Iron whiskers on asteroid Itokawa indicate sulfide destruction by space weathering

Extraterrestrial iron sulfide is a major mineral reservoir of the cosmochemically and astrobiologically important elements iron and sulfur. Sulfur depletion on asteroids is a long-standing, yet unresolved phenomenon that is of fundamental importance for asteroid evolution and sulfur delivery to the Earth. Understanding the chemistry of such environments requires insight into the behavior of iron sulfides exposed to space. Here we show that troilite (FeS) grains recovered from the regolith of asteroid 25143 Itokawa have lost sulfur during long-term space exposure. We report the wide-spread occurrence of metallic iron whiskers as a decomposition product formed through irradiation of the sulfide by energetic ions of the solar wind. Whisker growth by ion irradiation is a novel and unexpected aspect of space weathering. It implies that sulfur loss occurs rapidly and, furthermore, that ion irradiation plays an important role in the redistribution of sulfur between solids and gas of the interstellar medium.

nergetic ions from various cosmic sources can significantly alter solid mineral grains in interplanetary and interstellar space [1][2][3] . Within the solar system, solar wind ions are considered to be a main causes of space weathering, which alters the physical and chemical surface properties of airless bodies such as the Moon and asteroids 1,2 . Iron sulfides such as troilite (FeS) are common minerals in solar system materials. Although iron sulfides are important solid reservoirs of the cosmically abundant elements sulfur and iron, their behavior in space-exposed environments is poorly constrained. Remote-sensing of the S-type asteroid 433 Eros indicates significant sulfur depletion in its surface regolith by space weathering of iron sulfides [4][5][6][7] . One of the fundamental chemical characteristics of airless bodies are their volatile element abundances, including sulfur, which closely correlate with elemental fractionation during solid accretion in the solar nebula, subsequent thermal processing and gravitational differentiation 8,9 . Moreover, the sulfur abundance of small airless bodies is of great importance to estimate the amount of sulfur delivered to the early Earth, which likely became a major ingredient of a habitable surface and life 10 . However, the sulfur depletion by space weathering may obscure the detection of original sulfur abundances of airless bodies. Thus, the rates and modes of space weathering of iron sulfides are essential to understand the dynamics of asteroids, their relics of the early solar system and their impact on the terrestrial planets and life. Understanding the alteration of minerals by the solar wind is important for unveiling their surface evolution, because the altered regolith grains are key indicators of the timescales of geological processes on these dynamical bodies, such as motion, loss and replenishment of the regolith 11,12 . Furthermore, knowledge of the interaction between solids and energetic ions may help to understand the chemistry of cosmic radiation environments in the interstellar medium (ISM) 3,13 , where molecular clouds and new stars are born and deliver chemical components to emerging planetary systems, such as the early solar system.
Regolith particles recovered from the S-type asteroid 25143 Itokawa by the Hayabusa mission are key samples to examine the space weathering of primordial minerals, especially the effects of solar wind implantation 2,12,[14][15][16][17][18] . Their mineralogy corresponds to LL4-6 ordinary chondrites 19,20 , in which troilite is the major sulfide phase. The few previous studies of iron sulfides have reported little modifications by the solar wind 15,16 compared to the distinctly radiation-damaged rims of silicate minerals 2,14 , whereas earlier experimental studies have suggested chemical and structural changes of iron sulfides by ion irradiation [21][22][23] . So far, naturally altered iron sulfide has not been comprehensively examined. Here we present systematical analyses of the surface features of troilite-bearing Itokawa particles. Field emission scanning electron microscopy (SEM), focused ion beam (FIB) sectioning and transmission electron microscopy (TEM) are used to better understand the surface modifications of space-exposed troilite (see the Methods section, Supplementary Notes 1-4, and Supplementary Figs. 1-3). We show that the wide-spread occurrence of metallic iron whiskers as a decomposition product formed through solar wind irradiation on the troilite. It implies that sulfur loss occurs rapidly on airless bodies and, furthermore, that ion irradiation plays an important role in the partitioning of sulfur and iron in interplanetary and interstellar space, if small dust particles are involved.

