science.sciencemag.org/content/368/6491/654/suppl/DC1

Supplementary Materials for

Sample collection from asteroid (162173) Ryugu by Hayabusa2:

Implications for surface evolution T. Morota*, S. Sugita, Y. Cho, M. Kanamaru, E. Tatsumi, N. Sakatani, R. Honda, N.

Hirata, H. Kikuchi, M. Yamada, Y. Yokota, S. Kameda, M. Matsuoka, H. Sawada, C.

Honda, T. Kouyama, K. Ogawa, H. Suzuki, K. Yoshioka, M. Hayakawa, N. Hirata, M.

Hirabayashi, H. Miyamoto, T. Michikami, T. Hiroi, R. Hemmi, O. S. Barnouin, C. M.

Ernst, K. Kitazato, T. Nakamura, L. Riu, H. Senshu, H. Kobayashi, S. Sasaki, G.

Komatsu, N. Tanabe, Y. Fujii, T. Irie, M. Suemitsu, N. Takaki, C. Sugimoto, K. Yumoto,

M. Ishida, H. Kato, K. Moroi, D. Domingue, P. Michel, C. Pilorget, T. Iwata, M. Abe, M.

Ohtake, Y. Nakauchi, K. Tsumura, H. Yabuta, Y. Ishihara, R. Noguchi, K. Matsumoto,

A. Miura, N. Namiki, S. Tachibana, M. Arakawa, H. Ikeda, K. Wada, T. Mizuno, C.

Hirose, S. Hosoda, O. Mori, T. Shimada, S. Soldini, R. Tsukizaki, H. Yano, M. Ozaki, H.

Takeuchi, Y. Yamamoto, T. Okada, Y. Shimaki, K. Shirai, Y. Iijima, H. Noda, S.

Kikuchi, T. Yamaguchi, N. Ogawa, G. Ono, Y. Mimasu, K. Yoshikawa, T. Takahashi, Y.

Takei, A. Fujii, S. Nakazawa, F. Terui, S. Tanaka, M. Yoshikawa, T. Saiki, S. Watanabe,

Y. Tsuda

*Corresponding author. Email: morota@eps.s.u-tokyo.ac.jp

Published 8 May 2020, Science 368, 654 (2020)

DOI: 10.1126/science.aaz6306

This PDF file includes:

Materials and Methods

Figs. S1 to S14

Table S1

Caption for Movie S1

References

Other Supplementary Material for this manuscript includes the following:

(available at science.sciencemag.org/content/368/6491/654/suppl/DC1)

Movie S1 (.mp4)

2

Materials and Methods Spectral slopes of Ryugu’s surface

The radiometric calibration of the ONC-T data, its conversion to radiance factor (I/F), and the calculation of the spectral slope have been described elsewhere (2, 5, 26–29). We used the spectral slope between b- and x-bands (b-x slope) (2, 5). For the low-altitude (< 1 km altitude) observations during the touchdown operation, however, areas covered by all bands are limited because Ryugu’s surface moves due to Ryugu’s rotation during the rotation of the ONC-T filter wheel. Therefore, we used the p-b ratio Rpb defined as

𝑝 − 𝑏 ratio = 𝑅! / 𝑅! (S1),

where Rp and Rb are the p-band (0.95 µm) and b-band radiance factors. There is a strong correlation between the b-x slope and the p-b ratio. Figure S1 displays the global map of p-b ratio showing the same spatial distribution with that of the b-x slope (Fig. 1A).

To evaluate the precision of the b-x slope map (Fig. 1A), we compared the b-x slope maps derived by the Box-A (20 km altitude) and mid-altitude (~5 km altitude) observations (Fig. S2). The maps show similar spatial distribution. The standard deviation of differences between the maps is 0.010 µm-1, much smaller than the variation in b-x slope.

The b-x slope map (Fig. 1A) exhibits latitudinal variation; bluer materials are more concentrated at the equatorial ridge and in the polar regions, while redder materials are predominately found in the mid-latitude regions (Fig. S3). The polar regions have bluer spectra than the equatorial ridge. Similar latitudinal variations are found for the reflectance in both the visible band (5) and the near-infrared band (4). High resolution observations show a streaked pattern to the redder materials commensurate with ejecta deposits (Fig. S4B). Redder materials are also observed to have been dispersed by collision with boulders (Fig. S4F). Thus, redder materials show evidence for disruption and redistribution by impact and thermal fatigue processes.

