Main

Solar wind irradiation and high-velocity micrometeoroid bombardment dominate space weathering1,2 for all airless bodies. However, the effects of these processes vary substantially, depending on the specific class of body. The solar wind is a plasma composed mainly of low-energy protons and electrons streaming from our Sun1,2,3, which induces radiation damage, including amorphization of silicates and formation of nanophase metallic iron particles (npFe0). In contrast, micrometeoroids are interplanetary dust particles that impact airless surfaces at hypervelocities4, resulting in cratering, melting and vapour deposits, and sometimes also amorphous silicates and npFe0. Space-weathering products of two anhydrous bodies, the Moon and the S-type asteroid Itokawa, have been investigated extensively5,6,7,8,9,10,11,12,13,14. These studies revealed that nanometre-sized metallic Fe particles (npFe0), formed via space weathering, resulted in weakened absorption features in visible to near-infrared reflectance. In contrast, it has been unclear what role npFe0 plays in the reflectance properties of dark (C- and D-type) asteroids1,2.

Space-weathering modification of reflectance spectra features from airless bodies makes identifying a direct link between asteroids and specific meteorite classes based on composition and mineralogy difficult. The Hayabusa mission of the Japan Aerospace Exploration Agency (JAXA) revealed the connection between visible to near-infrared reflectance spectra from S-type asteroids and ordinary chondrite meteorites5, with the difference largely attributable to the role of nanophase particles. However, laboratory experiments that mimic solar wind irradiation and micrometeoroid impact on C-type asteroids show a lack of detectable production of npFe0, with some spectra reddening (a positive change in spectral slope) and others bluing (just the opposite)15,16,17,18,19,20. Thus, the observed change of spectral slope and absorption band in reflectance spectra of C-type asteroids compared with carbonaceous chondrites meteorites is difficult to interpret15,16,17,18,19,20. JAXA’s Hayabusa2 spacecraft observed spectral variation on asteroid Ryugu21,22,23,24 thought to be related to space weathering. Our studies of Ryugu samples offer the first opportunity to directly link the spectral variation to the space-weathering-induced physical and chemical alteration of regolith on C-type asteroids.

Results

Surface modifications found on Ryugu grains

The mineralogy of most Ryugu grains investigated by (scanning) transmission electron microscopy is similar to that of CI chondrites25, which are the most chemically primitive materials in the Solar System26, consistent with other recent studies27,28,29,30,31. Therefore, to understand the space weathering of Ryugu grains is to understand the weathering of the most chemically primitive Solar System material.

More than 500 grains (average diameter ~71 µm) collected at the first touchdown (landing) site (TD1) and >300 grains (average diameter ~57 µm) collected at the second touchdown site (TD2) were investigated for surface modifications potentially related to space weathering. Recognizable surface modifications of the phyllosilicate-rich matrix were found in ~6% of the observed grains from TD1 and ~7% from TD2 (Extended Data Fig. 1). The surface modifications of grains differ considerably from those from the Moon and Itokawa because the most abundant phases in Ryugu grains are hydrated sheet silicates (phyllosilicates), not anhydrous silicates (for example, olivine). Several distinct surface modifications are observed, including smooth layers, frothy layers, melt splashes and their combinations (Fig. 1 and Extended Figs. 2 and 3). We also examined three millimetre-sized grains (A0067, A0094 and A0058) that have surface modifications related to space weathering.

Fig. 1: Secondary electron images of Ryugu grains showing surface modifications related to space weathering.
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a, The grain C0105–03004800 was collected at the second touchdown site. It is composed of two parts showing different types of space weathering: a frothy layer and a smooth layer. Enlarged images of the two boxed areas on this grain are shown in the insets at the upper right (frothy layer) and the lower left (smooth layer) corners of a. b, The grain A0104–02203700 was collected at the first touchdown site. The frothy layer partially covers the smooth layer on the left-hand side of the image. The boundary between two types of layers is indicated by a dashed curve. The frothy layer has many burst vesicles. A melt splash, located at the lower centre of the image, is attached to the surface of the frothy layer.

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Smooth layers on Ryugu grains

Approximately 5% of the observed grains from TD1 and ~3% from TD2 have a smooth layer, evident as a thin (<100 nm) continuous smooth sheet covering the surface. Some of these layers contain vesciles of <50 nm diameter that intersect the surface (Fig. 2a and Extended Data Fig. 2a). Partial detachment of the smooth layers is observed in some grains (Extended Data Fig. 4). Electron diffraction reveals that smooth layers are almost completely amorphous (Extended Data Fig. 2d). Atomic ratios among major cations of the smooth layers are indistinguishable from those of the phyllosilicate-rich matrix (Fig. 3a). Most Fe in the smooth layer is Fe2+, but most Fe in the underlying phyllosilicates is Fe3+ based on the Fe L3-edge electron energy-loss spectroscopy (EELS), and Fe L3 and Fe K X-ray absorption near-edge spectroscopy (XANES) (Fig. 3b and Supplementary Table 1), indicating that the smooth layer is more reduced than the matrix.

