Slickenside as a record of shock metamorphism on asteroid Ryugu
M. Miyahara1, T. Noguchi2, T. Matsumoto2, N. Tomioka3, A. Miyake2, Y. Igami2, Y. Seto4, T. Nakamura5, H. Yurimoto6, R.
Okazaki7, H. Yabuta1, H. Naraoka7, S. Tachibana8, S. Watanabe9, Y. Tsuda10, and the Hayabusa2 Initial Analysis “Sand” Team1
Hiroshima Univ., Japan; 2Kyoto Univ., Japan; 3JAMSTEC, Japan; 4Osaka Metropolitan Univ., Japan; 5Tohoku Univ., Japan;
6
Hokkaido Univ., Japan; 7Kyushu Univ., Japan; 8Univ. of Tokyo, Japan; 9Nagoya Univ., Japan.; 10JAXA/ISAS, Japan.
Introduction: An asteroid is formed and evolved by repeated collisions. The Hayabusa2 spacecraft shows that boulders withlayered structures occur on the asteroid Ryugu (1). The layered structure can be regarded as parallel cracks, which is one of therepresentative features related to shock metamorphism. Evidence of shock metamorphism is found in some polished Ryugu
grains (C0055) (2). However, evidence of shock metamorphism has not been found on the surface of Ryugu grains. In the presentstudy, we scrutinize the surface morphology of several Ryugu grains to find features of shock metamorphism.
Samples and experimental methods: Numerous small Ryugu grains, ~100 µm across on average, were allocated to the M-P F
sub-team. Several grains from the chamber A (surface particles collected by the first touchdown) were attached on a gold (Au)plate (sample plate number: AP 042) with small amounts of epoxy glue in an N2 filled glove box for surface morphology
observation. The surface morphology of grains without any coating was observed by JEOL JSM-7100F and Hitachi S-5200 fieldemission gun scanning electron microscopes (FEG-SEM) at Tohoku Univ. and Hiroshima Univ., respectively. Some portions inthe grains were excavated and processed to be ultrathin foils by a Hitachi SMI4050 focused ion beam (FIB) system at Kochi
Institute for Core Sample Research, JAMSTEC after coating with osmium and carbon. The ultrathin foils were examined by aJEOL JEM-2100F transmission electron microscope (TEM) equipped with an energy dispersive spectrometer (EDS) at Tohoku
Univ. Chemical compositions were measured by EDS under scanning TEM (STEM) mode.
Results and discussion: More than 24 grains adhered on the sample
plate AP 042 were observed by secondary electron (SE) imaging (at alow accelerating voltage of 3.0–5.0 kV) to scrutinize the surface
morphology. Some grains were broken into finer grains when grains
were put on an Au plate, follows that fresh surface appeared newly.
Most grains had a bumped or rough surface, and some grains had
evidence of space weathering (3). On the other hand, two grains had asmooth surface on one side of the grain (Fig. 1). The smooth surface is
not a single crystal surface. An obscure liner texture was observed on
the smooth surface of one grain. An ultrathin foil was prepared fromthe smooth surface with a liner structure to observe its cross-section.
The smooth surface looks like a weathering vein consisting ofcarbonate or iron-hydro/oxide minerals which are found in somecarbonaceous chondrites. However, X-ray elemental maps indicatedthat there is no distinct difference in chemical compositions near thesmooth surface. Bright-field (BF)-TEM images showed that theultrathin foil consists mainly of fibrous phyllosilicates (saponite andserpentine) assemblage embedding small amounts of iron-sulfide andiron-oxide grains. The fibrous phyllosilicate assemblage is porous, andpores are filled with organic matter. These mineralogical features arelike the Ryugu grains investigated in other works (3, 4). However, there
is a distinct difference: the fibrous phyllosilicates assemblage near thesmooth surface is compacted and the compaction degree increases
toward the smooth surface. Lattice fringes corresponding tophyllosilicates become obscure with approaching the smooth surface,indicative of vitrification or dehydration. A ~200 nm layer from theFigure 1. SE images. a) A grain with a smooth surface
smooth surface is heavily compacted, where the (001) basal planes of(dashed line) and b) a high-magnification image of thefibrous phyllosilicates are sub-parallel to the smooth surface. The
smooth surface with a liner structure (dotted line).
smooth surface with a liner texture is a characteristic feature of a
Horizontal parallel stripes are artifact because of
slickenside, which is found in terrestrial fault rocks and shatter cones
electron charge-up.
around impact craters (5). The slickenside is formed by friction

between rocks along the two sides of a fault. Hence, we propose that the smooth surfaces appearing on some Ryugu grains are
the record of shock-induced shear deformation that occurred on the asteroid Ryugu. No evidence of decomposition is found in
fibrous phyllosilicates near the smooth surface of Ryugu grains. This suggests that friction heating temperature is below thedecomposition temperature of phyllosilicates (< ~700 °C) (6). A shock-melt vein is also formed by friction heating along afaulting zone in shocked meteorites (7). Such a melting texture is not observed in Ryugu grains. Ryugu's grains with slickensides
do not adhere well to each other because shock-induced melting and subsequent quenching did not occur. Therefore, grains withslickensides can be easily detached from each other during fall into the earth. In addition, a slickenside is too thin to survive
from atmospheric entry heating or terrestrial weathering. So, a slickenside could not be found in a meteorite.
References(1) S. Tachibana et al., Pebbles and sand on asteroid (162173) Ryugu: In situ observation and particles returned to Earth. Science375, 1011–1016 (2022).
(2) T. Nakamura et al., Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Scince
https://doi.org/10.1126/science.abn8671 (2022)(3) T. Noguchi et al., Mineralogy and space weathering of fine fraction recovered from asteroid (162173) Ryugu. the 53rd Lunarand Planetary Science Conference, Houston, 1747.pdf, 2022.
(4) T. Matsumoto et al., Mineralogical analysis of various lithologies in coarse Ryugu samples using transmission electron
microscopy the 53rd Lunar and Planetary Science Conference, Houston, 1344.pdf, 2022.
(5) D. Baratoux, W. U. Reimold, The current state of knowledge about shatter cones: Introduction to the special issue. Meteorit.
Planet. Sci. 51, 1389–1434 (2016).
(6) J. N. Weber, R. T. Greer, Dehydration of serpentine: Heat of reaction and reaction kinetics at PH2O=1 atm. Am. Min. 50,450–464 (1965).
(7) P. Gillet, A. E. Goresy, Shock events in the Solar System: The message from minerals in yerrestrial planets and asteroids.
AREPS 41, 257–285 (2013).