9:45 AM - 10:00 AM
[PPS03-09] Scale dependence of spatial variation of Ryugu materials based on optical multiband imaging of Earth-returned samples
Keywords:Ryugu, C-type asteroid
Ryugu is a C-type near-Earth asteroid investigated by the Hayabusa2 spacecraft. Hayabusa2 observed Ryugu with remote-sensing instruments and collected samples. C-type asteroids are thought to be sources for carbonaceous chondrites, containing water and organic matters, which may be ingredients for life and may have delivered these materials to Earth in the early history of Solar System.
Remote-sensing observations have revealed the shape or structure of Ryugu and the distribution of surface color 1,2,3. Color observations and crater counting have further shown that surface features with younger stratigraphic age and lower crater densities have bluer spectra 2,3. However, the process that changes the color of Ryugu has not be fully explained. Also, the average spectrum of returned samples are reported 4, but the correlation between the reflectance and spectrum of each sample or the inhomogeneity of spectra between samples are not shown.
In this study, we measured the spatial distribution of the reflectance and the color of samples using a newly developed optical multi-band imager 5 in JAXA’s curation facility, as a part of the curatorial description of returned samples from Ryugu. This system irradiates light through six bands of filters (ul:0.39 µm, b:0.48 µm, v:0.55 µm, Na:0.59 µm, w:0.70 µm, x:0.85 µm) equivalent to the Telescopic Optical Navigation Camera (ONC-T) of Hayabusa2 at 30 degrees of incident angle and obtain images (1.2 µm/pix) at zero emission angle (e.g., normal to surface). We calibrated images a reflectance standard (Spectralon/Labsphere), obtained reflectance maps at all the filter bands, and calculated spectral slope. We measured five large (>1 mm in diameter) particles and four bulk samples containing fine particles (<1 mm in diameter).
In particular, analysis of correlation between the reflectance and spectrum slope (b-x slope) of a bulk sample (sample ID C0205), which contains fine particles (<120 µm in diameter), revealed the trend that brighter particles have smaller spectrum slope (i.e., bluer) which is the same as the result of ONC measurements 2. This result suggests that originally bluer boulders of Ryugu redden and darken owing to such processes as space weathering, solar heating, and dust covering 2,3. These processes may hold a key for understanding the surface evolution of Ryugu. Comparison between the sub-mm scale color and reflectance distribution measured in this study and meter scale distribution observed by Hayabusa2 reveals interesting contrast. The distribution (1σ interval) of the reflectance is 0.53 to 4.0% and the spectrum slope is -1.2 to +2.0 (µm-1). These are far wider than those of the results of remote-sensing (reflectance: 1.7 to 2.1%, spectrum slope: 0.02 to 0.16 (µm-1)) 2. This result suggests that Ryugu is composed of well-mixed materials that have wider reflectance and spectrum slope distribution.
Another interesting property is that the range of distribution in color of samples become as narrow as 2 m/pix data when color is averaged over individual particles (~ mm in diameter). In other words, inter-particle variation in color is as small as those found in the global distribution of Ryugu observed at low resolutions (2 m/pix). The maximum size of samples we measured in this study is a few millimeters, so the scale is three digits apart from remote-sensing observation. If the range of distribution in color is constant between these scales, this would indicate that Ryugu experienced the process that separates materials and forms particles smaller than 1 mm in size. Also, the analysis of these samples will help us understand the materialistic process that may cause the color distribution of Ryugu.
References
1. S. Watanabe et al., 2019. Science. 364, 268-272
2. S. Sugita et al., 2019. Science. 364, eeaw0422.
3. T. Morota et al., 2020. Science. 368, 654-659
4. T. Yada et al., 2021. Nat. Astron. https://doi.org/10.1038/s41550-021-01550-6
5. Y. Cho et al., Submitted to Planet. Sp. Sci
Remote-sensing observations have revealed the shape or structure of Ryugu and the distribution of surface color 1,2,3. Color observations and crater counting have further shown that surface features with younger stratigraphic age and lower crater densities have bluer spectra 2,3. However, the process that changes the color of Ryugu has not be fully explained. Also, the average spectrum of returned samples are reported 4, but the correlation between the reflectance and spectrum of each sample or the inhomogeneity of spectra between samples are not shown.
In this study, we measured the spatial distribution of the reflectance and the color of samples using a newly developed optical multi-band imager 5 in JAXA’s curation facility, as a part of the curatorial description of returned samples from Ryugu. This system irradiates light through six bands of filters (ul:0.39 µm, b:0.48 µm, v:0.55 µm, Na:0.59 µm, w:0.70 µm, x:0.85 µm) equivalent to the Telescopic Optical Navigation Camera (ONC-T) of Hayabusa2 at 30 degrees of incident angle and obtain images (1.2 µm/pix) at zero emission angle (e.g., normal to surface). We calibrated images a reflectance standard (Spectralon/Labsphere), obtained reflectance maps at all the filter bands, and calculated spectral slope. We measured five large (>1 mm in diameter) particles and four bulk samples containing fine particles (<1 mm in diameter).
In particular, analysis of correlation between the reflectance and spectrum slope (b-x slope) of a bulk sample (sample ID C0205), which contains fine particles (<120 µm in diameter), revealed the trend that brighter particles have smaller spectrum slope (i.e., bluer) which is the same as the result of ONC measurements 2. This result suggests that originally bluer boulders of Ryugu redden and darken owing to such processes as space weathering, solar heating, and dust covering 2,3. These processes may hold a key for understanding the surface evolution of Ryugu. Comparison between the sub-mm scale color and reflectance distribution measured in this study and meter scale distribution observed by Hayabusa2 reveals interesting contrast. The distribution (1σ interval) of the reflectance is 0.53 to 4.0% and the spectrum slope is -1.2 to +2.0 (µm-1). These are far wider than those of the results of remote-sensing (reflectance: 1.7 to 2.1%, spectrum slope: 0.02 to 0.16 (µm-1)) 2. This result suggests that Ryugu is composed of well-mixed materials that have wider reflectance and spectrum slope distribution.
Another interesting property is that the range of distribution in color of samples become as narrow as 2 m/pix data when color is averaged over individual particles (~ mm in diameter). In other words, inter-particle variation in color is as small as those found in the global distribution of Ryugu observed at low resolutions (2 m/pix). The maximum size of samples we measured in this study is a few millimeters, so the scale is three digits apart from remote-sensing observation. If the range of distribution in color is constant between these scales, this would indicate that Ryugu experienced the process that separates materials and forms particles smaller than 1 mm in size. Also, the analysis of these samples will help us understand the materialistic process that may cause the color distribution of Ryugu.
References
1. S. Watanabe et al., 2019. Science. 364, 268-272
2. S. Sugita et al., 2019. Science. 364, eeaw0422.
3. T. Morota et al., 2020. Science. 368, 654-659
4. T. Yada et al., 2021. Nat. Astron. https://doi.org/10.1038/s41550-021-01550-6
5. Y. Cho et al., Submitted to Planet. Sp. Sci