# Space weathering record and pristine state of Ryugu samples from MicrOmega spectral analysis

T. Le Pivert-Jolivet, R. Brunetto, C. Pilorget, J.-P. Bibring, A. Nakato, V. Hamm, K. Hataakeda, C. Lantz, D. Loizeau, L. Riu, et al.

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T. Le Pivert-Jolivet, R. Brunetto, C. Pilorget, J.-P. Bibring, A. Nakato, et al. Space weathering record and pristine state of Ryugu samples from MicrOmega spectral analysis. Nature Astronomy, 2023, 7 (12), pp.1445-1453. 10.1038/s41550-023-02092-9. hal-04271792

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# Space weathering record and pristine state of Ryugu samples from MicrOmega spectral analysis

Received: 21 September 2022

Accepted: 4 September 2023

Published online: 5 October 2023

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T. Le Pivert-Jolivet Ⓕ<fnref n="1" />✉, R. Brunetto<fnref n="1" />, C. Pilorget<fnref n="1" />,², J.-P. Bibring<fnref n="1" />, A. Nakato³, V. Hamm<fnref n="1" />, K. Hatakeda³,⁴, C. Lantz Ⓕ<fnref n="1" />, D. Loizeau<fnref n="1" />, L. Riu<fnref n="1" />,⁵, K. Yogata Ⓕ³, D. Baklouti Ⓕ<fnref n="1" />, F. Poulet<fnref n="1" />, A. Aléon-Toppani Ⓕ<fnref n="1" />, J. Carter<fnref n="1" />, Y. Langevin<fnref n="1" />, T. Okada Ⓕ³,<fnref n="6" />, T. Yada Ⓕ³, Y. Hitomi³,⁴, K. Kumagai³,⁴, A. Miyazaki Ⓕ³, K. Nagashima³, M. Nishimura³, T. Usui Ⓕ³, M. Abe³,⁷, T. Saiki³, S. Tanaka³,⁷,⁸, S. Nakazawa Ⓕ³, Y. Tsuda³,⁷ & S. Watanabe Ⓕ⁹

The samples returned from asteroid Ryugu were collected both at its surface and at its subsurface by Hayabusa2 and can, thus, provide information on the space weathering of C-type asteroids at different depths without terrestrial contamination. The near-infrared hyperspectral microscope MicrOmega gathered data on the -OH feature at ~2.7 µm for 177 individual grains from the two collection sites. Here, through a spectral analysis of these data, we show that the position of the band peak can be used as an indicator of the degree of space weathering. Most subsurficial grains do not present space weathering features, indicating that Ryugu's subsurface layers have never been exposed to the interplanetary medium. Moreover, the ~2.7 µm feature for the Ryugu samples is narrower than that observed for CI chondrites, which are the closest meteorite analogues to Ryugu, suggesting that these contain more absorbed molecular water than Ryugu due to terrestrial aqueous contamination. We conclude that Ryugu samples should be considered as a reference for the primordial water abundance within primitive bodies.

In 2018, the Hayabusa2 spacecraft reached the C-type asteroid (162173) Ryugu. The spacecraft characterized it from orbit and in situ and collected samples from its surface<fnref n="1" />,². The aims of Hayabusa2 were to bring new insights on the formation and evolution of Ryugu, its parent body and the primordial grains in the early Solar System. Observations of Ryugu's surface by the NIRS3 spectrometer onboard Hayabusa2 revealed a very low albedo (0.017 ± 0.002 at 2.0 µm) and a narrow absorption feature at 2.72 µm, attributed to the stretching vibration of O-H coupled to a cation, most probably within phyllosilicates³.

This ~2.7 µm feature has been observed previously on other C-type asteroids⁴,⁵ and on Bennu, the target of the NASA OSIRIS-REx sample return mission. Bennu's ~2.7 µm feature has been observed peaking at 2.74 ± 0.01 µm and is broader and deeper than Ryugu's feature<fnref n="6" />.

Samples of Ryugu were collected from two different sites (TD1 and TD2), with the second near an artificial crater created by the Small Carry Impactor (SCI) experiment to excavate subsurface material⁷. The samples were successfully brought back to Earth in December 2020 and transferred for preliminary analyses within the Extraterrestrial

<footnote n="1">¹Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay, France. ²Institut Universitaire de France, Paris, France. ³Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan. ⁴Marine Works Japan, Ltd., Yokosuka, Japan. ⁵ESAC, ESA, Madrid, Spain.</footnote>

<footnote n="6">⁶University of Tokyo, Bunkyo, Tokyo, Japan. ⁷The Graduate University for Advanced Studies (SOKENDAI), Hayama, Japan. ⁸University of Tokyo, Kashiwa, Japan. ⁹Nagoya University, Nagoya, Japan. ✉e-mail: tania.lepivert@ias.u-psud.fr</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1 |** *Example of variations in peak position. a, Spectra of two grains with extreme positions at 2.716 µm for C0098 (blue-green) and 2.725 µm for A0067 (purple). The plus signs indicate the spectral channels. An offset and a factor were applied to the spectra to obtain the same band depths of 0.5 to allow a*</figcaption></figure>

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better comparison of the shifts and the shape of the bands. The uncertainty of the
relative shift is due to temperature differences between the two measurements
and is <1 nm (Methods). **b**, Ratio of scaled spectra of A0067 and C0098.

