Sulfide minerals bear witness to impacts across the solar system

Domain: earth_science
Published: 2025-07-01
Editorial outcome: accept

Abstract
Sulfide minerals are ubiquitous in extraterrestrial sample collections and serve as a unique record of the microstructural and chemical characteristics resulting from exposure to interplanetary space on airless planetary surfaces. Among these features are Fe-rich whiskers, identified in samples returned from asteroids and the Moon. While whisker production has previously been attributed to solar wind irradiation, the origin of these enigmatic features is poorly constrained. Here we perform in situ heating experiments to simulate micrometeoroid bombardment of sulfide minerals in the transmission electron microscope. Our results demonstrate that whiskers can form through impact events on airless surfaces and show that sulfides are recording evidence for micrometeoroid bombardment across the solar system. This work contributes to a better understanding of sulfur-depletion previously detected on asteroid surfaces and could be particularly important for the interpretation of data from upcoming missions to sulfide or metal-rich worlds, including asteroid Psyche.

Introduction
Planetary surfaces across the solar system experience weathering processes that fundamentally change their physical and chemical characteristics. On airless bodies, space weathering is driven by hypervelocity dust impacts and solar irradiation, cloaking the true mineralogy of these surfaces by irreparably modifying the composition and microstructure of grains in the regolith1,2. Our framework for space weathering processes is built predominantly from a lunar perspective, focused on silicate minerals that comprise the bulk of the Moon’s surface, and our returned sample collection. However, sulfide minerals are also abundant in extraterrestrial materials and serve as sensitive indicators of their formation conditions and the processing their parent bodies experienced throughout solar system history.

Sulfides exposed at the surface of primitive asteroids can provide a unique record of both the early solar system environment and the active processes operating on airless bodies today. A combination of recent remote sensing observations, laboratory experiments, and returned sample analyses demonstrated that sulfide minerals play a fundamental, yet unconstrained, role in how airless regoliths evolve with continued exposure to interplanetary space3.

Compositional measurements from the surface of S-type asteroid 433 Eros revealed that space weathering via solar wind irradiation can drive the depletion of S from surface-exposed sulfide minerals, an effect which was subsequently replicated in the laboratory4,5. Evidence of this depletion was also more recently identified in the returned sample record. Sulfide-bearing particles from asteroid 25143 Itokawa, collected by the JAXA Hayabusa mission, exhibit nanoscale space weathered rims substantially depleted in S, corroborating the remote sensing observations for Eros3,6,7. Further analyses of these sulfide minerals from Itokawa revealed unique and previously unseen microstructural characteristics attributed to space weathering: Fe-whiskers3. Comprised of S-depleted filamentous pillars, Fe-whiskers protrude up to several microns from sulfide grain surfaces and exhibit diagnostic morphologies. Subsequently identified in lunar sulfides as well8, the origin of these whiskers is enigmatic. Hypothesized to form via solar wind irradiation, together with S-depleted rims, these characteristics have pointed towards solar energetic ions as the dominant agent of change affecting sulfides on airless surfaces across the solar system. However, the role that micrometeoroid impacts could play in producing such characteristic textures is largely unexplored.

To evaluate alternative formation mechanisms for the unique microstructural and chemical signatures of space weathering in sulfides from returned samples, here we report on experiments to simulate the effects of micrometeoroid impacts in situ in the transmission electron microscope (TEM). Performing repeated, short-duration (millisecond) heating experiments up to 1100°C with natural pentlandite ([Fe, Ni]9S8) powders under vacuum, we simulate the thermal environment sulfides on airless surfaces experience during micrometeoroid bombardment (see Methods section). We show that hypervelocity impacts are a probable formation mechanism for Fe-whiskers, demonstrating that sulfide minerals are important recorders of the impact history of airless surfaces. Understanding space weathering of sulfides is critical for providing accurate constraints on the inventory of S in primitive bodies, revealing timescales for geological processes, and for evaluating the unique ways in which asteroids preserve a record of their evolution throughout solar system history.

