Icarus. Spectral decomposition of asteroid Itokawa based on principal component analysis

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1 Icarus 299 (2018) Contents lists available at ScienceDirect Icarus journal homepage: 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 a b s t r a c t 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 The heliocentric stratification of asteroid spectral types may hold important information on the early evolution 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 spaceweathering experiments on ordinary chondrites and olivine, suggesting that this trend may be a spaceweathering-induced spectral evolution track for S-type asteroids. Comparison with space-weathering experiments 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 weathering, 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 results 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 Elsevier Inc. All rights reserved. 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 ). Asteroid spectral types are often classified employing principal component analysis (PCA). In particular, the first and second principal 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. address: sumire.c.koga@gmail.com (S.C. Koga). principal-component (PC) space (e.g., Tholen 1984 ) shows that PCA is an effective approach for analyzing asteroid spectra. However, the relationship between the positions of asteroids in the PC space and physical and/or chemical states is not yet well understood. More specifically, multiple physical and chemical factors, such as the mineral composition, grain size, and surface age (i.e., space weathering), as well as observational conditions, such as phase angle and solar standard correction, are known to affect apparent PC scores of asteroid spectra (e.g., Binzel et al., 2004; Reddy et al., 2015 ), but which factor controls which PC is poorly understood. One approach that can be taken to resolve these issues is to apply PCA to disk-resolved spectra of an asteroid with geologic context / 2017 Elsevier Inc. All rights reserved.

2 S.C. Koga et al. / Icarus 299 (2018) The Japanese Hayabusa spacecraft made a rendezvous with an S-type asteroid Itokawa before obtaining samples from its surface (e.g., Fujiwara et al., 2006 ). During the rendezvous, the spacecraft obtained high spatial-resolution remote sensing data and revealed that the surface of asteroid Itokawa had notable spectral variation (e.g., Ishiguro et al., 2007 ). Detailed analyses of visible and near-infrared spectra of Itokawa have shown that space weathering contributes to spatial variation in reflectance spectra of the Itokawa surface ( Hiroi et al., 2006; Ishiguro et al., 2007 ). However, it remains unclear whether there is a component other than space weathering in the Itokawa spectral variation. Additionally, the degrees of space weathering on Itokawa have not been quantitatively compared with those of main-belt asteroids (MBAs) and laboratory experiments. Furthermore, the spectroscopic properties of the Hayabusa sampling site, such as the degree of space weathering, were not extensively addressed in previous remote sensing studies. In other words, the geologic context of the sampling site of the Hayabusa mission on the surface of Itokawa has not been fully explored with Hayabusa remote sensing data yet. In this study, we perform PCA on high-spatial-resolution multiband images of Itokawa and compare the multi-band spectra with the spectral distribution of main-belt asteroids and results of space-weathering simulation experiments using olivine and ordinary chondrites, which are the analog to Itokawa ( Abe et al., 2006; Tsuchiyama et al., 2011 ). 2. Image analysis methods 2.1. Data We used four image sets of six visible bands with central wavelengths of 0.381, 0.429, 0.553, 0.700, 0.861, and μm taken by the Asteroid Multi-band Imaging Camera (AMICA) ( Saito et al., 2006; Ishiguro et al., 2010 ) onboard the Hayabusa spacecraft. Images of each set were taken at approximately the same distance of 20 km from the asteroid mass center. These image sets cover the entire surface of Itokawa. The spatial resolution of the images used in this study is 2 m/pixel Image calibration and processing We first remove periodic electromagnetic noise imposed on some of the images, presumably generated by interference from other onboard instruments, by subtracting the superposition of two sine waves. Ishiguro et al. (2010) examined dark current and the linearity between light intensity and image count values, finding that the errors are negligible for a typical signal level of Itokawa images; dark current is less than 0.4% and the error in linearity is less than 0.3%. Thus, we did not conduct dark subtraction or linearity correction. Sensitivity uniformity was calibrated by dividing the images by flat-field images obtained before the flight. The count values of the images were converted to reflectance normalized at v-band (0.553 