Abundant microchondrules in 162173 Ryugu suggest a turbulent origin for primitive asteroids

Domain: earth_science
Published: 2025-07-23
Editorial outcome: minor_revision

Abstract
Chondrules are a characteristic feature of primitive Solar System materials and are common in all primitive meteorites except the CI-chondrites. They are thought to form owing to melting of solid dust aggregates by energetic processing within the solar nebula and thus record fundamental processes within protoplanetary disks. We report the discovery of abundant altered microchondrules (>350 ppm) with modal sizes of 6–8 µm within sample A0180 from C-type asteroid Ryugu. These microchondrules have similar log-normal size and shape distributions to normal-sized chondrules, implying evolution by similar size-sorting. We suggest here formation of microchondrules in an outer Solar System chondrule factory, located in the Jovian pressure-bump, followed by turbulent diffusion and concentration relative to chondrules by intense turbulence. Meridional flows could have also separated microchondrules from chondrules and deliver them sunwards of the pressure bump via Lindblad torque flows. Contrary to conventional wisdom we thus propose that the concentration of fine-grained, unprocessed grains could mean the most primitive asteroids did not have to form at the largest heliocentric distances.

Introduction
Chondrules are ubiquitous in primitive meteorites. In non-carbonaceous (NC) chondrites such as ordinary and enstatite chondrites, chondrules dominate and have abundances of ~85 vol% 1. In contrast, in carbonaceous chondrites (CCs) fine-grained matrix is abundant, with chondrules comprising <60 vol% of materials 1. In general, the closer the composition of the chondrite to solar, the fewer and smaller the chondrules it contains, with the CM2 chondrites containing less than 20 vol% chondrules 2, and the CI chondrites, which have the most primitive compositions, having no mm-sized igneous objects 1,3. Given that models suggest the formation of NC and CC chondrites as two discrete populations separated by the gap in the nebula generated by the early growth of Jupiter 4, the size and abundance of chondrules thus broadly decreases with heliocentric distance in the early Solar System from chondrule-rich NCs to chondrule-poor CCs.

Samples of carbonaceous asteroid 162173 Ryugu, returned by Japan Aerospace Exploration Agency’s Hayabusa 2 mission 5–8, have mineralogies, compositions and textures closely related to CI chondrites and thus have experienced significant aqueous alteration in the early Solar System 9–12. Hydration occurred owing to the melting of ice by internal heating due to the decay of short-lived radio-isotopes. Like CI chondrites, no mm-sized chondrules have been reported from Ryugu, even though remote sensing suggested these objects might be present 8. Their absence might suggest they were obscured by aqueous alteration, or were not accreted into its progenitor 2.

Small chondrule-like objects 10–20 µm in diameter have been identified in Ryugu samples, but have very low abundances of <20 ppm 13. These objects are dominated by magnesian olivine similar to Type I chondrules, whilst others have affinities to amoeboid olivine aggregates (AOAs). Their oxygen isotopes lie on the carbonaceous chondrite anhydrous mineral line (CCAM) similar to chondrules from chondrites 14. The occurrence of similar-sized chondrule fragments in STARDUST samples from comet Wild-2 15 and cluster IDPs 16, thought to be derived from comets, led Nakashima et al. 13, to suggest Ryugu chondrules represent early-formed chondrules that underwent outward radial transport to the formation region of Ryugu in the outer Solar System.

Results
Sub-spherical objects (SSOs) 5.4–30.2 µm in diameter were observed in sample A0180 and consist of phyllosilicate decorated with a rim of Ni-bearing sulphides (Fig. 1). Sixty one SSOs were observed in a series of polished sections of A0180 (Supplementary Data Fig. 1–3), whilst 108 were observed by Nano-XCT (Figs. 2, 3), where they were identified owing to the contrast provided by their sulphide rims.

In cross-section, all but one of the SSOs are characterised by near-complete rims of iron sulphide crystals surrounding hydrated silicates. The sulphides are too small for accurate quantitative microanalysis (<3 µm); however, elemental mapping indicates they are Ni-bearing, Fe-dominated sulphides similar to the larger pyrrhotite crystals present within the matrix sample (Fig. 1a–f). Most commonly (55% of SSOs) rims consist of tightly interlocking polycrystalline sulphide (Fig. 1g), with all but one of the remainders comprised of separated euhedral sulphide crystals up to 1.5 µm in diameter. Some of these exhibit tabular habits similar to sulphides in the matrix (Fig. 1a). Sulphide crystals can extend outwards into matrix (Fig. 1a, u) or inwards into the silicate core (Supplementary Fig. 1). The sub-spherical shapes of 11 SSOs are abruptly terminated with sulphide rims truncated, suggesting they are fragments of originally sub-spherical objects (Fig. 1s). The final SSO (C-03) lacks a discrete sulphide rim and instead has five sub-spherical Ni-rich sulphide grains located on the periphery of the object (Fig. 1m–r).

The interiors of the SSOs vary in texture and mineralogy. Thirty percent of SSOs have core materials indistinguishable from matrix in texture comprising porous, sub-micron phyllosilicates (Fig. 1g), with three examples containing micron-sized sheet-like crystals of phyllosilicate (Fig. 1t). The majority of SSOs have homogeneous cores of ferromagnesian silicate materials (Fig. 1a, m, s, u), sometimes with sub-micron inclusions of iron oxides (Fig. 1a). The presence of interior or circumference fractures suggests these are hydrated silicates that underwent volume change during hydration. Veins containing sub-micron iron-oxides occur within SSO C-41. One SSO (C-44) has a texture suggesting its core contains porphyritic anhydrous silicates in a mesostasis replaced by porous phyllosilicate (Fig. 1t).

Elemental mapping of SSOs C-01, C-02 and C-03 indicate their silicate cores have more homogeneous Mg and Si contents than the surrounding fine-grained matrix, whilst the interior of C-03 has areas depleted in Na compared to matrix (Fig. 1). Where large enough to analyse, the cores are broadly chondritic in composition (Supplementary Table 1).

The SSOs in A0180 can be identified in Nano-XCT data by the high X-ray attenuation of their sulphide rims compared to the surrounding matrix and their silicate interiors. The SSOs with sulphide rims have concentric structures in XCT slices allowing them to be segmented. Reconstructed 3D models of the objects confirm that sulphide rims form near complete concentric shells (Fig. 3b). Two further examples where sulphides form a hemisphere, were also identified. One SSO also feature two connected lobes, suggesting it has a compound structure (Fig. 2c). These three atypical examples were excluded from the size distribution measurements.