Samples Curation Center at the Institute of Space and Astronautical
Science (ISAS), Japan Aerospace Exploration Agency (JAXA), in
Sagamihara, Japan. Chambers A and C in the container, correspond-
ing to the collections from TD1 and TD2, respectively, were opened
in vacuo within the facility at ISAS. The samples were distributed over
six sample holders (A1 to A3 for chamber A samples and C1 to C3 for
chamber C samples) and labelled as ‘bulk’<sup>8</sup>, while being maintained in
an ultrapure nitrogen environment.

The initial description phase began in January 2021 with a non-destructive and non-invasive characterization of the samples, which had not been exposed to the terrestrial environment since their collection on Ryugu. The first studies of the six bulk samples, performed by the near-infrared hyperspectral microscope MicrOmega⁹ and a Fourier-transform infrared spectrometer⁸, confirmed optical microscope observations that the bulk material is extremely dark (geometric albedos <3%). Moreover, absorption features around 2.7, 3.1 and 3.4 µm were detected and interpreted as being due to the presence of OH-rich minerals, as already observed on Ryugu's surface, NH-rich compounds, and organic matter and/or carbonates, respectively. The bulk spectra have a very similar ~2.7 µm band position and shape⁸,⁹. These data exclude major chemical or mineralogical differences, at a centimetre scale, between the OH-bearing phases present in samples from the two chambers.

After the characterization of the bulk samples, individual grains
were extracted and placed on distinct sapphire sample holders, start-
ing with the largest grains, within the facility. Like the bulk samples,
the individual grains were characterized by MicrOmega and the
Fourier-transform infrared spectrometer.

This paper presents and discusses the measurements acquired for
individual grains extracted from the bulk material from both chambers,
with a focus on the variations of the spectral characteristics of the
~2.7 µm band. Specifically, we characterized the peak position and
the relative band depth of the ~2.7 µm feature in average spectra for
177 individual grains. This population includes 84 grains from cham-
ber A (named hereafter A#, where # corresponds to the grain ID) and

93 grains from chamber C (hereafter C#), measured by MicrOmega
between February 2021 and November 2021.

Results

The spectra averaged over several orientations of the 177 grains exhibit low reflectance factors, around 3% on average in the 2 µm region. Typical spectra are illustrated in Supplementary Fig. 1. The absorption band at ~2.7 µm was detected for each grain, but its shape, depth and position vary slightly from one grain to another. The ~2.7 µm band is asymmetric and peaks around 2.72 µm (Fig. 1).

The evolution of the peak position of the ~2.7 µm feature between
the grains was evaluated using a Gaussian fit. Because the band is
asymmetric, the Gaussian fit was performed on a restricted part of
the band, close to its peak and ignoring the right wing. We evaluated
the relative uncertainty to be equal to ±1 nm (Methods). The distribu-
tion of the peak position determined by the Gaussian fit is illustrated
in Fig. 2a. The peak position varies between ~2.715 and ~2.725 µm. The
most frequent peak position is similar for both chambers: ~2.717 and
~2.718 µm for chambers A and C, respectively. However, the shapes of
the two distributions are different; the number of grains with a peak
at longer wavelengths (>2.72 µm) is larger for chamber A, for which
they represent ~40% of the grains in this study (34 grains). In addi-
tion, the distribution exhibits a small secondary peak at ~2.723 µm. By
comparison, the proportion of grains from chamber C with a peak at
wavelengths higher than 2.72 µm is only ~5% (five grains). Two example
spectra with extreme peak positions are illustrated in Fig. 1. Notice-
ably, not only the absorption maximum position but also the entire
2.7 µm feature is shifted. The shape of the spectral ratio also suggests
that A0067's band is shifted towards longer wavelengths compared
to C0098's band. Moreover, the spectra of both grains and their ratio
show that there are slightly deeper features in the 2.74–2.80 µm range
in A0067's band.

The distribution of the band depth is illustrated in Fig. 2b. For both
chambers, the distributions are approximatively bimodal in shape. For
chamber A, there are two maxima, at band depths of ~0.11 and ~0.17,
<figure><img src="image_5.png" /><figcaption>**Fig. 2 | Distribution of spectral parameters.** *a*, Distribution of the peak position of grains from chamber A (red) and from chamber C (blue). The vertical dashed lines indicate MicrOmega spectral sampling. *b*, Distribution of relative band depth. Each distribution has been normalized to the total number of grains in each chamber.</figcaption></figure>

separated by a minimum at -0.15. For chamber C, the distribution also exhibits a minimum at ~0.15 but the two maxima are closer to each other; they occur at -0.13 and -0.16. The grains with a low band depth (<0.15) represent half of the total population of chamber A (-52%) and around two-thirds of the total population of chamber C (~63%). However, the width of the band depth distribution differs from one chamber to the other. The lowest -0.04 and the highest -0.23 band depths are both for grains from chamber A. By contrast, the distribution of band depth for chamber C is narrower, with depths varying between -0.08 and -0.20. An example of two spectra, with similar albedos but different depths is illustrated in Fig. 3.

