Icarus 299 (2018) 386–395

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Icarus

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Spectral decomposition of asteroid Itokawa based on principal

component analysis

Sumire C. Koga

a , ∗, Seiji Sugita

a , b , c , Shunichi Kamata

d , Masateru Ishiguro

e , Takahiro Hiroi f , Eri Tatsumi b , Sho Sasaki g

a Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba, Japan b Department of Earth and Planetary Science, Graduate School of Sciences, The University of Tokyo, Bunkyo, Tokyo, Japan c Research Center for Early Universe, Graduate School of Science, The University of Tokyo, Tokyo, Bunkyo, Japan d Creative Research Institution, Hokkaido University, Sapporo, Hokkaido, Japan e Department of Physics and Astronomy, Seoul National University, Gwanak, Seoul, Korea f Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA g Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan

a r t i c l e i n f o

Article history:

Received 7 May 2015

Revised 10 July 2017

Accepted 4 August 2017

Available online 18 August 2017

Keywords:

Asteroid Itokawa

Asteroids

surfaces

Spectroscopy

a b s t r a c t

The heliocentric stratification of asteroid spectral types may hold important information on the early evo-

lution of the Solar System. Asteroid spectral taxonomy is based largely on principal component analysis.

However, how the surface properties of asteroids, such as the composition and age, are projected in the

principal-component (PC) space is not understood well. We decompose multi-band disk-resolved visible

spectra of the Itokawa surface with principal component analysis (PCA) in comparison with main-belt

asteroids. The obtained distribution of Itokawa spectra projected in the PC space of main-belt asteroids

follows a linear trend linking the Q-type and S-type regions and is consistent with the results of space-

weathering experiments on ordinary chondrites and olivine, suggesting that this trend may be a space-

weathering-induced spectral evolution track for S-type asteroids. Comparison with space-weathering ex-

periments also yield a short average surface age ( < a few million years) for Itokawa, consistent with the

cosmic-ray-exposure time of returned samples from Itokawa. The Itokawa PC score distribution exhibits

asymmetry along the evolution track, strongly suggesting that space weathering has begun saturated on

this young asteroid. The freshest spectrum found on Itokawa exhibits a clear sign for space weather-

ing, indicating again that space weathering occurs very rapidly on this body. We also conducted PCA on

Itokawa spectra alone and compared the results with space-weathering experiments. The obtained re-

sults indicate that the first principal component of Itokawa surface spectra is consistent with spectral

change due to space weathering and that the spatial variation in the degree of space weathering is very

large (a factor of three in surface age), which would strongly suggest the presence of strong regional/local

resurfacing process(es) on this small asteroid.

© 2017 Elsevier Inc. All rights reserved.

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1. Introduction

The spectral distribution of different spectral types of asteroids

may place an important constraint on the early evolution of the

Solar System (e.g., Walsh et al., 2011; DeMeo and Carry, 2014 ). As-

teroid spectral types are often classified employing principal com-

ponent analysis (PCA). In particular, the first and second princi-

pal components (PC1, PC2) derived from PCA are widely used (e.g.,

Tholen, 1984; Bus and Binzel, 2002; DeMeo et al., 2009 ). The fact

that asteroid spectra form a number of well-defined clusters in the

∗ Corresponding author.

E-mail address: sumire.c.koga@gmail.com (S.C. Koga).

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http://dx.doi.org/10.1016/j.icarus.2017.08.016

0019-1035/© 2017 Elsevier Inc. All rights reserved.

rincipal-component (PC) space (e.g., Tholen 1984 ) shows that PCA

s an effective approach for analyzing asteroid spectra. However,

he relationship between the positions of asteroids in the PC space

nd physical and/or chemical states is not yet well understood.

ore specifically, multiple physical and chemical factors, such as

he mineral composition, grain size, and surface age (i.e., space

eathering), as well as observational conditions, such as phase

ngle and solar standard correction, are known to affect appar-

nt PC scores of asteroid spectra (e.g., Binzel et al., 2004; Reddy

t al., 2015 ), but which factor controls which PC is poorly under-

tood. One approach that can be taken to resolve these issues is

o apply PCA to disk-resolved spectra of an asteroid with geologic

ontext.

S.C. Koga et al. / Icarus 299 (2018) 386–395 387

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The Japanese Hayabusa spacecraft made a rendezvous with an

-type asteroid Itokawa before obtaining samples from its surface

e.g., Fujiwara et al., 2006 ). During the rendezvous, the spacecraft

btained high spatial-resolution remote sensing data and revealed

hat the surface of asteroid Itokawa had notable spectral varia-

ion (e.g., Ishiguro et al., 2007 ). Detailed analyses of visible and

ear-infrared spectra of Itokawa have shown that space weather-

ng contributes to spatial variation in reflectance spectra of the

tokawa surface ( Hiroi et al., 2006; Ishiguro et al., 2007 ). However,

t remains unclear whether there is a component other than space

eathering in the Itokawa spectral variation. Additionally, the de-

rees of space weathering on Itokawa have not been quantitatively

ompared with those of main-belt asteroids (MBAs) and labora-

ory experiments. Furthermore, the spectroscopic properties of the

ayabusa sampling site, such as the degree of space weathering,

ere not extensively addressed in previous remote sensing stud-

es. In other words, the geologic context of the sampling site of

he Hayabusa mission on the surface of Itokawa has not been fully

xplored with Hayabusa remote sensing data yet.

In this study, we perform PCA on high-spatial-resolution multi-

and images of Itokawa and compare the multi-band spectra with

he spectral distribution of main-belt asteroids and results of

pace-weathering simulation experiments using olivine and ordi-

ary chondrites, which are the analog to Itokawa ( Abe et al., 2006;

suchiyama et al., 2011 ).

. Image analysis methods

.1. Data

We used four image sets of six visible bands with central wave-

engths of 0.381, 0.429, 0.553, 0.700, 0.861, and 0.960 μm taken

y the Asteroid Multi-band Imaging Camera (AMICA) ( Saito et al.,

006; Ishiguro et al., 2010 ) onboard the Hayabusa spacecraft. Im-

ges of each set were taken at approximately the same distance of

20 km from the asteroid mass center. These image sets cover the

ntire surface of Itokawa. The spatial resolution of the images used

n this study is ∼2 m/pixel.