The 3D reconstruction of segmented SSOs suggests a modal mean diameter of 6–8 µm with a range from 5.4–30.2 µm (Fig. 4). The size distribution is broadly log-normal, with objects larger than the mode comprising 70% of the population. The SSOs have a small range of sphericity of 0.5–0.67 with a minor decrease with increasing diameter (Supplementary Fig. 4). Irregular step features on the reconstructed SSOs, owing to uncertainty in segmentation, results in some spread to lower values of sphericity, in particular for the smaller objects. The elongation and flatness of the SSOs indicates most are equant (88%) with nearly equal numbers of prolate (6.5%) and oblate (5.5%) shapes (Supplementary Fig. 4). The total volume fraction of the SSOs determined by XCT is 350 ppm. A total of 61 SSOs, however, were discovered during serial polishing of 7 vol% of the sample with a size range of 2–15 µm in diameter (Fig. 4) and a mode of 6–8 µm similar to the XCT size distribution, even though more smaller SSOs were observed. The abundance of SSOs determined by XCT is thus likely to be a minimum value.

Methods
Sample preparation
Sample A0180 is 1.592 × 0.756 × 0.985 mm in size and was captured in the Hayabusa 2 sample container chamber A from the surface of Ryugu (Supplementary Fig. 6). The sample was obtained from JAXA in a sealed container in a nitrogen atmosphere. The initial morphological inspection was conducted on the sample exterior through the container window, with the CLOXS digital optical microscope system on automated digital sample stages developed at JAXA/ISAS 48. The sample was decanted for Nano-XCT. During the mounting process the sample split into two along fractures (parts A and B). Sample B was mounted for observation.

After nano-XCT analysis the sample was embedded in a Specifix resin and polished under ethanediol with 0.1 µm aluminium oxide. The sample was stored in a desiccator prior to scanning electron microscope (SEM) analysis and in a sealed sample container after analysis. The polished block was carbon-coated prior to SEM analysis. The sample was then serially polished 37 times to expose new planes of section at separations of up to 1 µm. The depth of the polish was determined from the dimensions of spheroidal magnetite by assuming these were spherical and has an uncertainty estimated at 30%. Plucking out of SSOs was the main issue encountered during serial polishing, however, the presence of a cavity from which the SSO was removed can be used to distinguish objects that were hemispherical.

Nano-XCT analyses
The Ryugu sub-samples (A/B) were mounted in pipette tips and scanned using a Zeiss Versa X-ray micro-computed tomography scanner. X-rays were generated from a tungsten source, with a voltage of 90 kV and a current of 89 µA, using the inbuilt LE4 filter to reduce beam hardening effects. For each scan, 2401 projections were collected across a 360° rotation. Each projection was magnified by a 4x objective lens and recorded using a 2000 × 2000 CCD plane (16-bit pixel depth) at exposure times of 33 s and 28 s for A0180-A and A0180-B respectively, with no binning. Spatial resolutions (in voxels) were 0.625 µm for A-0180A and 0.672 µm for A-0180B. Image stacks were created using the Zeiss Reconstructor software.

CT segmentation and data processing. Nano-XCT data were segmented using Image/J and Avizo in order to generate 3D models of specific textural and mineralogical features. The plugin 3D Suite 49 was used to measure sizes from pre-segmented images. The segmentation techniques used varied depending on the feature analysed owing to X-ray attenuation contrast between phases. Initial manual segmentation was performed to mask the exterior of the sample to ensure noise outside the sample was not selected. This step greatly simplified 3D model reconstruction. An initial threshold segmentation was performed on the masked data to determine the volume of the particle, giving a value of 0.77 mm3.

Microchondrules were segmented by virtue of their sulphide rims, which form partial shells of high X-ray attenuation surrounding sub-rounded regions of low X-ray attenuation consistent with silicates. Segmentation by thresholding was used to generate detailed 3D models of larger microchondrules as shown in Fig. 4b. These models confirm that sulphides form partial to complete shells surrounding silicate cores. Thresholding, however, could not be used to evaluate the size and shape distribution of microchondrules since this required interpolation in examples with partial rims and inclusion of the central silicate core in the volume. Manual segmentation was used to delimit the exterior of microchondrules for size and shape factor measurements. The uncertainty of manual segmentation was determined by repeat measurement of a cavity in the sample, with ten manual measurements performed and suggests an uncertainty at 2σ of ~10% for diameter (Supplementary Table 2).

Scanning electron microscopy
Scanning electron microscopy was performed on a polished block using a Hitachi TM4000 Pro Desktop SEM at Imperial College London. Backscattered electron images were collected at an accelerating voltage of 15 kV and have resolutions of up to 7 nm per pixel with some variation owing to working distance. Semi-quantitative analyses were obtained by energy dispersive spectrometry (EDS) using an Oxford Instruments AZtec series, silicon drift detector (SDD) with an energy resolution [full width at half maximim (FWGM)] of 151 eV (Cu-Ka). Analyses were performed at an accelerating voltage of 20 kV using a beam current of 2 nA. No gain calibration was applied and thus analytical totals are not quantitative. Oxford instruments matrix corrections were used against factory standards. Uncertainty in elemental ratios was evaluated by analyses of San Carlo olivine and are <5% for major elements (Mg, Al, Fe, Si) and <20% for minor elements (Ca, Mn, Ni). Detection limits are ~0.1 wt% on carbon-coated, polished sections.

Qualitative elemental mapping was carried out at high spatial resolution using a FEI Quanta 650 field emission SEM equipped with a Bruker Quantax EDS system, Esprit 2.5 software, and an annular, four-channel XFlash FlatQUAD SDD (FWHM Mn Kα: 124 eV). The annular SDD is inserted between the pole piece and sample within the chamber of the SEM. The four integrated 15 mm2 SDD segments are arranged in radial quadrants around a hole through which the electron beam passes. This geometry allows data collection samples with substantial surface topography. To limit the interaction volume of emitted X-rays and enhance the spatial resolution for elemental analysis to the sub-micrometre scale, a low accelerating voltage of 6 kV has been applied. Hyperspectral imaging datasets that provide complete spectra for each pixel of the SEM image were acquired at pixel sizes ranging from 13 to 15 nm. The element distribution has been displayed as net intensity elemental maps. Here, the Bremsstrahlung background has been removed and the X-ray line families with overlapping peaks (e.g., Fe-L and Ni-L) were deconvolved using stored line profiles.