μm) using the result of Ishiguro et al. (2010), where the disk-integrated intensity was calibrated to match their telescope observation data of the diskintegrated average spectrum of Itokawa. Scattered-light correction was conducted employing the method given by Ishiguro (2014). The images of different bands were co-registered to v-band images by parallel translation of image shifting. The optimal translational shifts were chosen to minimize the total variance of the ratio (to v-band) image values in the area of interest in the image, since a ratio image with unsuccessful co-registration gives both very high and low values around shadows, where the count value changes discontinuously in original images. We used the minimum unit of 0.01 pixels for shifting because this small shift still resulted in a notable reduction in variance. Owing to the spin of the asteroid in the time between the taking of images with the six band filters, images are slightly distorted from each other. Thus, simple shifting cannot co-register all the pixels in the images. There were unavoidable mismatches particularly in peripheral areas and areas near shadows of large boulders. We evaluated the degree of coregistration mismatch by examining the spatial continuity obtained in ratio images. As discussed above, a mismatch in co-registration would lead to ratio images with large variations near the disk edge and shadows. For the nominal case, pixels with intensity more than 4% greater or smaller than the intensity of adjacent pixels are assumed to be resulted from unsuccessful co-registration and removed from further analyses. We used different threshold values (from 2% to 6%) and confirmed that the specific choice of this threshold for continuity does not affect the results of further analyses Comparison with MBA and meteorite spectra We first performed PCA on Itokawa surface spectra alone to extract the most dominant factor of the variation in the Itokawa surface spectra. We refer to the resulting PCs as Itokawa PCs. Second, PCA was performed on spectra of 533 asteroids (mainly MBAs) observed in the Eight-color Asteroid Survey (ECAS) ( Tedecso et al., 1982 ) (hereafter ECAS asteroids). Itokawa surface spectra were then decomposed with the PCs of ECAS asteroid spectra (i.e., PCA was performed on the ECAS dataset only, and then the eigenvectors were used to calculate PC scores for Itokawa surface and meteorites and olivine data). We refer to these PCs as ECAS PCs. The central wavelengths of AMICA filter bands are approximately the same as those used in the ECAS as shown in Table 1. The difference is much smaller than the bandwidth of AMICA and ECAS. Thus, we used reflectance data without any correction or spline interpolation. Third, spectra of eight ordinary chondrite samples irradiated by a pulse laser and one olivine sample irradiated by Ar + ion were taken from previous studies ( Hiroi et al., 2011; Brunetto et al., 2006b ) and NASA s RELAB database at Brown University. These spectra were decomposed with the ECAS PCs. It is noted that PCA Table 1 The central wavelengths (μm) and effective wavelength widths (μm) of the observational filters of AMICA and ECAS. Filter name of AMICA (ECAS) AMICA a ECAS b Central wavelength Effective wavelength width Central wavelength Effective wavelength width ul (u) b v w x p a Tedecso et al. (1982). b Ishiguro et al. (2010).

3 388 S.C. Koga et al. / Icarus 299 (2018) on different data sets (e.g., Itokawa surface points vs. MBAs) would yield different PCs. Thirty three spectra of different level of laserirradiation 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 to 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%, respectively, 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 effect 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 possible 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 results strongly suggest that Itokawa PC1 represents space weathering. The PC1 score map shows that the freshest terrains on Itokawa are found around both topographic highs, such as a few areas on the neck part, and rough terrain, such as several areas in Ohsumi Regio and a few areas on the western side of the head Comparison between Itokawa and MBA spectra The spectra of ECAS asteroids (mostly MBAs), the Itokawa surfaces, and meteorites are next compared in ECAS PC1-2 space ( Fig. 3 ). The spectra of Itokawa surfaces are distributed along a line in the ECAS PC space extending widely from the Q-type area to a high-population-density area in the S-type cluster. It is remarkable that spectral variation on asteroid Itokawa is comparable in size to the variation of the entire S-type-asteroid complex in the PC1-2 space. Another important property of the distribution of Itokawa spectra is its asymmetricity; the histogram along the Itokawa distribution line in the ECAS PC space ( Fig. 4 ) has a longer tail on one side than on the other. We conducted the same PCA with four sets of