Results
The characteristics of troilite-bearing Itokawa particles. A striking feature of the troilite surfaces is the appearance of elongated whiskers of metallic iron protruding from the space-exposed surfaces (Fig. 1). They are present on 8 of 10 Itokawa particles we have investigated, suggesting their common appearance (Supplementary Table 1, Supplementary Note 1). The whiskers are thinner at their tips, curved, and striated. Their length varies from several tens of nm to 2.8 µm. The whiskers often coexist with thin flat plates that are deficient in sulfur and possibly composed of iron (Fig. 1d) TEM observation showed that each whisker appears to be composed of a set of several sub-whiskers (Fig. 2a). Selected area diffraction (SAED) patterns of the whisker confirmed that they consist of bcc-structured metallic iron (Fig. 2b). Individual subwhiskers show the same diffraction contrast in TEM images (Fig. 2a), suggesting that single crystals of iron are predominant within them. SAED patterns and diffraction contrast of thinner whiskers (<300 nm in width) even show that they are composed of almost one single iron crystal (Fig. 2e) or several crystals with slightly different crystallographic orientations. The metal of the whiskers is typically almost pure iron (<0.7 wt% Ni), only in a few cases minor Ni was concentrated near the uppermost surfaces of some sub-whiskers (~4.1 wt%; Fig. 2d, Supplementary Table 2).
Open vesicles on troilite surfaces are delimited by subparallel, scale-like structures (Fig. 3a), which are elongate perpendicular to the c-axis of the troilite (Fig. 3a, b). Dark-field TEM images of the vesicular regions show many bright spots corresponding to small crystalline sub-domains (>10 nm) having slightly different crystallographic orientations compared to the substrate troilite (Fig. 2f). This occurs within 80-100 nm in depth. In the case of troilite apparently showing a non-vesicular texture in SEM images, many internal vesicles appear beneath the troilite surface, which extend to~50 nm in depth (Fig. 3c). Relatively large vesicles are concentrated at depths between 10 and 20 nm (Fig. 4b). SAED patterns of troilite in the vicinity of the vesicles indicate slight crystallographic misorientations (Fig. 3d). No discernable relationship in orientation between the iron whiskers and the troilite is evident in SAED patterns. The Fe/S ratio increases in the surface region with open vesicles as compared with the bulk troilite (Fig. 4a). In contrast, the Fe/S ratio of the surfaces with internal vesicles appears to be constant (Fig. 4b).
The whiskers, troilite, and silicate surfaces are covered with Oand Si-rich layers that are 7-15 nm thick (Fig. 2c, d). Such layers on troilite contain O, Si, and Mg (Fig. 4), and in some cases, Na, Cl, and Ca. These rims are probably vapor-deposited coatings generated by solar wind sputtering and/or micrometeorite impacts on nearby regolith grains 2 . The accumulation of vapordeposited materials appears to have occurred after whisker growth. The olivine and low-Ca pyroxene coexisting with troilite have outermost vapor-deposited rims and lower polynanocrystalline rims that are generally interpreted as products of solar wind damage 2,14 ( Supplementary Fig. 4). The residence times of the examined Itokawa grains within a few millimeters of the uppermost regolith surface were estimated to be~1000 tõ 8000 yr, calculated from solar flare track densities in silicates 24 (Supplementary Table 1 and Supplementary Fig. 5).

Discussion
Polynanocrystalline rims and vesicles are known to be typical damage features of experimentally ion-irradiated materials 25 and solar-wind-irradiated minerals 2,14 . Vesicles might expand through the coalescence of implanted atoms and/or by the clustering of induced vacancies 26 Fig. 6). These depths support the notion that vesicles developed through accumulation of solar wind gases.