Major and minor element compositions were determined using a JEOL 8530 FFE an electron probe microanalyzer (EPMA) equipped with five tuneable wavelength dispersive spectrometers hosted at the Natural History Museum in London. Operating conditions were 40 degrees take-off angle, acceleration voltage of 15 kV, a beam current of 20 nA and focused beam diameter. Elements were acquired using analysing crystals LIFL for Fe kα, Mn kα, Ni kα, Co kα and Cr kα, PETH for K kα, Ca kα, Ti kα and P kα, and TAPL for Na kα, Al kα, Si kα and Mg kα. The standards were orthoclase for K kα, wollastonite for Ca kα, rutile for Ti kα, Sc-phosphate for P kα, Mn-titanate for Mn kα, fayalite for Fe kα and Si kα, bunsenite for Ni kα, cobalt metal for Co kα, eskoalite for Cr kα, jadeite for Na kα, corundum for Al kα, forsterite for Mg kα. The on-peak counting times were 10 s for K kα and Na kα, 20 s for Ca kα, and 30 s for the rest of element peaks. Sodium and potassium were analysed first to minimise alkali migration during analyses. Background intensities were acquired before and after each peak position, with counting times equal to half of the respective on-peak’s acquisition time. Unknown and standard intensities were corrected for dead time using the normal (traditional single-term) correction method. Interference corrections were applied to Fe for interference by Mn, Mn for interference by Cr, and Co for interference by Fe. Oxygen was calculated by cation stoichiometry and included in the matrix correction. The matrix correction method was Phi-Rho-Z method, utilising the simplified Puchou-Pichor (XPP) algorithm. Mass absorption coefficients were taken from the FFAST Chantler dataset.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "See the attached file.", "reviewer_2": "What are the noteworthy results?\nYes, this study focused on samples returned from a spacecraft mission that sampled asteroid 162173 Ryugu.\n- Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work\nis not original, please provide relevant references.\nThe work is original, as the first observations and measurements were made from return samples from a primitive asteroid.\nThe discovery of abundant micro-chondrules is interesting for a broad community and understanding of the early Solar\nSystem history. The authors make the case for a higher turbulence as traditional models propose.\n- Does the work support the conclusions and claims, or is additional evidence needed?\nA discussion of the alteration process is needed for the non-specialist. Currently, the manuscript, while clear and concise,\ndoes not intrude on the goals of the study or the scientific question or hypothesis to test.\n- Are there any flaws in the data analysis, interpretation and conclusions? - Do these prohibit publication or require revision?\nI did not find the supplementary materials with the methods; it may have been something I missed, but I was not able to find\nthem in the documents available to me, and without these materials, it is not possible for me to answer this question.\n- Is the methodology sound? Does the work meet the expected standards in your field?\nMy recommendation for the authors is to extract mineral information from the nano-CT, as done recently on Martian\nmeteorites.\nThe study will benefit from a numeral model that supports Figure 5. It is unclear how this was constructed without a\nnumerical way to test these hypotheses.\n- Is there enough detail provided in the methods for the work to be reproduced?\nI did not find the supplementary materials with the methods. If this was my flaw, I would request the editor to forward them to\nme, otherwise the authors must re-submit with complete methods.", "reviewer_3": "Please see attached document."}, "author_response": "Response to Reviews\nAbundant microchondrules in 162173 Ryugu suggests a\nturbulent alternative to the distant origins of primitive asteroids\nGenge et al.,\nGeneral Statement\nWe have improved the manuscript significantly thanks to the reviewer’s comments. We have\nadded a large amount of new data to support our previous, largely as extended data. There\nhave been relatively few changes to the main text. The major changes are:\n1) We have performed serial sectioning of the specimen giving 37 new planes of section in\nwhich we found 61 SSOs. This is a significant increase from the 5 on which initially presented.\nThese include some lovely examples, which greatly strengthens the evidence that these are\nmicrochondrules. In particular there are some great fractured microchondrules that\ndemonstrate these are preaccretional.\n2) The newly discovered 61 SSOs were found in 8% volume of the sample, compared to the\n108 observed by XCT. This allows us to specify that the 350 ppm identified in XCT is a\nminimum estimate. We’ve measured the sizes of SSOs in section and found a similar size\ndistribution to the XCT, indeed the new data includes many smaller SSOs that confirm the log-\nnormal size distribution. The mode of the size distributions is similar.\n3) We have included a couple of sentences on objects in A0180 that resemble SSOs but are\nnot consistent with microchondrules (we’ve called the sulphide-rimmed objects). This allows\nus give more rigorous criteria for the identification of microchondrules.\n3) We have updated the astrophysical aspects of the paper according to reviewer 2's helpful\n(if very complex) suggestions.\nBelow we respond to all reviewer comments providing our response and details of changes in\nitalics. For your convenience we provide line numbers for changes. I tried to upload a tracked\nchanges version of the manuscript, however, the PDF conversion stripped out all the\nannotations.\nReviewer #1 Daisuke Nakashima\nComment 1\nThe argument that the SSOs are microchondrules is based only on their spherical shapes,\nwhich is weak. Since the SSOs and surrounding matrix are both composed of phyllosilicate,\nso that there is no mineralogical difference between them. In case of chondrule-like objects in\nthe Ryugu samples (Nakashima et al. 2023), they consist of olivine and Fe-Ni metal and have\n16Orich and -poor isotope ratios. Even if altered, it shows a barred olivine-like texture, so that\nit was recognized as a chondrule (Nakamura T. et al. 2022 Science). If the authors observe\nless-altered clasts in the Ryugu samples, unaltered microchondrules like those\n(cryptocrystalline?) in ordinary and CM chondrites may be found, though such an object was\nnot found in the clasts in the Ryugu samples as far as I have observed (Nakashima et al. 2023\nHayabusa Symposium).\n- The evidence that SSOs are microchondrules are: 1) sub-spherical shapes, 2) silicate-\ndominated objects with minor sulphide, 3) they have broadly chondritic compositions in major\nelements, 4) sulphide-rims (similar to type I chondrules in some chondrites), 5)\nmicrochondrules have been observed in Ryugu in small numbers indicating their presence.