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 theG-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 3σconfidence level, only two groups are evident, while going down to 2σmore 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.
Highlights
The Hayabusa2 JAXA asteroid sample return mission was launched on December 3, 2014 and recently arrived to the primitive asteroid (162173) Ryugu after 3.5 yr of cruise
The statistical analysis of the spectrophotometry by ONC-T and NIRS3 spectral data at 3σ confidence level clearly highlights a very homogeneous surface with slightly different small areas containing about 3% of the analyzed data
Decreasing the confidence level to 2σ, few small groups are detected with different average spectral slopes
Summary
The Hayabusa JAXA asteroid sample return mission was launched on December 3, 2014 and recently arrived to the primitive asteroid (162173) Ryugu after 3.5 yr of cruise. At that time the Hayabusa mission acquired a very large number of resolved images and spectra of the asteroid surface. The on-board instruments reveal Ryugu as a top-shape body characterized by an equatorial ridge with a radius of 502 ± 2 m, and a very low density of 1190 ± 3 kg m−3. This implies porosity larger than 50% and a rubble-pile nature (Watanabe et al 2019). Ryugu presents a very dark surface with the visible geometric albedo of 0.045 ± 0.002 (Sugita et al 2019)
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