Here we discuss the feasibility of iron whisker growth from the liquid, vapor and solid states. Aqueously mediated formation of iron whiskers appears highly unlikely, because of the dry  29 . Magnetite whiskers found in Martian meteorites were likely precipitated with carbonates from a supercritical fluid in fractures 30,31 . As for Itokawa grains, the walls of fractures/pores that formed during metamorphic equilibration can be identified as euhedral crystals on grain surfaces 12 . Although such equilibrated textures exist on the troilite surface (Supplementary Table 1), they do not correlate with whisker distribution. The majority of Itokawa grains display fractured surfaces that formed by the fragmentation of larger regolith grains 12 , observed troilite surfaces are no exception (Fig. 1). Hence, whiskers on the troilite must have developed after the formation of the regolith grains. If iron whiskers were produced by condensation from a vapor, then Itokawa's young formation age of~1.3-2.3 Ga 32-34 excludes the solar nebular gas (~4.5 Ga ago) as a vapor source being in contact with Itokawa's surface regolith. The origin of iron whiskers is therefore not comparable to that of graphite whiskers reported in CV chondrites, which are thought to have condensed from the nebular gas 35 . Impact-induced vaporization on Itokawa 2,14,15,17 is the most likely source of a vapor. However, assuming that the deposited rims have the bulk composition of LL chondrites, in which iron sulfides and metals account for only 6% of the total volume 36 , the Si-bearing rims on Itokawa grains do not balance the widespread occurrence of iron whiskers on troilite, because these rims are quite thin (<15 nm). Additionally, the whiskers do not occur on silicates and do not show any structures suggesting preferential iron condensation on troilite, such as droplets necessary for vapor-liquid-solid (VLS) growth 37 or epitaxy 31 . Moreover, heating of LL-chondrite materials forms sulfur-rich vapors that cause sulphurization rather than condensation of metallic iron 38 . Therefore, it appears unlikely that the whiskers are condensed materials. The space-exposed surface on Itokawa may contrast with lunar surface environments, where abundant melts and vapor-deposited materials (up to 200 nm thickness) cover the regolith grains 39,40 . Graphite whiskers 41 , optically transparent whiskers 42 , and elongated iron objects called stalks 43 have been reported in lunar samples, all of which were regarded as condensation products. Although the iron stalks consist of metallic iron, they do not correspond to iron whiskers on Itokawa, because they clearly show a droplet at their tip indicating VLS growth.
The remaining and most plausible mechanism for iron whisker formation is growth from a solid substrate that is supersaturated with cations, as proposed for the metallic whisker growth in sulfides or oxides [44][45][46] . This inference is supported by the fact that the iron whisker growth appears to be associated with the progressive irradiation-damage, given the lack of whiskers on troilite with minor damage 16 . Metallic iron may form when troilite comes into contact with H 2 gas 47 . The H 2 gas trapped in the vesicles may cause the reaction, FeS + H 2 = H 2 S + Fe 2+ + 2e − under solar heating (Supplementary Note 5, Supplementary  Fig. 7). Shifting the equilibrium towards H 2 S requires removal of the H 2 S produced from the vesicle. This process takes place most efficiently where an escape path for H 2 S gas is available such as along sub-grain boundaries, cracks, or open vesicles, while H 2 is replenished by solar wind irradiation. Additional heating by micrometeorite bombardment and larger impact events may play a role for increasing the replacement of these gases by accelerating the diffusion rate of hydrogen atoms and H 2 S in troilite. Furthermore, these heating events may raise the reaction rate and subsequent diffusion of iron atoms. The acceleration of the reaction can also happen by solar heating when the asteroid has an orbit with a lower heliocentric distance 16 . The action of the solar wind can selectively sputter sulfur atoms. The S/Fe ratio of troilite decreased to 0.55 ± 0.55 after experimental 4 keV He irradiation 7 with a dose of 17 × 10 17 ions/cm 2 , which corresponds to an exposure time of 4 × 10 4 yr to the solar wind at 1 AU. Energy transfer to sulfur atoms could result in the sulfur escape in the form of H-, S-bearing radicals and/or molecules 48,49 .