Based on the empirical power-law relationship between primary crater sizes and ray lengths for lunar craters (30), the primary crater diameter of the ejecta rays with length of a few tens of meters is estimated to be a few meters to ten meters. Using the relationship between the crater diameter and the excavation depth (31) the excavation depth is calculated to be a few tens of centimeters to meters. These estimates suggest that the redder material layer had a minimum original thickness of a few tens of centimeters.

3

Fig. S1. Same as Figure 1A, but for the p-b ratio.

4

Fig. S2. Comparison of b-x slope maps produced from Box-A data and mid-altitude data. (A) Box-A map, same as Fig. 1A. (B) Mid-altitude map; gray areas were not observed during the phase so show v-band mosaic produced from Box-A data. (C) The b-x slope profile along the equator. The red and blue lines indicate the Box-A and mid-altitude data, respectively. The spatial resolutions for Box-A and mid-altitude data are 2 m pixel-1 and 0.5 m pixel-1, respectively.

5

Fig. S3. Longitudinally averaged b-x slope of the Ryugu surface, showing the latitudinal variation. The horizontal dashed line indicates the global average of b-x slope.

6

Fig. S4. Spectral slope of selected regions on Ryugu’s surface. (A) v-band image (image hyb2_onc_20180801_195621_tvf) and (B) b-x slope image of the same region obtained from 5.1 km altitude. White arrows indicate examples of streaked patterns seen in redder materials distribution. (C) v-band image (image hyb2_onc_20180806_222148_tvf) and (D) b-x slope image obtained during the gravity measurement descent operations. (E) v-band image and (F) b-x slope image showing redder materials, which have been separated by flow around a boulder.

7

Digital elevation model of the touchdown area The digital elevation map (DEM) of L08-B and L08-E1 regions was produced from 18

ONC-T images taken on 15 and 25 October 2018 during descent operations to the asteroid. Resolutions of the images range from 0.0100 to 0.0154 m pixel-1. We used Agisoft Photoscan v.1.5, a commercial implementation of the Structure-from-Motion (SfM) technique, to construct the global shape model of Ryugu (2). The initial DEM output was projected onto a context image with wide coverage to scale and align to the asteroid’s body-fixed reference frame. The DEM covers ~1,000 m2 of the asteroid surface with 50,000 polygons. The average resolution of the DEM is ~0.14 m. Figure S5 is a local topographic map derived from the local DEM. The DEM stores local elevations that are determined by taking deviation of elevations from a quadric surface fitted to the local terrain of this area.

Fig S5. Local topographic map of the touchdown site. The local elevation is determined from deviation measurements from the quadric surface fitted to the local DEM. Solid and dashed contours represent 0.5 and 0.1 m intervals of elevation, respectively.

TM

L08-E1

Elevation [m]

L08-B

8

Color variations of boulders and albedo change of boulders before and after the touchdown Before and after the touchdown, 31 ONC-W1 images were obtained with a cadence of 2

seconds. Movie S1 shows an animation of these ONC-W1 images. ONC-T images were also acquired from ~50 m altitude after the ONC-W1 imaging was competed.

Several reddish spots are found within L08-E1 (Fig. S6A). The reddish spots tend to be slightly darker than bluer areas at same phase angle. In general, rough surfaces have redder spectra because of self-shadowing. However, the high-resolution image shows that some reddish spots are found on the smoother surfaces of boulders (RS1, RS2, and RS4 in Fig. S6B), suggesting that the variation of spectral slope cannot be explained only by differences of the surface conditions of boulders.

Figure S8 shows the albedo changes in boulders observed within these images. Before the touchdown, a ragged boulder nicknamed “Turtle Rock” (later lifted up by the RCS thrust) was originally darker than other surrounding boulders (Fig. S8A). Area T1 on Turtle Rock has a radiance factor of about 0.025, approximately 8% lower than Area S1 at the same phase angle (Fig. S8A). Immediately after the RCS thrust upon touchdown, the entire field-of-view (FOV) of ONC-W1 was darkened of about 18% by the dark fine grains lifted up by the RCS thrust and some boulders began to move (Fig. S8B). Area S1 was also darkened of about 18%, while Area T1 was darkened by only 11%. Based on the assumption that the entire FOV was uniformly darkened by lifted dust the observation suggests that Turtle Rock was brightened by about 8% by the removal of dust (Fig. S8D). Turtle Rock was lifted up by the RCS thrust 3–4 seconds after the touchdown (Fig. S8C). ONC-T image also shows that Turtle Rock became as bright as the surrounding, formerly brighter, boulders (Fig. S8E). The FOV of ONT-T is narrower than ONC-W1 images, and phase angle between different regions on the surface does not vary much (<5.7˚), allowing to compare the brightness of different areas. Area T2 on Turtle Rock has a mean radiance factor of about 0.026, comparable to that of Area S2 on the brightest boulder among the surrounding boulders (0.024). The phase angles for T2 and S2 are 16.0˚ and 19.7˚, respectively.