.2. Image calibration and processing

We first remove periodic electromagnetic noise imposed on

ome of the images, presumably generated by interference from

ther onboard instruments, by subtracting the superposition of

wo sine waves. Ishiguro et al. (2010) examined dark current and

he linearity between light intensity and image count values, find-

ng that the errors are negligible for a typical signal level of

tokawa images; dark current is less than 0.4% and the error in

inearity is less than 0.3%. Thus, we did not conduct dark sub-

raction or linearity correction. Sensitivity uniformity was cali-

rated by dividing the images by flat-field images obtained be-

ore the flight. The count values of the images were converted

Table 1

The central wavelengths (μm) and effective wavelength widths (μm) of th

Filter name of AMICA (ECAS) AMICA a

Central wavelength Effective wavele

ul (u) 0.381 0.045

b 0.429 0.108

v 0.533 0.072

w 0.7 0.070

x 0.861 0.081

p 0.96 0.075

a Tedecso et al. (1982) . b Ishiguro et al. (2010) .

o reflectance normalized at v-band (0.553 μm) using the result

f Ishiguro et al. (2010) , where the disk-integrated intensity was

alibrated to match their telescope observation data of the disk-

ntegrated average spectrum of Itokawa. Scattered-light correction

as conducted employing the method given by Ishiguro (2014) .

The images of different bands were co-registered to v-band im-

ges by parallel translation of image shifting. The optimal transla-

ional shifts were chosen to minimize the total variance of the ra-

io (to v-band) image values in the area of interest in the image,

ince a ratio image with unsuccessful co-registration gives both

ery high and low values around shadows, where the count value

hanges discontinuously in original images. We used the minimum

nit of 0.01 pixels for shifting because this small shift still resulted

n a notable reduction in variance. Owing to the spin of the aster-

id in the time between the taking of images with the six band

lters, images are slightly distorted from each other. Thus, simple

hifting cannot co-register all the pixels in the images. There were

navoidable mismatches particularly in peripheral areas and areas

ear shadows of large boulders. We evaluated the degree of co-

egistration mismatch by examining the spatial continuity obtained

n ratio images. As discussed above, a mismatch in co-registration

ould lead to ratio images with large variations near the disk edge

nd shadows. For the nominal case, pixels with intensity more

han 4% greater or smaller than the intensity of adjacent pixels

re assumed to be resulted from unsuccessful co-registration and

emoved from further analyses. We used different threshold val-

es (from 2% to 6%) and confirmed that the specific choice of

his threshold for continuity does not affect the results of further

nalyses.

.3. Comparison with MBA and meteorite spectra

We first performed PCA on Itokawa surface spectra alone to ex-

ract the most dominant factor of the variation in the Itokawa sur-

ace spectra. We refer to the resulting PCs as Itokawa PCs. Second,

CA was performed on spectra of 533 asteroids (mainly MBAs) ob-

erved in the Eight-color Asteroid Survey (ECAS) ( Tedecso et al.,

982 ) (hereafter ECAS asteroids). Itokawa surface spectra were then

ecomposed with the PCs of ECAS asteroid spectra (i.e., PCA was

erformed on the ECAS dataset only, and then the eigenvectors

ere used to calculate PC scores for Itokawa surface and mete-

rites and olivine data). We refer to these PCs as ECAS PCs. The

entral wavelengths of AMICA filter bands are approximately the

ame as those used in the ECAS as shown in Table 1 . The differ-

nce is much smaller than the bandwidth of AMICA and ECAS.

hus, we used reflectance data without any correction or spline

nterpolation.

Third, spectra of eight ordinary chondrite samples irradiated by

pulse laser and one olivine sample irradiated by Ar + ion were

aken from previous studies ( Hiroi et al., 2011; Brunetto et al.,

006b ) and NASA’s RELAB database at Brown University. These

pectra were decomposed with the ECAS PCs. It is noted that PCA

e observational filters of AMICA and ECAS.

ECAS b

ngth width Central wavelength Effective wavelength width

0.359 0.060

0.437 0.090

0.55 0.057

0.701 0.058

0.853 0.081

0.948 0.080

388 S.C. Koga et al. / Icarus 299 (2018) 386–395

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on different data sets (e.g., Itokawa surface points vs. MBAs) would

yield different PCs. Thirty three spectra of different level of laser-

irradiation conditions of eight meteorites and one olivine samples

were used for analysis. The number of spectra on an individual

Itokawa image ranges from 15,859–37,227; i.e., The array sizes of

Itokawa images are from ∼90 × 180 to ∼130 × 260 pixels.

3. Analysis results and discussion

3.1. PCA results from Itokawa spectra data

The result of PCA of Itokawa spectra indicates that Itokawa has

a dominant PC1 and a much weaker PC2. The contributions of

PC1 and PC2 to the total variance are 61%–79% and 22% −31%, re-

spectively, for all four sets of images. The spectral pattern of PC1

obtained from Itokawa spectra only (hereafter Itokawa PC1) has

a distinctive red slope consistent with the typical reddening ef-

fect of space weathering ( Fig. 1 ). Fig. 2 is a map of the Itokawa

PC1 score, showing that the score is higher in regions that have

regolith-covered areas (e.g., Muses-C) and lower areas within pos-

sible craters (e.g., Komaba crater). This trend is consistent with

the map of the degree of space weathering obtained employing

the spectral inflection method of Ishiguro et al. (2007) . These re-

sults strongly suggest that Itokawa PC1 represents space weather-

ing. The PC1 score map shows that the freshest terrains on Itokawa

are found around both topographic highs, such as a few areas on

Fig. 1. (a) Spectral pattern of the principal components of the six-band spectra of Itokaw

principal component (PC1) has a distinctive red spectral slope between wavelengths of 0

space weathering. (b) and (c) Comparison between Itokawa PC1 and a typical weathering

(the gray dots) is taken from Fig. 4 of Brunetto et al. (2007) , showing the spectral rati

Itokawa PC1 is scaled for the comparison ((b) 0.27 ∗PC1 + 0.48, (c) 0.29 ∗PC1 + 0.44). (For i

to the web version of this article.)

he “neck” part, and rough terrain, such as several areas in Ohsumi

egio and a few areas on the western side of the “head.”

.2. Comparison between Itokawa and MBA spectra

The spectra of ECAS asteroids (mostly MBAs), the Itokawa sur-

aces, and meteorites are next compared in ECAS PC1-2 space

Fig. 3 ). The spectra of Itokawa surfaces are distributed along a line

n the ECAS PC space extending widely from the Q-type area to a

igh-population-density area in the S-type cluster. It is remarkable

hat spectral variation on asteroid Itokawa is comparable in size

o the variation of the entire S-type-asteroid complex in the PC1-

space. Another important property of the distribution of Itokawa

pectra is its asymmetricity; the histogram along the Itokawa dis-

ribution line in the ECAS PC space ( Fig. 4 ) has a longer tail on one

ide than on the other.