Fig. 2: Cross-sections of three Ryugu grains showing typical surface modifications on the phyllosilicate-rich matrix.
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a, The cross-section A0104–02306901 was prepared from the grain A0104–02306900 collected at the first touchdown site. It has a smooth layer that forms a ~100-nm-thick continuous layer covering the surface of the grain. The phyllosilicate-rich matrix is present below the smooth layer. A yellow dashed curve indicates the cross-section of the sample surface. The boundary between the smooth layer and the phyllosilicate-rich interior is indicated by an orange dashed curve. b, The cross-section C0105–03003701 was prepared from the grain C0105–03003700 collected at the second touchdown site. A frothy layer containing abundant vesicles (darker circles) and <50-nm-size brighter spots (Fe-Ni sulfide beads) covers the surface of this grain. The thickness of the frothy layer varies considerably locally from <100 nm to >500 nm. A yellow dashed curve indicates the cross-section of the sample surface. C-depo denotes carbon depositions to protect the surface of the samples during FIB processing. c, An enlarged image of a frothy layer in a cross-section A0104–02802202. The frothy layer contains many tiny (<20 nm across) blisters (vesicles just below the surface) on its surface. These are high-anglular dark-field scanning transmission electron microscope images, in which materials with higher average atomic numbers are brighter than those with lower average atomic numbers.

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Fig. 3: Elemental compositions and redox states of Fe in a smooth layer, frothy layers and the interior phyllosilicates.
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a, The ternary [Si+Al]-Mg-Fe atomic-ratio diagram shows that elemental compositions of a smooth layer are indistinguishable from those of the interior phyllosilicates in the cross-section sample A0104–02306901. b, However, a Fe L3-edge peak in EELS spectra shows that Fe2+ is enriched in the smooth layer, which means that Fe3+ in the smooth layer is reduced to Fe2+. The EELS spectra were obtained from the upper (U) and lower (L) parts of the smooth layers, the upper (U) and lower (L) areas around the boundary between the smooth layer and the interior phyllosilicates, and the interior phyllosilicates. c, By contrast, the frothy layer in the cross-section sample C105–03003700 is more enriched in Fe relative to [Si+Al] and Mg than the interior phyllosilicates. d, The same compositional relationship is shown between the frothy layer in the cross-section sample A0058–C2001 and the interior phyllosilicates. The whole grain sizes of these samples are quite different. C105–03003700 and A0058–C2001 are ~30 µm and ~3 mm across, respectively. e–h, Fe3+ in the frothy layers is also reduced to Fe2+. e, Fe L3-edge peak spectra obtained by EELS. The spectra were obtained from the frothy layer, the boundary area between the frothy layer and the interior phyllosilicates, and the upper and lower areas of the interior phyllosilicates. f, Fe L3-edge peak spectra obtained by STXM–XANES. g,h, Fe K-edge spectra (g) and background-subtracted pre-edge peak spectra (h) obtained by XANES. Int. phyllosilicates, phyllosilicates in the interior of a sample.　Serp and Sap in a, c, and d are the abbreviations for serpentine and saponite, respectively.

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An unweathered Ryugu grain that was irradiated with 4 keV He+ at a fluence of 1.3 × 1018 ions cm−2 to simulate space weathering shows a surface morphology and an internal structure that are very similar to those of the smooth layers (Figs. 1a and 2a, and Extended Data Fig. 2) of the bonafide space-weathered grains. On lunar and Itokawa grains, smooth surfaces were formed by micrometeoroid impacts and subsequent redeposition. On Ryugu grains, in contrast, we found a ~10-nm-thick vapour deposit on top of a smooth layer of only one grain (Extended Data Fig. 5), suggesting that ‘vapour deposition’ does not play an important role in forming smooth layers. Instead, the laboratory-irradiated Ryugu grain indicates that solar wind irradiation probably played an important role in modifying the surface of the phyllosilicate-rich matrix, and the smooth layers represent space weathering induced by solar wind irradiation.

