Introduction

Carbonaceous asteroids record processes that occurred during the early evolution of the Solar System. They may have also critically contributed to the delivery of chemical compounds to the terrestrial planets (e.g.1,). Samples from these objects have been collected and successfully returned to Earth, first by the JAXA Hayabusa2 mission from the C-type asteroid Ryugu in December 20202, and more recently in September 2023 by the NASA OSIRIS-REx mission from the B-type asteroid Bennu3. For the first time, these collections offer a unique access to material directly collected on carbonaceous asteroids, presumed free from terrestrial contamination and alteration, contrary to meteorites.

Analyses of Ryugu samples have shown that their elemental, isotopic and mineralogical composition are most similar to CI-type chondrites (e.g.,2,4,5,6,7,8,9,10,11,12). As such, Ryugu mineralogical composition consists principally of phyllosilicates (serpentine- and saponite-type), carbonates, sulfides, magnetite and phosphates (e.g.,5,7,8). Various CHNOS-based organic materials and rare anhydrous silicates (olivine and pyroxene) have also been reported (e.g.,8,9,13,14). However, major differences could be observed between Ryugu samples and CI chondrites. In particular Ryugu samples are much darker than CI chondrites with reflectance as low as 2-3 % (e.g., 2,4,8) (vs 4-5% for CI chondrites). Analyses of the Ryugu samples have also highlighted the presence of Hydrated Ammonium-Magnesium-Phosphorus (HAMP)-rich compounds15, that were as of yet not observed in meteorites. Some others compounds such as sulfates and ferrihydrite were, however, absent (e.g.,5), as well as most of the interlayer water in the phyllosilicates7,12.

While some of the differences may reflect variations in the initial conditions (e.g., composition, formation environment) or the subsequent evolution of the parent bodies, exposure of asteroid material to the terrestrial atmosphere has been shown to significantly alter both its mineralogical properties16,17 and its water budget7,12,18. These results strongly support maintaining returned carbonaceous asteroids samples in pristine conditions, meaning free from terrestrial alteration and thus preserving their properties at collection, for at least initial characterization.

First results obtained from the Bennu samples have also shown some similarities with CI chondrites regarding elemental and mineralogical composition3. In particular, Mg-rich phyllosilicates (primarily serpentine and smectite), magnetite, sulfides, carbonates and phosphates have been identified in the bulk samples3,19. However, some differences were also observed from the Ryugu samples. Carbon and nitrogen abundances were found to be significantly higher in Bennu samples3,14. H and N isotopic analyses on Bennu samples have also shown some small but significant differences from CI chondrites and Ryugu material. While Ryugu’s δ15N and δD compositions (δ15N = 43.0 ± 9.0‰, δD = 252 ± 13‰14,) are consistent with those of CI chondrites, Bennu’s δ15N and δD compositions (δ15N = 82 ± 15‰, δD = 344 ± 13‰,3) are closer to those of the Tarda and Tagish Lake meteorites (C2-ungrouped), raising the question of the potential connection that exists between their parent bodies and the processes that drove their evolution.

Through an agreement between NASA and JAXA, samples from Ryugu and Bennu were exchanged between the two agencies. As a result, ~0.6 g of Bennu samples were extracted from the collection as five different bulk samples and delivered in August 2024 to the Extraterrestrial Sample Curation Center of JAXA (ESCuC, Sagamihara, Japan)[20](https://www.nature.com/articles/s41467-025-65438-z#ref-CR20 “Astromaterials Science Research Group, Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNES, S. Nakahara, et al. (2025) OSIRIS-REx Bennu Sample Curatorial Dataset. https://doi.org/10.17597/ISAS.DARTS/CUR-Bennu-description

.“). Similarly to Ryugu samples, pristine Bennu samples (never exposed to the terrestrial environment) are stored in a pure N2-purged controlled chamber. There they are submitted to a first round of preliminary examination with a suite of instruments including the near-IR hyperspectral microscope MicrOmega21. This instrument was already used for the characterization of the Ryugu sample collection4,11,12,15,18,22, enabling a direct comparison and unique cross-analysis between both collections of samples.

We present here an assessment of the IR spectral diversity observed in Bennu samples, while in their pristine state, and a first comparison with Ryugu samples.