9

Fig S6. Examples of reddish spots found within the L08-E region. (A) p-b ratio images (6) calculated from b-band and p-band (0.95 µm) images. These images were obtained from 116 m and 123 m altitudes in the touchdown rehearsal operation, respectively (hyb2_onc_20181015_134707_tbf and hyb2_onc_20181015_134655_tpf). (B) W1 image obtained from 3.6 m altitude during the spacecraft descent before the touchdown (hyb2_onc_20190221_222842_w1f). Red dashed circles indicate locations of phase angles (g) of 10˚, 20˚, and 30˚.

10

Fig S7. High-resolution versions of Fig. 2D and 2E. (A) ONC-W1 image obtained from 2 m altitude during the spacecraft descent before the touchdown (hyb2_onc_20190221_222859_w1f). A dark ragged boulder nicknamed “Turtle Rock” and an example of bright boulders with smooth surfaces (BB) are outlined by yellow and cyan dashed lines, respectively. (B) Close-up image of an area indicated by white dashed box in panel A. Yellow arrows indicate fresh bright spots at corners and a possible broken plane of boulders.

11

Fig S8. ONC-W1 and -T images before and after the touchdown. The right and left panels are same images displayed in gray scale and color scale, respectively. ONC-W1 images obtained (A) 3–4 seconds before the touchdown (hyb2_onc_20190221_222907_w1f), (B) immediately after the touchdown (hyb2_onc_20190221_222911_w1f), (C) 3–4 seconds after the touchdown (hyb2_onc_20190221_222913_w1f), and (D) 5–6 seconds after the touchdown (hyb2_onc_20190221_222917_w1f). (E) ONC-T image obtained about 100 seconds after the touchdown (hyb2_onc_20190221_223053_tvf). Red dashed circles indicate locations of phase angles (g) of 10˚, 20˚ and 30˚.

12

Estimates of total mass and size of dark fine grains Figure S6 shows ONC-W1 images obtained during the spacecraft’s ascent after

touchdown. The radiating cloud of dark fine grains lifted by the RCS thrust had a diameter of 6 m at 15 seconds after the touchdown (Fig. S9A), and expanded to 1.5 times that diameter at 35 seconds after the touchdown (Fig. S9C). The speed of the cloud’s expansion is estimated to be 0.075 m s-1.

After the touchdown, we found several dark spots in an image produced from stacking of the 31 ONC-W1 images obtained in the descent operation on March 6–8, 2019 (Fig. S10), suggesting that dust had adhered to the surface of the camera optics. There are two types of grains with diameters of about 82 pixel and 18 pixel, which are attached on the lens and charge-coupled devise cover (CCD) of ONC-W1, respectively. The dust attached on CCD may have been originally contained inside the camera optics. Because the CCD pixel size of ONC-W1 is 13 µm (26), the size of dust attached on CCD is calculated to be ~1.1 mm (~82×0.013), which is similar to the effective aperture diameter of ONC-W1 (1.08 mm) (26). Because the transmittance at the dark spot decreased by < ~10% due to the grains attached on lens, the cross section of the dust should be <~10% of the effective aperture area. Therefore, the diameter of the dust is estimated to be < ~0.34 mm (~0.11/2×1.1) by assuming the dust is spherical.

The shadow/sunlit boundaries underneath large part of the cloud of dust were invisible after it extended to 6 m in diameter (Fig. S9A), suggesting that the optical depth of this cloud is ≫ 1. Here we simply assume a disk-shaped dust cloud with a thickness equivalent to the dust diameter (0.34 mm). Because the surface area of dark cloud is ~30 m2, by assuming a grain density of 1190 kg m-3 (2), the total mass of the fine-grained cloud is estimated to be ~12 kg (~30×3.4×10-4×1190). The dust cloud should be thicker and the grain density may be higher, because the bulk density of Ryugu is 1190 kg m-3. Therefore, the estimate gives the minimum value of the cloud mass.

13

Fig S9. ONC-W1 after the touchdown. The right and left panels are same images displayed in gray scale and color scale, respectively. ONC-W1 images obtained (A) about 15 seconds (hyb2_onc_20190221_222925_w1f), (B) about 25 seconds (hyb2_onc_20190221_222935_w1f), and (C) about 35 seconds after the touchdown (hyb2_onc_20190221_222945_w1f). The red circles indicate areas centered at the touchdown point with diameters of 7 m (A), 8 m (B), and 9 m (C). The shadow of the spacecraft can also be seen in these images, decreasing in size as the spacecraft ascends from the surface. The other dark shadows are due to dusts. The yellow circles indicate a large boulder moved horizontally by >5 m.