The scatter diagram of the band depth and the peak position (Fig. 4) shows small variations of the ~2.7 µm feature among individual grains and between samples from the two chambers A and C. There is a moderate negative correlation (Pearson correlation coefficient of -0.62) between the two parameters. Grains with a peak at wavelengths below 2.720 µm (hereafter, we will call these grains 'blueshifted') tend to have diverse band depths (between 0.08 and 0.23, with most above 0.11). On the other hand, grains with a peak position above 2.720 µm (hereafter 'redshifted') have weak band depths (most below 0.14). Almost all grains from chamber C are blueshifted as are a high proportion of grains from chamber A, whereas redshifted grains are almost exclusively from chamber A. According to their average spectra and their spectral ratio (Supplementary Fig. 2), the blueshifted population from chamber A and the whole population of chamber C are similar in terms of peak position, although their band depths have slightly different distributions. Similar trends are observed with other spectral parameters, such as the effective single-particle absorption thickness (ESPAT) and the normalized optical path length (NOPL; Supplementary Material).

When the same parameters (peak position, band depth, NOPL and ESPAT) are calculated for the two spectra from the bulk⁹ (Fig. 4 and Supplementary Fig. 3), they exhibit similar values for bulk A and C. In Fig. 4, both bulk spectra have a peak position at ~2.719 µm and their band depths are similar: 0.18 for bulk A and 0.17 for bulk C.

The dispersion graphs between band position and reflectance factor at 2.5 µm (Supplementary Fig. 4b) and between spectral slope and band position (Supplementary Fig. 5b) show no clear correlation between the two parameters. The Pearson correlation coefficients

between peak position and reflectance factor and between peak position and slope are 0.16 and -0.05, respectively.

## Discussion

The ~2.7 µm band is ubiquitous in the near-infrared spectra characterized in this study. The band positions detected at millimetre scale for individual grains are consistent with the positions calculated for bulk samples at centimetre scale (Fig. 4) and with the position of the band determined from Ryugu's surface spectra measured at metre scale by NIRS3 (ref. 3).

The major OH-bearing phases in Ryugu's grain matrix were mainly identified by electron microscopy and X-ray diffraction as being saponite and serpentine-like textures¹⁰⁻¹². The modal abundance of the phyllosilicate-rich matrix, measured in 16 grains, is 88 ± 3 vol% for chamber A grains and 82 ± 9 vol% for chamber C¹⁰. The O–H band positions we presented in this work are all consistent with an attribution to hydroxyl moieties strongly bonded to magnesium atoms¹³. The ~2.7 µm band in phyllosilicates is composed of different sub-bands. The vibration frequency depends on the local cationic environment of the MgO–H group, resulting in a sharp and strong asymmetric feature peaking in the 2.7–2.8 µm range¹⁴. In carbonaceous chondrites, the variation of the ~2.7 µm feature position is partly due to the variation of the Mg/Fe ratio in the phyllosilicate phases; the position of the ~2.7 µm band is shifted towards longer wavelengths with an increase of the Fe content (for example, refs. 15–17). Mg-rich phyllosilicates, both terrestrial (for example, ref. 18) and in CM and CI chondrite matrixes¹⁹⁻²², show this typical hydroxyl stretching band around 2.72 µm. As an example, Takir et al.¹⁷ reported a position of 2.715 µm for Ivuna (CII), 2.723 µm for LAP 02277 (CM1) and 2.722 µm for MET 00639 (CM2). Moreover, the features between 2.74 and 2.8 µm in redshifted grains are slightly deeper compared to those in blueshifted grains (Fig. 1 and Supplementary Fig. 2). The global ~2.7 µm band shape is similar in both cases. Thus, the contributions between 2.74 and 2.8 µm are likely due to small variations of the cation environment of O–H bond; the progressive replacement of Mg by Fe in phyllosilicates decreases the depth of the sub-band corresponding to a pure Mg cationic environment and increases the depth of the sub-bands corresponding to Fe-richer cationic environments and occurring at higher wavelengths. The increase of the band intensity
<figure><img src="image_6.png" /><figcaption>**Fig. 3 | Example of variations in band depth.** Spectra of two grains with very similar reflectance levels but different depths: 21% for A0022 (green) and 9% for C0063 (orange). Neither normalization nor an offset were applied to the spectra to allow a direct comparison of their band depths. The coloured regions correspond to the standard deviation for the different orientations.</figcaption></figure>

in the 2.74–2.8 µm region with a decrease of the Mg content has been observed in synthetic phyllosilicates with different Mg/Ni ratios¹³. Thus, the slightly deeper features we observe in redshifted grains are also compatible with a decrease of the Mg/Fe ratio.

Nakamura et al.¹² measured variations of the Mg/(Mg + Fe) ratio with electron microscopy in millimetric-sized sections extracted from ten grains and one powder sample. In our data, the ten grains studied by Nakamura et al. have positions between 2.716 and 2.720 µm, and the ratio of phyllosilicates in the sections was 0.75–0.90. The variation in the Mg/Fe content in Ryugu grains is real and is consistent with our interpretation that the slight difference in the peak position between the redshifted and blueshifted populations, as well as the slight increase of the sub-band depths in the 2.74–2.8 µm region, reflects differences in the average Mg/Fe ratio in the phyllosilicates, with redshifted bands corresponding to a slightly smaller ratio.