We conducted the same PCA with four sets of six-band images

overing different areas on Itokawa and obtained the same results,

trongly suggesting the robustness of the above result ( Fig. 2 ). Fur-

hermore, the direction of the linear trend of the Itokawa surface

pectral distribution in the ECAS PC space and that of Itokawa PC1

gree well each other; the inner product of their direction vectors

s greater than 0.9.

.3. Comparison with laboratory simulation of space weathering

Pulsed laser irradiation experiments simulating space weath-

ring due to micrometeorite bombardment have shown that the

a surfaces. Error is from standard deviation for four different image sates. The first

.4 and 0.8 μm. This spectral pattern coincides with the typical reddening effect of

continuum obtained in laboratory experiments. The space weathering continuum

o of irradiated samples ((b) 27 J/cm

2 laser, (c) 52 J/cm

2 laser) to pristine samples.

nterpretation of the references to color in this figure legend, the reader is referred

S.C. Koga et al. / Icarus 299 (2018) 386–395 389

Fig. 2. (Top) V-band images of four data sets used in our analysis. The numbers shown in the figure refer to the following regions. 1: Shirakami slope, 2: Mountainview

boulders, 3: Noshiro smooth terrain, 4: Yatsugatake ridge, 5: Muses-C, 6: Komaba crater, 7: Uchinoura crater, 8: LINEAR crater, 9: Sagamihara, 10: Ohsumi crater, 11: Sanriku

ridge, 12: Miyabaru crater, 13: Arcoona crater. Analyzed rough and smooth terrains are indicated with light blue and pink fonts and lines, respectively. (Middle) Spatial map

of the PC1 score from PCA on only Itokawa surface spectra. The redder color indicates higher values of the PC1 score, and the bluer color lower values. (Bottom) Spatial map

of the Itokawa PC2 score. 2 × 2-pixel binning was applied. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this

article.)

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pectra of silicate mineral surfaces change as nanophase ion par-

icles form ( Sasaki et al., 2001; Brunetto et al., 2006a ). Heavy-ion

rradiation simulating solar-wind bombardment also leads to sim-

lar spectral changes ( Strazzulla et al., 2005 ). Comparison between

steroids and laboratory experiments suggests that space weath-

ring on asteroids may be dominated by ion irradiation during

he early stage and by micrometeorite bombardment during the

ate stage ( Vernazza et al., 2009 ). We compared spectra on the

tokawa surface analyzed in this study to those of ordinary chon-

rites with pulsed laser irradiation and olivine with heavy-ion irra-

iation ( Hiroi et al., 2011; Brunetto et al., 2006b ; RELAB database)

Fig. 3 ). The comparison indicates that the direction of movement

n meteorite and olivine spectra induced by laser and heavy-ion ir-

adiation coincides with that of the trend of Itokawa surface spec-

ra in the ECAS PC space. More specifically, the regression line is

iven by 0.58 × ECAS PC1 – 0.81 × ECAS PC2 = 1.5. Furthermore, the

ate of change in the meteorite and olivine spectra in the PC space

ecreases as irradiation dose increases. This strongly suggests sat-

ration effect of space weathering. The asymmetric distribution of

tokawa spectra ( Fig. 4 ) is also consistent with the saturation ef-

ect. More specifically, the half width for the half maximum of the

opulation along the above regression line is ∼0.3 unit for higher

egrees of space weathering and is ∼0.5 for lower degrees of space

eathering ( Fig. 4 ). Here, the unit is given by PC SW

= 0.81 × ECAS

C1 + 0.58 × ECAS PC2. The ratio of maximum deviations in the

rojected PC space measured from the median value ( �PC SW

∼0.3) to more mature spectra ( �PC SW

∼ 0.8) and less mature spec-

ra ( �PC SW

∼ −1.5). The fact that similar saturation effect is seen

n both Itokawa data and laboratory experiments suggests that the

ame spectral change observed in laboratories is actually proceed-

ng on the asteroid. We note that the distribution would be sum

f the results of both space weathering and resurfacing. The asym-

etric distribution of the spectra suggests that the timescales of

pace weathering and resurfacing are comparable.

Furthermore, we compared the spectra of areas on the Itokawa

urface whose spectral PC scores are similar to those of “space-

eathered” chondrite samples and the actual spectra of those sam-

les. The actual multi-band spectra of “space-weathered” chon-

rites and Itokawa spectra agree well with each other as shown

n Fig. 5 . Moreover, the spectral change obtained with laser irradi-

tion on olivine powder samples (Fig. 4 in Brunetto et al., 2007 )

xhibits a very similar pattern to the PC1 obtained by our analysis

f Itokawa surface spectra ( Fig. 1 b), supporting our interpretation

hat Itokawa PC1 represents space weathering. Here it is noted that

he spectral component of space weathering obtained by Brunetto

t al. (2007) is the ratio of spectra before and after the laser ex-

eriments, not the difference (i.e., subtract) between the two. The

ifference spectra (i.e., subtract) between before and after space-

eathering experiments by Brunetto et al. (2007) does not fit the

C1 very well. This is most likely because the amplitude of spec-

ral change due to experiments is too large to ignore the non-linear

ffect of logarithmic spectral evolution.

