Characterization of hydrated phosphorous-rich material within Bennu returned samples using near-infrared to mid-infrared spectra

Introduction: Samples returned from the asteroid Bennu by NASA’s OSIRIS-REx mission [1], and previously from the asteroid Ryugu by JAXA’s Hayabusa2 mission [2], offer unprecedented access to pristine carbonaceous asteroid material, preserved from terrestrial contamination. Ongoing laboratory investigations have shown that phosphates in Bennu occur in various forms, including isolated grains, surface coatings, and vein fillings, present primarily as Mg,Na-phosphates and Ca-phosphates [1]. These materials are often associated with high-reflectance phases in mottled particles and may be linked to bright veins seen in Bennu’s boulders. At the JAXA Extraterrestrial Sample Curation Center (ESCuC) in Sagamihara, Japan, 0.6 g of these samples are stored and analyzed in a controlled, pure N₂-purged environment, enabling high-resolution spectral analysis under conditions that prevent terrestrial contamination [3-6].

Characterizing hydrated phosphorus-bearing phases, particularly phosphates, is critical for understanding the role of fluid-mediated alteration processes in carbonaceous asteroids and the potential availability of bioessential elements like phosphorus in early Solar System materials.

Our study focuses on the diversity of hydrated phosphorus-bearing grains in Bennu, with particular interest in grains resembling Hydrated Ammonium-Magnesium-Phosphorus-rich (HAMP) phases previously detected in Ryugu material [3].

Methods: In September 2024, five bulk Bennu samples (~0.6 g total) were analyzed at ESCuC using the MicrOmega near-infrared hyperspectral microscope and µFTIR point spectroscopy while samples were held in an N₂-purged environment to prevent terrestrial contamination [4-5]. The same instrumentation and protocols used for MicrOmega observation of Ryugu [6] enabled comparison across the two bodies.

MicrOmega collects spectra spanning 0.99-3.6 μm with spatial resolution of 22.5 μm per pixel [7]. Spectral image cubes are collected of each bulk at multiple viewing geometries. At 4 azimuths (0, 90, 180, 270^o), ~15 MicrOmega images were collected over the bulk sample holder to span the entire bulk sample with overlap so no grains were visible only at image boundaries; this yields ~70 images per bulk sample which were analyzed for spectral variation.  The incident angle of 35^o allows for variation of illumination conditions as the viewing azimuths are changed. A LEICA visible microscope is coupled to MicrOmega at ESCuC which allows for high resolution visible imagery of the grains within the bulk samples. A secondary bulk campaign was carried out in April-May 2025 to reanalyze bulk samples after removal of the largest individual grains for grain analyses, yielding newly visible smaller grains.

µFTIR point spectra were collected in a N₂-purged chamber connected to the MicrOmega purged chamber so that samples could be transferred without terrestrial exposure. The µFTIR point spectrometer collects spectra spanning 2-12 μm over points of interest previously identified with MicrOmega, with spot size 70-100 μm. This spectral range allows for comparison of the 3 μm absorption visible in MicrOmega while also capturing the range of the P-O vibrations at longer wavelengths associated with phosphates [3,8]. 

Spectra were analyzed for H₂O features including a ~3 µm broad absorption in both MicrOmega and µFTIR data. µFTIR spectra were analyzed for the same HAMP-like absorptions as MicrOmega spectra, as well as spectral features visible at longer wavelengths including at 6.9 μm (N-H) and ~9.5 μm (P-O) [3]. The visible features of grains were analyzed using the LEICA visible microscope imagery.

Results and Discussion: We document the breadth of hydrated phosphorus-rich grains found in the 5 Bennu bulk samples returned to Earth by the OSIRIS-REx mission. We identified hydrated (strong absorptions at 3 µm) phosphorus rich (~9.5 µm feature) material, and investigated their visible features and spectral characteristics at multiple viewing geometries. The hydrated phosphorous-rich grains detected also form one population of Bennu’s bright grains.

 We compare these spectra of interest with spectra of possible phosphate analogs as well as Ryugu HAMPs. Phosphates are observed as bright grains or inclusions embedded within the low-albedo matrix. Many phosphate spectra are blue-sloped from 1.0-2.7 μm. A subset exhibits the absorptions spectrally consistent with HAMP grains from Ryugu [3], as seen in both MicrOmega and μFTIR spectra. Ongoing analysis will inform on further compositional diversity within Bennu samples.

Future work will expand this dataset with analyses of individually extracted grains. All results will contribute to the JAXA Bennu sample catalog, to be made accessible via AOs. Phosphorous is typically a minor component in carbonaceous material including Bennu [1], so while phosphates were not previously detected via remote sensing of the asteroid’s surface, their observation in returned samples can point to localized, chemically complex alteration histories within Bennu’s regolith.

References: [1] Lauretta D. S. et al., Met. and Planet. Sci. 59, 9, p. 2453-2486 (2024), [2] Yada T. et al., Nat. Astron. 6, p. 214-220 (2022), [3] Pilorget C. et al., Nat. Astron. 6, p. 221-225 (2024), [4] Fukai et al., Lunar and Planetary Science Conference, #1311, (2025), [5] Fukai et al. this conference, [6] Pilorget C., Okada, T. Hamm, V. et al. 2022, Nature Astronomy, 6, 221, [7] Bibring J.-P., Hamm V., Pilorget C., Vago J. L., & the MicrOmega Team. 2017, Astrobiology, 17, 621. [8] Jastrzebski W., Sitarz M., Rokita M., Bulat K. Spectrochimica Acta Part A, 79, 722-727.