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Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites

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Science  23 Aug 2019:
Vol. 365, Issue 6455, pp. 817-820
DOI: 10.1126/science.aaw8627

Landing on the surface of Ryugu

In October 2018, the Hayabusa2 spacecraft dropped the Mobile Asteroid Surface Scout (MASCOT) lander onto the surface of the asteroid (162173) Ryugu. Jaumann et al. analyzed images taken by the MASCOT camera during its descent and when resting on the surface. Colored light-emitting diodes were used to illuminate the lander's surroundings at night and produce color images. Ryugu's surface is dominated by two types of rock, but there is no evidence for fine-grained dust. Millimeter-sized inclusions in the rocks are similar to those present in carbonaceous chondrite meteorites. MASCOT operated for 17 hours on the surface before its nonrechargeable batteries ran out.

Science, this issue p. 817

Abstract

The near-Earth asteroid (162173) Ryugu is a 900-m-diameter dark object expected to contain primordial material from the solar nebula. The Mobile Asteroid Surface Scout (MASCOT) landed on Ryugu’s surface on 3 October 2018. We present images from the MASCOT camera (MASCam) taken during the descent and while on the surface. The surface is covered by decimeter- to meter-sized rocks, with no deposits of fine-grained material. Rocks appear either bright, with smooth faces and sharp edges, or dark, with a cauliflower-like, crumbly surface. Close-up images of a rock of the latter type reveal a dark matrix with small, bright, spectrally different inclusions, implying that it did not experience extensive aqueous alteration. The inclusions appear similar to those in carbonaceous chondrite meteorites.

Japan Aerospace Exploration Agency’s (JAXA’s) Hayabusa2 sample return mission (1), carrying the MASCOT lander, arrived at asteroid Ryugu on 27 June 2018 (2). Its initial measurements showed that Ryugu has an equatorial radius of 502 ± 2 m, a bulk density of 1.19 ± 0.02 g cm–3, and spectral properties typical of Cb-type carbonaceous asteroids (2). Ryugu’s visible geometric albedo is 4.5 ± 0.2%, making it one of the darkest objects in the Solar System (3). The German Aerospace Center (DLR) developed MASCOT in cooperation with the French Centre National d’Etudes Spatiales (CNES) (4).

MASCOT was released from the Hayabusa2 mother ship at 01:57:20 coordinated universal time (UTC) on 3 October 2018, from an altitude of 41 m above the surface of Ryugu (5). At 02:03:14 UTC, the lander made first contact with the surface (at contact point CP1), impacting the shadowed side of a large boulder. MASCOT then bounced backward and traveled a further 17 m over the surface to come to its first resting point (settlement point 1, SP1) in a local depression (Fig. 1, A to C). During the 6-min descent toward the surface and the bouncing phase, the MASCam instrument (6) took 20 images. In the same period, Hayabusa2’s Optical Navigation Camera (ONC) (3, 7) recorded both the shadow of MASCOT on the surface and the lander itself when its bright, white top was visible. These images allowed us to reconstruct the descent and bouncing trajectory (Fig. 1) (5). An up-raising maneuver (4, 5) at this first measurement position (MP1) (Fig. 1B) left the lander upside down, with most instruments aimed toward the sky. A sequence of five images acquired during the first night on the asteroid shows three moving objects, the angular velocities of which correspond to Ryugu’s rotation period of 7.63262 ± 0.00002 hours (2). We identified these objects as Jupiter, Saturn, and the 2.0 magnitude star σ Sagittarii. On the morning of the second day, MASCOT activated an internal mobility unit to reach a second location (MP2) (Fig. 1B), now properly orientated for its instruments to study the surface. Here, MASCam initiated a full day/night imaging cycle using an onboard array of light-emitting diodes (LEDs) to illuminate the area in front of the camera (Fig. 2). On the morning of the third day, the lander slipped ~5 cm sideways by executing a mini-move to enable stereo imaging for photogrammetric analysis, reaching a third location (MP3) (Figs. 1B and 2C) (5). Additional imaging over that day (Fig. 2D) enabled us to observe the surface at changing illumination and viewing geometries (5). ONC also imaged MASCOT close to the lander’s end of mission, allowing us to constrain its final location after the second relocation to MP4 as 22.31 ± 0.05°S, 317.16 ± 0.05°E (5). After 17 hours and 7 min of surface operations, MASCOT lost contact with Hayabusa2, shortly before its batteries ran out.