Comparison of the spectral change between space weathering

xperiments of ordinary chondrites and the actual Itokawa spec-

ral variation also allows us to estimate the time scale of space

eathering. The time required to “space-weather” pristine ordi-

ary chondrites to the average Itokawa spectrum is estimated to

e approximately 10 8 yr for the micrometeorite bombardment rate

sed by Sasaki et al. (2001) . We can also estimate the period of

pace weathering by the solar wind from ion irradiation experi-

ents. However, no ion irradiation experiments on L or LL chon-

rite samples that provide a good spectral match with Itokawa

pectra have been reported in the literature. Nevertheless, based

n ion irradiation experiments with different minerals, Brunetto

t al. (2006b) proposed a model to describe spectral change due

o cosmic-ray-induced space weathering. This empirical law gives

pace-weathered spectrum as a function of nuclear displacement d,

he number of nuclear displacements per unit surface area, propor-

ioned to ion fluence ( Brunetto and Strazzulla, 2005 ). Using these

odel and spectra, we obtained the shift distance �PC SW

of olivine

pectra in the ECAS PC space as a function of nuclear displace-

ents d . The obtained �PC SW

can be fit well with a power law

f d . We thus used this power law to estimate exposure time of

tokawa surface spectra. We then used three chondrite samples

sed in Fig. 3 as the pre-space-weathered material for Itokawa sur-

ace; we take �PC SW

from the average of the three L/LL chondrite

amples to the Itokawa average along the linear trend of Itokawa

pectra in the ECAS PC space ( Fig. 3 ) as for the spectral change

ue to space weathering. When the logarithm of �PC SW

is plot-

ed as a function of the logarithm of either the laser energy dose

r nuclear displacements ( d ), the data points exhibit a linear trend

Fig. 6 ). This strongly suggests a power-law relation between the

rradiation energy and �PC SW

. The best-fit power-law exponents

or the laser experiments and ion experiments are 0.60 ± 0.05 and

390 S.C. Koga et al. / Icarus 299 (2018) 386–395

Fig. 3. (a) Comparison among Itokawa spectra, ECAS spectra, and spectra of “space weathered” samples in laboratories. The letters indicate ECAS asteroids (Bus taxonomy).

ECAS asteroids with no type assignment are shown with black dots. Small orange dots indicate Itokawa surfaces and the blue circle indicates Itokawa average spectrum. The

circles, triangles, squares indicate laser-irradiated samples (Nulles (H6), Appley Bridge (LL6), Chateau-Renard (L6)), respectively. The blue diamond indicates the mean of the

three (pre-irradiation) chondrites. The orange diamonds indicate an olivine sample in the experiment by Brunetto et al. (2006b) . The blue dashed line and orange doted line

are the tracks of their spectral change calculated from the space weathering empirical model by Brunetto et al. (2006b) . Both laser energy (mJ) and ion fluence (Ar + /cm

2 ) are

shown. The black thin line is a regression line for Itokawa spectral data points, and given by 0.58 × ECAS PC1 - 0.81 × ECAS PC2 = 1.5. The arrowhead indicates the direction

for the positive sign of PC SW

. (b) The decomposition of the other five chondrites spectra that were irradiated with laser in laboratories ( Table 2 ) with PCs determined by

ECAS asteroids spectra. Symbols are the same as in (a), but asteroids are not shown for clarity of the figure. (For interpretation of the references to color in this figure legend,

the reader is referred to the web version of this article.)

Fig. 4. Histogram of Itokawa surface spectra along the Itokawa distribution line (the

black thin line in Fig. 3 ).

Table 2

The samples used in this study.

Chondrite/Mineral Sample form RELAB database ID/Referrence

Appley Bridge (LL6) < 125 μm-pellet Hiroi et al. (2011)

Chateau Renard (L6) < 125 μm-pellet Hiroi et al. (2011)

Nullus (H6) < 125 μm-pellet Hiroi et al. (2011)

Cynthiana (L/LL4) < 125 μm-pellet OC-TXH-015-D

Appley Bridge (LL6) Chip OC-TXH-012-A

Chateau-Renard (L6) Chip OC-TXH-011-A

Hamlet (LL4) Chip OC-TXH-002-A

Hedjaz (L3-6) Chip OC-TXH-016-A

Olivine Pressed powder Brunetto et al. (2006b)

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0.60 + 0.06/ −0.04, respectively. This coincidence in the power-law

exponents between laser and ion irradiation may reflect similar-

ity in the saturation process between these two space-weathering

echanisms; i.e., the two space weathering mechanisms may pro-

eed in a similar manner. The shift distance �PC SW

of the av-

rage spectrum of Itokawa from the average of the three chon-

rites used in the laser experiments is 3.8 ± 0.3. The corresponding

uclear displacement ( d ) is 2.0 × 10 19 (displacements/cm

2 ). The

S.C. Koga et al. / Icarus 299 (2018) 386–395 391

Fig. 5. Examples of the comparison between laser-irradiated chondrite spectra (red

dashed lines) and Itokawa spectra (solid lines) that have similar ECAS PC1 scores.

Black dashed lines indicate the spectrum of Cynthiana meteorite pellet sample

whose particle size is smaller than 125 μm. (a) Comparison between a spectrum

of Cynthiana pellet sample after 5-mJ laser irradiation and several Itokawa spectra

with low ECAS PC scores ( − 0.95 to − 0.42). Note that the Itokawa spectral curves

are overlapping each other in the figure. (b) Comparison between a spectrum of

Cynthiana pellet sample after 15-mJ laser irradiation and several Itokawa spectra

with high ECAS PC scores (0.69–0.70). Similarly to (a), the Itokawa spectral curves

are overlapping each other. (For interpretation of the references to color in this fig-

ure legend, the reader is referred to the web version of this article.)

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elationship between the irradiation time scale t (yr) and nuclear

isplacement d (displacements/cm

2 ) is t = 2.53 × 10 −13 d at 2.9 AU

Brunetto et al., 2006b ). Because Itokawa is at 1.3 AU from the Sun

nd the flux of solar wind decreases proportionally to the inverse

quare of distance from the Sun, the relationship at the Itokawa’s

ocation is t = 2.53 × 10 −13 d . Thus, the shift distance �PC SW

from

he pristine chondrite (the average of the three chondrites) to

tokawa average is (1 + 2/ −0.6) × 10 6 years. Here, the error is based

n uncertainty in both the shift distance �PC SW

and power-law

xponent. Uncertainty in �PC SW

is calculated from the scatter in

he pre-irradiation spectra of the three chondrites shown in Fig. 3 .

hat in power-law exponent is from upper and lower values in αnd β in the formula in Brunetto et al. (2006b) . It should be noted

hat this time scale gives an upper limit for the Itokawa average

pectral age, because the solar wind contains ions other than Ar +

nd the efficiency of space weathering induced by other ions, such

s He + , is likely high ( Loeffler et al., 2009 ).

Similarly, spatial distribution in surface exposure age can be

ssessed with the heterogeneity in the degree of space weather-

ng. The smallest ( ∼ −2) and largest ( ∼ 0.5) values of �PC SW

on

tokawa correspond to 0.45 Myr and 1.5 Myr, respectively. Thus, the

atio of oldest to youngest surface exposure ages is about three.

his ratio depends on the initial spectrum and model parameters

and β as discussed above for mean Itokawa surface age. An error

ropagation calculation yields based on uncertainty in the three

actors, α and β in the formula by Brunetto et al. (2006b) , and ini-

ial spectrum, lead to an estimate that the ratio of the oldest to

oungest surface exposure ages on Itokawa is 3.4 ± 0.8.