six-band images covering different areas on Itokawa and obtained the same results, strongly suggesting the robustness of the above result ( Fig. 2 ). Furthermore, the direction of the linear trend of the Itokawa surface spectral distribution in the ECAS PC space and that of Itokawa PC1 agree well each other; the inner product of their direction vectors is greater than Comparison with laboratory simulation of space weathering Pulsed laser irradiation experiments simulating space weathering due to micrometeorite bombardment have shown that the Fig. 1. (a) Spectral pattern of the principal components of the six-band spectra of Itokawa surfaces. Error is from standard deviation for four different image sates. The first principal component (PC1) has a distinctive red spectral slope between wavelengths of 0.4 and 0.8 μm. This spectral pattern coincides with the typical reddening effect of space weathering. (b) and (c) Comparison between Itokawa PC1 and a typical weathering continuum obtained in laboratory experiments. The space weathering continuum (the gray dots) is taken from Fig. 4 of Brunetto et al. (2007), showing the spectral ratio of irradiated samples ((b) 27 J/cm 2 laser, (c) 52 J/cm 2 laser) to pristine samples. Itokawa PC1 is scaled for the comparison ((b) 0.27 PC , (c) 0.29 PC ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4 S.C. Koga et al. / Icarus 299 (2018) 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.) spectra of silicate mineral surfaces change as nanophase ion particles form ( Sasaki et al., 2001; Brunetto et al., 2006a ). Heavy-ion irradiation simulating solar-wind bombardment also leads to similar spectral changes ( Strazzulla et al., 2005 ). Comparison between asteroids and laboratory experiments suggests that space weathering on asteroids may be dominated by ion irradiation during the early stage and by micrometeorite bombardment during the late stage ( Vernazza et al., 2009 ). We compared spectra on the Itokawa surface analyzed in this study to those of ordinary chondrites with pulsed laser irradiation and olivine with heavy-ion irradiation ( Hiroi et al., 2011; Brunetto et al., 2006b ; RELAB database) ( Fig. 3 ). The comparison indicates that the direction of movement in meteorite and olivine spectra induced by laser and heavy-ion irradiation coincides with that of the trend of Itokawa surface spectra in the ECAS PC space. More specifically, the regression line is given by 0.58 ECAS PC ECAS PC2 = 1.5. Furthermore, the rate of change in the meteorite and olivine spectra in the PC space decreases as irradiation dose increases. This strongly suggests saturation effect of space weathering. The asymmetric distribution of Itokawa spectra ( Fig. 4 ) is also consistent with the saturation effect. More specifically, the half width for the half maximum of the population along the above regression line is 0.3 unit for higher degrees of space weathering and is 0.5 for lower degrees of space weathering ( Fig. 4 ). Here, the unit is given by PC SW = 0.81 ECAS PC ECAS PC2. The ratio of maximum deviations in the projected PC space measured from the median value ( PC SW 0.3) to more mature spectra ( PC SW 0.8) and less mature spectra ( PC SW 1.5). The fact that similar saturation effect is seen in both Itokawa data and laboratory experiments suggests that the same spectral change observed in laboratories is actually proceeding on the asteroid. We note that the distribution would be sum of the results of both space weathering and resurfacing. The asymmetric distribution of the spectra suggests that the timescales of space weathering and resurfacing are comparable. Furthermore, we compared the spectra of areas on the Itokawa surface whose spectral PC scores are similar to those of spaceweathered chondrite samples and the actual spectra of those samples. The actual multi-band spectra of space-weathered chondrites and Itokawa spectra agree well with each other as shown in Fig. 5. Moreover, the spectral change obtained with laser irradiation on olivine powder samples (Fig. 4 in Brunetto et al., 2007 ) exhibits a very similar pattern to the PC1 obtained by our analysis of Itokawa surface spectra ( Fig. 1 b), supporting our interpretation that Itokawa PC1 represents space weathering. Here it is noted that the spectral component of space weathering obtained by Brunetto et al. (2007) is the ratio of spectra before and after the laser experiments, not the difference (i.e., subtract) between the two. The difference spectra (i.e., subtract) between before and after spaceweathering experiments by Brunetto et al. (2007) does not fit the PC1 very well. This is most likely because the amplitude of spectral change due to experiments is too large to ignore the non-linear effect of logarithmic spectral evolution. Comparison of the spectral change between space weathering experiments of ordinary chondrites and the actual Itokawa spectral variation also allows us to estimate the time scale of