Chemical composition of carbonaceous asteroid Ryugu from synchrotron spectroscopy in the mid- to far-infrared of Hayabusa2-returned samples

Abstract

Context. The current period is conducive to exploring our Solar System's origins with recent and future space sample return missions, which provide invaluable information from known Solar System asteroids and comets The Hayabusa2 mission of the Japan Aerospace Exploration Agency (JAXA) recently brought back samples from the surface of the Ryugu carbonaceous asteroid. Aims. We aim to identify the different forms of chemical composition of organic matter and minerals that constitute these Solar System primitive objects, to shed light on the Solar System's origins. Methods. In this work, we recorded infrared (IR) hyper-spectral maps of whole-rock Ryugu asteroid samples at the highest achievable spatial resolution with a synchrotron in the mid-IR (MIR). Additional global far-IR (FIR) spectra of each sample were also acquired. Results. The hyper-spectral maps reveal the variability of the functional groups at small scales and the intimate association of phyl-losilicates with the aliphatic components of the organic matter present in Ryugu. The relative proportion of column densities of the identified IR functional groups (aliphatics, hydroxyl + interlayer and/or physisorbed water, carbonyl, carbonates, and silicates) giving access to the composition of the Ryugu samples is estimated from these IR hyper-spectral maps. Phyllosilicate spectra reveal the presence of mixtures of serpentine and saponite. We do not detect anhydrous silicates in the samples analysed, at the scales probed. The carbonates are dominated by dolomite. Aliphatics organics are distributed over the whole samples at the micron scale probed with the synchrotron, and intimately mixed with the phyllosilicates. The aromatic C=C contribution could not be safely deconvolved from OH in most spectra, due to the ubiquitous presence of hydrated minerals. The peak intensity ratios of the organics methylene to methyl (CH2/CH3) of the Ryugu samples vary between about 1.5 and 2.5, and are compared to the ratios in chondrites from types 1 to 3. Overall, the mineralogical and organic characteristics of the Ryugu samples show similarities with those of CI chondrites, although with a noticeably higher CH2/CH3 in Ryugu than generally measured in C1 chondrites collected on Earth, and possibly a higher carbonate content.

Nanoscale Spectroscopic Identification and Characterization of Minerals and Organic Matter in Ryugu Particles

“…The conventional micro-FTIR spectra of Ryugu particles A0030 and C0034 show IR peaks and features due to OH in phyllosilicates, adsorbed/interlayer water, aliphatic hydrocarbons, aromatics and/or interlayer water, carbonates, and phyllosilicates (Figure 14). These IR absorption features are consistent with Yabuta et al (2023), Dartois et al (2023), and Kebukawa et al (2023). These IR spectra represent average compositions of typically a few tens of microns.…”

“…Alternatively, the lack of the 1,700 cm −1 peak in C0034 might also be due to the heterogeneous composition of the Ryugu samples. Particle‐to‐particle heterogeneity is commonly observed in Ryugu samples in terms of both organics (Dartois et al., 2023; Kebukawa et al., 2023‐a; Yabuta et al., 2023) and minerals (Ito et al., 2022; Nakamura, Matsumoto, et al., 2022; Yokoyama et al., 2022). Therefore, we favor the particle‐to‐particle heterogeneity of the Ryugu samples rather than the impact‐induced C=O loss.…”

“…The organic matter (submicrometer to nanometer in size) in the carbon-rich clasts (exogenous solid objects) have higher N than that of the Ryugu matrix, and these N-rich organic matter potentially formed in the outer solar system and likely in the different parts of the molecular cloud than the bulk Ryugu (Nguyen et al, 2023). Dartois et al (2023) reported conventional hyperspectral IR maps (at a 6 μm × 6 μm sampling window with a step size of ∼3 μm) to show the variations in the chemical functional groups and their spatial associations.…”

“…are even smaller (De Gregorio et al., 2013; Garvie et al., 2008; Nakamura‐Messenger et al., 2006; Yesiltas, Glotch, & Sava, 2021). Spatial resolution of conventional IR methods generally varies from >5 μm (Dartois et al., 2023) to 25 μm (Morlok et al., 2020), and it goes down to ∼1 μm if multi‐beam synchrotron radiation and array detectors are used (Nasse et al., 2011; Yesiltas et al., 2014; Yesiltas & Kebukawa, 2016). Thus, conventional IR methods cannot be used to detect and identify spectral signatures of nanoscale organic matter in situ in extraterrestrial samples because of their insufficient spatial resolution and various other technical limitations (such as optical diffraction limit, wavelength dependence).…”

Linking Cause and Effect: Nanoscale Vibrational Spectroscopy of Space Weathering from Asteroid Ryugu

“…Establishing a connection between remote sensing data (obtained at the millimeter to kilometer scale) and these nanometer-scale textures requires characterization techniques capable of measuring the spectral signature of Ryuguʼs space-weathered regions at submicrometer spatial resolution. Recently, new infrared techniques have been developed to overcome the diffraction limit, such as Nano-FTIR (Herman 2013;Kebukawa et al 2019;Yesiltas et al 2021) or photothermal atomic force microscopy-based infrared spectroscopy (AFM-IR; Dazzi & Prater 2017;Kebukawa et al 2019;Mathurin et al 2019Mathurin et al , 2022Phan et al 2022;Dartois et al 2023;Phan et al 2023). Both techniques can reach 50 nm spatial resolution with ∼8 cm −1 spectral resolution typically between 700 and 2000 cm −1 (Phan et al 2023).…”