Different degrees of aqueous alteration change the Mg/Fe composition in phyllosilicates and could partly be responsible for the shifts observed in our data. However, if aqueous alteration was responsible for the variations in the peak position that we detected between chambers A and C, this should have been traced by the Mg/Fe ratio in the phyllosilicates between the two chambers. In particular, we would have observed an Fe-rich population in chamber A, corresponding to the redshifted bands. Nakamura et al.¹⁰ measured the Mg/(Mg + Fe) ratio in interior phyllosilicates of grains that have almost extreme positions in our study (between 2.717 and 2.724 µm). Even when they observed variations in the Mg/(Fe + Mg) ratio among and within the grains, they did not detect an especially Fe-rich population of phyllosilicates in chamber A, whereas we clearly observed some grains from chamber A (among the grains studied by Nakamura et al.) with redshifted positions. The variations in composition they observed at small scale in the interior phyllosilicates are probably averaged out in our spectra. Thus, different degrees of aqueous alteration affecting the composition of the interior phyllosilicates cannot explain all the

<figure><img src="image_7.png" /><figcaption>**Fig. 4 | Dispersion diagram between band depth and peak position.** Band depth as a function of the band position for the 177 individual grains (represented by dots) from chamber A in red and from chamber C in blue. The same parameters calculated for the bulk spectra are represented by squares. Horizontal error bars correspond to the uncertainty determined for the two continua and the two fitting intervals. Vertical error bars correspond to the uncertainty determined with the two continua and are generally smaller than the symbol size.</figcaption></figure>

variations that we observed between the two chambers, especially the redshifted population of chamber A.

Chamber A grains originate from the surface of Ryugu while chamber C grains contains subsurface material excavated by the SCI experiment. Because the two chambers probed different places at different depths on Ryugu, the heterogeneities in the band position detected between the two chambers could translate into different exposure times to a surface alteration process (that is, space weathering). This alteration process, combining irradiation by the solar wind and solar cosmic rays and impacts from micrometeorites, affects all airless bodies, in particular asteroids, in the Solar System at short timescales (typically $10^4–10^6$ yr for the solar wind; for example, refs. 23,24). Experiments simulating irradiation by the solar wind and bombardment by micrometeorites onto the surfaces of primitive asteroids (for example, refs. 25–30) showed that space weathering affects the composition and the structure of the hydrated minerals within the primitive asteroid analogues.

Textures associated with space weathering, such as circular structures, melt droplets and smooth and 'frothy' layers, have been reported at the surface of small fragments (<220 µm)³¹ and five millimetric-sized grains³¹,³² by scanning or transmission electron microscopy. Interestingly, grains for which space weathering textures have been detected (that is, A0017, C0094, A0058, A0094 and A0067) are all redshifted in our diagram of band depth as a function of peak position (Fig. 5c). On the other hand, A0042 is a blueshifted grain and Nakato et al.³² reported fewer weathered textures on its surface compared to C0094 and A0017. Moreover, the authors show that the redshifted grain A0017 has two distinct areas: one where many space weathering features were detected (craters plus smooth and frothy layers) and a second area where very few space weathering features were detected (only
<figure><img src="image_8.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_10.png" /></figure>

**Fig. 5 | Variations of spectral parameters induced by space weathering.**

*a*, Identification of two areas with different degrees of space weathering on the surface of grain A0017. Few space weathering features have been detected in the area circled in red. *b*, Average spectra of the space-weathering-poor area (black) and space-weathering-rich area (grey) extracted from MicrOmega measurements of A0017. Both spectra have been scaled to a band depth of 0.5. *c*, Dispersion graph of the band depth as a function of the band position. The grains studied by Nakato et al.³² and Noguchi et al.³¹ are highlighted, respectively, in light purple and pink. The position and depth of the spectra of the space-weathering-rich and space-weathering-poor areas of grain A0017 are represented in dark purple. Horizontal and vertical error bars correspond to the uncertainty determined for the two continua and the two fitting intervals. Error bars have been removed for other grains (grey) for clarity. SpWe, space weathering. Panel *a* courtesy of the authors of ref. 32.

craters). We determined the average spectra of the two areas (Fig. 5) and calculated their ~2.7 µm band position and depth. Both regions have similar band depths. However, the area poor in space weathering features has a peak position at 2.718 ± 0.001 µm like the blueshifted population of chamber A, whereas the area showing numerous space weathering features has a peak position at 2.722 ± 0.001 µm, fitting into the redshifted population. Within the space-weathering-rich area, Nakato et al. detected larger variations of Fe content compared

to the typical unaltered matrix of Ryugu. These variations in the composition are fully consistent with the redshift we observed for the space-weathering-rich area. Noguchi et al.³¹ also observed a relative enrichment in Fe in frothy layers at the surface of grain A0058 compared to the interior phyllosilicates.

The redshift detected for the weathered population is consistent with the results of irradiation experiments of carbonaceous chondrites and phyllosilicates, that simulates the solar wind effects on the surfaces of primitive asteroids²⁵,²⁸. The shift detected by these studies translates into a decrease of the Mg/Fe ratio after irradiation but depends on the initial composition of the analogue and the experimental conditions. Rubino et al.²⁸ determined a shift of 14 and 19 nm for two serpentine s with different compositions, although they did not observe any shift for a saponite sample for the doses applied (up to 6 × 10¹⁶ ions cm⁻²). The shift detected in this experiment has the same order of magnitude as the shift observed between the blueshifted and the redshifted populations.

The precise measurement of the 2.7 µm peak position indicates the degree of space weathering experienced by the given grains: the blueshifted grains belong to a population that has experienced no, or almost no, space weathering, whereas the redshifted grains have faced different degrees of space weathering.

As a result, the collection of Ryugu samples offers the unique possibility of analysing the isotopic, physical and chemical properties of grains with and without space weathering. Ryugu samples also unlock the interpretation of remote-sensing observations of C-type asteroids: the ~2.7 µm feature is a robust record constraining the asteroid's exposure age, whereas the spectral slope and reflectance level are poor estimators of their space weathering degree.