space weathering. The time required to space-weather pristine ordinary chondrites to the average Itokawa spectrum is estimated to be approximately 10 8 yr for the micrometeorite bombardment rate used by Sasaki et al. (2001). We can also estimate the period of space weathering by the solar wind from ion irradiation experiments. However, no ion irradiation experiments on L or LL chondrite samples that provide a good spectral match with Itokawa spectra have been reported in the literature. Nevertheless, based on ion irradiation experiments with different minerals, Brunetto et al. (2006b) proposed a model to describe spectral change due to cosmic-ray-induced space weathering. This empirical law gives space-weathered spectrum as a function of nuclear displacement d, the number of nuclear displacements per unit surface area, proportioned to ion fluence ( Brunetto and Strazzulla, 2005 ). Using these model and spectra, we obtained the shift distance PC SW of olivine spectra in the ECAS PC space as a function of nuclear displacements d. The obtained PC SW can be fit well with a power law of d. We thus used this power law to estimate exposure time of Itokawa surface spectra. We then used three chondrite samples used in Fig. 3 as the pre-space-weathered material for Itokawa surface; we take PC SW from the average of the three L/LL chondrite samples to the Itokawa average along the linear trend of Itokawa spectra in the ECAS PC space ( Fig. 3 ) as for the spectral change due to space weathering. When the logarithm of PC SW is plotted as a function of the logarithm of either the laser energy dose or nuclear displacements ( d ), the data points exhibit a linear trend ( Fig. 6 ). This strongly suggests a power-law relation between the irradiation energy and PC SW. The best-fit power-law exponents for the laser experiments and ion experiments are 0.60 ± 0.05 and

5 390 S.C. Koga et al. / Icarus 299 (2018) 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 PC 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.) Table 2 The samples used in this study. Chondrite/Mineral Sample form RELAB database ID/Referrence Fig. 4. Histogram of Itokawa surface spectra along the Itokawa distribution line (the black thin line in Fig. 3 ) / 0.04, respectively. This coincidence in the power-law exponents between laser and ion irradiation may reflect similarity in the saturation process between these two space-weathering 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) mechanisms; i.e., the two space weathering mechanisms may proceed in a similar manner. The shift distance PC SW of the average spectrum of Itokawa from the average of the three chondrites used in the laser experiments is 3.8 ± 0.3. The corresponding nuclear displacement ( d ) is (displacements/cm 2 ). The

6 S.C. Koga et al. / Icarus 299 (2018) tial spectrum, lead to an estimate that the ratio of the oldest to youngest surface exposure ages on Itokawa is 3.4 ± Itokawa PC2 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 ( ). Similarly to (a), the Itokawa spectral curves are overlapping each other. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) relationship between the irradiation time scale t (yr) and nuclear displacement d (displacements/cm 2 ) is t = d at 2.9 AU ( Brunetto et al., 2006b ). Because Itokawa is at 1.3 AU from the Sun and the flux of solar wind decreases proportionally to the inverse square of distance from the Sun, the relationship at the Itokawa s location is t = d. Thus, the shift distance PC SW from the pristine chondrite (the average of the three chondrites) to Itokawa average is (1 + 2/ 0.6) 10 6 years. Here, the error is based on uncertainty in both the shift distance PC SW and power-law exponent. Uncertainty in PC SW is calculated from the scatter in the pre-irradiation spectra of the three chondrites shown in Fig. 3. That in power-law exponent is from upper and lower values in α and β in the formula in Brunetto et al. (2006b). It should be noted that this time scale gives an upper limit for the Itokawa average spectral age, because the solar wind contains ions other than Ar + and the efficiency of space weathering induced by other ions, such as He +, is likely high ( Loeffler et al., 2009 ). Similarly, spatial distribution in surface exposure age can be assessed with the heterogeneity in the degree of space weathering. The smallest ( 2) and largest ( 0.5) values of PC SW on Itokawa correspond to 0.45 Myr and 1.5 Myr, respectively. Thus, the ratio of oldest to youngest surface exposure ages is about three. This ratio depends on the initial spectrum and model parameters α and β as discussed above for mean Itokawa surface age. An error propagation calculation yields based on uncertainty in the three factors, α and β in the formula by Brunetto et al. (2006b), and ini- The interpretation of Itokawa PC2, which has a peak at 0.55 μm, is not straightforward as that of Itokawa PC1. The spectral pattern of PC2 itself does not