Results

Centimetre to millimetre spatial scale

At the scale of a few mm, MicrOmega analyses of the five Bennu bulk samples delivered to JAXA reveal that they are dominated by a dark matrix (~2–3% reflectance at 2.5 µm) with a red slope at shorter wavelengths, a ~ 2.72 µm absorption consistent with OH moieties (O–H stretching) strongly bonded to Mg (most likely in Mg-rich phyllosilicates) and a weaker ~3.4 µm feature attributable to the presence of organic compounds and/or carbonates (Fig. 1 a, Supplementary Fig. 1), consistent with[20](https://www.nature.com/articles/s41467-025-65438-z#ref-CR20 “Astromaterials Science Research Group, Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNES, S. Nakahara, et al. (2025) OSIRIS-REx Bennu Sample Curatorial Dataset. https://doi.org/10.17597/ISAS.DARTS/CUR-Bennu-description

.“).

Fig. 1: IR spectral characterization of bulk samples.

a Typical MicrOmega spectra at the scale of a few mm within Bennu bulks ORX-19000 and ORX-29000, compared to Ryugu bulk spectrum from chamber C4. b Normalized 2.7 µm absorption band. The average spectrum of mm-size grain ORX-10007 (extracted from bulk ORX-19000, Bennu) has been added for comparison. Both the band depth and the continuum were normalized to that of ORX-29000 spectrum to enable a comparison of the band shapes and positions. Vertical dashed lines are set at 2.71, 2.72 and 2.73 µm to highlight differences in peak positions. c Same as (b) with offset for clarity.

The ~2.7 µm absorption band exhibits an asymmetric shape with a more pronounced right wing. Importantly, there is no significant contribution of the O-H stretching mode of H2O molecules around 3 µm. All Bennu bulks show a similar 2.7 µm band profile (Fig. 1b, Supplementary Fig. 2). The peak position is estimated at 2.717 ± 0.005 µm and the relative band depth varies in the 10-12% range (Supplementary Fig. 2). Some slight variability is, however, observed in the band position (with typical shifts <10 nm to longer wavelengths), depth or shape (with slightly more pronounced right wings) in some mm-sized grains (Fig. 1b, c). The relative band depth of the ~3.4 µm feature typically varies in the 3-4% range (Fig. 1a, Supplementary Fig. 1). Its shape, more pronounced at ~3.42 µm, indicates that the signature of organics (aliphatic CH, e.g.9,) dominates over that of carbonates (which would show a doublet at ~3.3–3.5 µm) when integrating over areas a few mm wide. These organic compounds are presumably mixed with other matrix constituents at sub-micron scale, but local enrichments at larger scale could also be present, as within the Ryugu samples4.

Sub-millimetre spatial scale

At sub-mm scale, we identified within the dark matrix a great variety of grains and inclusions larger than a few tens of micrometres (MicrOmega spatial sampling is 22.5 × 22.5 µm2) with different spectral signatures. Most of them have a much higher reflectance factor (~5–20%) than the matrix. We can typically observe a few tens of them at the surface of each bulk sample (Fig. 2).

Fig. 2: Visible and IR images of bulk samples.

Bulk ORX-29000 observed by the Leica visible microscope and by the MicrOmega IR hyperspectral microscope (Mosaic of RGB composite images). The following wavelengths are used for the MicrOmega RGB image: R: 3.34 µm, G: 2.50 µm, B: 1.20 µm.