Briefly, we discuss the possibility that the solar wind has not been involved in the metallic whisker growth from troilite: Possible processes to form excess iron are heating events by solar irradiation, micrometeorite bombardments, and larger impacts.  Fig. 1). c-d was obtained from FIB section RA-QD02-0286-01 ( Supplementary Fig. 4).
Fe 2+ + 2e − ). Assuming that S 2 escape from the space-exposed troilite surface occurs at zero ambient S 2 partial pressure, the Hertz-Knudsen-Langmuir relationship results in an estimated a maximum evaporative flux of~3 × 10 −3 S atoms/cm 2 /yr at the maximum surface temperature of Itokawa of 130°C (Supplementary Note 6). This corresponds to~1 × 10 −18 nm/yr, which is an extremely low rate. Submicron-sized impact craters on troilite in Itokawa particles show no evidence for whisker growth 51 . Moreover, examined troilite surfaces with iron whiskers do not correspond to crater floors. Somewhat larger impacts might locally induce a transient temperature rise of the regolith and could increase the equilibrium pressure of S 2 , such that thermal evaporation could become more efficient. However, this process should take place at relatively greater depths in the regolith, where the temperature increases and the permeability required for S 2 loss are low. In addition, the temperature increase would be rather short in duration and long-term temperatures at depth are much lower than at the surface. Increased solar heating at possible lower heliocentric distances 16 might be insufficient for a fast recession of the troilite surface (Supplementary Note 6). Hence, selective sulfur loss appears not to be caused solely by heating events on the asteroid. Based on the discussion above, sulfur loss and supersaturation of Fe 2+ just beneath the surfaces of troilite grains very likely occurred as a consequence of solar wind  Fig. 1). b was obtained from FIB section RA-QD02-0286-01 ( Supplementary Fig. 4).  Table 3). The average characteristic length of iron diffusion in troilite on Itokawa's surface in its present-day orbit is the order of 10 nm/yr on average (Supplementary Note 8; Supplementary Fig. 9). This indicates that whisker growth is not controlled by cation diffusion, but rather by the production of excess cations. The rapid iron diffusion enables accumulation of sufficient excess Fe 2+ for whiskers growth. Hence, the total volume of whiskers at a given surface area might be a novel proxy for the duration of regolith space exposure that corresponds to the amount of excess iron accumulated. The net growth mechanism of iron whiskers on Itokawa appears to be unique among the extraterrestrial whiskers, which have been widely considered as vapor condensates 30,31,35,[41][42][43]53 . Solar wind irradiation has had a limited effect on the sulfur abundance on the surface of Itokawa, because the estimated sulfur loss is negligible compared with the total troilite volume. This is probably due to a short exposure time. Therefore, it is interesting that sulfur depletion was detected in the uppermost 10 µm of regolith on the S-type asteroid Eros that occupies heliocentric distances similar to Itokawa. Deeper sulfur loss is possible but not accessible to remote X-ray analysis 54 . Adopting the sulfur loss rate due to solar wind on Itokawa (0.04 nm/yr), sulfur depletion to 10 µm depth takes about 2.5 × 10 5 yr. Given that the regolith ages of Eros is 3-50 million years 55 , this indicate that substantial amounts of sulfur could have been lost through regolith gardening 5 . Sulfur loss through other processes, such as impact vaporization 4-6 may have occurred additionally. The solar wind may be most effective on bodies at low heliocentric distances, where the increased ion flux and surface temperature can exponentially enhance H 2 formation and sulfur loss. Alteration rims on iron sulfide might provide a clue for past orbital states of airless bodies with secular variations of their perihelia, as proposed for silicates 16 . Effective solar wind irradiation may also occur in small interplanetary dust due to their high surface-tovolume ratio. Sulfur loss by the irradiation could contribute to the sulfur abundance of the Earth, if irradiated small grains were involved in accretion of the Earth 56 .