Frothy layers and melt splashes on Ryugu grains

Frothy layers are found on ~1% of the observed grains from TD1 and ~2% of grains from TD2, which are composed of silicate glass containing abundant embedded vesicles ~0.1- to ~1 µm wide and numerous submicroscopic (<200 nm) rounded Fe-Ni sulfide beads (Fig. 2b and Extended Data Fig. 3). The internal structure suggests that silicate and Fe-Ni sulfides were melted and immiscibly separated into silicate and sulfide melts and that vesiculation occurred during melting. The frothy layers have higher Fe and lower Si+Al and Mg than the interior phyllosilicate-rich matrix (Fig. 3c,d), irrespective of the size of the grain on which they reside. The frothy layer is also more reduced in Fe than the underlying phyllosilicates (Fig. 3e–h). The Fe-Ni sulfide beads, composed of pyrrhotite and pentlandite with diameters from ~200 to <10 nm, are ubiquitous within frothy layers (Extended Data Fig. 6a–d). While most microphase sulfides are probably immiscibly separated as droplets during melting, the ~10-nm-sized nanophase sulfide shown in Extended Data Fig. 6d may be a vapour deposit, consistent with previous laser irradiation experiments15,16. We identified no npFe0 within the frothy layers investigated. However, on the surface of a frothy layer, we found an aggregate composed of npFe0 and troilite (stoichiometric FeS) (Extended Data Fig. 6e–h). Porous apatite, dolomite and magnetite occur in some frothy layers and are believed to be relict minerals that survived melting. In addition, a frothy layer is observed with abundant blisters (vesicles just below the surface) (Fig. 2c). Melt splashes (<10 µm across) are found on <1% of the observed grains from TD1 and ~1% of grains from TD2, and are attached to Ryugu grains with and without detectable surface modifications (Fig. 1b).

Exceptionally rare npFe0 on Ryugu

The exceptionally low abundance of npFe0 in Ryugu grains is in stark contrast to lunar and Itokawa surface samples, which contain abundant npFe0 in both radiation-damaged layers on ferromagnesian silicates7,8,9,11,13,14 and in vapour deposit layers produced by micrometeoroid impacts32,33. While nano- to microphase Fe-bearing sulfides are ubiquitous in all the frothy layers on the Ryugu grains investigated, no interior npFe0 was found. The reduction effect of space weathering might be insufficient to form npFe0 from abundant Fe3+ contained in phyllosilicates in Ryugu grains (Supplementary Table 1). In addition, the –OH in phyllosilicates may hinder the reduction of Fe. These results are similar to laboratory experiments where in situ X-ray photoelectron spectroscopic analyses of H+ and He+ irradiated Murchison CM2 chondrite showed that partial reduction of surface Fe to lower oxidation states occurred with simulated solar wind exposure19. Nano- to microphase Fe sulfide is also common in both H+ and He+ irradiated Murchison CM2 chondrite and laser-irradiated Murchison. These results are consistent with our observations, although Murchison is a CM chondrite with significantly different mineralogy and Fe content than Ryugu. We suggest that it is unlikely that npFe0 contributes significantly to the observed spectral variability on Ryugu, but submicroscopic Fe-Ni sulfides may contribute to this variability.

More abundant impact melts on Ryugu than on Itokawa

Among Itokawa grains, only 2 out of 590 grains (0.3%) show melted structures34 that resemble the frothy layers found on ~1% to 2% of Ryugu grains. The calculated dry and wet solidus temperatures of Ryugu material28 are approximately the same (862 and 867 °C) under ~105 Pa because of the low H2O solubility in the melt at low ambient pressures35,36, which indicates that high porosity may explain the higher abundance of impact melts among Ryugu grains relative to Itokawa grains. Ryugu grains have high average microporosity (~28%, measured by synchrotron radiation nanotomography) and may have experienced higher post-shock temperature than low-porosity (1.5–1.9%)37 Itokawa grains since porosity collapse by shock compression causes a large temperature increase38. Both the surface morphology and internal structure of laser irradiation products15,17 that simulate shock heating by micrometeoroid impacts are similar to those of the frothy layers (Figs. 1 and 2b, and Extended Data Fig. 3). Thus, one of the major formation mechanisms of the frothy layers might be frictional heating among loose regolith grains by meteoroid impact. In addition, in situ formation of melt by micrometeoroid impact onto the grains and deposition of melt formed by a neighbouring impact event would also contribute to the formation of the frothy layers. The observed small melt splashes might be ejecta formed during micrometeoroid cratering.

Discussion

We estimated the timescale of formation of the smooth layer on Ryugu grain surfaces. The fluence of the ion irradiation experiment (Extended Data Fig. 2c) is equivalent to ~3 × 103 years at 1.2 au (the semimajor axis of Ryugu’s orbit) by considering the solar wind flux density at 1 au, 3–5 × 108 ions cm−2 s−1 (ref. 39), and the average He/H ratio in the solar wind, 0.045 (ref. 40). We found an olivine crystal exhibiting a radiation-damaged rim and containing solar flare tracks with a number density of ~2 × 108 cm−2 (Extended Data Fig. 7), which corresponds to an ~6 × 103 year dwell time for the grain within ~1 mm from the surface, based on lunar sample studies41. A thin (~20 nm) smooth layer on the phyllosilicate matrix was found near the olivine grain in the same sample. These independent results suggest that it may take >3 × 103 years to form a detectable smooth layer on phyllosilicates. The exposure age of the smooth layer-covered surface of Ryugu grain A0067 is estimated to be 3 × 104 years, calculated by its crater population assuming they formed by interplanetary meteoroid impacts4 (Extended Data Fig. 8). In comparison, studies of craters on Itokawa grains42,43 showed that most submicrometre-scale craters were probably formed by secondary impacts of ejecta excavated from larger craters. If such impacts occurred on Ryugu, then the formation of the smooth layer would require less time. In either scenario, the upper limit on time required to develop the smooth layer is 3 × 104 years, which is consistent with the above estimate.

