The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes

Abstract

The near-Earth carbonaceous asteroid 162173 Ryugu is thought to have been produced from a parent body that contained water ice and organic molecules. The Hayabusa2 spacecraft has obtained global multicolor images of Ryugu. Geomorphological features present include a circum-equatorial ridge, east-west dichotomy, high boulder abundances across the entire surface, and impact craters. Age estimates from the craters indicate a resurfacing age of [Formula: see text] years for the top 1-meter layer. Ryugu is among the darkest known bodies in the Solar System. The high abundance and spectral properties of boulders are consistent with moderately dehydrated materials, analogous to thermally metamorphosed meteorites found on Earth. The general uniformity in color across Ryugu's surface supports partial dehydration due to internal heating of the asteroid's parent body.

Fig. 1.. Global map and images of Ryugu.

(A) Geologic map of Ryugu based on mosaicked v-band (0.55 μm) images. Impact craters and crater candidates are indicated with circles, color coded by confidence level (see text). There is greater latitudinal exaggeration of map-projected surface area on Ryugu than for a sphere, because of its diamond-like cross section. This leads to the apparent higher crater number density in the equatorial region of this map. (B) Oblique view of Ryugu (image hyb2_onc_20180824_102748_tvf_l2b), showing the circum-equatorial ridge (yellow arrows), trough (blue arrows) extending from the equatorial region through the south polar region to the other side of Ryugu, and the large and bright Otohime Saxum (red arrow) near the south pole. The location of the poles and the spin direction are indicated with white arrows. (C) Asymmetric regolith deposits on imbricated flat boulders on the northern slope of the circum-equatorial ridge of Ryugu (hyb2_onc_20181003_222509_tvf_l2b). Small yellow arrows at the edges of regolith deposits indicate the direction of mass wasting. The large yellow arrow indicates the current geopotential gradient from high to low (17). The direction of geopotential gradient is consistent with the mass wasting.

Fig. 2.. Craters on Ryugu.

(A) The largest crater, Urashima (290 m in diameter, 8.3°S, 92.5°E), on Ryugu (hyb2_onc_20180720_071230_tvf_l2b). Wall slumping is indicated with yellow arrows. (B) Kolobok crater (240 m, 1.5°S, 333.5°E), which has a deep floor, bowl-like shape, and a raised rim (hyb2_onc_20180720_100057_tvf_l2b). (C) LIDAR profiles of Urashima crater. Wall slumping is indicated with blue arrows. (D) LIDAR profiles of Kolobok crater. (E) CSFD on Ryugu and Itokawa and empirical saturation and crater production curves (54) with (orange) and without (green) dry-soil cohesion. Black crosses in (F) represent Itokawa crater candidates (11). Red and blue points indicate Ryugu craters with different crater CLs. (F) An R-plot (the CSFD normalized by D⁻², where D is diameter) for Ryugu (circles and squares) and Itokawa (crosses). The relative crater frequency R is defined as the differential crater frequency in a diameter range between D/k and kD, divided by D³, where k is 2^1/4. Saturation and crater production curves are the same as in (E). Ma, million years.

Fig. 3.. Multiband colors of Ryugu’s surface.

(A) Comparison between disk-averaged spectra (lines with squares, normalized at 0.55 μm) for Ryug at 12 different rotational phases and ground-based observations (lines without symbols) of Ryugu from (55) (blue) and from (21) (red). Data are also shown for the large main-belt asteroids Polana, Eulalia, and Erigone (56), each of which is the parent body of an asteroid family. Because of the similarity among the spectra taken at different phases, individual lines for Ryugu overlap. Spectra are offset by 0.1 for clarity. (B) Comparison between typical Ryugu surface colors (black) (reflectance factor at 30°, 0°, 30°) and those of dehydrated CCs (blue) and typical CCs (red). Individual meteorite names are indicated. The spectrum of a powder sample (≤155 μm) of Jbilet Winselwan was measured at 30°, 0°, 30° with the spectrometer system at Tohoku University (57). The rest of meteorite spectra are from (58). (C) Reflectance spectra of typical morphologic and color features on Ryugu. Locations of features (labeled 1 to 6) are shown in (E) and (F) and in fig. S12. Individual spectra are shifted vertically for clarity. Vertical-axis tick spacing is 0.05%. (D) Same as (C), but normalized by the Ryugu average spectrum. Vertical-axis tick spacing is 0.01. (E) b-x slope map (inverse micrometers) and (F) v-band reflectance factor map (percent) superposed on a v-band image map. The equatorial ridge and the western side (160°E to 290°E) have slightly higher v-band reflectances than other regions (see fig. S13 for statistical analysis).