.4. Itokawa PC2

The interpretation of Itokawa PC2, which has a peak at 0.55 μm,

s not straightforward as that of Itokawa PC1. The spectral pat-

ern of PC2 itself does not resemble the spectra of particular chon-

ritic minerals or other geologic materials. Comparison among lo-

al spectra with different PC2 scores ( Fig. 7 ) exhibits distinctive

ifference among them, but these end-member spectra or interme-

iate spectra do not resemble the spectra of particular chondritic

inerals or other geologic materials either. However, the spatial

istribution of the PC2 score appears to correlate to the rough-

ess/smoothness distribution on Itokawa. To examine the relation-

hip between the surface morphology and PC2 score, we measured

ean values of the PC2 score in six areas having smooth terrains

nd eight areas having rough terrains as indicated in Fig. 2 . Here,

he distinction between smooth and rough terrains is based on the

ork of Demura et al. (2006) . The results of the analysis indicate

hat all areas of smooth terrain have higher PC2 scores than all ar-

as of rough terrain ( Fig. 8 ).

One possible mechanism by which the roughness/smoothness

f terrain affects the reflectance spectra is the grain size effect.

hen the grain size increases or decreases, the spectral undulation

f the same silicate minerals generally decreases or increases, re-

pectively. If rough terrain has coarser grains, it would have higher

pectral peaks and deeper absorption bands, but the wavelength of

hese peaks and absorption bands do not change as a function of

rain size. Thus, the spectral component that reflects the grain size

ifference would resemble the spectrum of the mineral(s) whose

rain size(s) changes. More specifically, if grain size variation on

tokawa occurs regardless of the mineral species and spectral ab-

orption depth changes due to grain size change is independent

f mineral species, then the spectral pattern due to the grain size

hange should resemble the Itokawa spectrum. However, the spec-

rum of Itokawa has a peak around 0.7 μm, not around 0.55 μm as

een in PC2.

In contrast, if the change in grain size on Itokawa is dominated

y the grain size effect of a single species of mineral, then the

pectral pattern may resemble the spectrum of this mineral. In

act, olivines with Mg# between 86 and 90 exhibit a reflectance

eak around 0.55 μm ( Sunshine and Pieters, 1998 ). Chemical anal-

ses of particles retrieved from Itokawa revealed that 64% of min-

rals is olivine ( Tsuchiyama et al., 2011 ). If the grain size variation

f olivines is especially large and the olivines have a Mg# between

bout 86 and 90, then this may account for the spectral pattern

f PC2. However, more recent analyses of returned samples from

tokawa revealed that the chemical composition of Itokawa olivine

s more rich in Fe; Mg# of 70 to 73 ( Mikouchi et al., 2014; Komatsu

t al., 2015 ). The spectra of such olivine have a peak around 0.7 μm,

nconsistent with PC2. Thus, further investigation is necessary for

nderstanding the nature of PC2.

Nevertheless, we obtained the same spectral pattern of PC2

rom four different sets of images and the variance contribution

f PC2 was always significant ( ∼20–30%). These results support

hat PC2 is a real spectral component, strongly suggesting that

C2 reflects some real geologic processes and/or the properties of

tokawa surface materials.

. Implications for asteroid spectral evolution

The analysis results obtained in this study have a number of

mportant implications for the evolution of S-type asteroid spec-

ra and Itokawa samples brought back by Hayabusa. First, the

392 S.C. Koga et al. / Icarus 299 (2018) 386–395

Fig. 6. Shift distance �PC SW

in the ECAS PC space along the Itokawa distribution line versus the laser/ion irradiation energy and nuclear displacements ( d ) to samples.

Results for three meteorites in a laser-irradiation experiment and one olivine sample in an ion-irradiation experiment are plotted with two horizontal axes for the two

experiments.

i

2

(

w

M

I

p

i

s

c

f

(

w

w

2

(

g

I

l

s

o

r

w

f

o

w

b

e

p

results discussed in the previous section support that the domi-

nant fraction of the spectral variety seen on the Itokawa surface

is due to space weathering ( Abe et al., 2006; Hiroi et al., 2006 ).

The agreement between the distribution of Itokawa spectra and

spectral changes of meteorites in space weathering experiments in

terms of both direction and the pace of saturation strongly sug-

gests that the spectral distribution on the Itokawa surface in the PC

space may represent a spectral evolution track of asteroid Itokawa

due to space weathering. Furthermore, the fact that the distribu-

tion of Itokawa spectra in the ECAS-PC space span from the Q-

type region to the S-type region supports the hypothesis that Q-

type asteroids evolve to S-type asteroids through space weather-

ing ( Binzel et al., 2004 ). Here it is noted that S-type asteroids

cover a wide range of compositions, some of which may be signif-

icantly different from ordinary chondrites and that the way space

weathering occurs on all the S-type asteroids may not be the same

(e.g., Gaffey, 2010 ). Also, near-earth asteroids (NEA’s) may be bi-

ased samples of main-belt asteroids; LL-like compositions of as-

teroids may be preferentially collected in NEA’s (e.g., Dunn et al.,

2013 ). However, the continuous spectral distribution over Itokawa

surfaces found in this study clearly shows that ordinary-chondrite-

like spectra may evolve to Sq and S-types spectra through Q-type

spectra due to space weathering.

Second, our results strongly suggest that local resurfacing pro-

cesses play an important role on small asteroids. Global resurfacing

due to tidal forcing during Earth encounters has been proposed to

explain the young spectral age of Q-type asteroids ( Nesvorny et al.,

2005; Binzel et al., 2010 ). Such a global resurfacing process, how-

ever, may not explain the highly heterogeneous degree of space

weathering found on Itokawa. Regional or local processes, such as

impact cratering and landslides, on an asteroid may also play an

mportant role in resurfacing asteroid surfaces (e.g., Brunetto et al.,

015 ). More specifically, a model calculation by Shestopalov et al.

