The cosmic inventory of refractory elements, particularly the abundances of iron and nickel, is essential for understanding the thermal and chemical evolution of the Universe (Kimura et al., 2008; Leroux et al., 2000; Nittler and Ciesla, 2016; Reisener and Goldstein, 2003a; Reisener and Goldstein, 2003b). Iron, the most common refractory element, is primarily produced in Type Ia and core-collapse supernovae and is also present in AGB star outflows (Dwek, 2016). However, the physicochemical carriers and their behaviour in the interstellar medium (ISM) and early solar nebulae are poorly understood. Observations with ultraviolet and X-ray telescopes show that most of the iron produced in core-collapse supernovae is in the gas phase within the ISM (Dwek, 2016). Most of the escaped iron is thought to be trapped in interstellar dust, likely as silicates, carbon or composite grains, and metallic phases, and its distribution varies across different astrophysical environments (Dwek, 2016; Kimura et al., 2017).

Observations with ESO’s Very Large Telescope significantly advanced our understanding of metal vapor chemistry on comets at heliocentric distances between 0.68 and 3.25 AU (Guzik and Drahus, 2021; Hutsemékers et al., 2021; Manfroid et al., 2021; Opitom et al., 2021). Neutral iron (Fe I) and nickel (Ni I) emission lines are ubiquitous in 20 solar system comets, yet the surface temperature (150–340 K) is far too low to sublimate refractory minerals (Manfroid et al., 2021). The average Ni I/Fe I ratio is near unity. This higher Ni/Fe ratio suggests sublimation of materials or a sublimation process. Equilibrium thermodynamics and traditional models suggest that these missing vapours could be locked up in refractory mineral phases such as silicates, oxides or sulfides (Manfroid et al., 2021). This challenges the established models, because metallic iron sublimates at around 1200 K, whereas sulfides do so at 600 K. Additionally, the detection of gaseous neutral Ni I in the coma of comet 2I/Borisov (Guzik and Drahus, 2021), together with many-level fluorescence modelling of C/1996 B2 (Hyakutake) (Bromley et al., 2021), suggests volatile, metal-bearing sources, likely short-lived organometallic compounds or metal-bonded polycyclic aromatic hydrocarbons formed via a non-thermal mechanism rather than the sublimation of silicates (Bodewits and Bromley, 2021; Bromley et al., 2021; Manfroid et al., 2021).

To reconcile these observations, cosmochemists rely on the meteoritic record as the empirical basis for understanding the diversity of extraterrestrial materials. Nanophase iron (npFe) and kamacite (α-Fe,Ni) are widespread in primitive chondrites (Kimura et al., 2008; Leroux et al., 2000; Reisener and Goldstein, 2003a; Reisener and Goldstein, 2003b), interplanetary dust particles (IDPs) (Gainsforth et al., 2019; Keller et al., 1979; Zolensky et al., 2006), and lunar soils (Gu et al., 2025; He et al., 2024; Yan et al., 2022; Yin et al., 2025), where they record extreme non-equilibrium processing rather than the coarse equilibrated textures typical of differentiated iron meteorites. Sample-return missions such as Stardust (81P/Wild 2) (Gainsforth et al., 2019; Gu et al., 2025; He et al., 2024; Ishii et al., 1979; Keller et al., 1979; Matsumoto et al., 2024; Shen et al., 2024; Yan et al., 2024; Zolensky et al., 2006), Hayabusa2 (Ryugu) (Kimura et al., 2024; Matsumoto et al., 2024), and Chang’e 5 (Moon) (Gu et al., 2025; He et al., 2024; Shen et al., 2024; Yan et al., 2024; Yin et al., 2025) have significantly advanced the analytical frontier. They have revealed submicrometer Fe-rich particles within lunar impact glasses produced by high-velocity impacts that induce melting, liquid immiscibility, and ultrafast quenching (Gainsforth et al., 2019; Gu et al., 2025; He et al., 2024; Ishii et al., 1979; Keller et al., 1979; Matsumoto et al., 2024; Shen et al., 2024; Yan et al., 2024; Zolensky et al., 2006). Furthermore, these missions demonstrate that shock metamorphism in asteroidal parent bodies can promote recrystallization, lattice distortion, and mosaicism in Fe–Ni alloys—effects that differ from those resulting from simple thermal annealing (Gainsforth et al., 2019; Gu et al., 2025; He et al., 2024; Ishii et al., 1979; Keller et al., 1979; Matsumoto et al., 2024; Shen et al., 2024; Yan et al., 2024; Zolensky et al., 2006).