Finally, the fact that the peak position and the band depth of the 2.7 µm absorption change with the degree of space weathering has two major outcomes. Chamber A contains both mature (space-weathered) and less mature grains. At least one face of around 40% of the grains from chamber A show spectral features driven by space weathering. This implies that the fragmentation during the collection and the return of the samples³³ preserved the surface properties of a large fraction of millimetre-sized grains. Only the surface exposed to the solar wind of the initial (before being fragmented) blocks experienced space weathering. As a consequence, the samples collected in chamber A include grains exposed for a long time to the solar wind (typically 10⁴–10⁶ yr) as well as others from within the volume of the initial blocks that were protected from the space environment. Chamber C was filled with grains collected from the ejecta of the artificial crater, which, thus, originated from pre-impact subsurface locations. It contains a large number of less mature grains. This implies that the subsurface material of the second touchdown was never exposed to the space environment since the insertion of Ryugu into near-Earth orbits. That is, it did not experience any surface gardening processes. This is consistent with the absence of a regolith³⁴,³⁵, given its very low gravity.

The differences in band depth and position between the blueshifted and the redshifted populations are consistent with the spectroscopic observations of Ryugu's surface; the spectral ratio of the SCI crater region and the standard Ryugu surface measured by NIRS3 highlight spectral differences between the surface and the subsurface³⁶,³⁷. The crater and the ejecta, which were less exposed to space weathering, exhibit a deeper ~2.7 µm feature shifted towards shorter wavelengths compared to the exposed surrounding surface. Moreover, a spectroscopic study of craters³⁸ revealed that most craters, which are less exposed to space weathering than the surface, have a deeper band depth than the intercrater surface.

Although the peak position of the ~2.7 µm band in Ryugu grains is close to those found in CI chondrites, the shape of the band observed by MicrOmega in the Curation Facility is narrower compared to absorption bands of CI chondrites, measured in reflectance and under vacuum (for example, ref. 15). The absorption band in aqueously altered chondrites
is broader due to the presence of a wide feature centred around 3 µm and corresponding to HO–H stretching vibration. These water molecules can be adsorbed on the surface of the mineral grains constituting the chondrite sample or absorbed in the structure of phyllosilicates¹⁴.

Based on a thermogravimetric analysis combined with mass spectrometry of fragments from A0040 and A0094, Yokoyama et al.¹¹ suggested that Ryugu is depleted in both adsorbed and interlayer H₂O compared to Ivuna. The amounts of OH (that is hydroxyl groups bonded to metal atoms) in both Ryugu and Ivuna samples are similar. However, those authors found that the amount of interlayer water was 0.30 in wt% H₂O in Ryugu samples whereas it reached 6.58 wt% H₂O in an Ivuna sample. Notice that the average spectra of A0040 and A0094 have narrower bands than CI chondrites measured under vacuum and in dry conditions (in a chamber heated up to 475 K to remove adsorbed terrestrial water¹⁷,³⁹) (Supplementary Fig. 6), which is consistent with a lower content of interlayer water. All Ryugu grains contain essentially no molecular water whereas carbonaceous chondrites do; Ryugu samples have never been exposed to the terrestrial atmosphere until their preservation within the Curation Facility. One cannot exclude that a larger amount of interlayer water could originally have been present in Ryugu's parent body, which was then lost due to secondary processes such as solar heating. However, as Ryugu samples are not depleted in volatile elements compared to the CI chondrite reference¹¹, it seems more likely that a large part of the interlayer water found in carbonaceous chondrites comes from terrestrial contamination, as suggested by thermogravimetric analysis profiles¹¹. MicrOmega offers a way to identify grains with pristine properties, without the bias of the terrestrial environment. Thus, our results question previous interpretations of the H₂O content in carbonaceous chondrites. Thereby, Ryugu samples should be considered as a new reference to quantify the amount of hydroxyl and water within C-type asteroids.

In a few months, we will have an opportunity to analyse, within the same ISAS facility with the same instruments, Bennu samples returned by the OSIRIS-REx mission. This will be a truly coupled study of two carbon-rich near-Earth objects, Ryugu and Bennu. They have similar rubble piles shapes but different remote reflectance spectra (and specifically distinct ~2.7 µm bands³,⁶). Thanks to the spectral parameters identified in this study, which indicate the pristine properties of these objects, MicrOmega will enable us to decipher within the returned samples distinct records of their early history, shedding light on the evolution of their parent body.

## Methods

We characterized the ~2.7 µm feature of 177 grains extracted from bulk samples. The samples were measured between February 2021 and November 2021. This population includes 84 grains from chamber A and 93 grains from chamber C, with sizes between 1 and 7 mm and a median size of 2.5 mm. Each grain was positioned on an individual sapphire sample holder before being measured in a non-destructive and non-invasive way by the instruments within the Curation Facility.