2013) showed that the spectra of S-type asteroids are consistent

ith active surface rejuvenation due to regolith shaking. In fact,

iyamoto et al. (2007) found evidence for regolith migration on

tokawa, which would play an important role in resurfacing. Also,

revious multi-band spectral mapping of Itokawa has revealed that

mpact cratering would have contributed to resurfacing on this

mall asteroid ( Ishiguro et al., 2007 ; Yoshikawa et al., 2015 ). Lo-

al variations of spectral age related to morphology have also been

ound on other asteroids, such as Ida ( Chapman, 1996 ) and Eros

Murchie et al., 2002 ). Large spatial variation in the degree of space

eathering on Itokawa quantified in this study is also consistent

ith local resurfacing processes suggested by these studies.

Third, the PC1 score for the Hayabusa sampling site ( Yano et al.,

006 ) is near the average value for the entire Itokawa globe

Fig. 2 ). Thus, the samples returned by Hayabusa may be a

ood representation of the average degree of space weathering of

tokawa.

Fourth, the average spectral age of Itokawa estimated from so-

ar wind flux may be consistent with noble-gas age of returned

amples, suggesting that solar wind may be the dominant source

f space weathering on Itokawa. It has been argued that the

eason why most MBAs have only moderate degrees of space

eathering much lower than that of the Moon is that the ef-

ect of space weathering does not accumulate easily on aster-

ids. More specifically, a thin top surface layer may rejuvenate

ithout erasing craters ( Shestopalov et al., 2013 ), and there may

e efficient regolith migration on the asteroid surface ( Miyamoto

t al., 2007 ). Consideration of these factors may help us inter-

ret the surface ages of asteroids. Several different kinds of ages

S.C. Koga et al. / Icarus 299 (2018) 386–395 393

Fig. 7. Comparison among Itokawa local spectra with different PC2 scores. Spectra

with low PC1 values ( −3.0 < PC1 < −2.5) are chosen to highlight the effect of PC2 on

fresh surfaces. The black dashed line is the average of Itokawa surface spectra. The

gray thin lines are individual local spectra with PC2 scores in the range indicated

in the Figure, and the red lines are their averages. Note that spectra with high PC2

scores have shallower absorption bands in short and long wavelength ranges. (For

interpretation of the references to color in this figure legend, the reader is referred

to the web version of this article.)

h

p

f

c

1

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

No

shiro

Sa

ga

mih

ara

(1

Sa

ga

mih

ara

(2

Mu

ses-C

(1)

Mu

ses-C

(2)

Uch

ino

ura

Sa

nriku

LIN

EA

R

Ya

tsug

ata

ke

Oh

sum

i

Miya

ba

ru

Sh

iraka

mi

Arco

on

a

Mo

un

tain

view

Me

an

PC

2 s

core

Region Name

Smooth terrains Rough terrains

Fig. 8. Mean PC2 score values of six smooth terrains and eight rough terrains. Error

bars are the standard deviations of the PC2 score in the terrains.

b

5

(

m

s

s

t

t

y

e

t

s

m

o

l

b

t

e

o

l

i

o

c

t

a

5

s

e

s

O

s

f

w

i

ave been estimated for asteroid Itokawa using both returned sam-

les and AMICA image analyses. The thicknesses of the layers

or which relevant ages are estimated are different. More specifi-

ally, a possible parent-body catastrophic disruption age of about

.3 Gyr ( Park et al., 2014 ), crater retention age of 75–10 0 0 Myr

ased on strength scaling ( Michel et al., 2009 ), boulder age of

–75 Myr ( Basilevsky et al., 2014 ), and Ne isotope age of < 8 Myr

Nagao et al., 2011 ) and 1.5 Myr ( Meier et al., 2014 ) have been esti-

ated. Thus, the upper limit of time scale of space weathering by

olar wind estimated in section 4.3 is much shorter than the pos-

ible parent-body disruption age and crater age but comparable to

he noble-gas (cosmic-ray exposure) ages.

Fifth, no area on Itokawa exhibits a spectrum that matches pris-

ine chondrite free from space weathering in our meter-scale anal-

ses; the entire globe of Itokawa has some degree of space weath-

ring. More specifically, the longer tail of space weathering his-

ogram ( Fig. 4 ) does not reach pristine chondrite spectra in the PC

pace. This observation suggests that the entire surface of Itokawa

ay be covered with space-weathered grains. Such global coverage

f small space-weathered grains could be achieved by electrostatic

evitation ( Lee, 1996; Hartzell and Scheeres, 2011 ). Another possi-

le reason for the lack of pristine spectra on Itokawa may be ex-

remely rapid space weathering process observed in ion irradiation

xperiments (e.g., Loeffler et al., 2009 ). Comparison of microscopic

bservations between silicate samples treated with ion beams in

aboratories and Itokawa samples also suggests that space weather-

ng on individual grains on Itokawa could occur very fast on order

f 10 3 years ( Noguchi et al., 2014 ). Although we cannot reach a de-

isive conclusion on the mechanism for this lack of spectrally pris-

ine surface on Itokawa, understanding this observation may hold

key for the space weathering process on fresh silicates.

. Conclusions

We compared surface spectra of the asteroid Itokawa with the

pectra of MBAs and ordinary chondrites subjected to space weath-

ring experiments. Our analysis results strongly suggest that the

pectra of actual asteroids evolve as space weathering proceeds.

ur results also suggest that the range of space weathering ob-

erved on Itokawa can account for spectral differences among dif-

erent asteroid spectral types; the disk-resolved spectra of Itokawa

ere found to span continuously from around Q-type to S-type. It

s remarkable that the range of the spectral variation for Itokawa is

394 S.C. Koga et al. / Icarus 299 (2018) 386–395

D

D

D

D

F

G

H

H

H

I

I

I

K

L

L

M

M

M

M

M

N

N

N

R

S

S

S

S

T

T

comparable to the range of the distribution of the entire S cluster

in the ECAS PC space.

Furthermore, a distribution of the Itokawa PC1 score (i.e., space

weathering) consistent with the geologic context was found in the

spatial map of the PC1 score; e.g., craters have low scores. The lo-

cation of the touchdown point of Hayabusa ( Yano et al., 2006 ) in

the global map of our space-weathering score suggests that the de-

gree of space weathering at the Hayabusa sampling point is close

to the average for Itokawa, suggesting in turn that the surface

exposure age of the touchdown point may be close to the aver-

age surface age of Itokawa. The comparison of results from ion-

irradiation experiments and Itokawa spectra suggests that the time

required for such spectral evolution from ordinary chondrite spec-

tra to the average spectrum of Itokawa is approximately a few mil-

lion years or shorter, consistent with the surface age estimated

from returned samples ( Nagao et al., 2011; Meier et al., 2014 ).

