Volume 639, July 2020
|Number of page(s)||19|
|Section||Planets and planetary systems|
|Published online||13 July 2020|
Global photometric properties of (162173) Ryugu
Instituto de Astrofísica de Canarias (IAC), University of La Laguna,
2 Department of Astrophysics, University of La Laguna, La Laguna, Tenerife, Spain
3 Department of Earth and Planetary Science, The University of Tokyo, Bunkyo, Tokyo, Japan
4 Planetary Science Institute (PSI), Tucson, AZ, USA
5 Deutsches Zentrum für Luft- und Raumfahrt (DLR), Berlin, Germany
6 Institute of Space and Astronomical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Kanagawa, Japan
7 Kochi University, Kochi, Kochi, Japan
8 Okayama Observatory, Kyoto University, Asakuchi, Okayama, Japan
9 Seoul National University, Gwanak, Seoul, Korea
10 Brown University, Providence, RI, USA
11 The University Museum, The University of Tokyo, Bunkyo, Tokyo, Japan
12 Planetary Exploration Research Center (PERC), Chiba Institute of Technology, Narashino, Chiba, Japan
13 Rikkyo University, Toshima, Tokyo, Japan
14 National Institute of Advanced Industrial Science and Technology (AIST), Koto, Tokyo, Japan
15 Meiji University, Kawasaki, Kanagawa, Japan
16 Department of Complexity Science and Engineering, The University of Tokyo, Kashiwa, Chiba, Japan
17 The University of Aizu, Aizu-Wakamatsu, Fukushima, Japan
18 Kobe University, Kobe, Hyogo, Japan
19 National Astronomical Observatory of Japan (NAOJ), Mitaka, Tokyo, Japan
Accepted: 22 May 2020
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.
Key words: techniques: photometric / minor planets, asteroids: individual: Ryugu / space vehicles / methods: observational
© ESO 2020
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