A frothy layer with abundant blisters (Fig. 2c) suggests that, after irradiation by the solar wind, blisters formed during subsequent heating, probably related to micrometeoroid impact, which induced the release of trapped solar wind gas species. This is consistent with steady and continual solar wind irradiation through time, while micrometeoroid bombardment occurs sporadically.

Some grains have partially exfoliated smooth layers (Extended Data Fig. 4), which suggests that smooth layers can detach. Detachment of smooth layers may explain the low abundance (~7%) of investigated grains with space-weathered features. In addition, the fragility of Ryugu grains may be another important factor that reduces the abundance of grains with observable space weathering. Among 6 large (millimetre-sized) grains collected at TD1, ~66% (4 of 6) show evidence of space weathering based on the field emission scanning electron microscope (FE–SEM) observation at Kyoto and Tohoku Universities, which is much higher than 6–7% for <100-µm-sized grains (Extended Data Fig. 1). The difference can be interpreted as indicating that most fine-grained samples are fragments of larger grains. Exfoliation and destruction could occur by thermal fatigue and meteoroid impacts on Ryugu. In addition, they could also occur during sampling and transportation to Earth, or even during handling processes.

To quantify the amount of –OH in the Ryugu grain surfaces, we used energy dispersive X-ray spectroscope (EDS) measurements of focused ion beam (FIB) cross-sections of weathered and pristine grains (Fig. 4) to determine oxygen to cation ratios, correcting for S-bonded Fe and Ni. The ratio of oxygen to cations bonded with oxygen shows that, in pristine grains, interlayer H2O molecules in saponite are largely absent from a mixture of saponite and serpentine, but structural –OH groups in the phyllosilicates are retained (Fig. 4a). This estimation of –OH abundance is consistent with the thermogravimetric analysis of Ryugu grains28.

Fig. 4: Histograms of atomic ratios of oxygen to the cations bonded to oxygen in phyllosilicates, a smooth layer and frothy layers.
figure 4

A mixture of saponite without interlayer H2O molecules and serpentine has a range of ratios represented by green bands. If a mixture of saponite and serpentine is decomposed into an anhydrous compound, it has a range of ratios represented by red bands. In order to calculate the atomic ratios of oxygens to the cations bonded to oxygen in phyllosilicates, we subtracted the cations bonded to sulfur (S), which were calculated based on the assumption that the ratio of the S-bonded Fe and Ni ions to S is unity for simplicity. a, Phyllosilicates in a non-space-weathered grain contain almost no interlayer H2O but preserve structural –OH groups. b, A smooth layer lost a considerable amount of structural –OH groups and phyllosilicates just below the smooth layers partially lost structural –OH groups. c, Phyllosilicates just below the frothy layer have lost the structural –OH groups considerably. d, Phyllosilicates just below the frothy layer have lost almost all the structural –OH groups.Because the frothy layers have even lower ratios than the red bands, they are also anhydrous. Their very low ratios may be related to their high abundance of embedded Fe-Ni sulfide. The ratio at the right end of the green belts is 1.8, which is calculated from the generalized chemical formula of serpentine Y6Z4O10(OH)8. O/(Y + Z) = 18/10 = 1.8. The ratio at the left end of the green belts is 1.64, which is calculated from the generalized chemical formula of saponite with no interlayer H2O molecules X0.6Y6Z8O20(OH)4. O/(X + Y + Z) = 24/14.6 = 1.64. The ratio at the right end of the red belts is 1.5, which is calculated from the generalized chemical formula of the dehydrated decomposition product of saponite X0.6Y6Z8O22. O/(X + Y + Z) = 22/14.6 = 1.5. The ratio at the left end of the red belts is 1.4, which is calculated from the generalized chemical formula of the dehydrated decomposition product of serpentine Y6Z4O14. O/(Y + Z) = 14/10 = 1.4.

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Space weathering over the length of Ryugu’s residence time at near-Earth orbits after its orbital shift from the main belt, which is thought to be several megayears based on noble gas data44, may also play an important role in the removal of interlayer H2O from saponite. During the prolonged exposure to interplanetary space, even structural –OH groups might be removed from a mixture of serpentine and saponite that has lost its interlayer H2O. This may occur by decomposition of saponite and serpentine by dehydroxylation, that is, a decomposition to anhydrous compounds and liberated H2O molecules. However, we note that the solar wind would probably penetrate grains very heterogeneously owing to the high microporosity of the phyllosilicate-rich matrix of Ryugu grains.