Results
In situ heating experiments
Prior to simulated micrometeoroid impacts, the pentlandite particles are angular in morphology and range in size from tens of nanometers to several microns in their longest dimension (Fig. 1a). Energy dispersive X-ray spectroscopy (EDS) analyses indicate that their compositions are uniform and stoichiometrically pentlandite (25 at% Fe, 26 at% Ni, 47 at% S) across the grain prior to the heating events. After the samples experienced a single thermal pulse to 1100°C, the morphologies of the samples changed significantly. Secondary electron images reveal that the grains became rounded and sub-spherical in shape after heating, indicating they experienced at least partial melting. In addition to changes in grain morphology, there are unique microstructures, referred to as whiskers, that protrude from exterior grain surfaces, up to 3µm in length (Fig. 1b). Larger grains exhibit more whiskers, with the highest concentration of these structures appearing on pentlandite grains that were originally >4µm in their longest dimension. The whiskers themselves have variable morphologies, with some exhibiting a striated or filamentous structure with blunt ends, and others displaying pointed, narrow tips (Fig. 2c, d, f). All whiskers exhibit some degree of curvature, appear evenly distributed across all visible surfaces of the particles, and range in diameter from tens to hundreds of nanometers. EDS analyses indicate that the whiskers are composed entirely of Fe and Ni, and completely depleted in S. The morphology and distribution of the whiskers produced in this experiment are consistent with observations of Fe-rich whiskers on the surfaces of sulfide grains collected on both asteroid Itokawa and the Moon3,8.

The pentlandite grains were then exposed to a second thermal pulse to 1100°C to simulate multiple micrometeoroid impact events and to determine the effect of progressive impacts on the production and retention of the Fe–Ni whiskers. After the second heating event, most grains still exhibit some degree of whisker-like structures on their surfaces, though their morphologies and distribution changed significantly (Fig. 2b, g). Some whiskers disappeared altogether, and others appeared to coalesce into ridge-like structures in more muted relief from the grain surfaces. The majority of whiskers that are retained exhibit a more truncated, squat morphology, absent of the striated, filamented structures present after the initial thermal pulse. However, there are a limited number of whiskers that do retain the pointed, conical structures that protrude microns from the grain surfaces and are reminiscent of the morphologies present after the first thermal pulse. EDS mapping of the whiskers with consistent morphologies after the second thermal pulse indicates they are still composed of Fe and Ni and remain completely depleted in S (Fig. 2i–k).

Whisker microstructure and chemistry
A focused-ion-beam (FIB) scanning-electron-microscope section was prepared to transect both a remnant whisker and an exposed ridge on the surface of a pentlandite grain that had experienced a double thermal event (Fig. 3a). A selected area electron diffraction (SAED) pattern demonstrates that the grain interior is crystalline pentlandite (Fig. 3b). TEM analysis reveals that the whisker protrudes 200nm from the surface and is nearly 500nm in width. The ridge-like structure spans almost 2µm in length along the uppermost surface of the grain and extends 200nm into the grain interior. The ridge appears structurally complex, composed of multiple layers, some containing densely packed nanoparticles (Fig. 3a). In addition, there are unique, loop-like microstructures in the grain interior, several microns below the grain surface, that are comprised of an accumulation of euhedral, crystalline nanoparticles (Supplementary Information). EDS maps of the entire FIB section indicate that the whisker, the ridge structure, and the interior loop have unique compositions compared to the surrounding pentlandite. The nanoparticle-rich regions of the loop and the ridge are enriched in Ni compared to the surrounding material, have an Fe concentration consistent with pentlandite, and are strongly depleted in S. The ridge has an Fe–Ni nanoparticle-rich layer that is depleted in S but is surrounded by a S-rich matrix.