The first family of spectra that are commonly found (Type 1) exhibits a ~ 3.3-3.5 µm absorption feature diagnostic of carbonates (combination modes and overtones of the fundamental stretching and bending modes in CO3) (Fig. 3). These spectra are associated with bright grains and inclusions (reflectance factor of ~5–15%) ranging in size from a few tens to a few hundred of micrometres. They appear variously as clusters of small inclusions (tens of micrometres in size) on grains dominated by the phyllosilicate-rich matrix, as long vein-like features, or as more diffuse distributions throughout the grain. Additional ~2.3 µm, ~2.5 µm and ~2.8 µm features are also sometimes observed in the largest detections. These weaker features are associated with combination modes of carbonates (3ν3, ν1 + 2ν3 and 2ν3 + ν4, respectively, with ν1 the symmetric CO3 stretching mode, ν3 the asymmetric stretching and ν4 the in plane-bending). A strong red slope below 1.5 µm can also be observed for a fraction of the carbonates, attesting to the presence of Fe2+ among the cations. Notably, a ~ 2.7 µm feature is also present. The position and shape of the ~3.3-3.5 µm feature are indicative of the main cations coupled to CO32- (e.g.,23,24). Following the methods developed by refs. 11 and [25](https://www.nature.com/articles/s41467-025-65438-z#ref-CR25 “Mahlke, M. et al. Aqueous alteration of Ryugu’s parent body: Insights from carbonates seen by MicrOmega. Europlanet Science Congress 2024, held 8-13 September, 2024 at Freie Universität, Berlin, Germany. id. EPSC2024-174. https://doi.org/10.5194/epsc2024-174

(2024).“), we were able to identify at least two families of significant importance: dolomite (CaMg(CO3)2) and breunnerite ((Mg,Fe)CO3) (Fig. 3). Such carbonates have also been identified by complementary methods by refs. 3,17,19. Both types of carbonates exhibit spectra very similar to those of the Ryugu samples, in particular the presence of a 2.7 µm feature, which may reflect small-scale mixing of matrix material with the carbonates11.

Fig. 3: Examples of Type 1 spectra (carbonates) observed by MicrOmega within Bennu bulk samples.

a–c Visible images show the context where these carbonate inclusions are detected. In MicrOmega RGB composite images of the same areas (d–f), carbonates appear in shades of green and blue (R: 3.34 µm, G: 2.50 µm, B: 1.20 µm). g Average MicrOmega spectra of the carbonate-rich regions highlighted by the dashed circles on the MicrOmega RGB images (A1 averages 19 pixels, A2: 10 pixels, B: 50 pixels, C: 89 pixels). Examples of carbonates identified in Ryugu pristine samples by MicrOmega under similar conditions11 are shown for comparison. Reference spectra of terrestrial carbonates are also displayed (dolomite BKR1CB017a spectrum from RELAB and a terrestrial breunnerite spectrum acquired at IAS, Orsay11).

A second common family of spectra (Type 2) exhibits a deep and broad ~3 µm absorption feature that we attribute to the O-H stretching mode of H2O molecules (e.g.,26). In most cases, either a sharp peak or a shoulder at ~2.7 µm can be observed on the left edge of the band (Fig. 4i - spectra A,B,C,D). Weak absorptions between ~3.0 and 3.3 µm are also sometimes present. Such spectral features are generally associated with bright, detached grains (reflectance factor of ~10–20%), but inclusions in millimetre-sized matrix-dominated grains have also been observed. Grain sizes range from a few tens to a few hundreds of micrometres. Many detached grains exhibit a whitish, uniform appearance in optical images, punctuated by smaller, dark micrometre-sized grains that seem either embedded within the whitish material or stuck to the surface (Fig. 4a). These smaller grains may be leftovers from the surrounding matrix when the bright grain was detached, but we cannot rule out the presence of deeper inclusions. Other detached grains might appear as clusters of smaller bright units and dark material. The bright inclusions found in millimetre-sized grains and that exhibit a deep and broad ~3 µm absorption have various complex morphologies, sometimes as veins running through the grain (Fig. 4d), and sometimes as aggregates of bright inclusions a few tens to a few hundreds of micrometres in size (Fig. 4b). Notably, they tend to have a whitish to whitish-bluish appearance in the optical images, suggesting some variability in their composition. Complementary mid-IR spectra obtained with a µ-FT-IR spectrometer (see Methods) on inclusions/grains displaying such a signature exhibit a ~ 6.1 µm as well as a ~ 6.9 µm absorption band that we attribute respectively to the presence of H2O (bending mode ν2, e.g.,27,28), and NH4+ (out of plane bending mode ν4, e.g.,29,30). In particular, the presence of NH4+ could explain the spectral bands observed between 3.0 and 3.3 µm in MicrOmega data (e.g.,30). Additionally, a broad feature between ~9 µm and 10 µm attributed to P-O stretching vibrations (e.g.,31,32) can also be observed. Similar spectral signatures have also been observed in Ryugu samples15, indicating the presence of Hydrated Ammonium-Magnesium-Phosphorus (HAMP)-rich compounds. The specific and diagnostic ~3 µm (on pristine samples), ~6.9 and ~9.5 µm band shape, position, and depth are similar between Ryugu and Bennu HAMP-rich compounds.