Reflectance hyperspectral imaging of the individual grains was performed by MicrOmega, a hyperspectral microscope operating in the near-infrared (0.99–3.65 µm)⁴⁰. The average spectrum of each grain assessed in this study is available from the Ryugu Sample Database System (https://darts.isas.jaxa.jp/curation/hayabusa2/). The instrument chamber and the sample chamber are separated by a sapphire window. Both chambers are purged with an ultrapure nitrogen flux to prevent any contamination from the terrestrial atmosphere⁴¹, except for ultraclean N₂ gas. Thus, the analysed grains have not experienced any contamination between their collection on Ryugu's surface and their analysis by MicrOmega. MicrOmega acquires images of 256 × 250 pixels with a spatial sampling of 22.5 µm and a total field of view covering ~5.7 × 5.7 mm². The spectral resolution of the instrument is 20 cm⁻¹. The typical spectral sampling over the spectral range is 20 cm⁻¹ and is 10 cm⁻¹ in the ~2.7 µm region, which correspond to ~7 nm. The spectral

calibration provides an uncertainty on the absolute position of the spectral feature of ±2.5 nm (ref. 41). Additionally, the spectral calibration depends on the temperature of the dispersive system at the time of the measurement. Thus, we could expect an additional spectral shift between measurements due to operational temperature differences. The expected shift remains below 1 nm for temperature variations of ~10 °C between measurements⁴¹. For the dataset used in this study, the typical variation in the temperature between measurements did not exceed 2–3 °C. We, thus, expected the impact of temperature to be negligible compared to the shifts of the ~2.7 µm band positions (typically of 2–10 nm) observed among grains in our dataset. The radio-metric calibration has an accuracy of ~20% for the absolute reflectance level and of 1% for the relative (spectral channel to spectral channel) reflectance level⁴¹. Due to the illumination geometry (incidence of 35° and emission of 0°) and the shapes of the grains, some areas on sample surfaces were in shadow. Measurements of the individual grains were, therefore, performed at several orientations, typically with azimuth angles of 0°, 90°, 180° and 270°, to obtain full coverage of a grain.

We used the following procedure to extract a representative mean spectrum from the hyperspectral data for each grain. First, thermal emission maps measured by MicrOmega were used to discriminate the pixels within the grain from the total field of view. The difference in the thermal emission between the grain and the bottom of the sapphire dish allowed us to create a mask of pixels for the grain (Supplementary Fig. 7). For a few grains, it was impossible to generate a mask automatically because they were too close to the edge of the sample holder or too large. In this case, the pixels belonging to the grains were discriminated manually after visual inspection of the thermal emission maps.

Then, the spectra of all pixels within the masks were averaged to obtain a mean spectrum at one orientation. Finally, several orientations (between two and four) were averaged to give a spectrum less affected by shadowing and, thus, more representative of the grain composition. A typical final spectrum is an average for about 3,000–100,000 pixels. An example of spectra for grain C0008 before and after averaging the several orientations is illustrated in Supplementary Fig. 1.

We computed two parameters to characterize the ~2.7 µm feature: the peak position of the band and its depth, as described below. As the spectral sampling is ~7 nm, we performed a Gaussian fit to increase the sensitivity to band shifts and to the noise in the data.

Each spectrum was first divided by a linear continuum, defined as a linear regression from both side of the ~2.7 µm feature, between 2.6 µm and 3.2 µm. The boundaries were defined so as to avoid regions with absorption bands in all spectra. This excluded the spectral regions near 2.3 µm and 2.5 µm where absorption bands associated with carbonates are present in the average spectrum of grain C0181 (ref. 42), the spectral region around 2.9 µm with a H₂O feature, the region near 3.06 µm with a NH feature and the spectral region between 3.3 and 3.6 µm with a contribution from organics or carbonates⁹. Two sets of intervals, representing the continuum and fulfilling the previous constraints, were tested for the linear regression. The first set was defined as: 2.57–2.61 µm (3,888–3,831 cm⁻¹) for the left boundary and 3.18–3.24 µm (3,142–3,085 cm⁻¹) for the right boundary. The second set was 2.61–2.64 µm (3,831–3,783 cm⁻¹) for the left boundary and 3.13–3.18 µm (3,199–3,142 cm⁻¹) for the right boundary (Supplementary Fig. 8). We performed an additional independent test by calculating the depth and the position without any linear continuum. For this test, the spectra were simply normalized to the average reflectance level between 2.56 and 2.65 µm. Overall, the variations of depth and position between the two continuums and the normalization are small, around 0.5 nm on average for the position and around 0.005 on average for the band depth. This means that the way the continuum is defined has a limited influence on the two parameters.

The profile of the ~2.7 µm O–H stretching band is determined by the superimposition of different stretching vibrations at different frequencies depending on the local cationic environment of the
hydroxyl groups¹³. Since we used a single Gaussian as a fitting model to estimate the variations of the peak position of the feature, the fit was performed only for the major contribution, that is, the left part of the band. We fitted the band with one Gaussian with a wavenumber (energy) scale over a restricted interval around 2.72 µm (3,704 cm⁻¹). Several intervals were tested so that we could find the best compromise between an interval large enough to obtain a result as robust as possible and an interval small enough to minimize the contribution of the asymmetric part of the absorption band where a single Gaussian becomes inadequate for fitting the data. Among all tested intervals, two were retained: 2.656–2.736 µm (3,765–3,655 cm⁻¹) and 2.656–2.743 µm (3,765–3,645 cm⁻¹) (Supplementary Fig. 9).

The average position of the peak was defined as $(\lambda_{max} + \lambda_{min})/2$ and its uncertainty as $(\lambda_{max} - \lambda_{min})/2$, with $\lambda_{max}$ and $\lambda_{min}$ corresponding to the maximum and the minimum values among the four results (two fitting intervals and two continua). Notice that the difference in the peak positions between the two fits, around 2 nm on average, was more important than the difference in the band positions between the two continua.