A substantial amount of structural –OH has been lost in smooth layers (Fig. 4b). Almost all the structural –OH has been lost in frothy layers and also in the phyllosilicates just below frothy layers (Fig. 4c,d). These data suggest that more structural –OH in phyllosilicates is removed through dehydroxylation as space weathering proceeds (Fig. 5). A portion of structural –OH in phyllosilicates just below the smooth layer also appears to be absent (Fig. 4b), and sporadic amorphization of phyllosilicates is observed. Solar wind particles could penetrate to such depths, in regions of highest porosity of the phyllosilicate-rich matrix (Fig. 2a), and potentially promote dehydroxylation reactions. In addition, phyllosilicates just below the frothy layer lost most of their structural –OH (Fig. 4c, d). Frictional heating induced by meteoroid impacts and the formation of new surfaces by thermal stress45 could promote dehydration by dehydroxylation.

A conceptual illustration (Fig. 5) shows the development of solar wind implantation, dehydration by dehydroxylation of phyllosilicates and progressive coverage of anhydrous silicate-rich melt on a Ryugu grain. Once a surface of a Ryugu grain is exposed to interplanetary space, the effects of solar wind irradiation start to accumulate at and near the surface. As time passes, the gradual accumulation of solar wind radiation damage and phyllosilicate dehydroxylation form the smooth layer on its surface, which means that partial dehydration occurs in the smooth layer as shown in Fig. 4b. Because the effects of solar wind are constrained by the limited kinetic energy of solar wind particles3, the thickness of the smooth layer seldom exceeds ~100 nm. Sometimes, a very thin (~10 nm) vapour deposition may be formed on its surface as shown in Extended Data Fig. 5, although it is not illustrated in Fig. 5.

In contrast, formation of impact melts (frothy layers and melt splashes) is an intermittent process. The impact melt can be formed in several ways: in situ formation of melt by micrometeoroid impact melt onto the grain, deposition of melt formed by a neighbour impact event and in situ melting by frictional heating among porous regolith. In this conceptual illustration, partial coverage by impact melt occurred twice, at times I and II. After the coverage by a frothy layer, the effects of solar wind irradiation start to accumulate in the frothy layer. If another subsequent heating event or impact occurs, resulting in the deposition of another melt deposit, the implanted solar wind gases in the frothy layer may form blisters (Fig. 2c). The frothy layers (impact melts) are almost anhydrous because they were formed by high-temperature processing, which is indicated in Fig. 4c,d.

In Fig. 5, the change of colour from light blue via orange to yellow represents the progress of dehydration. The surface material of asteroid Ryugu becomes covered by nearly anhydrous material over time. After a long period of space exposure, dehydration by dehydroxylation of the phyllosilicates proceeds below both the smooth layers and the frothy ones, as shown in Fig. 4. We measured the chemical compositions of the frothy and smooth layers and the underlying phyllosilicate-rich matrix from the surface of grains to ~1.5 µm below the surface, demonstrating that the effects of dehydration by dehydroxylation extend to at least that depth in space-weathered Ryugu grains. Note that the natural overturn, or gardening, of regolith grains on the asteroid parent body interrupt the schematic history of space weathering so that the space-weathering processes on any one grain do not necessarily progress as shown in Fig. 5.

Fig. 5: A conceptual illustration showing the development of two types of space weathering with dehydration by dehydroxylation observed on a Ryugu grain.
figure 5

Once a surface of a Ryugu grain is exposed to interplanetary space, the effects of solar wind irradiation start to accumulate at and near the surface, which is shown as hatched areas labelled as the ‘Solar wind implanted zone’ in the figure. As time passes, the gradual accumulation of solar wind radiation damage and phyllosilicate dehydroxylation form the smooth layer on its surface with a thickness that seldom exceeds ~100 nm. In contrast, the formation of impact melts (frothy layer, cratering and melt splash) is an intermittent process. In this conceptual illustration, partial coverage by impact melt occurred twice at times I and II. The change of colour from light blue via orange to yellow represents the progress of dehydration. As shown in Fig. 4, the impact melts are almost anhydrous. Therefore, the surface of the model grain is covered by both nearly anhydrous impact melts and dehydroxylated amorphized phyllosilicates. As a result, the surface of the asteroid Ryugu becomes covered by anhydrous material over time. After a long period of space exposure, dehydration by dehydroxylation of the phyllosilicates proceeds below both the smooth layers and the frothy ones. Note that the natural overturn, or gardening, of regolith grains on the asteroid parent body interrupt the schematic history of space weathering so that the space-weathering processes on any one grain do not necessarily progress as shown in Fig. 5.