These nanophase metals are associated with high-energy shock events in the turbulent ISM. Shock processing involves rapid, high-energy events from supernovae, stellar winds, or impacts—capable of briefly (over microseconds to milliseconds) reaching extreme pressures and temperatures. In this extreme environment, refractory materials are vapourised by intense thermal spikes (Jones et al., 1996). As the vapour cools instantaneously, it recondenses into nanophase solids that preserve the imprint of kinetic control. The classical equilibrium condensation models effectively reproduce the mineralogical sequences under slow-cooling nebular conditions. Still, they failed to capture the non-equilibrium behaviour of minerals crystallising under shock-driven processes (Keller and Messenger, 2011).

Laboratory shock-wave experiments are used to address this gap directly, constraining how metal vapour behaves under the relevant conditions. Studies of extraterrestrial samples, especially meteorites, show that shock pressures of about 15–45 GPa induce partial to complete melting and form shock veins in recrystallised metallic phases (Gillet and Goresy, 2013; Xie and Sharp, 2007). However, comparatively low shock-pressure regimes (<10 GPa) have received less attention, particularly regarding vapor-phase processes. Recent analyses of mildly shocked Ryugu asteroid samples show nanophase kamacite without extensive melting of phyllosilicate-rich matrices or dehydration textures (Kimura et al., 2024), underscoring the significance of this comparatively less-explored regime. In many astrophysical environments, particularly within the ISM, low-velocity shock waves are characterised by velocities of the order of 1–2 km/s (Jones et al., 1996; Rudnitskij, 1997). Under such low-velocity conditions, dust grains are subjected to thermal processing rather than destruction or complete evaporation (Biennier et al., 2017). In addition, recent laboratory studies on the rapid formation of interstellar minerals and mineral quantum dots show that intense shock events can melt precursor materials within milliseconds (Roy et al., 2025). These mineral assemblages produced by shocks are particularly significant for interpreting ISM and cometary environments, especially with respect to condensation from the vapour phase.

In this study, we demonstrate that shock-wave-induced nucleation of Fe-Ni alloys occurs from solid-state Fe and Ni powder precursors (<10 μm) under conditions similar to those in turbulent, metal-rich regions of the ISM. The resulting nanophase metals were thoroughly analysed using advanced electron microscopy and spectroscopy techniques. The findings show that nanophase kamacite (α-Fe-Ni) with a composition of Fe∼₀.₉₀Ni∼₀.₁₀ can nucleate and grow within transient shocks reaching a reflected shock temperature of about 6,000 K and a reflected shock pressure of around 14.5 bar for ∼2 ms. These results substantiate that shock-induced nucleation offers a plausible natural pathway for producing nanophase Fe-Ni metal, which is vital for interpreting in-situ observations of metallic nanoparticles in extraterrestrial materials such as lunar regolith and cometary dust, as well as remote sensing detection of gaseous Fe I and Ni I in comets.