High-resolution TEM (HRTEM) images of the whisker reveal a complex microstructure. The whisker is intermittently amorphous, but there are zones of crystallinity that include individual crystalline strands that transect the structure from its base to the surface (Fig. 4g). The regions of the whisker that are crystalline exhibit d-spacings of 1.75 Å, consistent with (002) for awaruite, an Fe–Ni metal (Ni3Fe to Ni2Fe). The isolated strands exhibit a microstructural orientation that is consistent with the underlying, crystalline pentlandite (311), suggestive of heteroepitaxial growth. The boundary between the crystalline pentlandite grain and the whisker is sharp, and the uppermost 100nm of the grain beneath the whisker has a high concentration of planar defects (e.g., stacking faults) (Fig. 4g). These defects propagate through to the crystalline strands. Quantitative EDS linescans show that S content steeply decreases from 47 at% in the grain interior to 0 at% in the whisker over the upper 100nm of the grain surface. In contrast, the Ni content increases over the same upper 100nm, from 26 at% to ~75 at% at the base of the whisker, and closer to 65 at% at its tip. The Fe content stays more consistent, only increasing from 27 at% to 35 at% at the tip of the whisker (Fig. 4f). The measured chemistry in the whisker is consistent with an awaruite composition. In contrast, the crystalline strands maintain their S content and a pentlandite composition as they transect the whisker.