\nFinally an abstract at MetSoc describes the least altered clast found so far in Ryugu and it\nincludes “chondrule-like” glassy spheres of ~10 µm in size (Matsumoto et al., 2024). I believe\nit is against Nature Coms policy to reference abstracts (?), however, it should give the\nreviewers more confidence on our identification of microchondrules. The much larger number\nof SSO examples we now give are hopefully much more convincing.\nDaisuke quite rightly points out that previously identified microchondrules from Ryugu have\neither porphyritic or barred olivine textures, even if altered, suggesting these were glassy.\nHowever, we only can observe microchondrules in XCT if they have sulphide-rims, these rarer\nporphyritic and barred microchondrules may well be present. In non-carbonaceous chondrites,\nin which glassy microchondrules are common. Thus homogeneous microchondrules formed\nby replacement of glass are entirely consistent with microchondrules, indeed those with barred\nand porphyritic textures are slightly anomalous since such objects should cool more rapidly.\nFinally, the one example of a microchondrule found in carbonaceous chondrites is an\naqueously altered homogeneous sphere - thus there is a precedent for identifying such objects\nas microchondrules.\nTo address Daisuke's comments I have added the following:\nLine 126 \"There is, therefore, excellent evidence that all these sub-spherical silicate objects\nSSOs reported here represent hydrated microchondrules since they are: 1) sub-spherical in\nshape, 2) have broadly chondritic compositions, 3) are similar to altered microchondrules in\nCM2 chondrites, and 4) microchondrules with barred textures have been reported in Ryugu.\nThe inferred glassy textures of SSO precursors in contrast are dissimilar to microchondrules\nreported from Ryugu [12,13], however, glassy microchondrules are common in chondritic\nmeteorites owing to the more rapid cooling of these small droplets [17-20].\"\nWe have also added EMPA analyses of the cores of microchondrules to the extended data to\ndemonstrate these objects are chondritic.\nFinally, I have updated the figure captions to describe objects as SSOs rather than\nmicrochondrules. None of the reviewers commented on this, however, it is bad practice to bias\nresults.\nComment 2\nThere are some observations suggesting that the Ryugu original parent body have formed\nfarther location from the Sun (farther than any other chondrite parent bodies and/or comet\nforming regions); carbonated water in a sulfide grain (Nakamura T. et al. 2022), 15N anomaly\n(Ito et al. 2022 Nat. Astron.), and Fe isotope anomaly along with Cr and Ti isotope anomalies\n(Hoppe et al. 2022 Sci. Adv.). Although the authors discussed about carbonated water in a\nsulfide grain, how about remaining two observations?\n- The evidence from N and Fe-Cr-Ti isotope data for the highly primitive nature of Ryugu can\nbe explained in the same way as the chemical composition. The fine-grained material is the\nleast processed and preserved the primitive components. I have added both these papers to\nthe penultimate paragraph as:\nLine 288 \" Previous studies of Ryugu have found higher δ18O, Δ17O, δ15N, ε54Cr [9,11] than CI\nchondrites suggesting Ryugu is more primitive, and that the Fe, Ti and Cr isotope systematics\nof Ryugu and CI chondrites are distinct from other carbonaceous chondrites, but similar to\neach other [46]. These observations have been interpreted to mean formation of Ryugu at\ngreater heliocentric distance of formation, based on the assumption that processing of presolar\nmaterials, the carriers of nucleosynthetic signatures, and volatile materials increases\nsunwards in the early Solar System.” – this is followed by our existing suggestion that fine-\ngrained materials, concentrated by turbulence, have experienced less processing.\nComment 3\n(Not a major comment) Accretion location of the Ryugu original parent body should be\nexplicitly indicated in Fig. 5.\n- Added\nReviewer #2 (Remarks to the Author):\nComment 1\nCurrently, the manuscript, while clear and concise, does not intrude on the goals of the study\nor the scientific question or hypothesis to test.\nI disagree, this is a rather rigid definition of the structure of a scientific paper, some do indeed\nhave a goal and a clear hypothesis, others, like this one, are serendipitous discoveries. I think\nit is clear from the start what this paper is about. Nature Communications is full of excellent\npapers with similar structure (\"we found this, isn't it significant\") so there is precedent.\nComment 2\nA discussion of the alteration process is needed for the non-specialist.\n- I don't think a lengthy discussion is necessary. I have added a sentence on the alteration\nprocess for non-specialists:\nLine 44 \"Hydration occurred owing to the melting of ice by internal heating due to the decay\nof short-lived radio-isotopes. \"\nComment 3\nAre the supplementary materials missing.\nI apologise, I realise that there were a couple of references to extended data, and none\nprovided. These were ancillary, merely showing the shape factors. I have now added them.\nWe have now had the opportunity to collect much more data and there is much more in the\nextended data than there was before. This should all add considerable weight to our\nidentification of microchondrules.\nComment 4\nMy recommendation for the authors is to extract mineral information from the nano-CT, as\ndone recently on Martian meteorites.\n- This is unnecessary, we have characterised the composition of phases through elemental\nmapping of microchondrules in section. X-ray attenuation gives non-unique identifications in\nparticular for micron-sized grains where resolution and edge effects makes attenuation\naffected by the surrounding matrix. EDS analyses are far superior.\nComment 5\nThe study will benefit from a numeral model that supports Figure 5. It is unclear how this was\nconstructed without a numerical way to test these hypotheses.\n- Apologies, I mean't to specify what this figure is based upon. The figure caption now ends\nwith:\n\"The figure is based on observations of a protoplanetary disk by Teague et al. [47].\"\nReviewer 3\nThe study is compelling, and I look forward to seeing it published.\nComment 1\nTurbulent concentration and size-sorting of fines of primitive carbonaceous materials may\ndemand high levels of turbulence, higher than expected at 15-30 AU, rather than large\nheliocentric distance.\" It would be useful at this point of the discussion to indicate actual α\nnumbers thought to be relevant for the inner and outer regions referenced.