Fig. 1 Reconstructed MASCOT trajectory.

(A) Hayabusa2/ONC context image with its location on Ryugu shown in the inset. The MASCOT descent and bouncing trajectory is indicated in yellow. The MASCam field of view for two descent images shown below (“E” and “D”) are indicated by the white lines. The first contact point was CP1. (B) Schematic MASCOT mission timeline starting from release at T0. After bouncing, MASCOT reached SP1 and acquired data in several measurement positions (MP1 to MP4) until the end of mission (EoM) at MP4. (C) A 50 m by 50 m digital terrain model of the landing area with color-coded height (defined in a local topographic coordinate frame, relative to the local tangential plane) (3, 5). “MR” is the MASCOT release point. Yellow dots indicate MASCOT positions observed in ONC images. (D) Image taken during descent and (E) image of the boulders at CP1 [scale changes with distance; the boulder in (D), lower left, is ~1 m].

Fig. 2 Images of a type 1 rock at the second and third MASCOT locations (5).

(A) Image taken during late morning from MP2a. (B) Nighttime LED-illuminated image from MP2b. (C) Noon image from MP3a. The bright square is a reflection of sunlight off the MASCam alignment cube. (D) MASCam’s last image of the surface, taken in the late afternoon from position MP3b. The bright square has moved to the right because of the change in position of the Sun. The scale is derived from a distance map (5). The numbers under each image are the observation IDs.

The MASCam images acquired during descent and bouncing reveal a surface covered with rocks and boulders of different lithologies. Four types of rock have been identified in ONC images (3). Two of these types are readily identified in MASCam images (Fig. 3): dark and rough (type 1) and bright and smooth (type 2). Type 3 rocks (bright and mottled) were not observed. In the highest-resolution MASCam images, the surfaces of rock type 1 have a cauliflower-like appearance with a very small-scale rough and crumbly surface (Fig. 1 and Fig. 2, A and C). Type 2 rocks appear flat and possess elongated, nondendritic fractures that divide the rock into planar slabs with smooth surfaces. MASCam observed rocks with sizes from decimeters to a few tens of meters. The distribution of both rock types is spread almost evenly over the surface in the areas imaged by MASCam. This observation is consistent with the interpretation that Ryugu originated either from two parent bodies after collision, breakup, and reaccumulation, resulting in two types of material, or formed within the same parent body at different interior temperature and pressure conditions as a result of catastrophic disruption and redistribution, also resulting in two types of material (3). Ryugu’s bulk composition is expected to be similar to that of carbonaceous chondrite meteorites containing phyllosilicates and organics (CI or CM meteorites) (2, 3, 8, 9). Exposure of such materials to the diurnal temperature changes of ~100 K on Ryugu (2, 10) may trigger thermal fatigue, leading to crack growth and potentially breakdown of consolidated material (11, 12). Measurements by the MASCOT radiometer (MARA) instrument indicate that the rock in front of the lander has a bulk porosity of at least 28% and a low tensile strength (10). Type 1 rocks may respond to thermal fatigue by crumbling, resulting in their cauliflower-like appearances at small scales. Type 2 rocks may instead split in planar slabs, indicating preferential fracturing along weaker dimensions.

Fig. 3 Rock morphologies present on Ryugu rock.

(A to E) MASCam images acquired during descent showing at least two types of rocks. Rocks of type 1 appear dark with rough surfaces with a cauliflower-like, crumbly structure, whereas rocks of type 2 are smoother and brighter. (F to K) Color images (0.465, 0.523, and 0.633 μm) of a type 1 rock taken during the second night revealing bright inclusions. We estimated the distance to the area in (G) as ~25 cm (5), from which we estimate the size of the inclusions. Magnified images of two areas (H and J) show that many inclusions are either bluish (dark orange arrows) or reddish (bright green arrows) in the blue (0.47 μm) and infrared (0.81 μm) ratio images (I and K). (G) shows the size and coverage of inclusions (red contours).