14

Fig S10. Stack image of ONC-W1 images obtained in the descent operation on March 6–8, 2019. There are two types of grains with diameters of about 82 pixel and 18 pixel, which are attached on the lens and CCD of ONC-W1, respectively.

15

Spectral change of the touchdown site before and after the first touchdown The ONC image data covering the touchdown site and its surrounding regions before

and after the first touchdown were obtained from about 1.8 km and 2.2 km altitudes during the spacecraft descents in the MASCOT deployment operation on October 2, 2018, and in the small carry-on impactor operation, respectively on April 4, 2019 (Fig. S11). Also, the pre- and post-touchdown Near Infrared Spectrometer (NIRS3) spectra were obtained in the Box-C (5–7 km altitude) operations on October 30, 2018 and on February 28, 2019, respectively (Fig. S12). The spatial resolutions are about 11 to 12 m. The phase angles for the pre- and post-touchdown NIRS3 spectra are about 8˚ and 15˚, respectively.

Fig. S11. Color change around the TD spot before and after the touchdown. ONC-T v-band image (A) and b-x slope image (B) before the touchdown (hyb2_onc_20181002_204148_tvf). ONC-T v-band image (C) and b-x slope image (D) after the touchdown (hyb2_onc_20190404_205307_tvf). Dashed circle indicate the area covered by dark fine grains after the touchdown. The TD spot, which was slightly bluer than the surrounding regions before the touchdown, reddened to the same level as the surrounding regions after the touchdown.

16

Fig. S12. Comparison of NIRS3 spectra of the touchdown site before and after the touchdown. (A) The average spectra after the photometry correction (4), calculated from 8 and 3 spectra before (red) and after (blue) the touchdown, respectively. The error bars show the standard deviation of those spectra. (B) Reflectance ratio (after/before).

Before TD (Oct. 30, 2018)After TD (Feb. 28, 2019)

A

B

Wavelength [nm]1800 2000 2200 2400 2600 2800 3000 3200

Refle

ctan

ce

0.014

0.015

0.016

0.017

0.018

0.019

0.020

0.8

0.9

1.0

1.1

1.2

Ratio

ed R

eflec

tanc

e (af

ter/b

efor

e)

17

Spectral slope of Ryugu craters 86 crater candidates on Ryugu’s surface were identified using the ONC image data

obtained in Box-A and Box-B (20 km altitude), Box-C, and the Mid-altitude operations (5, 16). The spatial resolutions of the images are 2 m pixel-1, 0.5–0.7 m pixel-1, and 0.5 m pixel-1, respectively. Craters were categorized into 4 classes based on the confidence level (CL) definitions (5, 16), CL1: circular depression with rim, CL2: circular depression, CL3: quasi-circular depression, CL4: Quasi-circular feature. We adopted the list of crater candidates from Hirata et al. (16).

To calculate the difference in spectral slopes between the interiors and surroundings of these craters, we used the b-x slope images calculated from the multi-band images obtained during the mid-altitude operations. We calculated the difference between mean spectral slopes in the crater interior and the surrounding areas (defined as the area within a distance of one crater radius from the crater rim) by the mean b-x slope in the surrounding area minus that in the crater interior. We used craters classified into CL1, CL2, and CL3 to determine the model ages from the formation of spinning top-shape to the surface reddening event. Craters in the mid- to high-latitude regions were excluded from our analysis because they were not observed in the mid-altitude operation images.

The histogram of the contrast in b-x spectral slope shows a bimodal distribution (Fig. 3), indicating that craters on Ryugu can be divided into two groups: redder craters and bluer craters. This suggests that the surface reddening occurred mainly within a short period after the formation of redder craters and before the formation of bluer craters. However, the red crater peak in the histogram is not centered at zero, indicating that even the red craters as well are slightly bluer than their surroundings. A potential explanation is that wall slumping exposed fresh bluer subsurface materials (5), consistent with the fact that the inner walls of redder craters are slightly bluer than the floors (e.g., craters Urashima (7.19S, 92.99E) and Kolobok (0.70S, 330.28E) in Fig. 1).