\nI don't think the introduction is the best place for this kind of detail. Later in the discussion we\ndo give alpha values. There is obviously no precise numbers to give, only estimates, but what\nI have added is:\nLine 228 \"whilst Desch et al [28] estimates values in the inner Solar System (<1 au) of α =\n4x10-3 with the outer Solar System (>10 AU) having α <10-5 with α falling as a power law in\nbetween.\"\nLater on in the discussion we already give values of alpha for the turbulent wake of Jupiter.\nComment 2\nPerhaps they might consider writing instead \"studies may have under-estimated their\nabundance.\"\nChanged, quite right.\nComment 3\nIn lines 184-186 the authors write that \"The smallest eddies in a turbulent system are defined\nby the Kolmogorov scale, at which kinetic energy is dissipated by viscosity into heat, and are\nthose with the lowest vorticity.\" If the eddy overtime is being identified with the inverse vorticity\n(which is reasonable) then the opposite is true in Kolmogorov turbulence. The vorticity of an\neddy of lengthscale ℓ in the inertial range of classic 3D isotropic turbulence goes like ωℓ ∼\n(L/ℓ)2/3. Maybe the authors intended to say that the overturn times shorten with decreasing\nlengthscale?\nApologies, this is a typo, I of course mean the highest vorticity.\nComment 4\nThe authors write on lines 186-187 that \"Particles with stopping times equivalent to the\noverturn time of the Kolmogorov scale eddy are preferentially concentrated by turbulence\n[24,25]\" The principle the authors are getting at are correct, but there have been several\nimportant advances in this picture since the publication of references [24, Leshin et al.] and\n[25, Cuzzi et al. 2006]. The key point is that preferential concentration does not necessarily\noccur at the Kolmogorov scale. Two recent studies, Hartlep et al. (2017) and Hartlep &\nCuzzi (2020), demonstrate that for a given level of turbulence, α, and a specific particle size, d\n(with Epstein regime stopping time ts ≡ ρs(d/2) ρgcs where ρs is the solid particle density, ρg is the\nambient gas density, cs is the local sound speed), there exists a length scale within the inertial\nrange of presumed 3D isotropic turbulence where enhanced clustering due to turbulent\nconcentration is expected. The preferred length scale, denoted as ℓclust, is identified at the\nscale where the turbulent Stokes number, Stℓ ≡ ωℓ(ℓclust)ts is ≈ 0.3.\nThis differs from the traditional expectation that the clustering scale should be the Kolmogorov\nscale, ℓη ∼ Re−3/4H, where H is the local gas scale height and Re is the molecular Reynolds\nnumber. Equation (1), which the authors use to estimate a preferred d (derived from Cuzzi et\nal. 2001), is based on the assumption that the clustering scale is ℓη, thereby determining the\ncorresponding particle size. In other words, the case where ℓclust = ℓη represents a special\nlimiting scenario in isotropic turbulence. Clustering does not have to happen on that scale.\nTherefore, I am concerned that the inferences regarding the implied particle size and/or high\nlevels of turbulence may be inaccurate if based solely on the application of Eq. (1): for any d\nthere exists a lengthscale ℓ = ℓclust in which clustering is maximal. The authors need to\nconfront this possibility or state clear caveats as with regards to the use of Eq. (1).\n- As a meteoriticist I am afraid I find Hartlep et al 2017 particularly inaccessible and difficult for\na non-specialist to follow. The argument seems to be that direct numerical simulations of\nparticle concentration in turbulence suggest particles of larger sizes are concentrated by\nturbulence than suggested by equation 1, specifically that equation 1 assumes concentration\nat a stokes number of 1, whilst Hartlep suggests 0.3. This result seems to be based on\noptimisation to produce scale invarience that is valid in the inertial range. It is unclear to me\nwhy this is necessarily the favoured solution. Whilst Desch et al. 2018 predicts the\nconcentration of chondrule-size particles, which matches observation, Hartlep et al., 2017\npredicts concentration of clusters of thousands of chondrules, meaning there is no process to\nconcentrate chondrules, which is contrary to observation - although admittedly this is\nsomewhat of a chicken and the egg argument. I would be much more comfortable if it were\nobjections based on experimental daa, rather than a choice between two models, both with\ninherent assumptions. This said, the reviewer has a valid point, there are two papers, by the\nsame author casting doubt on the precision of Eqn 1. I acknowledge this issue in the paper by\nsaying:\nLine 218 “Equation 1 is based on the assumption that the clustering scale is the Kolmogorov\nscale and is thus a special limiting scenario in isotropic turbulence that leads to concentration\nof chondrule-sized objects in the inner Solar System. Direct numerical simulations of particle\nclustering in turbulence in contrast suggests clustering of larger particles in turbulence [29].”\nComment 5\nIn the discussion containing Lines 199-210, it is difficult to gauge the relative coupling of the\nparticles to the gas. For the dynamicists it would be beneficial to write out an estimate for the\ncorresponding Stokes number, St ≡ tsΩK, where ΩK is the local Keplerian rotation frequency.\n- This is somewhat problematic since it is not as simple as it sounds. The stokes number\nmeasures the coupling between gas and dust and thus depends on gas surface density at any\nheliocentric distance and is influenced by collisions and fragmentation (Batygin et al., 2022).\nGiven these uncertainties I think it not useful to estimate a value subject to such uncertainty.\nAll this paper requires is the concept that with increasing turbulent intensity concentration of\nincreasingly small particles occurs. We've relied on a simple analytical equation in order to\ndispense with these complexities. I hope the reviewer agrees this is more than most\nmeteorites studies attempt. If there is a simple and rigorous way to specify the turbulent stokes\nnumber with heliocentric distance in the early solar system, without pages of exposition and\ncaveats, then please let the reviewer educate me and I will be more than happy to include\nthem.