To verify that MicrOmega spectral sampling is not a limiting factor when estimating the peak position, we compared the peak positions determined for the same samples with the single Gaussian fit for two different spectral sampling. We used spectra of CI and CM chondrites from Takir et al.¹⁷,³⁹. The samples were measured by reflectance spectroscopy under vacuum and dry conditions (that is, the chamber was heated to 475 K to adsorbed remove terrestrial water). These spectra have a spectral resolution of between 2 and 4 cm⁻¹. We resampled the Takir et al. spectra to MicrOmega spectral sampling of 10 cm⁻¹ using a linear interpolation, and we calculated the peak positions before and after the resampling. For the fitting, we used the same interval as for Ryugu grains. We obtained the same position when we applied the single Gaussian fit to the two spectra with different spectral sampling (Supplementary Fig. 10). Thus, we conclude that MicrOmega spectral sampling of 10 cm⁻¹ is not a limiting factor when estimating the peak position.

For all grains assessed in our study, we also compared the position estimated by a single Gaussian fit with the position estimated by a fit of an asymmetric function (that is, a fit with two Gaussians instead of one). We used a fit with two Gaussian curves, with the following intervals: 2.656–2.755 µm (3,765–3,630 cm⁻¹), 2.656–2.778 µm (3,765–3,600 cm⁻¹) and 2.656–2.786 µm (3,765–3,590 cm⁻¹) and the two previous continua (Supplementary Fig. 11). The position of the band was then estimated by resampling the fit and calculating the position of the absorption maximum. The average position of the band and its uncertainty were defined as $(\lambda_{max} + \lambda_{min})/2$ and $(\lambda_{max} - \lambda_{min})/2$, respectively. The dispersion graph between the peak position estimated from two Gaussians and the band depth is illustrated in Supplementary Fig. 12. Compared to the single Gaussian method (Fig. 4), the peak position covers a larger interval, between 2.712 and 2.724. We can see that the grains with initial position (position with single Gaussian fit) around 2.716 µm are shifted up to 3 nm towards shorter wavelengths with the two Gaussian fits, whereas the grains with position around 2.725 µm are shifted only by 1 nm towards shorter wavelengths. This means that the single Gaussian fit is less sensitive than the fit with two Gaussians when determining the peak position of grains with a -2.7 µm band at short wavelengths. However, the variations observed between band depth and band position between Ryugu samples are similar. Specifically, the shifts between the spectra for chambers A and C are independent of the choice of the fitting function. Thus, the heterogeneities in peak position are described well by the single Gaussian fit.

To estimate the relative uncertainty (that is, the precision when evaluating the shift from one grain to another), we compared grains whose positions were different distances from the average position (Supplementary Fig. 13 and Supplementary Table 1) and calculated their spectral ratio (Supplementary Fig. 14). In theory, when a blueshifted

band is divided by a redshifted band, the spectral ratio behaves in the following way from lower to higher wavelengths: first, it falls for values <1, then it increases dramatically, passing through 1, before reaching a maximum and, finally, it gently decreases to 1 again (Supplementary Fig. 15). When a redshifted band is divided by a blueshifted band, the opposite trend is observed (Supplementary Fig. 16). It appears that for grains separated by more than 2 nm, the spectral ratio shows clear evidence of a shift, with an increase/decrease followed by a decrease/increase of the ratio, depending on the direction of the shift (redshift/blueshift). We also studied individual grains shifted from each other by steps of 2 nm according to the Gaussian fit (Supplementary Table 2). A progressive shift is visible in the -2.7 µm band of those grains (Supplementary Fig. 17). These two tests confirm that we can estimate the shift between the peak position with an accuracy of ~2 nm. Thus, we consider that the relative uncertainty of ±1 nm for our relative peak positions and that the variations of 2–10 nm detected between the grains in this study are real.

The band depth of the 2.7 µm feature was calculated at the minimum of the reflectance position and relative to the linear continuum. The depth is computed as:

$$Bd = 1 - \frac{R_{min}}{R_c}$$

where $R_{min}$ is the minimum of the reflectance for the 2.7 µm band and $R_c$ is the reflectance of the continuum at the same wavelength⁴³. Like the peak position, the band depth was calculated for two continua (Supplementary Fig. 8).

For carbon-rich materials like Ryugu samples, the albedo and, thus, the band depth of the -2.7 µm OH feature may be affected by the presence of opaque phases, such as organic matter or oxides. We verified that the variation of the band depth was not due only to differences in the proportion of opaque phases and, thus, the variation in the albedo between the grains. Milliken and Mustard⁴⁴,⁴⁵ showed that ESPAT, calculated for single-scattering albedo (SSA) spectra from which the continuum had been removed, was less dependent on the variations of the albedo than the relative band depth. Thus, ESPAT and NOPL are often used in studies that aim to quantify the water content in dark samples from their reflectance spectra, such as carbonaceous chondrites⁴⁶ and C-type asteroids like Ryugu¹⁷. If the trends observed between band depth and band position (Fig. 4) are also observed for NOPL and ESPAT, then we can assume that the variations observed for band depth are not due only to the albedo but are a characteristic of the OH-bearing phase.

NOPL is defined as:

$$NOPL = \frac{-\ln[R(\lambda) + 1 - R_c(\lambda)]}{-\ln[1 - R_c(\lambda)]}$$

where $R_c$ is the reflectance factor of the continuum and R the reflectance factor. Here, the NOPL was calculated at the minimum of the reflectance position.