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Strong absorption in the 3 µm region of the reflectance spectra1,2 is attributed to phyllosilicates and other –OH-rich minerals as well as H2O ice. Among these materials, the 2.7 µm band is ascribed to –OH1,2. Dehydroxylation from phyllosilicates may weaken the band. Spacecraft-based measurements of the reflectance spectra obtained at the artificial crater at TD2 on Ryugu normalized to a surface standard spectrum show a 2.7 µm band depth inside the crater that is ~5% stronger than that of the standard spectrum and those from outside the crater46. These observations suggest that more –OH is preserved in the subsurface material that was exposed by the formation of the artificial crater. Additional factors such as grain size, porosity and viewing geometry can also affect reflectance spectra47,48,49. Considering all these factors together, we hypothesize that the 2.7 µm band features are potential indicators of the degree of space weathering for C-type asteroids.

In addition to the band depth, band shift could be an important indicator of the degree of space weathering, but a definitive interpretation of shift is so far elusive, with different experiments indicating different shift directions. Measurements of the Ryugu artificial crater showed a small shift, but it is difficult to link this to measurements at the scale of individual particles. Several experiments have tried to simulate space weathering induced by solar wind irradiation or micrometeoroid impact to assess impact on spectral features15,16,17,18,19,20,48,49,50. However, none yet fully satisfy realistic space-weathering conditions and include coordinated near-infrared spectroscopy, scanning electron microscopy and transmission electron microscopy on the irradiated samples. Further studies are greatly needed to accurately reproduce the features of space weathering of Ryugu samples.

In the case of Ryugu, strong thermal alteration on the parent body23,46 can be excluded as the cause for surface dehydration because phyllosilicates in nearly all Ryugu samples preserve their structural –OH27,28. Therefore, either space weathering, solar radiative heating or both could have caused these differences22. If the surface material of Ryugu was heated by solar radiation23, all the grains and boulders from the surface to 10 to 100 cm deep must have experienced heating and dehydration throughout their interiors23. However, no heavily heated grains were found in this and other studies28,29,30,31,44. Therefore, solar radiation heating is unlikely to have caused the decrease of the 2.7 µm feature. As described previously, space-weathered Ryugu grains have an almost completely dehydrated surface and an interior with no evidence of thermal metamorphism. If the surface material contains a higher number of space-weathered grains than the subsurface material, as expected based on regolith gardening processes, the spectral differences before and after crater formation can be explained by different amounts of space-weathered grains in the surface and subsurface material of Ryugu. Therefore, it is likely that space weathering played an important role in the dehydration of Ryugu’s surface.

We note that ~40% of C-type asteroids do not show the 2.7 µm band features (sometimes generally referred to as the 3 µm band) and several interpretations were proposed for their origins51,52,53. Based on our data from Ryugu grains, we propose that the absence of the 2.7 µm absorption band can be at least partly explained by surface dehydration due to space weathering. Gradual covering by anhydrous amorphous silicate with longer exposure to space weathering was proposed by a radiative transfer study of Bennu20. The clear 2.7 µm feature on carbonaceous asteroid Bennu54 explored by NASA’s OSIRIS-REx spacecraft may be related to weaker space weathering experienced by Bennu than Ryugu or due to differences in phyllosilicate species and their chemical compositions. Suppression of the ‘water band’ by space weathering of C-type asteroid surfaces has implications for interpreting remote spectra, the first and least expensive tool for identifying water resources for eventual in situ resource utilization in space. Asteroids that appear dry on the surface may be water-rich, potentially requiring revision of our understanding of the abundances of asteroid types and the formation history of the asteroid belt.

Methods

Sample transfer and preparation for analyses

To preserve the pristine nature of the returned samples, the samples were prepared and analysed without cleaning, washing or other procedures that could introduce terrestrial contamination. The sample catcher has three separate chambers to store samples collected in the different locations on Ryugu. Chambers A and C contain samples collected from the first and second touchdown sites (TD1 and TD2), respectively. Air-tight sample transfer holders55 were used to bring samples from JAXA to Kyoto University. The allocated grains were handled in an N2-filled glove box at Kyoto University. Ryugu grains from both chambers were attached to Au plates on pin stubs with small amounts of epoxy glue. About 250 grains and about 40 thin foil sections prepared by FIB were investigated at the 21 hub universities and laboratories.

FIB–scanning electron microscopy

Observation and sample preparation at Kyoto University are as follows. Surface morphology of about 300 grains was observed by a JEOL JSM-7001F FE–SEM. We observed them 15 pA current and 2 kV acceleration voltage. FIB sections were prepared using a Thermo Fisher Helios FIB–SEM. Selected areas were cut out with a 30 kV Ga+ ion beam. Before the extraction, the target areas were Pt-C coated by a 2 kV electron beam. Then, Pt-C was deposited on the target areas by 16 or 30 kV Ga+ ion beams. The sections mounted on the TEM grids were thinned to a thickness of 50 to 200 nm on the 12 or 16 kV Ga+ ion beams. The damaged layers were removed using a 2 kV Ga+ ion beam. About 70 FIB sections were prepared and investigated by the team. In parallel with the above work, we also performed FIB and (scanning) transmission electron microscopy under air-free conditions using an air-tight FIB–SEM sample transfer holder and a double tilt LN2 Atmos Defend Holder (Mel-Build Corporation) at Kyushu University. Another air-tight sample holder was used to transfer the samples. An Ar-filled glove box was used for sample handling. A Thermo Fisher Scios FIB–SEM was used for the observation of about 500 grains and for FIB processing of space-weathered grains. The conditions of the FIB psrocessing are similar to those at Kyoto University.