Our observations have strong implications for the alteration of iron sulfides in the interstellar environments. Iron sulfides have been detected in the outflows of evolved stars 57 and are likely injected into the interstellar medium (ISM) 58 . An unsolved question regarding the chemical composition of the ISM is the unique role of sulfur in the gas phase. In the diffuse ISM sulfur is barely depleted from the gas phase 59,60 . On the contrary, in dense molecular clouds sulfur is strongly depleted in the gas, which has been attributed to complex astrochemical reactions involving organo-sulfur compounds 61 . Here it is not entirely clear how sulfur can be transferred between inorganic sulfides, the gas and, eventually, organics in these environments. Submicron-sized silicate dust in the diffuse ISM has been considered to become amorphous by the irradiation of H + and He ++ accelerated by shockwaves from supernovae 3 . The flux of ions with energies of 1 keV/nucleon in the shockwave models (10 7 -10 9 ions/cm 2 /s) and their total ion dose (up to 10 18 -10 19 H + ions/cm 2 ) 13 are similar to the solar wind (flux 62 = 2 × 10 8 H + ions/cm 2 /AU 2 /s, dose = 6 × 10 18 H + ions/cm 2 , assuming an exposure age of 10 3 yr at 1 AU). Our observation supports the hypothesis 60 that sulfur is supplied to the gas phase of the ISM via the destruction of iron sulfides due to ion irradiation. Rapid whisker growth as opposed to the formation of a protective iron shell implies that sulfides are easily destroyed under such conditions. Metallic iron is predicted to appear as a byproduct of this alteration process in the ISM and might exists there even though its primary formation by homogeneous nucleation from supernova ejecta appears to be highly ineffective 63 . Metallic iron in the ISM can be an important catalysts for molecule formation 64 and can produce magnetic dipole emissions that disturb the cosmic microwave background 65 . Therefore, the alteration of iron sulfides by ion implantation could have a large impact on the chemistry and electromagnetic properties of the ISM.
From our study, we conclude that, contrary to previous evidence, iron sulfides are highly susceptible to decomposition by ion irradiation in space. Sulfur loss from iron sulfides by solar wind could cause a remarkable decrease of sulfur abundances on airless bodies at low heliocentric distances. Their susceptibility to this process implies that ion irradiation plays an important role for the partitioning of iron and sulfur in interplanetary and interstellar environments. Iron whiskers are newly recognized features of space weathering and could become distinctive indicators to interpret the history of airless bodies, such as the Cgroup asteroids currently studied by Hayabusa2 and OSIRIS-REx. The appearance of whiskers induced by ion irradiation has never before been found in the fields of mineralogy and material science -potentially their further study will provide new insights into whisker growth mechanisms and corrosion processes due to ion irradiation.

Methods
Samples. The Itokawa particles investigated in this study were collected from room A of the sample catcher and were captured during the second touchdown in the MUSES-C Region on Itokawa. The mineral phases and average diameters of the Itokawa particles were investigated during their initial description 66 at the Extraterrestrial Sample Curation Center (ESCuC) of the Japan Aerospace Exploration Agency (JAXA). SEM analysis. We examined surface features of 10 regolith particles (average diameters = 68-241 μm) including iron sulfide (Supplementary Table. 1) using a field emission SEM (FE-SEM; Hitachi SU6600) at the ESCuC after the initial routine description. The particles were placed on an Au-coated holder for SEM observations, without a conductive coating, using an electrostatically controlled micromanipulation system. Until these SEM observation, the Itokawa particles had never been exposed to an atmospheric environment, thus minimizing the contamination and alteration of the studied particles 66 . We performed secondary electron (SE) imaging at accelerating voltages of 1.5 and/or 2.0 kV under high vacuum with an electron beam current of~10 pA.
