4 1-59.
HAPKEB. W. (1 986) Bidirectional reflectance spectroscopyW. The extinction
coefficientand the opposition effect. Icarus 67,264280.
HAPKEB. (1 993) Theory of Reflectance and Emittance Spectroscopy.
Cambridge University Press, New York, New York, USA. 455 pp.
HAPKEB. (2000) How to turn OC's into S's: Space weathering in the
asteroid belt (abstract). Lunar Planet. Sci. 31, #1087, Lunar and
Planetary Institute, Houston, Texas, USA (CD-ROM).
HAPKEB. (2001) Space weathering from Mercury to the asteroid
belt. J. Geophys. Res. 106, 10 039-10 073.
HELFENSTEIN
P. ET AL. ( 1 994) Gahleo photometry of asteroid 95 1
Gaspra. Icarus 107, 37-60.
HELFENSTEIN
P. ET AL. ( 1 996) Galileo photometry of asteroid 243
Ida. Icarus 120, 48-65.
HIROIT. AND SASAKIS. (2001) Compositional dependency of space
weathering on the SIAIRIV asteroids. Meteorit. Planet. Sci. 36,
15 87- 1596.
HOE F. AND CINTALAM. (1996) Impact experiments related to the
evolution of planetary regoliths. Meteorit. Planet. Sci. 32, 179-209.
HOUSESK. R. AND WILKENING L. L. (1982) Regoliths on small bodies
in the solar system. Ann. Rev. Earth Planet. Sci. 10, 355-376.
IZENBERG N. I. ET AL. (2000) In-flight calibration of the near Earth
asteroid rendezvous mission's near infrared spectrometer I. Initial
calibrations. Icarus 148,550-571.
JOHNSONT. A N D FANALEF. P. (1973) Optical properties of
carbonaceous chondrites and their relationship to asteroids. J.
Geophys. Res. 78,8507-8515.
KEILK. AND FREDRIKSSON
K. (1 964) The iron, magnesium, and
calcium distribution in coexisting olivines and rhombic pyroxenes
of chondrites. J. Geophys. Res. 69,3487-35 15.
KEILK., BELLJ. F. AND BRITTD. T. (1992) Reflection spectra of
shocked ordinary chondrites and their relationship to asteroids.
Icarus 98,43-53.
J., THOMAS
P. C., HELENSTEINP., BELTONM. J. S.,
LEE P., VEVERKA
CHAPMAN
C., GREELEYR., PAPPALARDO
R., SULLIVAN
R. AND
HEADJ., III (1996) Ejecta blocks on 243 Ida and on other
asteroids. Icarus 120, 87-105.
1637
LUCEYP. G. (1 998) Model near-infrared optical constants of olivine
and pyroxene as a function of iron content. J. Geophys. Res.
103,1703-1713.
McCoy T., NITTLERL., BURBINET. H., TROMBKA
J. I., CLARKP. E.
AND MURPHY
M. E. (2000) Anatomy of a partially-differentiated
asteroid: A "NEAR'-sighted view of Acapulcoites and Lodranites.
lcarus 148,29-36.
MCKAY D., SWINDLE T. AND GREENBERG R. (1989) Asteroidal
regoliths: What we do not know. In Asteroids II (eds. R. Binzel,
T. Gehrels and M. S. Matthews), pp. 617-642. Univ. Arizona
Press, Tucson, Arizona, USA.
MEIBOM
A. AND CLARK B. E. ( I 999) Evidence for the insignificance
of ordinary chondrite material in the asteroid belt. Meteorit.
Planet. Sci. 34, 7-24.
MOROZL., FISENKO
A,, FEMJONOVA
L., PIETERS c . AND KOROTAEVA
N. (1996) Optical effects of regolith processes on S-asteroids as
simulated by laser shots on ordinary chondrite and other mafic
materials. lcarus 122, 366-382.
MURCHIES. L. AND PIETERSC. M. (1996) Spectral properties and
rotational spectral heterogeneity of 433 Eros. J. Geophys. Res.
101,2201-2214.
MURCHIES. L. ET AL. (2002) Color variations on Eros from NEAR
multispectral imaging. Icarus (in press).
PIETERSC. M. (1983) Strength of mineral absorption features in the
transmitted component of near-infrared light: First results from
RELAB. J. Geophys. Res. 88,9534-9544.
PIETERSC. M. AND MCFADDENL. (1994) Meteorite and asteroid
reflectance spectroscopy: Clues to early solar system processes.
Ann. Rev. Earth Planet. Sci. 22, 457-497.
PIETERSC., TAYLORL. A., NOBLE S. K., KELLERL. P., HAPKE B.,
MORRIS R. V., ALLEN C. C., MCKAYD. S. AND WENTWORTH s.
(2000) Space weathering on airless bodies: Resolving a mystery
with lunar samples. Meteorit. Planet. Sci. 35, 1101-1 108.
SASAKI S., NAKAMURA
K., HAMABEY., KURAHASHI
E. AND HIROIT.
(2001) Production of iron nanoparticles by laser irradiation in a
simulation of lunar-like space weathering. Nature 410,555-557.
SEARS D. AND DODD R. ( 1 988) Overview and classification of
meteorites. In Meteorites and the Early Solar System (eds. J. F.
Kerridge and M. S. Matthews), pp. 3-3 1. Univ. Arizona Press,
Tucson, Arizona, USA.
SULLIVAN R. ETAL. (1996) Geology of243 Ida. kurus 120, 119-139.
SUNSHINE J., PIETERS c. AND PRATT s. (1990) Deconvolution of
mineral absorption bands: An improved approach. J. Geophys.
Res. 95,6955-6966.
TEDESCOE. F., ED. (1 992) The IRAS Minor Planet Survey. Tech.
Rep. PL-TR-92-2049, Philips Laboratory, Hanscom Air Force
Base, Massachusetts, USA. 220 pp.
THOMASP. C., JOSEPHJ., ROBINSON
M., MURCHIES., VEVERKA
J.,
CLARKB. E. AND CHAPMAN C. (2002) Shape, slopes, and slope
processes on Eros. Icarus (in press).
TROMBKAJ. I. E T A L . (2000) The elemental composition of asteroid
433 Eros: Results of the NEAR-Shoemaker x-ray spectrometer.
Science 289,2101-2105.
VEVERKA J. ETAL. (2000) NEAR at Eros: Imaging and spectral results.
Science 289, 1993-2228.
VEVERKA J. ETAL. (2001) Imaging of small scale features on 433 Eros
from NEAR: Evidence for a deep regolith. Science 292,484-488.