Fig. 4: Examples of Type 2 spectra (hydration feature at 3 µm) observed by MicrOmega within Bennu bulk samples.

a–d Visible images show the context where these inclusions/grains are detected. In MicrOmega RGB composite images of the same areas (e–h), they appear in shades of green and blue (R: 3.34 µm, G: 2.50 µm, B: 1.20 µm). i Average MicrOmega spectra of the regions highlighted by the dashed circles on the MicrOmega RGB images (region A: 31 pixels, region B: 28 pixels, region C: 60 pixels, region D: 63 pixels). HAMP spectra, measured in Ryugu samples by the same MicrOmega instrument while the samples were kept pristine in their preservation chamber are shown for comparison15. j Complementary mid-IR spectral data obtained on Areas A and C with a µ-FT-IR spectrometer. These combined near-IR-mid-IR analyses did not require any extraction from the facility, preventing any exposure of the sample to the terrestrial environment. A HAMP spectrum, measured ex-situ in Ryugu samples by a µ-FT-IR instrument, is shown for comparison15. The latter displays a slightly broader 3 µm band due to absorption/adsorption of atmospheric water. An insert with rescaled reflectance factor provides a better view at the ~9.5 µm feature attributed to the P-O stretching vibration. Indeed, the intensity of this feature is quite sensitive to local geometric conditions and might significantly vary between the samples. The weak features between 3.3 and 3.6 µm in spectrum B are artifacts.

A third family of spectra (Type 3) – quite rare (about a dozen of occurrences in the bulk samples) - exhibits both ~2.7 µm and ~3.05 µm features (Fig. 5). The ~3.05 µm band is attributed to the N-H stretching vibration (e.g.,30), in NH4+ or in NH-rich organics for instance. The 2.72 µm feature is attributed to the stretching vibration of O-H strongly bonded to Mg atoms that we typically find in Mg-rich phyllosilicates (e.g.,33). Together, these indicate an NH-bearing species associated with a MgOH-rich material, plausibly a phyllosilicate similar to that of the matrix, as shown by the shape and position of the band. A weak ~2.32 µm feature can also sometimes be observed which we attribute to combination vibrations involving the O-H stretch and the bend of O-H strongly bonded to Mg atoms (e.g.,34). Its presence is fully consistent with that of the 2.72 µm feature, in particular in the context of high albedo which increases the visibility of spectral features compared to dark material. This spectral family is generally associated with bright whitish to yellowish inclusions (reflectance factor of ~5–20% typically), with sizes from a few tens to a few hundreds of micrometres (Fig. 5a). The spectral properties of these inclusions closely resemble those observed in Ryugu samples.

Fig. 5: Example of a Type 3 spectrum (NH-/MgOH-rich inclusion) observed by MicrOmega within Bennu samples.

a Visible image shows the context where the inclusion is detected. In MicrOmega RGB composite image of the same area (b), it appears in shades of blue (R: 3.34 µm, G: 2.50 µm, B: 1.20 µm). c MicrOmega average spectrum indicated by the dashed circle (120 pixels). A ‘Type 3’ spectrum measured in Ryugu samples by the same MicrOmega instrument in similar conditions is shown for comparison (from Ryugu Bulk C4,[44](https://www.nature.com/articles/s41467-025-65438-z#ref-CR44 “Jiang, T. et al. Detection of NH-rich compounds in Bennu pristine samples via IR characterization at JAXA Curation Center and comparison with Ryugu, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16881. https://doi.org/10.5194/egusphere-egu25-16881

2025.“),).

