Boulder size and shape distributions on asteroid Ryugu

Michikami, Tatsuhiro; Honda, Chikatoshi; Miyamoto, H.; Hirabayashi, Masatoshi; Hagermann, A.; Irie, Terunori; Nomura, Keita; Ernst, C. M.; Kawamura, Masaki; Sugimoto, K; Tatsumi, Eri; Morota, Tomokatsu; Hirata, Naru; Noguchi, Takaaki; Cho, Yuichiro; Kameda, Shingo; Kouyama, T.; Yokota, Yasuhiro; Noguchi, Rina; Hayakawa, Masahiko; Hirata, Naoyuki; Honda, Rie; Matsuoka, Moe; Sakatani, Naoya; Suzuki, Hidehiko; Yamada, Manabu; Yoshioka, Kazuo; Sawada, Hirotaka; Hemmi, Ryodo; Kikuchi, Hiroshi; Ogawa, Kazunori; Watanabe, S.; Tanaka, Satoshi; Yoshikawa, Makoto; Tsuda, Yuichi; Sugita, Seiji

doi:10.1016/j.icarus.2019.05.019

Cited by 126 publications

(178 citation statements)

References 41 publications

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“…The difference of albedo could reflect either a compositional and/or structural difference of regolith between east and west. For example, Michikami et al (2019) reported the difference in the large boulder density distribution on east and west hemispheres. Therefore, the grain size of hemispheres could also be different.…”

Section: Albedo and Color Mapsmentioning

confidence: 99%

Global photometric properties of (162173) Ryugu

Tatsumi

Domingue

Schröder

et al. 2020

A&A

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Context. The Hayabusa2 spacecraft launched by Japan Aerospace Exploration Agency has been conducting observations of the asteroid (162173) Ryugu since June 2018. The Telescopic Optical Navigation Camera (ONC-T) onboard Hayabusa2 has obtained thousands of images under a variety of illumination and viewing conditions. Aims. Our objective is to examine and validate the camera calibration, derive a photometric correction for creating global albedo maps, and to interpret the photometric modeling results to characterize the surface of Ryugu. Methods. We observed (162173) Ryugu with the Gemini-South telescope, and combined these measurements with other published ground-based observations of the asteroid. The ground-based observations were compared with the data obtained by ONC-T in order to validate the radiometric calibration mutually. We used a combination of the Hapke disk-integrated and disk-resolved model equations to simultaneously analyze the combined ground- and spacecraft-based data. Results. The average spectrum of Ryugu was classified as Cb-type following the SMASSII taxonomy and C/F-type following the Tholen taxonomy based on spacecraft observations. We derived Hapke model parameters for all seven color filters, which allowed us to photometrically correct images to within an error of <10% for ~80% of the image pixels used in the modeling effort. Using this model, we derived a geometric albedo of 4.0 ± 0.5% (v band) for Ryugu. The average reflectance factor at the standard illumination condition was 1.87 ± 0.14% in the v band. Moreover we measured a phase reddening of (2.0 ± 0.7) × 10−3 μm−1 deg−1 for Ryugu, similar to that observed for the asteroid (101955) Bennu. Conclusions. The global color map showed that the general trend was for darker regions to also be redder regions, however there were some distinct exceptions to this trend. For example, Otohime Saxum was bright and red while Kibidango crater was dark and blue. The darkness and flatness of Ryugu’s reflectance might be caused by a high abundance of organic materials.

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Section: Albedo and Color Mapsmentioning

confidence: 99%

Global photometric properties of (162173) Ryugu

Tatsumi

Domingue

Schröder

et al. 2020

A&A

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“…On the other hand, sphericity of a triaxial ellipsoid with the same axis ratios is 0.913. Therefore, sphericity depends not only on axis ratios, but also on particle shape, and we will use Ψ = 0.85 ± 0.06 as an average sphericity rather than the average sphericity derived from the shape data in Michikami et al (2019) when calculating interparticle forces and initial porosities below.…”

Section: Particle Size and Shape Distributionsmentioning

confidence: 99%

Macroporosity and Grain Density of Rubble Pile Asteroid (162173) Ryugu

et al. 2020

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Upon arrival of the Hayabusa2 spacecraft, the C-complex asteroid (162173) Ryugu was found to be a spinning top-shaped rubble pile (Watanabe et al., 2019) with Cb-type spectrum and very low albedo around 0.045, consistent with thermally metamorphosed CM/CI meteorites (Sugita et al., 2019). Observations further show that a weak 2.7-μm absorption is present, suggesting a small amount of hydrated minerals exist on the surface (Kitazato et al., 2019). Furthermore, the surface was found to be dominated by blocks and boulders (Michikami et al., 2019; Sugita et al., 2019), and 50% of the surface is covered by boulders with diameters exceeding 0.5 m. A bulk density of 1,190 ± 20 kg m −3 was determined using the SFM20180804 shape model (Watanabe et al., 2019), which allowed for an estimate of asteroid porosity. Assuming typical grain densities for carbonaceous chondrites (

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“…Consolidated objects can fragment efficiently (34) with fragments extending up to 10 to 50 m in size (35,36). Loosely bound rubble piles convey impact energy inefficiently (37) and have boulders up to similar sizes (38,39) that will survive impact. Integrating the distributions from the smallest up to the largest fragments to determine total masses yields similar results for both cases.…”

Section: Summary Of Modeled Dust Cloud Dynamicsmentioning

confidence: 99%

New HST data and modeling reveal a massive planetesimal collision around Fomalhaut

Gáspár

Rieke

2020

Proc. Natl. Acad. Sci. U.S.A.

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The apparent detection of an exoplanet orbiting Fomalhaut was announced in 2008. However, subsequent observations of Fomalhaut b raised questions about its status: Unlike other exoplanets, it is bright in the optical and nondetected in the infrared, and its orbit appears to cross the debris ring around the star without the expected gravitational perturbations. We revisit previously published data and analyze additional Hubble Space Telescope (HST) data, finding that the source is likely on a radial trajectory and has faded and become extended. Dynamical and collisional modeling of a recently produced dust cloud yields results consistent with the observations. Fomalhaut b appears to be a directly imaged catastrophic collision between two large planetesimals in an extrasolar planetary system. Similar events should be very rare in quiescent planetary systems of the age of Fomalhaut, suggesting that we are possibly witnessing the effects of gravitational stirring due to the orbital evolution of hypothetical planet(s) around the star.

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Boulder size and shape distributions on asteroid Ryugu

Cited by 126 publications

References 41 publications

Global photometric properties of (162173) Ryugu

Global photometric properties of (162173) Ryugu

Macroporosity and Grain Density of Rubble Pile Asteroid (162173) Ryugu

New HST data and modeling reveal a massive planetesimal collision around Fomalhaut

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