Fig. 4.. Statistics and morphologies of boulders on Ryugu.

(A) Distribution in longitude of boulders with diameters of 20 to 30 m and ≥30 m. (B) Cumulative size distribution of large boulders, compared between different latitudinal zones. (C) A type 1 boulder, which is dark and rugged (hyb2_onc_20181004_042509_tvf_l2b). A close-up view of its layered structure is shown in fig. S11D. (D) A type 2 bright boulder with smooth surfaces and thin layered structure (hyb2_onc_20181004_012509_tvf_l2b). A close-up view of its layered structure is shown in fig. S11E. (E) A type 3 bright and mottled boulder (hyb2_onc_20180801_213221_tvf_l2b). (F) The sole type 4 boulder, Otohime Saxum, has concentric (yellow arrows) and radial (blue arrows) fractures, consistent with a fracture system generated by an impact (hyb2_onc_20180719_124256_tvf_l2b). In (C) to (F), the brightness of each image is stretched independently.The yellow and white scale bars are 10 and 100 m, respectively.

Fig. 5.. Colors of surface features on Ryugu.

Colors measured from ONC-T images are compared between areas of regolith (gray-black contour) and the four types of boulders (solid, monotone squares) on Ryugu. The legend applies to both panels. (A) Comparison of v-reflectance factor and b-x slope distribution. The average value of Ryugu’s surface is indicated with a white cross. Contours indicate 95 and 68% of the surface area. (B) Comparison of principal component space (PC2-PC3) and main-belt C-complex asteroids (56) (colored circles), a moderately dehydrated CC [Y-86029, orange diamond (58)], Murchison (CM2) samples with heating [black line (58)] and laser irradiation [light green (59) and gray lines (58)], and heated Ivuna (CI) samples [blue line (58)]. Parent bodies of major asteroid families in the inner main belt, Polana (open blue star), Eulalia (solid light blue star), and Erigone (open green star), are also shown (56). Images of the four types of boulders are shown in Fig. 4 and fig. S11. Thick black arrows denote locations of end-member spectra (spectra with deep 0.7-μm absorption, flat spectra, and spectra with deep ultraviolet absorption) in this PC space.

Fig. 6.. Thermal infrared camera measurement results.

(A) Brightness temperature image taken with TIR at 06:07:11 UTC on 10 July 2018 (hyb2_tir_20180710_060711_l2). (B to D) The image in (A) compared with calculated thermal images by using the structure-from-motion shape model (17), assuming uniform thermal inertia of (B) 50, (C) 200, and (D) 500 J m⁻² s^−0.5 K⁻¹, respectively. (E) An ONC-T image of large boulders (6.4°S, 148.4°E), taken during low-altitude (5 to 7 km) observations (hyb2_onc_20180801_144909_tvf_l2b). Surface area (open circle) not covered with regolith was chosen for temperature analysis. (F) As in (E), but for a boulder at (20.9°S, 27.8°E) (hyb2_onc_20180801_174157_tvf_l2b). (G) Temperature profile of the location indicated with the circle in (E) observed with TIR at 20 km from the Ryugu center (open circles). Theoretical temperature profiles for uniform thermal inertias of 200 and 600 J m⁻² K⁻¹ s^−0.5 are shown with curves. Solid curves are for a horizonal plane that starts to receive solar light at local time 7.5 hours; dashed curves represent a tilted plane that receives sunlight at later times. The observed data are largely enclosed by the upper envelopes of time-shifted curves for 200 and 600 J m⁻² K⁻¹ s^−0.5. (H) Same as (G), but for the location indicated by the circle in (F).

Fig. 7.. Close-up observation results of surfaces on Ryugu.

(A) A boulder partially buried with regolith (yellow arrows) and a smaller boulder with angular fragments having different brightness (blue arrow) near the MINERVA-II landing site (9 mm per pixel, hyb2_onc_20180921_040154_tvf_l2b). (B) A rugged boulder with layered structure (yellow arrows) near the MASCOT landing site (6 cm per pixel, hyb2_onc_20181003_003036_tvf_l2b).

Fig. 8.. Schematic illustration of Ryugu’s formation.

Ryugu formed from the reaccumulation of material ejected from an original parent body by an impact, possibly by way of an intermediate parent body (bottom). Three scenarios to explain Ryugu’s low hydration and thermal processing may have occurred before disruption of the original parent body (top).