Three Itokawa particles (RA-QD02-0286, RA-QD02-0292, and RA-QD02-0325) were transferred onto an adhesive carbon-conductive tape for further analysis. We determined the elemental compositions of their surfaces with an energy-dispersive X-ray spectrometer (EDX) using an FE-SEM (Hitachi SU6600) equipped with a X-Max N 150 mm 2 (Oxford Instruments) in JAXA and an FE-SEM (Hitachi SU6600) equipped with a Bruker XFlash ® FlatQUAD detector at the Institute for Molecular Science (IMS, Higashi-Okazaki, Japan). The accelerating voltage for SE imaging was 1.5 kV, whereas for EDS analysis we used 5 and 10 kV. TEM analysis. We then extracted electron-transparent sections for transmission electron microscopy (TEM) studies of regions of interest on iron sulfide surfaces, using focused ion beam (FIB) systems. The FIB systems were a FEI Quanta 200 3DS at Kyoto University, Japan, and a FEI Quanta3D FEG at the University of Jena, Germany. To protect the particle surfaces during FIB processing and enhance the electrical conductivity, the particles were coated with carbon (~20 nm thick). We then coated the particle surfaces with an electron-beam-deposited Pt layer (at 5 kV) followed by a Ga ion-beam-deposited Pt layer (at 30 kV). For each particle, sections that were a few tens of micrometers in size were extracted using a microsampling manipulator and mounted onto TEM copper grids. After the target samples were attached to the TEM grids, they were thinned to~100 nm using a 30 kV Ga + beam and were finally cleaned using a 5 kV Ga + beam at 40-80 pA.
Thin sections from the three particles were labeled RA-QD02-0286-01, RA-QD02-0292-01, and RA-QD02-0325-01. These sections were prepared in order to sample surface features of the iron sulfide initially detected by SEM, and also typically included also space-exposed silicates. We extracted an additional section (RA-QD02-0325-02) from pyroxene next to the iron sulfide sampled by RA-QD02-0325-01, because the latter did not sample any exposed silicates.
Prepared sections were examined using a FE-TEM (FEI Tecnai G 2 FEG) equipped with an Oxford 80 mm 2 energy-dispersive SDD X-ray detector and a Gatan UltraScan 2k CCD camera at the Institute for Geosciences, University of Jena, Germany. EDX analysis was performed in scanning TEM (STEM) mode aided by high-angle annular dark-field (HAADF) imaging. Quantitative elemental abundances within the whisker metal were calculated using the Cliff-Lorimer thin film approximation with k-factors determined from an artificial Fe-Ni-Co-Cr alloy of known composition 67 . High-resolution elemental profiles (Fig. 4) were obtained by acquiring laterally extended multispectral X-ray images in STEM mode, followed by segmenting, lateral integration, and fitting of extracted X-ray spectra. These spectra were evaluated semi-quantitatively to provide X-ray intensities and intensity ratios across surface layers with spatial resolutions of a few nanometer in the direction perpendicular to the surface. Compared with spot measurements, this approach minimizes beam damage. We performed further quantitative EDX analysis of the iron sulfides using a FE-TEM (JEOL JEM-3200FSK) equipped with an EDX detector at Kyushu University, Kyushu, Japan. The elemental compositions of the iron sulfides were calculated using the Cliff-Lorimer thin film approximation. We used troilite nodules in the Cape York iron meteorite and a terrestrial millerite from Sanany, Ural in Russia, as standards for the k-factors. EDX detection limits depend on the counting statistics and we achieved down to 0.1 wt% for Cr, Ni and P in metal and sulfide (Cr and P were not detected at this level). Co and Cu have substantially higher detections limits (at least one order of magnitude) due to overlap of X-ray lines and spurious contributions from the sample environment (TEM column and Cu TEM grids).
Densities of solar flare tracks (SFT) in silicate grains of each particle were determined using low-angle annular dark-field (LAADF) imaging in the STEM mode at a long camera length. SFTs were counted in the resulting images and we estimated the ratio of SFTs to the visible area of the track-sensitive mineral.

Data availability
The data that support this study are available from the corresponding author upon request.