講演情報
[PPS07-P08]リュウグウ表面のボルダーの割れ目の異方性
*佐々木 晶1、神田 志穂1、菊地 紘2、道上 達広3、諸田 智克4、本田 親寿5、宮本 英昭6,7、逸見 良道7、杉田 精司4、巽 瑛理8、金丸 仁明1、坂谷 尚哉2、渡邊 誠一郎9、並木 則行10、入江 輝紀9、Michel Patrick11、平林 正稔12、平田 成5、中村 智樹13、野口 高明14、廣井 孝弘15、松本 晃治10、野田 寛大10、亀田 真吾16、神山 徹17、鈴木 秀彦18、山田 学19、本田 理恵20、長 勇一郎4、吉岡 和夫4、早川 雅彦2、松岡 萌2、澤田 弘崇2、横田 康弘20、吉川 真2 (1.大阪大学大学院理学研究科宇宙地球科学専攻、2.宇宙航空研究開発機構宇宙科学研究所、3.近畿大学工学部、4.東京大学大学院理学系研究科地球惑星科学専攻、5.会津大学、6.東京大学大学院工学系研究科、7.東京大学総合研究博物館、8.カナリア天文物理学研究所、9.名古屋大学大学院環境学研究科地球環境科学専攻、10.国立天文台RISE月惑星探査検討室、11.コート・ダジュール天文台、12.オーバーン大学、13.東北大学大学院理学研究科地学専攻、14.九州大学基幹教育院 、15.ブラウン大学、16.立教大学理学部物理学科、17.産業技術総合研究所、18.明治大学理工学部物理学科、19.千葉工業大学惑星探査研究センター、20.高知大学自然科学系理工学部門)
キーワード：
はやぶさ２、リュウグウ、熱疲労、割れ目の方向
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.
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.