Spectral variations of phyllosilicate features in Bennu grains: evidence from NIR and MIR measurements of pristine returned samples at ISAS Curation Facility.

Introduction: Carbonaceous asteroid samples offer a unique opportunity to access primitive materials from the early solar system. Alteration processes that occurred during the evolution of asteroids and their parent bodies have affected the structural and spectral characteristics of their surface materials. The JAXA Hayabusa2 and NASA OSIRIS-REx missions successfully returned samples from the asteroids Ryugu and Bennu [1,2]. Using returned samples—preserved from terrestrial contamination— can significantly improve our understanding of alteration mechanisms that occur during the early evolution of the solar system.

The OSIRIS-REx mission returned approximately 120 g of surface material from Bennu in September 2023 [2]. Preliminary analyses of NASA's Bennu samples have revealed similarities with Ryugu samples and CI chondrites, notably a composition rich in phyllosilicates [2]. Here, we present a first characterization of these phyllosilicates using NIR and MIR measurements on Bennu pristine samples, without exposure to the terrestrial environment that could affect their properties.

Samples and Methods: Through a sample exchange agreement between NASA and JAXA, a 0.6 g portion of the Bennu sample, divided into five aggregate samples, was delivered to JAXA’s Extraterrestrial Sample Curation Center (ESCuC) in Sagamihara in August 2024. The Bennu – and Ryugu – samples are now stored there in dedicated ultraclean and N₂-purged facilities to prevent from any terrestrial contamination. For this specific study, initial characterization was performed within their preservation chamber first using MicrOmega, a near-infrared hyperspectral imaging microscope developed by the Institut d’Astrophysique Spatiale (Université Paris-Saclay/CNRS) [3] coupled with a LEICA visible microscope. This instrument operates in the 0.99-3.65 µm spectral range with a spatial resolution of 22.5 x 22.5 µm² per pixel. Additional measurements were then conducted with a µ-FTIR point by point spectrometer with a spectral range between 2 and 12 µm on chosen 70-100 µm spots. Here, we focus on individual mm-sized grains of Bennu samples that were extracted from initial aggregate samples ORX-19000 and ORX-29000 [5]. We investigated the OH vibration band at ~2.7 µm with MicrOmega on ~100 individual grains and the Si-O band at ~10 µm with the µ-FTIR on about half of these grains. For each grain, several spectral parameters were extracted. For the OH band, in the NIR, these include peak position, band depth, reflectance factor at 2.5 µm, and NIR spectral slope. For the Si-O band, in the MIR, key parameters include the positions of the main Reststrahlen band, the Christiansen feature and the first derivative peak, along with the relative distances between them. The results were compared with MicrOmega measurements of Ryugu samples performed within the Curation Facility (e.g., [5]), as well as with external laboratory measurements conducted on Ryugu samples [6,7] and on meteorites [8].

Results and discussion: The OH band properties derived from the average spectra of individual Bennu grains measured with MicrOmega reveal some similarities with those of the Ryugu grains initially analyzed by [5]. Approximately 10% of the Bennu grains exhibit a more redshifted OH band peak position than the rest of the population. In particular, this subset includes grain ORX-10001 [9], measured on both sides with the instruments, which shows a clear spectral dichotomy between its two faces. For these grains, the Si–O stretching band appears broader and presents a redshifted Reststrahlen peak, while the Christiansen feature remains unchanged, compared to the rest of the Bennu collection and to laboratory measurements of Ryugu samples [6,7].

It has been previously suggested that the redshifted Ryugu grains from Chamber A (from the first sampling site) experienced more space weathering [5]. This may indicate varying degrees of surface alteration among the Bennu samples, efficiently probed by the variable position of the OH band. The shift observed in the Si-O band follows the same trend as that seen in ion irradiation in laboratory on carbonaceous chondrites, such as Alais and Tagish Lake [8]. This spectral behavior, however, not identified in Ryugu samples, may indicate a distinct alteration process specific to a fraction of the Bennu material.

These findings suggest peculiar alteration histories of the two bodies, offering new insights into the formation and evolution of planetesimals. Ongoing investigations aim to further understand the alteration processes responsible for this heterogeneity in Bennu, which, at this stage, appears to align with irradiation-induced changes observed in the laboratory on CI meteorites.

Acknowledgments: We thank the Curation Facility team of JAXA for their work on the samples. We also thank the Graduate school of physics of Paris-Saclay University and the CNES for their financial support.

References: [1] Yada, T. et al. (2022) Nat. Astron. 6, p. 214-220. [2] Lauretta, D.S. et al. (2024) Meteorit Planet Sci, 59: 2453-2486. [3] Bibring et al. (2017) Astrobiology 17, 621-626. [4] Fukai, R. et al. (2025) LPSC #1311. [5] Le Pivert-Jolivet, T. et al. (2023) Nature Astronomy 7, 1445–1453. [6] Dionnet, Z. et al. (2023) Meteorit Planet Sci, 59,10.1111/maps. 14068. [7] Amano, K. et al. (2023), Sci Adv. 9, eadi3789. [8] Lantz, C. et al. (2024) Planet. Sci. J. 5 201. [9] Nardelli, L. et al. (2025) LPSC #1928.