IR spectra of Ryugu's anhydrous ingredients compared with primitive dust from the outer solar system

Abstract

Introduction: Ryugu is a second-generation C-type asteroid formed by the reassembly of fragments of a previous larger body in the main belt. While the majority of Ryugu samples returned by Hayabusa2 are composed of a lithology dominated by aqueously altered minerals, clasts of a more pristine olivine-pyroxene lithology remain in the least-altered samples [1]. These clasts are objects of prime interest for revealing the composition of the original building blocks of Ryugu’s parent asteroid and of the dust from which they formed.We used infrared hyperspectral imaging to analyze four mm-sized sections of Ryugu samples extracted from Chamber A (A0026) and Chamber C (C0002, C0023, C0025). We compare the Ryugu IR spectra to observations of asteroids, comets, meteorites and interplanetary dust particles (IDPs), to study the potential links between the original building blocks of Ryugu’s parent asteroid and objects that retained dust from the outer Solar System.Methods: We used different FTIR microscopes at the SMIS beamline of synchrotron SOLEIL [2]: (1) a synchrotron-radiation-fed microscope equipped with a large mid-IR range MCT/B-detector, (2) a far-IR bolometer-equipped microscope, (3) an imaging microscope equipped with a 128×128 pixels focal plane array detector. For (1) and (2), different apertures were used from 5 to 100 µm. For (3), we used a field of view of ~420 µm and pixel size of 3.3 µm, and several IR tiles were accumulated in mosaic IR hyperspectral images, to analyze mm-sized areas in Ryugu stones. Complementary point-to-point micro-IR measurements were performed at Tohoku University (globar source), with spots of ~100-200 µm.Results and discussion: While a large fraction of the matrix of C0002 shows the 10-µm feature of phyllosilicates similar to aqueously altered chondrites, very different silicate spectral features are observed at some clasts detected by Nakamura et al. [1], because of the presence of anhydrous inclusions. Most of these inclusions have a double-peak structure, due to the intimate mixture of hydrated and anhydrous silicates, in particular olivine with a band at ~880 cm-1 (11.36 µm). Some of them show the signature of pure olivine, while others show a strong spectral contribution of pyroxene at ~1075 cm-1 (9.3 µm). A very few grains have a large spectral profile indicating a significant contribution of amorphous phases. Several olivine grains are also observed in C0023 and C0025, in the less-altered lithologies. These features are absent from A0026.In a principal component analysis, spectra of Ryugu samples, CI chondrites and hydrated IDPs reveal a pattern that correlates with increasing alteration, from the least altered clasts of Ryugu stones, to weakly altered Ryugu clasts, and then major (most altered) lithology of Ryugu, Alais, Tagish-Lake, Orgueil, and the hydrated IDPs.Some grains enriched in amorphous silicates discovered in C0002 have IR spectra similar to D-type asteroid Hektor (a Jupiter Trojan) [3], to comet Hale-Bopp [4], and to anhydrous chondritic porous IDPs of cometary origin [5]. They can be identified with anhydrous grains rich in GEMS (Glass with Embedded Metal and Sulfides) [1], which in turn are similar in texture and composition to the GEMS found in IDPs of probably cometary origin, formed in the protoplanetary disk.Conclusions: The amorphous-rich grains in C0002 are one of the most interesting reservoirs of anhydrous “cometary-like” dust found in Ryugu. Their IR spectra suggest a possible link between at least one of the reservoirs from which Ryugu’s parent asteroid originated and the reservoir that formed comets and D-type asteroids in the outer protoplanetary disk [6].Acknowledgments: This work is part of the multi-analytical sequence of the Hayabusa2 “Stone” MIN-PET group, led by T. Nakamura. It was supported by the Centre National d’Etudes Spatiales (CNES-France, Hayabusa2 mission) and by the ANR project LARCAS (Grant ANR-22-CE49-0009-01) of the French Agence Nationale de la Recherche. The micro-spectroscopy measurements were supported by grants from Region Ile-de-France (DIM-ACAV) and SOLEIL.