resemble the spectra of particular chondritic minerals or other geologic materials. Comparison among local spectra with different PC2 scores ( Fig. 7 ) exhibits distinctive difference among them, but these end-member spectra or intermediate spectra do not resemble the spectra of particular chondritic minerals or other geologic materials either. However, the spatial distribution of the PC2 score appears to correlate to the roughness/smoothness distribution on Itokawa. To examine the relationship between the surface morphology and PC2 score, we measured mean values of the PC2 score in six areas having smooth terrains and eight areas having rough terrains as indicated in Fig. 2. Here, the distinction between smooth and rough terrains is based on the work of Demura et al. (2006). The results of the analysis indicate that all areas of smooth terrain have higher PC2 scores than all areas of rough terrain ( Fig. 8 ). One possible mechanism by which the roughness/smoothness of terrain affects the reflectance spectra is the grain size effect. When the grain size increases or decreases, the spectral undulation of the same silicate minerals generally decreases or increases, respectively. If rough terrain has coarser grains, it would have higher spectral peaks and deeper absorption bands, but the wavelength of these peaks and absorption bands do not change as a function of grain size. Thus, the spectral component that reflects the grain size difference would resemble the spectrum of the mineral(s) whose grain size(s) changes. More specifically, if grain size variation on Itokawa occurs regardless of the mineral species and spectral absorption depth changes due to grain size change is independent of mineral species, then the spectral pattern due to the grain size change should resemble the Itokawa spectrum. However, the spectrum of Itokawa has a peak around 0.7 μm, not around 0.55 μm as seen in PC2. In contrast, if the change in grain size on Itokawa is dominated by the grain size effect of a single species of mineral, then the spectral pattern may resemble the spectrum of this mineral. In fact, olivines with Mg# between 86 and 90 exhibit a reflectance peak around 0.55 μm ( Sunshine and Pieters, 1998 ). Chemical analyses of particles retrieved from Itokawa revealed that 64% of minerals is olivine ( Tsuchiyama et al., 2011 ). If the grain size variation of olivines is especially large and the olivines have a Mg# between about 86 and 90, then this may account for the spectral pattern of PC2. However, more recent analyses of returned samples from Itokawa revealed that the chemical composition of Itokawa olivine is more rich in Fe; Mg# of 70 to 73 ( Mikouchi et al., 2014; Komatsu et al., 2015 ). The spectra of such olivine have a peak around 0.7 μm, inconsistent with PC2. Thus, further investigation is necessary for understanding the nature of PC2. Nevertheless, we obtained the same spectral pattern of PC2 from four different sets of images and the variance contribution of PC2 was always significant ( 20 30%). These results support that PC2 is a real spectral component, strongly suggesting that PC2 reflects some real geologic processes and/or the properties of Itokawa surface materials. 4. Implications for asteroid spectral evolution The analysis results obtained in this study have a number of important implications for the evolution of S-type asteroid spectra and Itokawa samples brought back by Hayabusa. First, the

7 392 S.C. Koga et al. / Icarus 299 (2018) 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. results discussed in the previous section support that the dominant 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 suggests 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 distribution 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 weathering ( Binzel et al., 2004 ). Here it is noted that S-type asteroids cover a wide range of compositions, some of which may be significantly 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 biased samples of main-belt asteroids; LL-like compositions of asteroids 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-chondritelike 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 processes 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, however, 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 important role in resurfacing asteroid surfaces (e.g., Brunetto et al., 2015 ). More specifically, a model calculation by Shestopalov et al. (2013) showed that the spectra of S-type asteroids are consistent with active surface rejuvenation due to regolith shaking. In fact, Miyamoto et al. (2007) found evidence for regolith migration on Itokawa, which would play an important role in resurfacing. Also, previous multi-band spectral mapping of Itokawa has revealed that impact cratering would have contributed to resurfacing on this small asteroid ( Ishiguro et al., 2007 ; Yoshikawa et al., 2015 ). Local variations of spectral age related to morphology