A fourth kind of spectral family (Type 4a) – very rare (only a few occurrences) - exhibits a strong <1.6 µm slope and a flat continuum at longer wavelengths which is consistent with olivine (Fig. 6e - spectrum A). This signature was observed for several ~50-100 µm large, detached bright grains (reflectance factor of ~5–10 %). Complementary mid-IR spectral data were also obtained for one such grain. It shows a broad feature at ~10 µm with 3 distinct sub-features at ~9.8, ~10.3 and ~10.8 µm, confirming the olivine attribution (Fig. 6e – spectrum A). Additional features between ~4.8 and 6 µm are also interpreted as Si-O combination and overtone modes of crystalline olivine (e.g.,35,36). The position of the 3 sub-features around 10 µm in this grain is indicative of Mg/(Fe+Mg)#~60%37,[38](#ref-CR38 “Clark, R. N. et al. USGS digital spectral library splib06a. US Geological Survey, Digital Data Series 231, http://speclab.cr.usgs.gov/spectral.lib06

(2007).“),39. Such specific composition might, however, not be representative of all the olivine detections. In addition, a small feature at ~2.7 µm due to OH moieties strongly bonded to Mg atoms can be noticed in the mid-IR spectrum. As the grain size is close to the observation spot size, such a signature most likely originates from the neighboring grains.

Fig. 6: Examples of type 4 spectra observed by MicrOmega within Bennu samples.

a, b Visible images show the context where the grains/inclusions are detected. In MicrOmega RGB composite image of the same areas (c, d), they are highlighted by a dashed circle (R: 3.34 µm, G: 2.50 µm, B: 1.20 µm). e Corresponding MicrOmega average spectra (15 pixels for spectrum A in blue and 135 pixels for spectrum B in red) and complementary mid-IR spectra obtained with a µ-FT-IR spectrometer (spot size of 50 µm spot for spectrum A in cyan and 100 µm for spectrum B in purple). The footprint of the mid-IR observation for Region B is indicated by a black square in the visible image. These combined near-IR-mid-IR analyses did not require any extraction from the facility, preventing any exposure of the sample to the terrestrial environment.

In one particular case (Type 4b), we observe both a strong <1.6 µm slope and a broad asymmetrical 3 µm feature with an additional and prominent component at ~2.8 µm compared to Type 2 spectra (Fig. 6e – spectrum B). A small feature at ~3.5 µm can also be observed. This signature has been detected in a whitish-yellowish inclusion about 300 µm in size with a complex morphology (Fig. 6b). A complementary mid-IR spectrum of the inclusion exhibits two sharp peaks at ~9.5 and ~10.6 µm, as well as a smaller feature at ~10.3 µm. Possible small absorption bands between ~4.8 and ~6 µm are also observed. The combination of the <1.6 µm slope, the features between ~9.5 and ~10.6 µm and that between ~4.8 and ~6 µm are consistent with olivine (though with a smaller ~10.3 µm feature compared to the two main peaks). The additional presence of a ~ 3 µm absorption band, due to the O-H stretching mode of H2O, is quite surprising. The explanation can come from the ~2.8 µm absorption feature which can be due to the SiO-H vibration (e.g.40,41,). This would then indicate that this olivine, in which H2O is present, probably as ad/absorbed molecules, is hydroxylated, i.e. slightly altered compared to that shown in spectrum A of Fig. 6e. The position of the main peaks at ~9.5 and ~10.6 µm, would also point toward a composition richer in Mg (Mg/(Mg+Fe)#~70%).

Discussion

At the scale of a few millimetres, the Bennu samples analyzed in this work exhibit spectral properties in the near-IR range that are similar to the ones of Ryugu pristine samples: a reflectance factor of 2-3 % at 2.5 µm, a red slope from ~1.0 to 2.5 µm and two main absorption bands at ~2.7 µm and ~3.4 µm4. The absence of a significant ~3 µm contribution to the matrix spectra, similar to the one observed in the spectra of CI chondrites and air exposed Ryugu samples (e.g.12,18,33,), confirms that the samples have never been exposed to the terrestrial environment since their collection from the asteroid (i.e. pristine). This also indicates that the phyllosilicates, notably the smectites, are comparatively poor in interlayer water molecules. The position of the ~2.7 µm, at 2.717 ± 0.005 µm, is consistent with the presence of Mg-rich phyllosilicates (e.g.,33,42). The ~2.7 µm band is also similar to the one observed on Ryugu samples in terms of peak position (within the uncertainties) suggesting similarities in the overall composition of the phyllosilicates, in particular regarding their Mg/Fe content12. Bennu and Ryugu samples also display similarly a weak feature at ~3.4 µm, indicative of the presence of aliphatic CH in organics and of carbonates. This feature is slightly more pronounced in Bennu samples than in Ryugu samples (~3–4% for Bennu samples vs. ~2% for Ryugu sample, Fig. 1). Its interpretation in terms of enhanced abundance of a given constituent (organics or carbonates) must be consistent with both elemental abundances of carbon slightly higher in Bennu than in Ryugu samples (4.7 % in Bennu vs. 3.8 % in Ryugu3,14,) and a higher H/C atomic ratio in Ryugu samples14,43.

