Multivariable statistical analysis of spectrophotometry and spectra of (162173) Ryugu as observed by JAXA Hayabusa2 mission

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, Sorbonne Paris Cité, UPMC Université Paris 06, Sorbonne Universités, 5 place Jules Janssen, 92195 Meudon, France
e-mail: antonella.barucci@obspm.fr
² Kochi University, Kochi 780-8520, Japan
³ ISAS, JAXA, Sagamihara 252-5210, Japan
⁴ The University of Tokyo, Tokyo 113-0033, Japan
⁵ The University of Aizu, Aizu-Wakamatsu 965-8580, Japan
⁶ INAF, Osservatorio Astronomico di Roma, Monte Porzio Catone 00078, Italy
⁷ Nagoya University, Nagoya 464-8601, Japan
⁸ Rikkyo University, Tokyo 171-8501, Japan
⁹ Kwansei Gakuin University, Hyogo 662-8501, Japan
¹⁰ Brown University, Providence, RI 02912, USA
¹¹ Tohoku University, Sendai 980-8578, Japan
¹² AIST, Tokyo 135-0064, Japan
¹³ Meiji University, Kawasaki 214-8571, Japan
¹⁴ Chiba Institute of Technology, Narashino 275-0016, Japan
¹⁵ Kobe University, Kobe 657-8501, Japan
¹⁶ Jacobs/NASA Johnson Space Center, Houston, TX 77058, USA
¹⁷ Planetary Science Institute, Tucson, AZ 85719, USA
¹⁸ SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193, Japan
¹⁹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain

Received: 6 May 2019
Accepted: 4 July 2019

Abstract

Context. Starting from late June 2018, the JAXA asteroid sample return mission Hayabusa2 acquired a large quantity of resolved images and spectra of the surface of the asteroid (162173) Ryugu.

Aims. By studying the visible and near-infrared spectral behavior across the surface of Ryugu using a statistical analysis, we aim to distinguish spectral homogeneous groups and to detect the small heterogeneities. This allows us to better constrain the surface composition variations.

Methods. In order to isolate and interpret the difference in the asteroid surface spectral behavior, we applied the G-mode multivariate statistical analysis to a set of pixels containing information of (i) the visible ONC-T spectrophotometry, and (ii) the near-infrared NIRS3 spectra thereby obtaining automatic statistical clustering at different confidence levels.

Results. The analysis of both ONC-T and NIRS3 data allows us to highlight small spectral variations on the Ryugu surface. At a 3σ confidence level, only two groups are evident, while going down to 2σ more groups are obtained with differences in spectral slope and band depth.

Conclusions. The identified groups have been associated with main morphological surface features. The spectral slope variations that characterize the small groups obtained by ONC-T data analysis, are interpreted as a consequence of space weathering with the presence of more or less fresh material and/or the different grain sizes of the regolith. The variations found analyzing the NIRS3 data are attributed to slightly different contents of hydrated material and different regolith sizes. The distribution on the Ryugu surface of the groups obtained by the analysis of the two instruments indicates a clear spectral dichotomy both between the east and west, and the north and south hemispheres. Small sized regolith grains associated to the redder spectra seem concentrated in the southwestern part of the body.