Finally, the large regional variation in the degree of space

weathering on Itokawa suggests that the resurfacing activity for

near-Earth asteroids may not be restricted to global resurfacing

due to close encounter with large planets, such as Earth, but

that local resurfacing, such as impact gardening, may also be

important.

Acknowledgments

The multi-band image data of Itokawa are available from the

Data Archives and Transmission System (DARTS, http://darts.jaxa.

jp/index.html.en ). Dataset name: hayamica. The ECAS data are

available from the NASA Planetary Data System (PDS, http://pds.

nasa.gov/tools/data-search/ ). Dataset name: EIGHT COLOR ASTER-

OID SURVEY V4.0. The spectral data of meteorites are available

from the NASA RELAB facility at Brown University ( http://www.

planetary.brown.edu/relab/ ). Dataset names are listed in Table 2 .

This work was supported by a Grant-in-Aid (No. 17H01175 ) and

Core -to- Core program “International Network of Planetary Sci-

ences” from the Japan Society for the Promotion of Science (JSPS).

References

Abe, M., et al., 2006. Near-infrared spectral results of asteroid Itokawa from theHayabusa spacecraft. Science 312, 1334–1338. doi: 10.1126/science.1125718 .

Basilevsky, A.T. , Head, J.W. , Horz, F. , Ramsley, K. , 2014. Survival time of meter-sizedrock boulders on the surface of Asteroid Itokawa. In: Lunar and Planetary Sci-

ence Conference, vol. 45, p. 1688. Abstract . Binzel, R.P., Rivkin, A.S., Stuart, J.S., Harris, A.W., Bus, S.J., Burbine, T.H., 2004. Ob-

served spectral properties of near-Earth objects: results for population distri-

bution, source regions, and space weathering processes. Icarus 170, 259–294.doi: 10.1016/j.icarus.2004.04.004 .

Binzel, R.P., Morbidelli, A., Merouane, S., DeMeo, F.E., Birlan, M., Vernazza, P.,Thomas, C.A ., Rivkin, A .S., Bus, S.J., Tokunaga, A .T., 2010. Earth encounters as

the origin of fresh surfaces on near-Earth asteroids. Nature 463, 331–334.doi: 10.1038/nature08709 .

Brunetto, R., Strazzulla, G., 2005. Elastic collisions in ion irradiation experiments: a

mechanism for space weathering of silicates. Icarus 179, 265–273. doi: 10.1016/j.icarus.20 05.07.0 01 .

Brunetto, R., Romano, F., Blanco, A., Fonti, S., Martino, M., Orofino, V., Verrienti, C.,2006a. Space weathering of silicates simulated by nanosecond pulse UV excimer

laser. Icarus 180, 546–554. doi: 10.1016/j.icarus.2005.10.016 . Brunetto, R., Vernazza, P., Marchi, S., Birlan, M., Fulchignoni, M., Orofino, V., Straz-

zulla, G., 2006b. Modeling asteroid surfaces from observations and irradiation

experiments: the case of 832 Karin. Icarus 184, 327–337. doi: 10.1016/j.icarus.2006.05.019 .

Brunetto, R., Roush, T.L., Marra, A.C., Orofino, V., 2007. Optical characterization oflaser ablated silicates. Icarus 191, 381–393. doi: 10.1016/j.icarus.2007.04.023 .

Brunetto, R., Loeffler, M.J., Nesvorny ,́ D., Sasaki, S., Strazzulla, G., 2015. Aster-oid surface alteration by space weathering processes. In: Michel, P., De-

Meo, F.E., Bottke, W.F. (Eds.), Asteoids IV, pp. 597–616. doi: 10.2458/azu _ uapress _9780816532131-ch031 .

Bus, S.J., Binzel, R.P., 2002. Phase II of the Small Main-Belt Asteroid Spectroscopic

Survey - A feature-based taxonomy. Icarus 158, 146–177. doi: 10.10 06/icar.20 02.6856 .

Chapman, C.R. , 1996. S-type asteroids, ordinary chondrites, and space weathering:the evidence from Galileo’s fly-bys of Gaspra and Ida. Meteor. Planet. Sci. 31,

699–725 .

eMeo, F.E., Binzel, R.P., Slivan, S.M., Bus, S.J., 2009. An extension of the Bus asteroidtaxonomy into the near-infrared. Icarus 202, 160–180. doi: 10.1016/j.icarus.2009.

02.005 . eMeo, F.E., Carry, B., 2014. Solar System evolution from compositional mapping of

the asteroid belt. Nature 505, 629–634. doi: 10.1038/nature12908 . emura, H., et al., 2006. Pole and global shape of 25143 Itokawa. Science 312, 1347–

1349. doi: 10.1126/science.1126574 . unn, T.L., Burbine, T.H., Bottke, W.F., Clark, J.P., 2013. Mineralogies and source re-

gions of near-Earth asteroids. Icarus 222, 273–282. doi: 10.1016/j.icarus.2012.11.

007 . ujiwara, A., et al., 2006. The rubble-pile asteroid Itokawa as observed by Hayabusa.

Science 312, 1330–1334. doi: 10.1126/science.1125841 . affey, M.J., 2010. Space weathering and the interpretation of asteroid reflectance

spectra. Icarus 209, 564–574. doi: 10.1016/j.icarus.2010.05.006 . artzell, C.M., Scheeres, D.J., 2011. The role of cohesive forces in particle launching

on the Moon and asteroids. Planet. Space Sci. 59, 1758–1768. doi: 10.1016/j.pss.

2011.04.017 . iroi, T., Abe, M., Kitazato, K., Abe, S., Clark, B.E., Sasaki, S., Ishiguro, M., Barnouin-

Jha, O.S., 2006. Developing space weathering on the asteroid 25143 Itokawa. Na-ture 443, 56–58. doi: 10.1038/nature05073 .

iroi, T. , Sasaki, S. , Noble, S.K. , Pieters, C.M. , 2011. Space weathering of ordinarychondrite parent bodies, its impact on the method of distinguishing H, L, and LL

types, and implications for Itokawa samples returned by the Hayabusa mission.