\nComment 6a\nIn the paragraph starting on line 238, the authors suggest that sunward of the inner edge,\nspiral wave shedding could trigger the Streaming Instability (SI). While this might be plausible,\nit is only likely if the turbulence is weak enough or if the particle Stokes numbers (see below)\nfall within the ideal range for SI to operate. In terms of turbulence, it has become increasingly\nevident in recent years that SI may not be triggered if the background turbulence is high and\nthe Stokes numbers are low (below 0.01). The authors should consider adding a cautionary\nnote in this regard by citing recent studies on the matter, such as Estrada & Umurhan (2023),\nChen & Lin (2020), and Umurhan et al. (2020). More importantly, the authors should note that\nSI can indeed be triggered when St = 0.1−1 (see Li & Youdin, 2021 and Estrada & Umurhan,\n2023), but this occurs only under the right turbulence conditions. Is the Stokes number of the\nd = 7μm particle mentioned in line 203 within this preferred range? What is the precise value?\nAgain this is a very complex question. Estrada & Umurhan (2023) model suggests turbulence\nat alpha of 10-3 to 10-4 in the early solar system precludes streaming instability in the first few\nmillion years, contrary to the evidence for early formed planetesimals by meteorites. Of course\nanother mechanism, such as pebble accretion, may be responsible for planetesimal growth,\nbut these are subject to some of the barriers to growth that are circumvented by streaming\ninstability. However, other papers such as Gerosa et al., (2023) show that radial shear in\nturbulent disks can favour anticyclones that concentrate particles into hierarchical clusters that\nmerge with time. This doesn't seem to be considered by Estrada & Umurhan (2023) and might\nchange their results. It seems the behaviour of particles in turbulent systems is very complex\nand small differences in assumptions can result in very different outcomes. As described\nabove turbulent stokes numbers are somewhat problematic.\nTo acknowledge these issues I have added:\nLine 277 \"although recent models caution that turbulence can also resist the formation of\ndense enough regions to cause instability [43].\"\nComment 6b\nIn lines 246-250, the authors also mention the possibility of very large turbulence, α ∼ 10−2,\ntriggering the SI. Specifically, I am referring to the statement in lines 249-250: \"Turbulence is\nnot only able to concentrate particles of the same size as microchondrules, it is likely to\nconcentrate them to the levels needed to trigger streaming instability.\" This has not yet been\ndemonstrated in any published numerical study of the SI (see the previously cited references).\nThe authors should either revise this statement or include qualifying language to indicate that\nthis remains an unverified hypothesis.\nI have changed \"likely\" to \"might\" as suggested.\nResponse to Reviewers Comments\nAbundant microchondrules in 162173 Ryugu suggests a turbulent\nalternative to the distant origins of primitive asteroids\nIn the following we respond to each of the reviewers comments and provide details of the\nchanges made in italics.\nReviewer 1\nThe authors addressed my comments by modifying the main text and adding more images\nand EMPA data, so that one of my two concerns is cleared. But another one remains.\nI recommend publication after addressing the concern and additional comments.\nComment 1\nThe argument that the Ryugu original parent body formed at ~3.2AU is not yet convincing.\nAn alternative scenario explaining the Ryugu isotope anomalies is quantitative and hard to\nimagine. Kawasaki et aal. (2022) found that the ratios of 16O-rich to 16O-poor minerals in\nRyugu and CI chondrites are higher than in other carbonaceous chondrites groups but are\nsimilar to that of comet 81P/Wild2 and suggested that Rugu and CI chondrites accreted in\nthe outer Solar System closer to the region of comets. How do you explain this observation\nin your scenario?\nResponse 1\nThis concern makes the assumption that the accreted parent bodies become increasingly\nprimitive with distance from the sun. In our paper we propose that the concentration of fine-\ngrained materials preferentially concentrates primitive materials that have escaped\nprocessing by virtue of their smaller grain size. This also clearly applies to 16O-rich minerals.\nWe already stated:\nLine 311 “An alternative scenario for the formation of Ryugu is the inward cycling and\nconcentration of fine-grained materials in or near the gap formed by Jupiter. The primitive\nnature of CI-like materials and the large abundance of presolar nuclides could be explained\nby their formation in regions that preferentially trap fine, largely unprocessed, primitive\ngrains, and expel processed larger grains such as chondrules, whilst retaining\nmicrochondrules. Thus, CI chondrites might not be fine-grained because they are primitive,\nbut they may be primitive because they are fine-grained.”\nTo address the reviewers comment we have now mentioned Kawasaki et al (2022):\nLine 301 “Furthermore, the ratio of 16O-rich/16O-poor minerals within Ryugu and CI\nchondrites is higher than other carbonaceous chondrites and similar to comet 81/P Wild-2\n[48].”\nLine 311 We have also added the words “The primitive chemical and isotopic nature of CI-\nlike” to the paragraph on our alternative scenario.\nSpecific comments\nL26 – Meridonal corrected to Meridional\n- Corrected\nL46 – objectsi corrected to objects\n- Corrected\nL46-47 Reference added to Weisberg et al 2006 Messi\n- Added\nL67 Commas added between …) and 5.4-…and ….diameter and were…\nI wouldn’t put comma’s there…the copy editor can decide\nL69 Period added between….Fig and 1-3…\nPeriod added.\nL78:What is 2-u?\nApologies it should read (Fig. 1a,u).\nL85: What does CC in Fig. 1t stand for?\nFigure caption 1. Definition now added: “(t) SSO C-44 exhibits a core with a facetted\nanhydrous silicate crystals set in a cryptocrystalline matrix (CC) consisting of phyllosilicate”\nL133-136: Add a reference(s), if any\nThere are no references these are our alternative explanations. I think the lack of references\nmakes this clear.\nL190-191: Is it possible to describe the lithologies where the SSOs are observed. Since the\nSSOS are all replaced by phyllosilicate, the lithologies may not be the least- and less-\naltered.\nLine 199. I have added “Sample A0180 lacks anhydrous silicates and is more already than\nmany Ryugu samples, however, it retains evidence for freeze-thaw processes and may have\nexperienced low-temperature alteration [23].” A more in-depth description of A0180 is given\nin the references.\nL229: Add a period between…in between and This…\nAdded\nL243: Analysis conditions of EMPA should be described.\nNow added to the methods\nReviewer 2\nNo further suggestions\nReviewer 3\n“Equation 1 is based on the assumption that the clustering scale is the Kolmogorov scale\nand is this a special limiting scenario in isotropic turbulence that leads to concentration of\nchondrule-sized objects in the inner Solar System. Direct numerical simulations of particle\nclustering in turbulence in contrast suggest clustering of larger particles in turbulence [29].”