Iron-nickel alloys are fundamental building blocks of planetary materials. Yet their formation pathways under the low-pressure, low-temperature conditions typical of the ISM and the outer solar system remain unresolved. Kamacite (α-(Fe, Ni) containing 4–7.5 wt% Ni) is the dominant metallic phase in many extraterrestrial materials. In typical metallurgical processes and during slow cooling in differentiated asteroid cores, the evolution of Fe-Ni alloys is largely governed by the Fe-Ni phase diagram. Sintering mixed Fe and Ni powders at temperatures of about 1,000 °C–1,250 °C promotes diffusion-controlled formation of taenite (γ-Fe, Ni), a nickel-rich fcc phase stabilised at elevated temperatures (Singh et al., 2018). As the alloys cool very slowly, taenite partially transforms into kamacite (α-(Fe, Ni)), a bcc phase, producing the distinctive Widmanstätten pattern in iron meteorites (Yang and Goldstein, 2005). The crystallographic relationship between kamacite and taenite commonly follows Kurdjumov–Sachs or Nishiyama–Wassermann orientation relationships. The associated phase transformation can proceed over millions of years (cooling rates of 1–100 K Myr^-1), allowing coarse, chemically zoned kamacite-taenite intergrowth to form in octahedrites (Yang and Goldstein, 2005). Recent advances in mapping the Fe-Ni phase diagram through ab initio modelling, combined with diamond-anvil cell experiments, provide vital insights into both ambient and planetary core conditions (Sun et al., 2024; Torchio et al., 2020; Wei et al., 2025). The bcc solid solutions dominate at core-like pressures above about 330 GPa and >6000 K, with Ni alloying stabilising the bcc structure and elevating the melting curve (Sun et al., 2024; Torchio et al., 2020; Wei et al., 2025). These high-pressure and high-temperature phase relations help contextualise the prevalence of bcc Fe-Ni observed in rapidly quenched samples formed at far lower pressures.

Compared with slow cooling over millions of years in planetary cores, shock processing imposes extreme kinetic constraints that fundamentally alter the evolution of the Fe-Ni phase. In the present experiment, reflected shocks heat the sample to peak temperatures of >6000 K at reflected pressures of about 14.5 bar for only 2–3 milliseconds. The sample is subsequently cooled at rates of about 10⁶ K/s. Under such nonequilibrium conditions, the thermodynamic path through the Fe-Ni phase diagram is effectively near-vertical, thereby narrowing the time-temperature window in which solid-state diffusion-controlled transformations and exsolution normally occur. This rapid quenching regime favours direct condensation of bcc kamacite nanoparticles from the vapour phase, bypassing the usual fcc (taenite) → bcc (kamacite) transformation sequence and suppressing large-scale chemical zoning. The resulting fine-grained bcc metal microstructures closely resemble those documented in shock-metamorphosed meteoritic metals, where transient high-temperature, high-pressure pulses and rapid cooling also produce nonequilibrium kamacite textures.

The nanophase Fe-Ni alloy produced in these shock experiments shows striking similarities to metal grains found in the most primitive extraterrestrial materials (Vandemoortele et al., 2007). Cometary dust contains abundant amorphous silicates known as GEMS (Glass with Embedded Metal and Sulfides), submicron particles regarded as essential constituents of the outer solar system (Bradley, 1979; Keller and Messenger, 2011). The origin of GEMS remains debated: one view interprets them as chemically homogenised, irradiated interstellar grains later incorporated into the solar system (Bradley, 1979), whereas another favours formation as non-equilibrium condensates in the early solar nebula (Keller and Messenger, 2011). Isotopic studies indicate that only a small percentage (1%–6%) can be confidently identified as surviving presolar circumstellar grains (Keller and Messenger, 2011). In GEMS, nanophase Fe-Ni metals, such as kamacite, are thought to act as nucleation centres around which the amorphous silicate matrix forms the bulk of GEMS (Keller and Messenger, 2011). Rare instances of GEMS with crystalline kamacite cores support this hypothesis, implying that metallic seeds served as substrates for subsequent silicate accumulation. In chondritic porous interplanetary dust particles (IDPs), kamacite inclusions, typically 5–50 nm across, are common, with compositions in the range Fe₀._93-0.₉₇Ni₀._03-0.₀₇ (Keller and Messenger, 2011). The shock-synthesised kamacite reported here has a slightly larger mean size (∼33 nm) and is marginally more Ni-rich, yet remains within the compositional and size range observed for natural kamacite in GEMS-bearing IDPs.