In: Lunar and Planetary Science Conference, vol. 42, p. 1264. Abstract . shiguro, M. , 2014. Scattered light correction of HAYABUSA/AMICA data and quanti-

tative spectral comparisons of Itokawa. Publ. Astron. Soc. Japan 66 (3), 55 . shiguro, M., et al., 2007. Global mapping of the degree of space weathering on as-

teroid 25143 Itokawa by Hayabusa/AMICA observations. Meteor. Planet. Sci. 42(10), 1791–1800. doi: 10.1111/j.1945-510 0.20 07.tb0 0538.x .

shiguro, M., et al., 2010. The Hayabusa Spacecraft Asteroid Multi-band Imaging

Camera (AMICA). Icarus 207, 714–731. doi: 10.1016/j.icarus.2009.12.035 . omatsu, M. , Mikouchi, T. , Arai, T. , Fagan, T.J. , Zolensky, M. , Hagiya, K. , Ohsumi, K. ,

Karouji, Y. , 2015. Petrology of two Itokawa particles: comparison with equi-librated LL chondrites. In: Lunar and Planetary Science Conference, vol. 46,

p. 1884. Abstract . ee, P., 1996. Dust levitation on asteroids. Icarus 124, 181–194. doi: 10.1006/icar.

1996.0917 .

oeffler, M.J., Dukes, C.A ., Baragiola, R.A ., 2009. Irradiation of olivine by 4 keV He + :simulation of space weathering by the solar wind. J. Geophys. Res. 114, E03003.

doi: 10.1029/20 08JE0 03249 . eier, M.M.M. , et al. , 2014. A precise cosmic-ray exposure age for an olivine grain

from the surface of near-Earth asteroid (25143) Itokawa. In: Lunar and PlanetaryScience Conference, vol. 45, p. 1247. Abstract .

ichel, P., O’Brien, D.P., Abe, S., Hirata, N., 2009. Itokawa’s cratering record as ob-

served by Hayabusa: implications for its age and collisional history. Icarus 200,503–513. doi: 10.1016/j.icarus.20 08.04.0 02 .

ikouchi, T., et al., 2014. Mineralogy and crystallography of some Itokawa particlesreturned by the Hayabusa asteroidal sample return mission. Earth Planets Space

66, 82. doi: 10.1186/1880- 5981- 66- 82 . iyamoto, H., et al., 2007. Regolith migration and sorting on Asteroid Itokawa. Sci-

ence 316, 1011–1014. doi: 10.1126/science.1134390 . urchie, S., et al., 2002. Color variations on Eros from NEAR multispectral imaging.

Icarus 155, 145–168. doi: 10.10 06/icar.20 01.6756 .

agao, K., et al., 2011. Irradiation history of Itokawa regolith material deducedfrom noble gases in the Hayabusa samples. Science 333, 1128–1131. doi: 10.1126/

science.1207785 . esvorny, D., Jedicke, R., Whiteley, R.J., Ivezic, Z., 2005. Evidence for asteroid space

weathering from the Sloan Digital Sky Survey. Icarus 173, 132–152. doi: 10.1016/j.icarus.2004.07.026 .

oguchi, T., et al., 2014. Space weathered rims found on the surfaces of the Itokawa

dust particles. Meteor. Planet. Sci. 49, 188–214. doi: 10.1111/maps.12111 . Park, J. , et al. , 2014. 40 Ar/ 39 Ar age of grains returned from asteroid 25143 Itokawa.

In: Annual Meteoritical Society Meeting 77, p. 5190. Abstract . eddy, V., Dunn, T.L., Thomas, C.A., Moskovitz, N.A., Burbine, T.H., 2015. Mineralogy

and surface composition of asteroids. In: Michel, P., DeMeo, F.E., Bottke, W.F.(Eds.), Asteroids IV, pp. 43–63. doi: 10.2458/azu _ uapress _ 9780816532131-ch003 .

aito, J., et al., 2006. Detailed images of Asteroid 25143 Itokawa from Hayabusa.

Science 312, 1341–1344. doi: 10.1126/science.1125722 . asaki, S., Nakamura, K., Hamabe, Y., Kurahashi, E., Hiroi, T., 2001. Production of iron

nanoparticles by laser irradiation in a simulation of lunar-like space weathering.Nature 410, 555–557. doi: 10.1038/35069013 .

hestopalov, D.I., Golubeva, L.F., Cloutis, E.A., 2013. Optical maturation of asteroidsurfaces. Icarus 225, 781–793. doi: 10.1016/j.icarus.2013.05.002 .

Strazzulla, G., Dotto, E., Binzel, R., Brunetto, R., Barucci, M.A., Blanco, A., Orofi-

noet, V., 2005. Spectral alteration of the Meteorite Epinal (H5) induced by heavyion irradiation: a simulation of space weathering effects on near-Earth asteroids.

Icarus 174, 31–35. doi: 10.1016/j.icarus.2004.09.013 . unshine, J.M., Pieters, C.M., 1998. Determining the composition of olivine from

reflectance spectroscopy. J. Geophys. Res. 103, 13675–13688. doi: 10.1029/98JE01217 .

Tedecso, E.F., Tholen, D.J., Zellner, B., 1982. The eight-color asteroid survey - stan-

dard stars. Astron. J. 87, 1585–1592. doi: 10.1086/113248 . holen, D.J. , 1984. Asteroid Taxonomy from Cluster Analysis of Photometry Ph.D.

thesis. Dep. of Planetary Sciences, Univ. of Arizona, Arizona, U.S.A . suchiyama, A., et al., 2011. Three-dimensional structure of Hayabusa samples: ori-

gin and evolution of Itokawa regolith. Science 333, 1125–1128. doi: 10.1126/science.1207807 .

S.C. Koga et al. / Icarus 299 (2018) 386–395 395

V

W

Y

Y

ernazza, P., Binzel, R.P., Rossi, A., Fulchignoni, M., Birlan, M., 2009. Solar wind asthe origin of rapid reddening of asteroid surfaces. Nature 458, 993–995. doi: 10.

1038/nature07956 . alsh, K.J., Morbidelli, A., Raymond, S.N., O’Brien, D.P., Mandell, A.M., 2011. A low

mass for Mars from Jupiter’s early gas-driven migration. Nature 475, 206–209.doi: 10.1038/nature10201 .

ano, H., et al., 2006. Touchdown of the Hayabusa Spacecraft at the Muses Sea onItokawa. Science 312, 1350–1353. doi: 10.1126/science.1126164 .

oshikawa, M., Kawaguchi, J., Fujiwara, A ., Tsuchiyama, A ., 2015. Hayabusa sam-ple return mission. In: Michel, P., DeMeo, F.E., Bottke, W.F. (Eds.), Asteoids IV,

pp. 397–418. doi: 10.2458/azu _ uapress _ 9780816532131-ch021 .