Forming the first solids precursors in the Solar System through kinetic condensation

 Abstract

The three major classes of chondritic meteorites—enstatite, ordinary, and carbonaceous—exhibit distinct oxidation states and mineral compositions. Traditionally, this diversity has been attributed to formation under different redox conditions or varying gas compositions in the protoplanetary disk. However, such explanations require fine-tuned scenarios that are difficult to reconcile with a nebula of nearly solar composition.

We present results from KineCond, a time-dependent condensation model that simulates the time-dependant condensation of a cooling gas of solar composition under a wide range of pressures and cooling rates. Our simulations reveal that kinetic (i.e., non-equilibrium) condensation naturally leads to three mineralogical types only, with redox states that match those of the enstatite, ordinary, and carbonaceous chondrites when plotted in at Urey-Craig diagram -without requiring any changes in gas composition.

This suggests that the redox and mineralogical diversity of primitive Solar System solids may be the direct result of local thermodynamical conditions during condensation (cooling rates, pressure), rather than local compositional heterogeneity of fO₂. Our results offer a new physical framework for understanding the early chemical architecture and oxidation state of the Solar Nebula.

Context

Chondrites, the most primitive meteorites, preserve a record of the Solar System’s early chemical environment. Chondrites are made of diverse components, namely refractory inclusions (CAIs or AOA), chondrules, metallic inclusions, and matrix. They are traditionally grouped into enstatite (EC), ordinary (OC), and carbonaceous (CC) classes, which differ both in oxidation state and bulk composition. These classes fall into distinct regions on the Urey–Craig diagram, a classical plot of Fe oxidation state. By increasing order of oxidation we have EC, OC and CC. Traditional models explain these differences via the varying C/O ratio or O/H ratio in the protoplanetary disk, but no plausible astrophysical process has been shown to generate the necessary vast range of oxygen fugacities necessary within a solar composition disk. We propose that non-equilibrium condensation in the Solar Nebula a simple, physical explanation for this redox diversity. We show that chondrites mineral precursors formed by kinetic condensation naturally define 3 mineralogical groups, then may represent the 3 main chondrites populations due to their close match in the Urey-Craig diagram but also in bulk mineralogy.

Method

We developed KineCond, a time-dependent condensation model that simulates the cooling of a solar-composition gas from 2000 to 130 K in a closed system. The model tracks 39 direct condensation/evaporation reactions and 38 gas–solid nebular (generalized) reactions, across pressures from 10⁻⁹ to 10⁻¹ bar and cooling timescales from 0.01 to 1000 years. Solids evolve through competition between condensation, evaporation, and solid-gas exchange. Lack of kinetic data on solid-gas reactions are compensated by an exploration of reactions constants on several orders of magnitude. We find that at first order, condensation processes dominate over gas-grain reactions for determining the final mineralogies of precursors.

We define an empirical condensation index X = log₁₀(P/bar) + log₁₀(T_c/yr), which captures the influence of pressure and cooling rate. For each simulation, the resulting mineralogy is projected onto the Urey–Craig diagram and compared to known chondrite groups. X >−5 correspond close-to equilibrium condensation, whereas X<−5 correspond to fast and out-of equilibrium condensation.

Results

Our simulations show that condensates naturally segregate into three distinct mineralogical and redox groups:

• Type A: Metal-rich, reduced, enstatite-rich (X > –2), with sulfides. Recalling EH bulk mineralogy
• Type B: Intermediate oxidation, fayalite with forsterite, Recalling OC bulk mineralogy
• Type C: Oxidized, containing fayalite, magnetite and phyllosilicates (X < –6), Recalling CO-CV bulk mineralogy.

This behavior is remarkably robust: varying elemental ratios (e.g., Mg/Si, Al/Si) affects mineral proportions but not the redox state. Cooling rate and pressure, however, are decisive. Highpressure, slow-cooling conditions reproduce equilibrium results, and close to EC mineralogy, but under realistic nebular conditions (e.g., P ∼ 10⁻⁴ bar at 1 AU), kinetic effects dominate. Fast condensation leads to oxidation because under rapid cooling, atoms and molecules in the gas phase do not have enough time to fully equilibrate with condensates. Oxygen, being the most abundant

Figure 1: Urey-Craig diagram. X axis: Number oxidized Fe atoms/Si and normalized to solar Fe/Si, Y axis : fraction of metal (Fe+FeS) and normalized to Solar . Colored shapes shows measurements  in chondrites populations. Solids lines show results of Kinecond condensation simulations. In blue : slow condensation (type A condensates), in blue : moderately fast condensation (type B condensates), in red : very fast and out of equilibrium condensation (type C condensates). Oxidation state of CM and CO are never matched (parent body water circulation could explain it).

condensable specie, is rapidly incorporated into solid phases as they supersaturate and condense upon rapid cooling conditions. In contrast, reduced minerals are typically produced in equilibrium condensation, that require slower reaction times or higher temperatures to establish. As a result, fast condensation ”freezes in” more oxidized mineral phases than would form under equilibrium or slower kinetics. We find that minerals typical of CAIs, AOAs and chondrules, metal nuggets, and also phillosilicates are formed under specific condensation conditions, depending on X.

Transient heating followed by recondensation (e.g., via bow shocks) leads to similar groupings.

Discussion

Kinetic condensation offers a natural explanation for the redox and mineralogical diversity of chondrite precursors. Unlike equilibrium models, it does not require ad hoc parameters (like the ”isolation factors”) or large-scale chemical heterogeneities to reproduce at the same time reduced and oxidized mineralogies. The key factor is the pressure and the condensation time. Fast condensation at low pressure favors oxidized forms, whereas slow condensation at high pressure favors reduced minerals. The emergence of three stable mineralogical regimes from solar gas alone reproduces the key structure of the Urey–Craig diagram (Fig. 1).

This suggests that the redox properties of chondrites may reflect early condensation kinetics rather than later alteration.  Our results suggest a paradigm in which the redox properties of planetary building blocks are inherited from non-equilibrium condensation.

Acknowledgments

This work was supported by the DISKBUILD project (ANR-20- CE49-0006), the LabEx UnivEarthS initiative (ANR-10-LABX-0023 and ANR-18-IDEX-0001) and by the French space agency CNES (Centre National d’Etudes Spatiales), the Swiss National Science Foundation (SNSF) , Eccellenza Professorship (203668) , SERI contract No. MB22.00033, a SERI-funded ERC Starting grant “2ATMO”