Spectral variations of the 2.7 µm feature at sub-mm scales in Ryugu samples, within the ISAS Curation Facility

In December 2020, the Hayabusa2/JAXA mission returned to Earth ~5.4g of primitive material from the carbonaceous asteroid (162173) Ryugu. Since their return, the samples are stored and analysed in the ISAS (Institute of Space and Astronautical Science) Curation Facility. All the analyses in the Curation Facility are conducted under clean and controlled environment [Yada et al., 2023]. The measurements in the Curation Facility offer the opportunity to characterise the physical and chemical properties of Ryugu samples without any bias from the terrestrial atmosphere.

A systematic non-destructive characterisation of bulk samples and individual grains (a few mm in size) have been performed by the near-infrared (0.99 – 3.65 µm) hyperspectral microscope MicrOmega in the Curation Facility [Bibring et al., 2017, Riu et al., 2022]. To be specific, MicrOmega is able to characterise the feature around 2.7 µm, associated with the O-H stretching vibration in phyllosilicates. This absorption band has been detected at the surface of Ryugu by the NIRS3 spectrometer [Kitazato et al., 2019], and was also observed by MicrOmega at cm-scale, in the bulk samples [Pilorget et al., 2022]. With a spatial resolution of 22.5 x 22.5 µm² and a total field of view of ~ 5.7 x 5.7 mm², MicrOmega enables a study of the variations of spectral features at small scales. For example, variations in the depth and the position of the ~2.7 µm feature have been detected between the average spectra of ~200 mm-sized grains [Le Pivert-Jolivet et al., 2023]. In this study, we aim at characterising the spatial variations of the O-H feature at the surface of the grains. To study the feature at sub-mm scales, we averaged groups of 8x8 pixels at the surface of 233 individual mm-sized grains. We used different spectral parameters to estimate the position and the depth of the ~2.7 µm feature.

At sub-mm scales, we detect heterogeneities in the peak position and depth of the ~2.7 µm feature. In most cases, the spectral heterogeneities observed are at small scales (~200 µm). However, out of the 233 mm-sized grains characterised in this study, we observed large-scale heterogeneities on ~30 of them (some detections are to be confirmed). On these grains, the variations of the spectral parameters are consistent on several contiguous pixels (i.e. areas sizes ranging from several hundreds of microns to ~1.5 mm) and are observed at different illumination angles of the grains. The large-scale spectral heterogeneities exhibit variations in peak position, in band depth, or in both parameters. In some cases, the large-scale spectral heterogeneities are spatially correlated with a variation in the surface aspect of the grain or the reflectance level. For all grains, we observe variations of the degree of heterogeneity of the peak position from one grain to another. Grains with average peak position at lower wavelengths tend to have more homogeneous peak positions (i.e., the peak position varies little at the surface) and have stronger values of band depth than the grains with average peak position at higher wavelengths.

 

Previous work has shown that some Ryugu grains exhibit various sub-mm (<500 µm) lithologies with different degrees of aqueous alteration [Nakamura et al., 2022]. The next step of this work is to investigate whether our results could be partly related to these lithologies, or if the variation in heterogeneity degree and large-scale dichotomies reflects other physical or chemical processes. For example, a variation in the abundance of opaque phases could also affect the spectral parameters.

 

References:

Yada, T. et al., 2023, Earth, Planets and Space. 75, 170; Bibring, J.-P., et al., 2017, Space Sci. Rev. 208, 401-412; Riu, L. et al., 2022, Rev. Sci. Instrum. 93, 054503 ;Kitazato, K. et al., 2019, Science. 364, 272-275; Pilorget, C. et al., 2022, Nature Astronomy, 6, 221-225; Le Pivert-Jolivet, T. et al., 2023, Nature Astronomy, 7, 1445-1453; Nakamura, T. et al., 2022, Science, 379, 6634