Methods
To perform in situ heating experiments in the transmission electron microscope (TEM), we used microelectromechanical systems (MEMS) heating chips with SiN support films. Pentlandite was purchased from Ward’s Science, and grain compositions were confirmed in the scanning electron microscope (SEM). In situ heating experiments have been shown to successfully simulate micrometeoroid impacts in previous studies43. Sulfides were crushed with an agate mortar and pestle and then immediately suspended in isopropanol. Fluid containing the grains was extracted with a micropipette and then drop-cast onto MEMS chips compatible with the Hitachi Blaze heating holder. Heating experiments were performed under vacuum in the aberration-corrected 200keV Hitachi HF5000 scanning TEM (STEM) in the Kuiper-Arizona Laboratory for Astromaterials Analysis (K-ALFAA) at the University of Arizona. The HF5000 is equipped with bright-field (BF), dark-field (DF), high-angle annular dark-field (HAADF), and secondary electron (SE) detectors as well as dual Oxford 100mm2 windowless silicon-drift energy dispersive X-ray spectroscopy (EDS) detectors. We subjected sulfide grains to two distinct thermal regimes: (1) fast (<0.5s) repeated thermal pulses from room temperature to 1100°C and back to simulate the short-duration high-temperature effects of micrometeoroid impacts, and; (2) slow, step-heating with a ramp rate of 100°C/s, to investigate the behavior of sulfides at several known, relevant exsolution and eutectic temperatures, e.g., 16, 21, 23, 44–54. For the rapid heating protocol, we performed three individual heating experiments for the same sample. For the slow heating experiments (Supplementary Information), we held the sample for two minutes at each of the following temperatures: 200°C, 550°C, 610°C, 750°C, 950°C, and 988°C. For both thermal profiles, we imaged the samples before and after heating, recorded videos of the samples during the thermal events, and analyzed changes in grain chemical composition by collecting quantitative EDS maps. We then used the FEI Helios NanoLab 660 focused ion beam scanning electron microscope (FIB-SEM) at the K-ALFAA to extract an electron-transparent thin section spanning multiple whisker structures that developed on the sample surface during heating. We performed high-resolution (HRTEM) imaging and selected area electron diffraction (SAED) to understand the whisker microstructure. We also collected quantitative EDS maps and linescans to understand changes in sample chemistry.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Summary: The authors present a synthesis of iron sulfide nano-whiskers, analogous to space weathering features found on regolith grains returned from the Moon and multiple asteroids. Remarkably, these whiskers are reproduced by rapidly heating and cooling the sulfide grain, simulating the heat from a micrometeoroid impact, rather than previously-invoked mechanisms involving solar wind ion irradiation. In-situ characterization of the lab-grown whiskers using EDS, FIB-TEM, and electron diffraction demonstrates close similarity with those found in regolith samples. The authors propose a novel whisker growth mechanism, in which the rapidly melted surface of the sulfide grain loses sulfur to space by diffusion, exsolving FeNi metal which contracts faster than the underlying sulfide as it quenches, resulting in compression-driven whisker growth similar to metallic thin films. Sulfide mineral grains may thus record planetary impact histories to an extent not previously understood.\nSignificance: I have been looking forward to a paper like this ever since Matsumoto and colleagues first reported iron nano-whiskers in Itokawa grains five years ago. To my knowledge, this is the first attempt to replicate nano-whiskers in the laboratory using temperature, and the results are quite compelling. It would be one thing if an experiment like this simply produced whisker-like growths that appear similar to sulfide regolith particles under microscope imaging, but far more relevant is the detailed microstructural analysis showing the chemical variation of the whiskers from base to tip and the presence of crystalline defects, both of which evidence the proposed formation mechanism as a diffusive and thermo-mechanical process. I suspect the planetary materials community would take interest in both the specific mechanism described, as well as the broader use of thin films and synthetic alloys as analogs for nano- and micro-scale processes in the space environment, as we continue finding more diverse mineral systems to be useful indicators of planetary surface evolution.\nSoundness: I do not find any faults in the methodology, data analysis, or interpretations of this paper. I expect another laboratory capable of rapidly heating and cooling a grain with in-situ microanalysis tools would be able to replicate the procedures the authors employ, although I would be curious to see how results might differ if an alternate heating mechanism were employed, e.g. pulsed laser irradiation. The images provided in both the main and supplemental documents clearly show the features described in the body of the text, enhanced by minimal but nonetheless effective annotations and descriptive captions. In discussing their findings, the authors suggest an extraordinary whisker formation mechanism, but are careful to do so only after thoroughly assessing the shortcomings of alternative hypotheses, and maintain focus on the proposed mechanism’s consistency with observed chemical, microstructural, and morphological features of the whiskers. Overall, I find that the conclusions are well-supported by the empirical evidence presented.