18

Cratering chronology model for Ryugu We used existing cratering chronology models for Ryugu (5, 32). The models were

calculated based on the intrinsic collision probabilities Pi for both main belt asteroids (MBAs) and near-Earth asteroids (NEAs) (18, 19), the population models of MBAs and NEAs (18, 19), the mean impact velocities for the main belt (5.3 km s-1) and NEAs (18 km s-1) (33, 34), and the impact crater size scaling law for a coarse grain target ((32), their equation 8). Crater size is strongly dependent on the strength of target materials. Although the effective strength of Ryugu is unknown, we selected a strengthless target for Ryugu’s materials consistent with Hayabusa2’s artificial impact experiment (17). The target grain size, target disruption energy, and the scaling parameters K1, K2, µ1, and µ2 for angular-grained targets are listed in Table S1. The derived chronology models are shown in Figs. S13 and S14. Table S1. Parameters for Ryugu cratering chronology models.

Parameters Values Reference Intrinsic collision probability Pi (10-18 km-2 yr-1)

2.86 (MBA), and 15.3 (NEA) (33, 34)

Impact velocity (km s-1) 5.3 (MBA), and 18.0 (NEA) (33, 34) Surface gravity (m s-2) 0.00015 (2) Target density (kg m-3) 1190 (2) Impactor density (kg m-3) 2300 Scaling parameters (35)

µ1 0.41 µ2 1.23 K1 0.24 K2 0.01 Effective strength (MPa) 0.0

Target grain size (m) 3 (5) Target disruption energy (J kg-1) 1000 (36)

19

Fig. S13. Final crater size on Ryugu calculated for coarse grain target. Final crater diameters on Ryugu calculated based on the impact crater size scaling law for a coarse grain target (5, 32) are shown by red (MBAs) and blue (NEAs) solid curves. Dashed curves show the final crater diameters calculated based on the impact crater size scaling law for a dry soil target (35).

Fig. S14. Cratering chronology model of Ryugu. (A) MBA chronology model. (B) NEA chronology model. Red and blue curves indicate the cratering chronology models for main-belt asteroids and for near-Earth asteroids, respectively. Black, red and blue squares indicate crater frequencies of all craters, red craters and blue craters, respectively. Dashed line indicates the empirical saturation.

Impactor Diameter [m]

Crat

er D

iamet

er [m

]

0.1 1.0 10.010

100

1000

NEA Holssaple

MBA Holssaple

MBA Tatsu

mi & Sugita

NEA Tatsu

mi & Sugita

10-110-210-3 101100

103

102

101

100

10-1

10-2

Crater Diameter [km]

Cumu

lative

Cra

ter F

requ

ency

[km-

2 ]

10-110-210-3 101100

103

102

101

100

10-1

10-2

Crater Diameter [km]

Cumu

lative

Cra

ter F

requ

ency

[km-

2 ]

MBA chronology model NEA chronology modelA B

20

Comparison with laboratory measurement on carbonaceous chondrites The color variations observed on Ryugu’s surface suggest that solar heating or space

weathering may have caused the reddening on Ryugu. Laboratory experiments using compressed powder pellet samples have shown that low-albedo carbonaceous chondrite meteorites (CCs), such as types CM and CI, become bluer upon ion irradiation (9). However, laboratory experiments on uncompressed chip surface of the same class of CCs indicate that spectral changes are completely different depending on the physical properties of the CCs and iron irradiation energy (37). The uncompressed CCs slightly redden and darken by low-energy (4 keV) He exposure. Because Ryugu’s surfaces are mostly covered with boulders and pebble (5, 8) the latter experimental conditions by (37) are probably closer to the conditions on Ryugu. Thus, observed reddening may be due to space weathering on uncompressed chip surfaces on low-albedo CCs. Heating experiments on the Murchison meteorite indicate that it reddens at very high temperatures (600˚C to 800˚C) after bluing at moderately high temperatures (400˚C to 600˚C) (10). The OH band depth in the near-infrared band changes slightly at very high temperature (600˚C to 800˚C). These laboratory experiments may be analogous to the observed reddening on Ryugu. Nevertheless, we cannot conclude which process(s) is responsible for the color change found on Ryugu because of the inconsistent results of laboratory experiments of space weathering and heating of CCs (9, 37, 38).

21

Caption for Movie S1. Animation created from ONC-W1 continuous imaging data obtained during the touchdown operation. Before and after the touchdown, 31 ONC-W1 images were obtained by continuous imaging with a cadence of 2 seconds. The animation was created from the ONC-W1 Level 2C images (distortion-corrected radiance data). The lower and upper limits of pixel values in each image are set to 0.0 and 1.7 W m-2 sr-1, respectively.

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