(Scanning) transmission electron microscopy

At Kyoto University, a JEOL JEM 2100 F (scanning) transmission electron microscope ((S)TEM) operating at 200 kV equipped with a JEOL JED-2300T EDS was used. The ζ-factor method56 was used for quantitative analysis. Electron diffraction maps with quasi-parallel illumination were acquired using a Gatan Orius200D camera. At Kyushu University, a monochromatized and Cs aberration-corrected Thermo Fisher Titan Cubed G2 operating at 300 kV, equipped with four-quadrant windowless super-X silicon drift detector EDS and Gatan Quantum 965 image filter (GIF) for EELS was used. The probe current was less than 200 pA for TEM observation and ~60 pA for STEM observation as well as energy dispersive X-ray spectroscopy and EELS. The typical energy resolution of this EELS analysis was 0.4 eV. The energy dispersion was 0.1 eV per channel at the camera for EELS, which was calibrated by using standard samples of fayalite (Fe2SiO4) and synthetic Co olivine (Co2SiO4). EELS mapping was conducted using ~6 nm × ~6 nm square pixels and the acquisition time per pixel was 100 ms. The obtained EELS spectra were averaged over several hundred pixels (~10,000 nm2) to improve the signal-to-noise ratio. The Fe3+/ΣFe ratio was quantified by the Fe L3 peak as follows. The background was first subtracted using a power-law fit over an energy range of 20 eV, then the spectrum was decomposed to Fe2+ and Fe3+ components by multiple linear least-squares fitting based on two standard spectra obtained from fayalite (Fe2SiO4) and Fe2O3 in the energy range corresponding to the Fe L3 peak (705–715 eV). The Fe3+/ΣFe ratio was calculated from the ratio of the integrated spectral intensity of the Fe3+ component to the sum of the integrated spectral intensity of both components. The EDS acquisition time per pixel was 10 µs. For quantitative analysis, Cliff–Lorimer correction was used. K-factors were determined using many mineral standards. At Tohoku University, a JEOL JEM-2100F (S)TEM operating at 200 kV was used. K-factor correction was based on several mineral standards. At the University of Hawai’i at Mānoa, a monochromatized and Cs aberration-corrected Thermo Fisher Titan G2 (S)TEM operating at 300 kV, equipped with EDAX® thin-window EDS and Gatan Tridium EELS was used. A ‘TitanX’ ChemiSTEM and a Thermo Fisher Titan G2 STEM were also used for additional analysis at the Molecular Foundry, Lawrence Berkeley National Laboratory. At Université de Lille, a monochromatized and Cs aberration-corrected Thermo Fisher Titan Themis (S)TEM operating at 300 kV, equipped with four-quadrant windowless super-X SDD and Gatan Quantum 966 ERS GIF for EELS was used. For quantitative analysis, Cliff–Lorimer correction was used. K-factor correction was based on several mineral standards. At the University of Arizona, a Cs aberration-corrected Hitachi HF5000 (S)TEM, equipped with an Oxford Instruments X-Max 100 TLE EDS system and a Gatan GIF Quantum ER (model 965) electron energy-loss spectroscope was used. The microscope was operated at 200 kV using a 100 pm probe. The energy dispersion was set to 0.25 eV per channel. Maps were acquired by averaging three frames with a relatively large pixel time of 0.2 s for core loss and 0.001 s for the low loss. To quantify the Fe3+/Fe2+ ratio, FeO, Fe2O3 and Fe3O4 were used as standards. The samples were measured with a dispersion of 0.25 eV per channel. The background was subtracted using a power-law fit over an energy range of 100 eV. Then, a linear fit was applied in the pre-edge region for the removal of any residuals to ensure a null background intensity. After the background removal, the continuum intensity beneath the edge was subtracted, using a double arctan function57. The Fe L3a and Fe L3b peak maxima of FeO and Fe2O3 were shifted to 708.7 eV and 710.25 eV, respectively, by systematically applying an offset of 3.19 eV to match the energies described in the literature58. The spectrum images were acquired over an area measuring 76 × 20 pixels and 2.83 × 0.75 µm. The Fe L2,3-edge was quantified using the methods described in the literature58,59. The obtained calibration curve was applied to each pixel of the spectrum image to determine the Fe3+/ΣFe. At the Naval Research Laboratory, the Nion UltraSTEM200-X equipped with a Gatan Enfinium ER electron energy-loss spectroscope and a windowless Bruker SDD EDS was used. Bright-field TEM images were collected on a JEOL2200FS TEM, equipped with a Gatan OneView camera. Quantification of STEM-EDS data was performed with the Cliff–Lorimer method. The EELS measurements have a typical energy resolution of 0.5 eV.