ESPAT was calculated from the SSA spectrum using the same assumptions as Garenne et al.⁴⁶ (no opposition effects, isotropic scattering of the particles and similar porosity between samples and standards). The SSA spectra were derived from the reflectance spectra, following the relation between the bidirectional reflectance R and the SSA w:

$$R = \frac{w}{(1 + 2\gamma\mu_0)(1 + 2\gamma\mu)}$$

$\gamma$ is the albedo factor and is defined as $w(\lambda) = 1 - \gamma(\lambda)^2$. Here, $\mu = \cos(e) = \cos(0^\circ)$ and $\mu_0 = \cos(i) = \cos(35^\circ)$. The SSA spectra were divided by a linear continuum using the same boundaries as for the
reflectance spectra continuum. Then, the ESPAT function was calculated for each spectrum at the position of the SSA minimum:

$$ESPAT(\lambda) = \frac{1 - [w(\lambda)/w_{\max}(\lambda)]}{w(\lambda)/w_{\max}(\lambda)}$$

with $w(\lambda)$ and $w_{\max}(\lambda)$ corresponding to the minimum of the SSA and the SSA of the continuum at the same wavelength, respectively.

The reflectance factor at -2.5 µm was calculated as the average reflectance between 2.45 and 2.55 µm. The uncertainty for the reflectance factor was computed as the standard deviation between 2.45 and 2.55 µm. The spectral slope was defined as a linear regression of the spectrum. Two intervals were tested: 1.4-2.5 µm and 1.6-2.3 µm. The final value of the slope corresponds to $(s_{\max} + s_{\min})/2$ and its uncertainty as $(s_{\max} - s_{\min})/2$, with $s_{\max}$ and $s_{\min}$ corresponding to the maximum and the minimum slopes. Distributions of the reflectance factor at 2.5 µm and the spectral slope are presented in Supplementary Figs 4a and 5a. There is no difference in the shape of the distribution of the reflectance factor or the slope between the two chambers.

We performed a two-sample Kolmogorov-Smirnov (KS) statistic test to quantify the heterogeneities between the two chambers for each spectral parameter (peak position, band depth, reflectance factor at 2.5 µm and spectral slope). The KS value is the maximum distance between the cumulative distributions of two datasets$^{48}$. A high KS value indicates that the two datasets do not come from the same distribution. We also calculated the *P* value of the KS statistic test, which describes the probability that two datasets have the same distribution. We summarize in Supplementary Table 3 the KS and the *P* values associated with each spectral parameter.

The two parameters with the weakest *P* value are the peak position and the band depth. This means that they better highlight heterogeneities between the two collection sites than the reflectance factor at 2.5 µm or the spectral slope.

To compare our results, we used as references KS and *P* values calculated on two datasets that originated from the same distribution. The two datasets are the whole chamber C (93 grains) and a subpopulation of 28 (that is, ~30%) randomly selected grains from chamber C itself. We performed the random selection 1,000 times and averaged the results to obtain an accurate value that describes the heterogeneities between a subpopulation and the whole population of chamber C (Supplementary Table 4). For all spectral parameters, the KS and the *P* values were equal to ~0.15 and ~0.60, respectively.

All spectral parameters from the previous test (chamber A versus chamber C) have KS values higher than the reference (~0.15) and a *P* value lower than the reference (~0.60). The peak position is the parameter with the highest difference between the *P* values of the two tests. This means that the distributions of the peak positions of grains from chambers A and C are substantially and statistically different; they are produced by different physico-chemical properties and not by a sampling effect. The peak position is, thus, the better proxy for describing the heterogeneity between the two chambers. The peak position is a unique criterion to identify the given properties of a new grain from the collection, specifically its degree of space weathering.

## Data availability

The average spectrum of each grain assessed in this study is available from the Ryugu Sample Database System (JAXA) at https://darts.isas.jaxa.jp/curation/hayabusa2/. Source data are provided with this paper.

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## Acknowledgements

We thank the whole Hayabusa2 team for their scientific and technical contributions to this successful mission. We thank the National Centre for Space Studies, which is the French space agency, for its support. T.Y. received support from the Japan Society for the Promotion of Science, grant number JP18K03830. We warmly thank P. Beck and D. Takir for providing us with near-infrared spectra of carbonaceous chondrites.

## Author contributions

R.B., C.P., J.-P.B., A.N., V.H., K.H., D.L., L.R., K.Y., T.O., T.Y., T.U., M.A., T.S., S.T., S.N., Y.T. and S.W. conceived and designed the experiments. T.L.P.-J., R.B., C.P., J.-P.B., A.N., K.H., C.L., D.L., L.R., K.Y., T.O., T.Y., Y.H., K.K., A.M., K.N. and M.N. performed the experiments. T.L.P.-J., R.B., C.P., J.-P.B., A.N., C.L., D.L., L.R., D.B., F.P., A.A.-T., J.C. and Y.L. analysed the data. T.L.P.-J., R.B., C.P., A.N., V.H., K.H., C.L., D.L., L.R. and K.Y. contributed materials and analysis tools. T.L.P.-J., R.B., C.P., J.-P.B., A.N., C.L., L.R., D.B., F.P. and T.O. wrote, discussed and commented on the paper.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41550-023-02092-9.

**Correspondence and requests for materials** should be addressed to T. Le Pivert-Jolivet.

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