Bradley and co-workers argued that GEMS are ancient, likely presolar materials that largely escaped substantial parent-body thermal metamorphism (Bradley and Dai, 2004). Their embedded metal grains show no evidence of prolonged annealing and instead point to rapid, strongly non-equilibrium formation histories (Bradley, 1979; Ishii et al., 1979). The high dislocation densities, deformation twins, and compositional homogeneity documented in the shock-synthesised kamacite closely parallel the microstructural traits of GEMS-hosted metals, indicating that both may have formed through analogous rapid processes, such as vapour-phase condensation or quench crystallisation from a transient melt.

The Fe-Ni powders used in this study have particle sizes <10 µm and serve as the starting material. This size range was chosen to approximate the dominant fine fraction of lunar-type regolith: particle-size analyses of Chang’E5 soils show that 95% of grains by number fall between 1.40 and 9.35 µm, indicating that micron-scale particles constitute the primary dispersed population on airless planetary surfaces (He et al., 2024; Li et al., 2022). These fine grains, which host nanophase Fe and Fe-Ni-S within impact-melt glass and agglutinates, are the primary carriers of space weathering signatures, including the Moon’s optical darkening and reddening (Gu et al., 2025; Moretti et al., 2005; Pieters and Noble, 2016). From a shock-physics perspective, micron-scale grains are large enough to retain heat and form melt pockets under brief, high-temperature pulses, yet small enough to be heated quasi-uniformly during a single shock event, as demonstrated by in situ micrometeoroid-analogue heating experiments on pentlandite that produce Fe-Ni whiskers and S-depleted rims on micron-scale grains (Thompson et al., 2025). In our experiments, the primary and reflected shocks together impose a two-step thermal history that efficiently vapourises and recondenses the material into kamacite nanoparticles. The incident shock first compresses and heats the Ar gas to about 2800 K while accelerating it to a post-shock velocity of ∼1.68 km/s, leaving much of the energy in bulk motion rather than internal heating. When this supersonic flow impinges on the end flange, a reflected shock wave forms and propagates upstream, decelerating the gas and converting kinetic energy into internal energy, thereby raising the temperature to ∼6000 K. Under these extreme conditions, refractory Fe and Ni, with boiling points near 3134 K and 3003 K, respectively, undergo partial melting during the primary shock and then rapid, nonequilibrium evaporation during the reflected shock pulse. The combination of rapid heating and subsequent quenching favours direct vapour-phase condensation of bcc kamacite nanoparticles, bypassing diffusion-controlled transformations and yielding particle sizes, compositions, and a defect-rich microstructure comparable to those inferred for kamacite in GEMS-bearing cometary and interplanetary dust.

NASA’s Stardust mission’s return of samples from comet 81P/Wild 2 provides additional constraints, as they contain nanophase Fe-Ni metal similar to that seen in cometary IDPs, although many grains show modifications associated with hypervelocity capture in silica aerogel (Ishii et al., 1979; Keller et al., 1979; Zolensky et al., 2006). Where minimally altered metal is preserved, compositions cluster around Fe₀.₉₀Ni₀.₁₀ indistinguishable from our synthetic kamacite and systematically offset from solar Fe₀.₉₃Ni₀.₀₇, reinforcing the link between experimental products and cometary metals.

The consistent Ni enrichment relative to solar abundances probably reflects vapor-phase fractionation processes. Observations of cometary comae report Ni/Fe ratios near unity, roughly an order of magnitude above solar, indicating that Ni can be strongly overrepresented in the gas phase (Manfroid et al., 2021). Experiments and models of metal vapor condensation under reducing, moderately supersaturated conditions show that Ni nucleates more easily than Fe, offering a natural explanation for the observed enrichment when kamacite condenses from such vapors (Bromley et al., 2021).

The detection of neutral Fe I and Ni I emissions in comets at heliocentric distances of one to three AU, where surface temperatures are only ∼150–340 K, rules out conventional sublimation of refractory minerals (stability >1,200 K) or metallic Fe–Ni (melting ∼1,400–1,500 K) as the dominant source (Guzik and Drahus, 2021; Hutsemékers et al., 2021; Manfroid et al., 2021). Yet production rates of ∼10²²–10²³ atoms s^-1 for Fe and Ni imply substantial metal release into the coma, pointing instead to mechanisms such as non-thermal desorption, chemical erosion, transient grain heating, superheating of submicron particles, impact vaporization reaching ∼10⁷ K, or photodissociation of volatile metal-bearing species such as Fe(CO)₅ and Ni(CO)₄ (Guzik and Drahus, 2021; Hutsemékers et al., 2021; Manfroid et al., 2021).