\nThis statement mischaracterizes the conclusions of Hartlep & Cuzzi (2020). Their work does\nnot claim that larger-sized particles are necessarily the ones preferentially concentrated. The\ncore idea the Turbulent Concentration mechanism is that the strongest intermittency in\nparticle accumulation occurs when the turbulent eddy Stokes number is approximately 0.3.\nThis condition can arise under a broad range of local parameters, including turbulent\nintensity, length-scale of interest, local disk mass, particle size and other factors that\ncollectively determine St_neta.\nThe point emphasized by Hartlep & Cussi (2020) is that while Desch et al (2020) assume a\npriori that maximal clustering occurs at the Kolmogorov scale, this is only a subset of the\nbroader conditions that can yield St_neta = 0.3. Consequently, the revised text should clarify\nthis point more accurately – and simply saying that HC20 predicts larger sized particles is\nincorrect.\nResponse 1\nSteve Desch has suggested the following changes. Firstly we acknowledge that we base our\nmodel on Desch et al. (2018) rather than HC20 and then give a little more detail:\nLine 208. “Particles with stopping times equivalent to the overturn time of the Kolmogorov\nscale eddy––i.e., those with Stokes numbers St ~1 relative to the Kolmogorov scale or St\n~10-4 relative to the largest-scale eddies––are preferentially concentrated by turbulence\n[24,25,26].”\nLine 214 “Recently [29] argued for a different cascade model that predicts different sizes of\nparticles concentrated in different sizes of eddies, and questioned whether particles with St <\n10-2 could be concentrated at all, due to disruption of clumps by ram pressure. Given that\nthe size of chondrule concentrated in asteroids appears universally uniform, and given the\nsuccess of disk models predicting the size of chondrules that would be concentrated [28], we\nwill continue to use the findings of Cuzzi et al. [26].”\nHowever, HC20 do state in their abstract “An important difference between these results and\nthose of Cuzzi et al. is that particle growth by sticking must proceed to a radius range of at\nleast one to a few centimeters for the IMF and meteoritical properties to be most plausibly\nsatisfied. That is, as far as the inner nebula goes, the predominant \"particles\" must be\naggregates of chondrules (or chondrule-size precursors) rather than individual chondrules\nthemselves.” This statement would seem to be consistent with our characterisation of that\npaper. To address this issue we modified the statement to:\nLine 228. “As mentioned above, direct numerical simulations of particle clustering in\nturbulence in contrast suggests clustering of larger particles in turbulence under the most\nlikely conditions for planetestimal formation [29].”\nThose interested should read HC20 and Desch et al 2018.\nComment 2\nPlease add turbulent stokes numbers\nResponse 2\nWe’ve now added the following for an estimate of the turbulent stokes number:\nLine 278. “The sunward inner edge of the Jovian pressure bump is an intriguing possible\nenvironment for the concentration of small particles. At 3.2 AU and 4 Myr, the model of [28]\npredicts Σ ≈ 8 g cm-2, T = 90 K, which for α ≈ 10-2 would predict d = 5 μm using Equation 1.\nThe Stokes numbers of such particles are St = (2π)1/2 ρs a / Σ ≈ 2 x 10-4.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "See the attached file.", "reviewer_2": "The authors made all the requested changes, and I think the paper should be accepted for publication. I have no further\nsuggestions.", "reviewer_3": "I appreciate the authors' efforts in addressing my concerns, and they have done so for the majority of my suggestions.\nHowever, I still have some remaining issues that I would like to ensure they fully grasp. Regarding their revision in response\nto my comment #5, they added the following sentence on line 218 of their manuscript:\n\"Equation 1 is based on the assumption that the clustering scale is the Kolmogorov scale and is thus a special limiting\nscenario in isotropic turbulence that leads to concentration of chondrule-sized objects in the inner Solar System. Direct\nnumerical simulations of particle clustering in turbulence in contrast suggest clustering of larger particles in turbulence [29].\"\nThis statement mischaracterizes the conclusions of Hartlep & Cuzzi (2020). Their work does not claim that larger-sized\nparticles are necessarily the ones preferentially concentrated. The core idea behind the Turbulent Concentration mechanism\nis that the strongest intermittency in particle accumulation occurs when the turbulent eddy Stokes number St_η s\napproximately 0.3 (as I noted in my previous review). This condition can arise under a broad range of local parameters,\nincluding turbulent intensity (α), length-scale of interest (λ) local disk mass (Σ), particle size (d), and other factors that\ncollectively determine St_η(α,λ,Σ,d,...).\nThe point emphasized by Hartlep & Cuzzi (2020) (though admittedly, their paper is rather dense in its exposition) is that\nwhile Desch et al. (2020) assume a priori that maximal clustering occurs at the Kolmogorov scale (L_kol) this is only a\nsubset of the broader conditions that can yield St_η = 0.3. Consequently, the revised sentence's implication that \"larger\nparticles\" are necessarily concentrated in the inertial range of turbulence is not an inherent outcome of turbulence itself.\nInstead, the key insight of Hartlep & Cuzzi (2020) is that for a given particle size, d, significant clustering can occur at scales\nλ that is much larger than L_kol. The revised text should clarify this point more accurately -- and simply saying that the HC20\nit predicts larger sized particles is incorrect.\nThe authors response to my comment #5 is insufficient. They can quite easily make an estimate for the more tranditional\nStokes number, St_K, as defined by the stopping time (ts = d*ρg/(cs*ρs); where ρs is the density of the individual solid dust\nparticle, and cs is the local sound speed) multiplied by the local Keplerian frequency (ΩK). They have the requisite model\nassumptions already in place, as ρg can be derived from Σ (i.e., the same input value range used in their Eq. 1), and cs from\nT. Providing a range of implied St_K values based on their findings should be a straightforward extension and at least\nmentioned in a sentence in the main text and then followed by a short exposition in the supplemental materials.\nThis is crucial because an estimated range of effective Stokes numbers is necessary for particle-gas turbulence modeling.\nUnderstanding the implied conditions for clumping based on observational data allows us to better test formation dynamics\nand compare theoretical predictions with actual data. This is why I ask this."}, "author_response": ""}]