\nIn summary, I believe this work meets the expected standards for Nature Communications, and am pleased to recommend it for publication. I would like to congratulate the authors on their success, and look forward to future work expanding on their ideas.", "reviewer_2": "This study conducts in-situ heating experiments to simulate micrometeorite bombardment on pentlandite grains, and concludes that Fe-whisker formation could be driven by impact events rather than primarily solar wind irradiation as previously thought. The methods are technically sound, the data is of a high quality, and the results are presented in detail. It is clear to see obvious whiskers forming after a single thermal pulse to 1100 oC, followed by a degradation of whiskers with subsequent thermal pulses. The formation mechanism of Fe-whiskers via thermal events is well explained. The conclusions of this manuscript are well supported by the data shown, and are highly relevant for both past and future missions to sulfide/metal-rich bodies.\nComments:\nMy only substantial comment/question is: Why was pentlandite used in the experiments rather than troilite/pyrrhotite? I’m not very familiar with the Fe-whisker literature, but from what I’ve seen the whiskers are usually Ni-poor and form around pyrrhotite grains? It would be good to justify the use of pentlandite in the experiments. Also, it would be useful to add in a few speculative comments somewhere to state whether the same mechanism of whisker formation from a pentlandite grain would also apply to a pyrrhotite grain (how does the melting point of pyrrhotite and thermal expansion properties of pyrrhotite compare to pentlandite? Would the starting composition make any difference to the whiskers? How might whisker formation rates and morphologies from a Ni-poor grain be different to a Ni-rich grain, if at all?). It would then be clearer whether this mechanism could form the Ni-poor Fe-whiskers observed in Itokawa (Matsumoto et al., 2020).\nLines 93 – 95: It’s great that the whiskers formed after one thermal pulse are morphologically consistent with whiskers observed in samples from Itokawa and the Moon! Are there no naturally produced whisker morphologies in the literature that are consistent with those after multiple experimental thermal pulses? It’s strange that this is the case, as you’d expect the sulfides in Itokawa and the Moon would have experienced multiple impacts?\nFigure 2: while it is relatively obvious to spot that, for example, C) corresponds to the pink outlined region in A), I think it would be useful to add labels inside the boxes outlined in A) to make it even clearer how the panels correspond to each other (if this change is made here, then similar labels should be added to all other figures where appropriate).\nFigure 5: in the caption: “through which whiskers during simulated…”, needs changing to “through which whiskers formed during simulated…”?"}, "author_response": "We thank the reviewers for their comments, which have greatly improved the manuscript. We have provided responses to their comments in green text below.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nSummary: The authors present a synthesis of iron sulfide nano-whiskers, analogous to space weathering features found on regolith grains returned from the Moon and multiple asteroids. Remarkably, these whiskers are reproduced by rapidly heating and cooling the sulfide grain, simulating the heat from a micrometeoroid impact, rather than previously-invoked mechanisms involving solar wind ion irradiation. In-situ characterization of the lab-grown whiskers using EDS, FIB-TEM, and electron diffraction demonstrates close similarity with those found in regolith samples. The authors propose a novel whisker growth mechanism, in which the rapidly melted surface of the sulfide grain loses sulfur to space by diffusion, exsolving FeNi metal which contracts faster than the underlying sulfide as it quenches, resulting in compression-driven whisker growth similar to metallic thin films. Sulfide mineral grains may thus record planetary impact histories to an extent not previously understood.\nSignificance: I have been looking forward to a paper like this ever since Matsumoto and colleagues first reported iron nano-whiskers in Itokawa grains five years ago. To my knowledge, this is the first attempt to replicate nano-whiskers in the laboratory using temperature, and the results are quite compelling. It would be one thing if an experiment like this simply produced whisker-like growths that appear similar to sulfide regolith particles under microscope imaging, but far more relevant is the detailed microstructural analysis showing the chemical variation of the whiskers from base to tip and the presence of crystalline defects, both of which evidence the proposed formation mechanism as a diffusive and thermo-mechanical process. I suspect the planetary materials community would take interest in both the specific mechanism described, as well as the broader use of thin films and synthetic alloys as analogs for nano- and micro-scale processes in the space environment, as we continue finding more diverse mineral systems to be useful indicators of planetary surface evolution.\nWe thank the reviewer for their positive commentary.\nSoundness: I do not find any faults in the methodology, data analysis, or interpretations of this paper. I expect another laboratory capable of rapidly heating and cooling a grain with in-situ microanalysis tools would be able to replicate the procedures the authors employ, although I would be curious to see how results might differ if an alternate heating mechanism were employed, e.g. pulsed laser irradiation. The images provided in both the main and supplemental documents clearly show the features described in the body of the text, enhanced by minimal but nonetheless effective annotations and descriptive captions. In discussing their findings, the authors suggest an extraordinary whisker formation mechanism, but are careful to do so only after thoroughly assessing the shortcomings of alternative hypotheses, and maintain focus on the proposed mechanism’s consistency with observed chemical, microstructural, and morphological features of the whiskers. Overall, I find that the conclusions are well-supported by the empirical evidence presented.\nIn summary, I believe this work meets the expected standards for Nature Communications, and am pleased to recommend it for publication. I would like to congratulate the authors on their success, and look forward to future work expanding on their ideas.\nWe sincerely thank the reviewer for their positive commentary.