At the second location, MASCam imaged a small rock in front of the lander. Nighttime images were acquired with illumination provided by onboard LEDs (6). Because of the small phase angle (~5°), they reveal bright inclusions embedded in a dark matrix (Fig. 3). Most inclusions are smaller than 1 mm, but some are several millimeters across (5). In one location (Fig. 3E), the areal coverage of the inclusions is ~10%. The size of the inclusions is similar to those of chondrules (submillimeter range), calcium- and aluminum-rich inclusions, and other refractory inclusions in carbonaceous chondrite meteorites (8, 9). Typical carbonaceous chondrites of subtypes CI2, CM, CO, and CV possess refractory inclusions that appear similar to those in Ryugu’s rock (5). However, CI1 meteorites are not a good analog because of their very low abundance of inclusions (11, 12). The destruction of inclusions in carbonaceous chondrites is thought to result from extensive aqueous alteration (13, 14), so their preservation on Ryugu is consistent with the notion that the asteroid was only moderately hydrated in the past (3). Ryugu’s spectrum is similar to that of certain moderately heated CI and CM meteorites (3, 15). The CI2 type meteorite Tagish Lake, which did not experience extensive heating, has a very low albedo and abundant inclusions, like Ryugu (5, 16).

Judging by their color, we can distinguish two broad types of inclusions, blue and red, although inclusions of different colors are also abundant (Fig. 4). Blue inclusions feature a moderately negative spectral slope of ~10% over the visible wavelength range. The positive spectral slope of the red inclusions is much steeper, in the 20 to 50% range. Their reflectance in the near-infrared spectrum (0.8 μm) is variable. Inclusions appear typically twice as bright as the rock matrix, although the smallest may be brighter as they are not spatially resolved. The spectral shape of unresolved inclusions is affected by the surrounding matrix to a minor degree, but because of their brightness they dominate the signal in each pixel. The inclusions shown in Fig. 4B are well registered over the four color channels and we expect no spectral artifacts as a result (5). The very flat nature of the matrix reflectance spectrum is consistent with the ground-based spectra of Ryugu (1719) and the average ONC reflectance spectrum (3). The matrix reflectance has a slight excess at 0.63 μm that is responsible for the reddish appearance of the rock in the color images (Fig. 3, E to J) but it is within the calibration uncertainty. The visible colors of the inclusions provide compositional clues but their interpretation is limited by the lack of spectral data >1 μm in wavelength, where most diagnostic bands for mineralogy are located. However, we may infer the composition by comparing the Ryugu inclusions with those in carbonaceous chondrites. For example, the decreasing reflectance between 0.5 and 0.8 μm of the bright chondrules of the Murchison meteorite is due to olivine (9, 20). The similar drop in reflectance toward 0.8 μm displayed by some Ryugu rock inclusions could also be due to olivine. The origin of the orange color of some of Murchison’s larger inclusions is not known (20). The MASCam observation of abundant multicolor, millimeter-sized inclusions suggests that C-type asteroids are linked to carbonaceous chondrites.

Fig. 4 Spectral variations in type 1 rock.

(A) These MASCam images were taken in LED light during the nighttime. In the image center, the rock appears gray with a reddish tint and many small, bright inclusions are visible. The insets enlarge an area in the center at the full brightness range of the LEDs to show albedo and color variations. The brightness in the true color composite (0.465, 0.523, and 0.633 μm) on the left is enhanced (5) to show details of the surface. (B) Area in the white box at the full brightness range. (C) Same area in saturated colors. The spectra of several bright inclusions (“b” to “f”) are compared with that of the matrix (“a”) in (D) and (E), both as absolute reflectance (D) and as reflectance normalized at 0.55 μm (E). The spectra are averages over the last three image sets of the night.