Scanning transmission X-ray microscopy

The HERMES scanning transmission X-ray microscopy (STXM) beamline at the synchrotron SOLEIL was used. Analytical conditions for X-ray absorption near-edge structure (XANES) analysis using STXM is as follows. Energy calibration was done using the 3p Rydberg peak of gaseous CO2 at 294.96 eV as well as an internal haematite standard. The microscope operates under a high vacuum at 10–5 mbar. Stacks of images are collected at the Fe L2,3-edge in the energy range 680–720 eV, with an energy increment of 0.15 eV in the spectral range of the two main peaks associated with Fe3+ and Fe2+ absorption. The dwell time per pixel was fixed to 1 ms. The beam is focused onto the sample using a Fresnel zone plate of 25 nm. We selected pixel sizes of ~40 nm. The hyperspectral dataset was extracted and processed using the Hyperspy Python-based package60. The Fe3+/ΣFe ratio was quantified at each pixel58,59. The background is first subtracted, then a double arctangent is fitted and subtracted to take into account the iron content variation. Then the spectrum is integrated over the energy range corresponding to the Fe3+ absorption (708.8–712 eV) and the retrieved value is divided by the spectrum integrated over the full energy range (705–712 eV). This ratio is converted into the Fe3+/ΣFe ratio using calibration curves obtained on silicate standards. Ultimately, the component map was created by a linear least-squares fitting of the hyperspectral dataset, using component end-member spectra as inputs (oxidized silicates, melted silicate and sulfides).

Synchrotron Fe K X-ray absorption spectroscopy

The I 14 X-ray Nanoprobe Beamline at Diamond Light Source, UK was used to achieve X-ray absorption spectroscopy mapping and X-ray fluorescence maps were obtained, each map measured at varied energies ranging from 7,050 to 7,350 eV with a higher energy resolution range over the XANES features (~7,100–7,150 eV). The XANES maps were processed using Mantis v.2.3.02 (ref. 61) and isolated spectra normalized in Athena v.0.8.056 (ref. 62).

Microcrater measurement

Forty impact craters ranging from 0.5 to 8.5 µm in average diameters on a millimetre-sized grain (A0067) were measured using a FIB–SEM at Kyoto University. The surface area of 3.6 × 105 µm2 was observed. The cumulative impactor flux F(m) was calculated as F(m) = N(m)/ST, where m is the mass of the impactor, N(m) is the cumulative distribution of the impactor, S = 3.6 × 105 µm2 is the total investigated area of the Ryugu grain and T is the exposure time needed for craters to accumulate in the space environment. The mass of the impactor (m) is calculated from the diameter of the craters, assuming that an impactor is a spherical object having a density of 3 g cm−3 and that the ratio of the crater diameter D to the impactor diameter d (D/d) is assumed to be 1.60 based on laboratory impact experiments63,64. For comparison, interplanetary meteoroid flux is calculated using the models in the literature4,64,65.

Microporosity measurements using X-ray nanotomography

Microporosity was estimated from the results of scanning imaging X-ray microscopy66 performed at SPring-8 BL47XU on Ryugu regolith samples (~15–80 µm; 27 particles). The samples were mounted on Ti needles using a FIB–SEM at Kyoto University, and differential phase images were obtained by scanning X-rays at 8 keV. Then 180° images were obtained every 0.4–1.2°, followed by phase recovery and tomographic reconstruction to obtain three-dimensional images of phase contrast with refractive index decrements (RIDs). The pixel size was ~100 × 100 × 100–200 nm. The grain surfaces were defined by automatic segmentation67 and manual modification from the three-dimensional images, and their average RIDs were obtained. The RIDs (δ) are approximately proportional to the material density (ρ), as expressed by the following equation68: δ = aρb, where a = 3.7174, b = 0.87132. In this manner, the average RIDs were converted to the material density. Microporosity is the ratio of bulk density to particle density subtracted from 1. Since the particle density of the Ryugu has not been measured, the grain density of the Orgueil meteorite (2.42 × 103 kg m−3)69 was used to estimate the microporosity.

Helium irradiation experiments

The irradiation experiments were performed at ISAS/JAXA. Irradiation experiments of 4 keV He ions onto a Ryugu grain C0107–HE01 (~300 × ~200 µm) were performed in a vacuum. The grains were fixed on a gold substrate with a small amount of epoxy glue. The ion flux was kept at ~1.5 × 1013 ions cm–2 s and the total dose of the irradiated ions was 1.3 × 1018 ions cm–2. The surface textures before and after irradiation were observed with a JEOL JSM-7000F FE–SEM at the University of Tokyo without carbon deposition under low acceleration voltage and low current conditions (2 kV and 50 pA). FIB thin foil preparation and (S)TEM observation were performed at Kyoto University.