Other similarities are also seen for a variety of OH-, H2O-, NH- (a fraction of which are in the form of NH4+)-rich components, as well as carbonates, which can be observed at sub-millimetre scale. Both the HAMPs and the other NH-bearing phases show strong spectral similarities with those observed in Ryugu samples4,15,[44](https://www.nature.com/articles/s41467-025-65438-z#ref-CR44 “Jiang, T. et al. Detection of NH-rich compounds in Bennu pristine samples via IR characterization at JAXA Curation Center and comparison with Ryugu, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16881. https://doi.org/10.5194/egusphere-egu25-16881

2025.“). Carbonates also appear to be dominated at MicrOmega scale by (Mg,Ca) and (Mg,Fe) types, as in Ryugu samples11. Altogether, the spectral similarities between alteration products such as carbonates, HAMPs and the dominating Mg-rich phyllosilicates, as well as the NH-rich components in both Ryugu and Bennu samples suggest resembling, though not necessarily identical, initial material, environmental conditions and formation processes. In particular, the detection of NH4+ as part of HAMP-rich compounds indicates that the parent body of Bennu formed or accreted a part of its material in the outer Solar System, in agreement with43. The preservation of HAMPs both in Bennu and Ryugu samples also indicates that their further evolution has remained at low temperature (<~70 °C if the struvite is considered as analog45), in agreement with17 that derived a temperature range of 20–55 °C based on the salt formation sequence, and similar to what has been proposed for Ryugu (e.g.,8).

Some significant differences could nonetheless be observed between Bennu and Ryugu samples. The large amounts of samples analyzed in this study, combined with the extensive set of similarities identified between two distinct objects reduce the likelihood that these differences translate possible biases from the collection process. The band depth of the ~2.7 µm feature that dominates down to the millimetre scale is weaker in Bennu samples compared to Ryugu samples (10–12 % vs. 12–18% in Ryugu bulk samples4). The grain size does not seem to be the cause of this discrepancy as the Bennu bulk samples show variations of similar magnitude, whether measured in areas dominated by larger (mm-size) grains or by finer fractions (Supplementary Fig. 2). Both the Ryugu and Bennu samples exhibit a similar band shape (Fig. 1), which would argue for a similar mineralogical composition. The lower ~2.7 µm band depth could then be due to 1) a less extended aqueous alteration of the initial anhydrous precursor (i.e., less crystallized Mg-rich phyllosilicates, e.g.,46), 2) a more space weathered material12, or 3) the presence of more opaque material that could lower (or even hide) the signature. No systematic band shift, which would indicate a stronger contribution from space weathered material (e.g.,47), is observed on average relative to Ryugu samples. The reflectance level, acquired in identical observational conditions, is also similar between Ryugu and Bennu samples (Fig. 1). The first hypothesis is, thus, favored over the two others. Additionally, a greater amount of anhydrous silicates has been observed in Bennu samples (~2 vol%3) compared to Ryugu samples (<0.5 vol%48,). Here we also show that large olivine grains (larger than a few tens of microns), either intact or slightly altered, are present in Bennu samples. Such grains/inclusions have not, so far, been identified in the Ryugu bulk samples in similar observations by MicrOmega4. We cannot rule out that this constitutes one of the potential minor biases induced by the collection process of Hayabusa2. The differences could also translate a possible exogenous origin of these compounds, as exogenic bright boulders with spectral properties similar to ordinary chondrites meteorites have been observed at the surface of Bennu[49](https://www.nature.com/articles