The visible inclusions suggest that the rock in front of MASCOT is not covered by microscopic (submillimeter) particles, which the MARA results confirm (10). MASCam observed no deposits of fine material during the descent. Such deposits were also absent from the surface of Itokawa (21), another rubble pile asteroid, whereas they are thought to be present on the surface of Eros (22). We expect dust to be formed continuously on the surface of Ryugu through exposure to the space environment (2325). However, all boulder appears clean, and therefore dust particles have likely been removed, lost either to space or the porous interior (2). The fate of fine particles is determined by a balance of forces. Cohesive forces are thought to dominate over electrostatic, gravitational, and solar radiation pressure forces for particles up to a decimeter in size for a body as small as Ryugu (26). Although the cohesive barrier may be overcome by means of electrostatic levitation (25), the physical conditions leading to this process on small bodies are not well understood. On Ryugu, larger particles (>100 μm) are more likely to be removed from the surface than is finer dust (27). Alternatively, cohesion may be overcome by means of physical forces such as micrometeorite impact, seismic shaking, boulder migration, collision, drag by sublimation of volatiles, and/or thermal fracturing (25). The absence of fine deposits on the surface of small rubble piles such as Ryugu and Itokawa implies the existence of an efficient removal mechanism for small particles. Gravel-sized particles may collect in the interior under the influence of gravity, but the absence of dust is not easily explained because of the strong cohesive forces involved.

Supplementary Materials

science.sciencemag.org/content/365/6455/817/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S5

Tables S1 and S2

References (2839)

References and Notes

  1. Materials and methods are available as supplementary materials.
Acknowledgments: The MASCOT lander on the Hayabusa2 mission of JAXA is a DLR/CNES cooperation. MASCam was developed and built under the leadership of the DLR Institute of Planetary Research with contracted contributions of Astrium GmbH and is operated by the DLR Institute of Planetary Research in Berlin in cooperation with the DLR Institute of Space Systems in Bremen and DLR-MUSC in Cologne. This study was supported by the JSPS International Planetary Network. The authors thank H. Sadada, M. Yamada, E. Tatsumi, N. Sakatani, C. Honda, K. Ogawa, M. Hayakawa, H. Suzuki, Y. Cho, and M. Matsuoka for Hayabusa2 ONC development and science observations. We cordially thank Mike Zolensky for providing fig. S5. The Hayabusa2 mission is operated by JAXA. Funding: The team acknowledges funding by DLR and CNES. Author contributions: R.J. coordinated and wrote the paper, R.J., N.S., S.E.S., A.K., F.T., K.D.M., and T.R. contributed to camera development, operations, and data processing. R.J., N.S., S.E., F.W., F.P., F.S., K.A.O., S.K., H.Y., S.E.S., S.W., M.Y., S.S., and H.R. contributed to the reconstruction of the trajectory, stereo processing, MASCOT location, comparison with ONC data, and Hayabusa operations. R.J., N.S., K.A.O., K.K., R.P., J.B.V., R.W., S.S., and P.M. contributed to the geomorphological analysis and discussion of Ryugu’s origin. R.J., S.E.S., K.St., N.S., S.M., J.P.B., M.G., K.A.O., and K.D.M. contributed to the spectral and photometric analysis. R.J., T.M.H., S.U., C.K., J.B., N.S., A.M.S., F.W., J.R., J.T.G., L.L., Y.T., T.O., T.Y., Y.M., A.K., F.T., S.S., and T.S. contributed to the MASCOT development, landing site selection, and on-surface operations. Competing interests: The authors declare no competing interests. Data and materials availability: All images and input data used in this paper and supplementary materials are available at http://europlanet.dlr.de/MASCAM until they become available at the long-term archive of the Planetary Data System at https://pdssbn.astro.umd.edu/. Additional data from the mission will be delivered to the DARTS archive at www.darts.isas.jaxa.jp/planet/project/hayabusa2/ and higher-level data products will be available in the Small Bodies Node of the Planetary Data System https://pds-smallbodies.astro.umd.edu/ 1 year after landing on the asteroid.
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