AFM-IR measurements on Ryugu grains and other natural and synthetic samples

Since the Hayabusa2 mission was sent, the understanding of Ryugu nature, i.e., size, shape, mass (Watanabe et al. 2019), grain morphology (Nakato et al. 2023), composition (Noguchi et al. 2023; Potiszil et al. 2023), parent-body origin (de Len et al. 2018; Tatsumi et al. 2021) and evolution (Morota et al. 2020; Nakamura et al. 2023), have been enlarged thanks to onboard measurements, as well as laboratory measurements on returned samples. Herein, we are interested in the mineral and the organic composition of Ryugu. For this interest, laboratory spectral measurements are of great use.In this study, we are focused on which functional groups are present in Ryugu returned samples, therefore in the infrared wavelength range. For this, transmission IR measurements can be performed with conventional Fourier transform infrared spectrometers, FTIR, which can be coupled with IR microscopes, micro-FTIR. Both IR techniques have been largely used for compositional studies on extraterrestrial matter, as chondrites and IOM of chondrites (e.g., Beck et al., 2010, 2014; Kebukawa et al., 2011; Orthous-Daunay et al., 2013; Quirico et al., 2014, 2018). However, these techniques are constrained by their diffraction limit affecting the spatial resolution and leading to a diffraction-limited spatial resolution of 0.61l/NA, being l the wavelength and NA the numerical aperture (Centrone 2015). In the case of a source of wavelength between 2.5 to 16.7 mm, this resolution is in the first tens of micrometers.In the search for overcoming the diffraction limit, other techniques have been developed as is the case of the infrared atomic force microscope AFM-IR (A. Dazzi et al. 2005). The AFM-IR instead of measuring the absorbed light with optical detectors, it measures the mechanical expansion of the sample caused by an IR source that is directly proportional to the absorbance. The expansion is then detected by the AFM cantilever tip and the mechanical motion (deflection) of the whole cantilever (which carries the sample photothermal expansion information) is detected. Then the deflection amplitude is transformed (by Fourier transform) at each wavenumber giving the infrared spectrum (Mathurin et al. 2022). Since the AFM tip is the direct detector of the sample expansion, the resolution is limited by the tip radius of curvature (smaller or equal to 20 nm) as well as the sample thermo-mechanical properties and thickness, rather than the wavelength (A. Dazzi et al. 2010). This indicates that AFM-IR has a lateral resolution at the nanometer scale, being able to analyze from micro- to nanoscale heterogeneities on a sample (A. Dazzi et al. 2017).AFM-IR measurements have been performed on extraterrestrial matter as is the case for chondrites, for example on Orgueil and EET 92042 (Phan et al. 2022), on ultra-carbonaceous Antarctic micrometeorites (Mathurin et al. 2019), on Murchison and Bell (Kebukawa et al. 2019); and on one asteroid, the one of our interest, Ryugu (Dartois et al. 2023; Kebukawa et al. 2023; Phan et al. 2024). Based on the different IR spectral measurements that have been done on Ryugu returned samples (Dartois et al. 2023; Kebukawa et al. 2023; Phan et al. 2024; Quirico et al. 2023; Yabuta et al. 2023), three main functional groups on organic matter have been identified (carbonyl, aromatic and aliphatic groups) and three others on the mineral composition (carbonate, silicate and sulfate groups, the last one being possibly due to terrestrial oxidation).Herein, we present an AFM-IR and micro-FTIR studies on one Ryugu returned sample prepared as FIB sections and a grain of the same sample prepared in two different ways; first, one portion of the grain is pressed between two diamond windows, and second, another part is used for HF/HCL extractions to analyze its insoluble organic matter (IOM). Another part of our work consists in investigating AFM-IR measurements issues. 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