have also been found on other asteroids, such as Ida ( Chapman, 1996 ) and Eros ( Murchie et al., 2002 ). Large spatial variation in the degree of space weathering on Itokawa quantified in this study is also consistent with local resurfacing processes suggested by these studies. Third, the PC1 score for the Hayabusa sampling site ( Yano et al., 2006 ) is near the average value for the entire Itokawa globe ( Fig. 2 ). Thus, the samples returned by Hayabusa may be a good representation of the average degree of space weathering of Itokawa. Fourth, the average spectral age of Itokawa estimated from solar wind flux may be consistent with noble-gas age of returned samples, suggesting that solar wind may be the dominant source of space weathering on Itokawa. It has been argued that the reason why most MBAs have only moderate degrees of space weathering much lower than that of the Moon is that the effect of space weathering does not accumulate easily on asteroids. More specifically, a thin top surface layer may rejuvenate without erasing craters ( Shestopalov et al., 2013 ), and there may be efficient regolith migration on the asteroid surface ( Miyamoto et al., 2007 ). Consideration of these factors may help us interpret the surface ages of asteroids. Several different kinds of ages

8 S.C. Koga et al. / Icarus 299 (2018) Mean PC2 score Mountainview Arcoona Shirakami Miyabaru Ohsumi Yatsugatake LINEAR Sanriku Uchinoura Muses-C (2) Muses-C (1) Sagamihara (2 Sagamihara (1 Noshiro Smooth terrains Region Name 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. 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.) have been estimated for asteroid Itokawa using both returned samples and AMICA image analyses. The thicknesses of the layers for which relevant ages are estimated are different. More specifically, a possible parent-body catastrophic disruption age of about 1.3 Gyr ( Park et al., 2014 ), crater retention age of Myr based on strength scaling ( Michel et al., 2009 ), boulder age of 5 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 estimated. Thus, the upper limit of time scale of space weathering by solar wind estimated in section 4.3 is much shorter than the possible parent-body disruption age and crater age but comparable to the noble-gas (cosmic-ray exposure) ages. Fifth, no area on Itokawa exhibits a spectrum that matches pristine chondrite free from space weathering in our meter-scale analyses; the entire globe of Itokawa has some degree of space weathering. More specifically, the longer tail of space weathering histogram ( Fig. 4 ) does not reach pristine chondrite spectra in the PC space. This observation suggests that the entire surface of Itokawa may be covered with space-weathered grains. Such global coverage of small space-weathered grains could be achieved by electrostatic levitation ( Lee, 1996; Hartzell and Scheeres, 2011 ). Another possible reason for the lack of pristine spectra on Itokawa may be extremely rapid space weathering process observed in ion irradiation experiments (e.g., Loeffler et al., 2009 ). Comparison of microscopic observations between silicate samples treated with ion beams in laboratories and Itokawa samples also suggests that space weathering on individual grains on Itokawa could occur very fast on order of 10 3 years ( Noguchi et al., 2014 ). Although we cannot reach a decisive conclusion on the mechanism for this lack of spectrally pristine surface on Itokawa, understanding this observation may hold a key for the space weathering process on fresh silicates. 5. Conclusions We compared surface spectra of the asteroid Itokawa with the spectra of MBAs and ordinary chondrites subjected to space weathering experiments. Our analysis results strongly suggest that the spectra of actual asteroids evolve as space weathering proceeds. Our results also suggest that the range of space weathering observed on Itokawa can account for spectral differences among different asteroid spectral types; the disk-resolved spectra of Itokawa were found to span continuously from around Q-type to S-type. It is remarkable that the range of the spectral variation for Itokawa is

9 394 S.C. Koga et al. / Icarus 299 (2018) 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 location of the touchdown point of Hayabusa ( Yano et al., 2006 ) in the global map of our space-weathering score suggests that the degree 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 average surface age of Itokawa. The comparison of results from ionirradiation experiments and Itokawa spectra suggests that the time required for such spectral evolution from ordinary chondrite spectra to the average spectrum of Itokawa is approximately a few million 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, jp/index.html.en ). Dataset name: hayamica. The ECAS data are available from the NASA Planetary Data System (PDS, nasa.gov/tools/data-search/ ). 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