Decision letter
Nature Communications
Manuscript Number: NCOMMS-24-55952
Title: Abundant microchondrules in 162173 Ryugu suggests a turbulent alternative to the distant
origins of primitive asteroids
Summary: In the present paper, sub-spherical objects (SSOs), i.e., microchondrules, are observed
in the Ryugu sample A0180 using Nano X-ray CT and on the polished surface of the sample using
SEM. The microchondrules (5.4 – 30.2 µm in size) consist of phyllosilicate and sulfide rims and
are more abundant (350 ppm) than chondrule-like objects found in Nakashima et al. (2023 Nat.
Commun.). The authors argue that the microchondrules have formed in the outer solar system
chondrule factory beyond the Jupiter orbit at around 3 au and were transported inward near the
Jupiter orbit, where the original parent body of Ryugu accreted. This means that the accretion
location of the Ryugu original parent body is closer to the Sun than expected in the previous studies.
General comment: The present paper provides new insights into the evolution of solar system
materials, especially the origin of microchondrules in solar system bodies beyond the Jupiter orbit
and deserves to be published in Nature Communications. However, I have two concerns about the
present paper, which are shown below. I recommend publication after addressing the concern and
additional comments.
1. The argument that the SSOs are microchondrules is based only on their spherical shapes, which
is weak. Since the SSOs and surrounding matrix are both composed of phyllosilicate, so that
there is no mineralogical difference between them. In case of chondrule-like objects in
the Ryugu samples (Nakashima et al. 2023), they consist of olivine and Fe-Ni metal and have 16O-
rich and -poor isotope ratios. Even if altered, it shows a barred olivine-like texture, so that it
was recognized as a chondrule (Nakamura T. et al. 2022 Science).
If the authors observe less-altered clasts in the Ryugu samples, unaltered microchondrules
like those (cryptocrystalline?) in ordinary and CM chondrites may be found, though such an
object was not found in the clasts in the Ryugu samples as far as I have observed (Nakashima
et al. 2023 Hayabusa Symposium).
2. There are some observations suggesting that the Ryugu original parent body have formed
farther location from the Sun (farther than any other chondrite parent bodies and/or comet
forming regions); carbonated water in a sulfide grain (Nakamura T. et al. 2022), 15N anomaly
(Ito et al. 2022 Nat. Astron.), and Fe isotope anomaly along with Cr and Ti isotope anomalies
(Hoppe et al. 2022 Sci. Adv.). Although the authors discussed about carbonated water in a
sulfide grain, how about remaining two observations?
3. (Not a major comment) Accretion location of the Ryugu original parent body should be
explicitly indicated in Fig. 5.
9/24/2024
Daisuke Nakashima
Review of “Abundant microchondrules in 162173 Ryugu
suggests a turbulent alternative to the distant origins of
primitive asteroids" by Genge et al., submitted to Nature
Communications
October 2024
The authors report the discovery of altered microchondrules, 6-8 µm in size, in the
A0180sampleobtainedfromtheC-typeasteroidRyugu. Theirsizeandshapedistri-
butions mirror those of regular chondrules, suggesting similar formation processes.
The authors go on to propose that these microchondrules formed in the outer Solar
System near a Jovian pressure bump and were later transported sunward – all occur-
ring under rigorous turbulent conditions. This finding challenges the notion that the
most primitive asteroids necessarily formed at the farthest heliocentric distances.
IwanttoclarifythatIamnotanexpertinthesampleanalysistechniquesextensively
discussed. I am operating under the assumption that these methods were executed
properly and that the findings are accurate. My comments will focus solely on the
nature of disk turbulence and the turbulence-related physical inferences presented
by the authors.
Thestudyiscompelling,andIlookforwardtoseeingitpublished. However,Ibelieve
there are significant issues with the turbulence estimates in their model, as well as
with the broader discussion on turbulence in disks. I strongly encourage a revision
that addresses these concerns, as doing so will only strengthen the manuscript and
better convey the importance of their findings.
All of the following in no particular order of importance
1. Lines 61-63, the authors write, "Turbulent concentration and size-sorting of
finesofprimitivecarbonaceousmaterialsmaydemandhighlevelsofturbulence,
higher than expected at 15-30 AU, rather than large heliocentric distance." It
would be useful at this point of the discussion to indicate actual α numbers
thought to be relevant for the inner and outer regions referenced.
2. Aminorwordingmatter, theauthorswriteinlines172-174that"However, only
five microchondrules were observed on the polished surface in sample A0180,
1
with the majority detected by Nano-XCT, thus previous studies have likely
under- estimated their abundance." The tone suggested here is that the authors
are certain that these are understimates, but they haven’t demonstrated that
this is the case either. Perhaps they might consider writing instead "sutdies
may have underestimated their abundance."
3. In lines 184-186 the authors write that "The smallest eddies in a turbulent
system are defined by the Kolmogorov scale, at which kinetic energy is dissi-
pated by viscosity into heat, and are those with the lowest vorticity." If the
eddy overtime is being identified with the inverse vorticity (which is reason-
able) then the opposite is true in Kolmogorov turbulence. The vorticity of an
eddy of lengthscale ℓ in the inertial range of classic 3D isotropic turbulence
goes like ω ∼ (L/ℓ)2/3. Maybe the authors intended to say that the overturn
ℓ
times shorten with decreasing lengthscale?
4. The authors write on lines 186-187 that "Particles with stopping times equiva-
lent to the overturn time of the Kolmogorov scale eddy are preferentially con-
centrated by turbulence [24,25]" The principle the authors are getting at are
correct, but there have been several important advances in this picture since
the publication of references [24, Leshin et al.] and [25, Cuzzi et al. 2006].
The key point is that preferential concentration does not necessarily
occur at the Kolmogorov scale. Two recent studies, Hartlep et al. (2017) and Hartlep &
Cuzzi (2020), demonstrate that for a given level of turbulence, α, and a specific
(cid:14)
particle size, d (with Epstein regime stopping time t ≡ ρ (d/2) ρ c where ρ
s s g s s
is the solid particle density, ρ is the ambient gas density, c is the local sound
g s
speed), there exists a length scale within the inertial range of presumed 3D
isotropic turbulence where enhanced clustering due to turbulent concentration
is expected. The preferred length scale, denoted as ℓ ,
clust
is identified at the
scale where the turbulent Stokes number, St ≡ ω (ℓ )t is ≈ 0.3.
ℓ ℓ clust s
This differs from the traditional expectation that the clustering scale should
be the Kolmogorov scale, ℓ ∼ Re−3/4H, where H is the local gas scale height
η
and Re is the molecular Reynolds number. Equation (1), which the authors
use to estimate a preferred d (derived from Cuzzi et al. 2001), is based on
the assumption that the clustering scale is ℓ ,
η
thereby determining the corre-
sponding particle size. In other words, the case where ℓ = ℓ represents a
clust η
special limiting scenario in isotropic turbulence. Clustering does not have
to happen on that scale.
Therefore, I am concerned that the inferences regarding the implied particle
size and/or high levels of turbulence may be inaccurate if based solely on the
application of Eq. (1): for any d there exists a lengthscale ℓ = ℓ in which
clust
clustering is maximal. The authors need to confront this possibility or state
clear caveats as with regards to the use of Eq. (1).
5. In the discussion containing Lines 199-210, it is difficult to gauge the relative
coupling of the particles to the gas. For the dynamicists it would be beneficial
to write out an estimate for the corresponding Stokes number, St ≡ t Ω ,
s K
where Ω is the local Keplerian rotation frequency.
K
6. In the paragraph starting on line 238, the authors suggest that sunward of the
inner edge, spiral wave shedding could trigger the Streaming Instability (SI).
While this might be plausible, it is only likely if the turbulence is weak enough
2
or if the particle Stokes numbers (see below) fall within the ideal range for SI
to operate. In terms of turbulence, it has become increasingly evident in recent
years that SI may not be triggered if the background turbulence is high and
the Stokes numbers are low (below 0.01). The authors should consider adding
a cautionary note in this regard by citing recent studies on the matter, such as
Estrada & Umurhan (2023), Chen & Lin (2020), and Umurhan et al. (2020).
More importantly, the authors should note that
SI can indeed be triggered
when St = 0.1−1 (see Li & Youdin, 2021 and Estrada & Umurhan, 2023), but
this occurs only under the right turbulence conditions. Is the Stokes number of
the d = 7µm particle mentioned in line 203 within this preferred range? What
is the precise value?
In lines 246-250, the authors also mention the possibility of very large turbu-
lence, α ∼ 10−2,
triggering the SI. Specifically, I am referring to the statement
in lines 249-250: "Turbulence is not only able to concentrate particles of the
same size as microchondrules, it is likely to concentrate them to the levels
needed to trigger streaming instability." This has not yet been demonstrated in
any published numerical study of the SI (see the previously cited references).
The authors should either revise this statement or include qualifying language
to indicate that this remains an unverified hypothesis.
3