Among these possibilities, the organometallic hypothesis is particularly attractive because Fe(CO)₅ and Ni(CO)₄ have sublimation temperatures only slightly above that of CO₂ (∼150–200 K), consistent with metal release far from the Sun, and Ni(CO)₄ sublimates roughly ten times faster than Fe(CO)₅ near 300 K, naturally producing Ni-rich gas (Bodewits and Bromley, 2021; Manfroid et al., 2021). However, metal carbonyls have not yet been directly detected in comets and their formation pathways remain uncertain, motivating an alternative picture in which metal atoms liberated by any of the above mechanisms recondense as nanophase kamacite when local vapor densities and cooling rates are favorable, with kinetic effects during condensation preferentially incorporating Ni.

The radial behaviour of Fe and Ni emissions also constrains the nature of their parent species: a 1/ρ dependence of the brightness profile implies short-lived parents with destruction scales on the order of the nucleus size, and Haser modelling yields parent scales of about 170 km at 1 AU for Ni-bearing species, corresponding to lifetimes of roughly 140–600 s (Guzik and Drahus, 2021). These lifetimes are comparable to the effective interaction time in the shock experiments once differing expansion speeds are taken into account, with shock-processed material expanding at about 1.68 km/s in the laboratory versus around 0.5 km/s in typical cometary comae (Guzik and Drahus, 2021), suggesting that broadly similar kinetic and thermal regimes may operate in both environments.

Astronomical observations of infrared spectra of dust in supernova remnants and at shocked molecular cloud interfaces show contributions from both amorphous silicates and crystalline metal oxides (Rho et al., 2008), indicating ongoing dust reprocessing in these high-temperature regions. If metallic Fe-Ni nanoparticles form there and subsequently oxidise, they could plausibly evolve into the meta-oxide populations responsible for part of the observed infrared emission features (Bouvis et al., 2025). Conversely, in environments that remain sufficiently reducing, some fraction of these metal grains may avoid oxidation, survive as zerovalent Fe-Ni, and later be incorporated into nascent planetary systems as refractory metallic components (Bouvis et al., 2025).

Finally, the characteristic size range of the shock-synthesised nanophase kamacite grains (tens of nanometres) places them in a regime where radiation pressure can strongly influence their dynamics. For such sub-micron particles, the ratio of radiation pressure to stellar gravity can approach or exceed unity, leading to efficient outward acceleration and potential removal from circumstellar regions, preferentially injecting them into the ISM (Burns et al., 1979). This could establish a circulation pattern where metal nanoparticles formed in star-forming regions are expelled into the ISM, mix with gas and dust over gigayear timescales, and ultimately recycled into subsequent generations of protoplanetary disks, where they may reappear as GEMS-like inclusions and related metal phases.

Our shock-wave synthesis of nanophase Fe-Ni alloy at millisecond timescales, combined with recent astronomical detections of atomic Fe I and Ni I vapors in comets and comprehensive characterization of the synthetic products, establishes a plausible formation pathway for metal nanoparticles in various cosmic environments. The key findings of this study are:

Nanophase kamacite with a composition of Fe_∼0.90Ni_∼0.10, grain sizes of 20–200 nm (mean 33 nm), and a bcc crystal structure forms within milliseconds under reflected shock pressure of 14.5 bar and temperatures of ∼6000 K, demonstrating that extended thermal processing is not necessary for metal alloy formation. High dislocation densities, deformation twins, compositional homogeneity, and the lack of chemical ordering in the shock-synthesised kamacite suggest it formed under far-from-equilibrium conditions with ultrarapid cooling (∼10⁶ K s^-1). This is consistent with vapour condensation or quench crystallisation.