\nReviewer #2 (Remarks to the Author):\nThis study conducts in-situ heating experiments to simulate micrometeorite bombardment on pentlandite grains, and concludes that Fe-whisker formation could be driven by impact events rather than primarily solar wind irradiation as previously thought. The methods are technically sound, the data is of a high quality, and the results are presented in detail. It is clear to see obvious whiskers forming after a single thermal pulse to 1100 oC, followed by a degradation of whiskers with subsequent thermal pulses. The formation mechanism of Fe-whiskers via thermal events is well explained. The conclusions of this manuscript are well supported by the data shown, and are highly relevant for both past and future missions to sulfide/metal-rich bodies.\nComments:\nMy only substantial comment/question is: Why was pentlandite used in the experiments rather than troilite/pyrrhotite? I’m not very familiar with the Fe-whisker literature, but from what I’ve seen the whiskers are usually Ni-poor and form around pyrrhotite grains? It would be good to justify the use of pentlandite in the experiments. Also, it would be useful to add in a few speculative comments somewhere to state whether the same mechanism of whisker formation from a pentlandite grain would also apply to a pyrrhotite grain (how does the melting point of pyrrhotite and thermal expansion properties of pyrrhotite compare to pentlandite? Would the starting composition make any difference to the whiskers? How might whisker formation rates and morphologies from a Ni-poor grain be different to a Ni-rich grain, if at all?). It would then be clearer whether this mechanism could form the Ni-poor Fe-whiskers observed in Itokawa (Matsumoto et al., 2020).\nWe thank the reviewer for their comments. Using pentlandite as opposed to troilite or pyrrhotite was driven by observations of whiskers features in other returned samples, namely in material from asteroid Bennu collected by the OSIRIS-REx mission (see Keller et al. 2024 Meteoritical Society Abstract #3036). In those samples, the whiskers are FeNi-rich, as opposed to predominantly Fe-rich as has been observed most commonly in samples from Itokawa and the Moon. These are the first whiskers that the authors observed themselves in returned samples, contributing to the motivation for this study.\nWhen comparing whisker formation in pyrrhotite versus pentlandite, we suggest that our proposed mechanism could also drive whisker formation in sulfides with Ni-poor compositions. From Kellerud (1963; citation 21 in manuscript text) the maximum melting temperature for pyrrhotite is 1190 °C, which is close to the temperature achieved in these in situ heating experiments and is relevant to thermal conditions experienced by sulfide minerals during true micrometeoroid impacts. This melting temperature is still lower than that of Fe-metal, suggesting that pyrrhotite would experience similar conditions to pentlandite, wherein S would preferentially escape from the outer rim of the grain, resulting in quenched Fe-metal shell surrounding a predominantly molten core of pyrrhotite. As a result, the same compressive stresses might develop in the pyrrhotite samples during thermal events, subsequently prompting analogous whisker growth. Without Ni, however, the whiskers would be composed of strictly Fe, as is observed in some samples from Itokawa and the Moon. In terms of thermal expansion, some compositions of pyrrhotite have thermal expansion coefficients that are lower, but within range of measurements made for pentlandite (Rajamani and Prewitt 1975; citation 29 in the manuscript text). Though whisker formation is probable in sulfides with both Ni-rich and Ni-poor compositions, the larger differences in melting temperature and thermal expansion coefficients between pentlandite and FeNi-metal compared to pyrrhotite and Fe-metal suggest that whisker formation might be a more efficient process in Ni-rich sulfides.\nWe have added several sentences to the discussion section (see green text in manuscript file) to reflect that this would likely be a viable mechanism for the formation of whiskers as observed in pyrrhotite as well.\nLines 93 – 95: It’s great that the whiskers formed after one thermal pulse are morphologically consistent with whiskers observed in samples from Itokawa and the Moon! Are there no naturally produced whisker morphologies in the literature that are consistent with those after multiple experimental thermal pulses? It’s strange that this is the case, as you’d expect the sulfides in Itokawa and the Moon would have experienced multiple impacts?\nThere are indeed features in returned samples that appear more similar to the whisker morphologies we see in the samples that experienced multiple thermal pulses in our experiments. In Matsumoto et al. (2020), Figure 1d (copied below) shows some surface features that are more linear in morphology and protrude less from the grain surfaces compared to the more discrete whiskers. These ridge-like features are reminiscent of the degraded whiskers in experimental samples that experienced multiple thermal pulses. This similarity could suggest returned samples are recording the effects of repeated impact processing after all.\nWe have added a sentence reflecting the similarity in these observations to the discussion section (see green text in the manuscript file).\n(Matsumoto et al. 2020)\nFigure 2: while it is relatively obvious to spot that, for example, C) corresponds to the pink outlined region in A), I think it would be useful to add labels inside the boxes outlined in A) to make it even clearer how the panels correspond to each other (if this change is made here, then similar labels should be added to all other figures where appropriate).\nWe thank the reviewer for the suggestion. To prevent obscuring some of the features in the figures with superimposed labels, we added text to the captions of Figures 2 and 4 to indicate that colored insets correspond to the location of features identified in subsequent panels.\nFigure 5: in the caption: “through which whiskers during simulated…”, needs changing to “through which whiskers formed during simulated…”?\nCorrected."}]

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