[P-PS07] Solar System Small Bodies: Explorations of Ryugu, Bennu, and the Solar System at Large

[PPS07-P08] Crack orientation of surface boulders on Ryugu

*Sho Sasaki¹, Shiho Kanda¹, Hiroshi Kikuchi², Tatsuhiro Michikami³, Tomokatsu Morota⁴, Chikatoshi Honda⁵, Hideaki Miyamoto^6,7, Ryodo Hemmi⁷, Seiji Sugita⁴, Eri Tatsumi⁸, Masanori Kanamaru¹, Naoya Sakatani², Sei-ichiro Watanabe⁹, Noriyuki Namiki¹⁰, Terunori Irie⁹, Patrick Michel¹¹, Masatoshi Hirabayashi¹², Naru Hirata⁵, Tomoki Nakamura¹³, Takaaki Noguchi¹⁴, Takahiro Hiroi¹⁵, Koji Matsumoto¹⁰, Hirotomo Noda¹⁰, Shingo Kameda¹⁶, Toru Kouyama¹⁷, Hidehiko Suzuki¹⁸, Manabu Yamada¹⁹, Rie Honda²⁰, Yuichiro Cho⁴, Kazuo Yoshioka⁴, Masahiko Hayakawa², Moe Matsuoka², Hirotaka Sawada², Yokota Yasuhiro²⁰, Makoto Yoshikawa² (1.Department of Earth and Space Sciences, School of Science, Osaka University, 2.ISAS/JAXA, 3.Faculty of Engineering, Kindai University, 4.Department of Earth and Planetary Science, The University of Tokyo, 5.The University of Aizu, 6.School of Engineering, The University of Tokyo, 7.The University Museum, The University of Tokyo, 8.Instituto de Astrofisica de Canarias, 9.Division of Earth and Planetary Sciences, Nagoya University, 10.RISE Project, National Astronomical Observatory of Japan, 11.Observatoire De La Cote D'Azur, 12.Auburn University, 13.Department of Earth and Planetary Materials Sciences, Tohoku University, 14.Department of Arts and Science, Kyushu University, 15.Brown University, 16.Department of Physics, Rikkyo University, 17.National Institute of Advanced Industrial Science and Technology, 18.Department of Physics, Meiji University, 19.Planetary Exploration Research Center, Chiba Institute of Technology, 20.Department of Science and Technology, Kochi University)

Keywords:Hayabusa-2, Ryugu, thermal fatigue, crack orientation

Hayabusa 2 spacecraft revealed that a small carbonaceous asteroid (162173) Ryugu is a rubble pile with overall density 1.19 x 10³kg/m³ [1].
The surface of Ryugu is covered with various sizes of boulders. On Ryugu, the relative abundance of large boulders (>20m) is about twice as that of Itokawa or Bennu [2,3]. On the surface of Itokawa, several cracked boulders are observed and compared with cracked fragments from impact experiments [4]; impacts on Itokawa or its parent body would form boulder cracks.

Thermal fatigue is advocated for the disintegration process of surface rocks [5], where diurnal / annual thermal cycle may promote crack growth in the rocks on regolith over various spatial and temporal scales [6]. Growth of crack is rapid enough to fracture a few 10 cm size rock [7].

In preliminary analysis, we noticed that cracks on Ryugu boulders have preferred meridional orientation [8]. Desert rocks of the Earth and Mars have preferred orientation of cracks [9,10]. Here in this study, we analyzed more than 500 cracks on Ryugu boulders and checked their orientations. We analyzed 101 images (taken from 29-4083m height at proximity operation phase by Hayabusa-2 ONC-T. Image resolution is 3mm/pixel at best. We confirm the image position and resolution from shape model matching (SPC) and/or altimetry data by LIDAR. Hayabusa 2 usually observes the surface from the direction of the sun, which provide low phase angle data with short shadow width. We carefully check images so that we do not pick up the shadowed surface structure as a crack. Some cracks are confirmed using the image with different (larger) solar phase angle.

To check if a rock has a crack or not, 15-20 pixels are necessary. At the highest resolution, we may check a rock as small as 20cm. Assuming the same range size, about 2-5% of boulders have cracks. So far, we do not observe changes of the abundance ratio of cracked rocks on the Ryugu surface.

We classified cracks into four styles (as shown in the Figure):

(a) Straight cracks: Some cracks are running linearly without bending or kinking. (b) Sinuous cracks: Some cracks have bowing, bending, and wavy structure. (c) Arrested cracks: We observed many rocks have a crack which does not go through. (d) Complex (typically branched) cracks.

We separated the strike of cracks into 18 directions with 10deg bin. We analyzed 538 boulders and found 60% of their cracks have the meridional direction (+-15deg from N-S) except complex type. This trend is common among crack types as well as rock size.

If boulder cracks on Ryugu are formed by impact processes, whether impacts occur before or after Ryugu formation, the direction of cracks should distribute more randomly. So far, solar-induced thermal stress on a surface boulder by diurnal rotation and annual revolution of Ryugu might be a possible process for the growth of boulder cracks in the meridional direction.

Ref: [1] Watanabe, S. et al (2019) Science 364. [2] Sugita, S. et al (2019) Science 364. [3] Michikami, T. et al (2019) Icarus, 331, 179-191 [4] Nakamura et al. (2008) EPS 60, 7-12. [5] Delbo M. et al. (2014) Nature 508, 233–236. [6] Molaro, J. L. et al. (2017) Icarus 294, 247-261. [7] El Mir, C., et al. (2019) Icarus, [8] Sasaki, S. et al. (2019) LPSC L, #1368. [9] MacFadden et al., (2005) Geol. Soc. Am. Bull. 117, 161-173. [10] Eppes, M. C. et al. (2015) Nature Comm., 6, 6712, [11] Delbo, M. et al (2019) EPSC-DPS-176-2.