Atmospheric entry of hydrated, phyllosilicate-rich micrometeorites: experiment and numerical model

Numerical modelling is crucial for understanding micrometeorite atmospheric entry, yet most existing models treat cosmic dust grains as chemically inert, anhydrous particles. However, empirical studies of micrometeorites recovered on Earth reveal that hydrated, phyllosilicate-bearing particles dominate the cosmic dust flux at size fractions above ~100 µm. The thermal decomposition of phyllosilicates is expected to play a significant role in reducing peak temperatures during entry, thereby increasing the chances of their survival to the Earth's surface, but this process is currently not incorporated in most models. To address this, we developed the first numerical model simulating the thermal behaviour of phyllosilicate-dominated micrometeorites during atmospheric entry. Building on the Love and Brownlee model, we include both sub-solidus decomposition and supra-solidus evaporation processes, as constrained by thermogravimetric analysis data from heating experiments on cronstedtite and saponite, the main phyllosilicate species in CM, CR and CI chondrites. Three particle-specific factors govern decomposition behaviour of phyllosilicate-dominated micrometeorites during entry: (1) grain density, (2) enthalpy of dehydration, and (3) the volatile budget. The sub-solidus loss of water helps reduce peak temperatures in phyllosilicate micrometeorites, but the effect is relatively modest compared to anhydrous olivine. Furthermore, saponite experiences lower peak temperatures than cronstedtite, despite cronstedtite having a higher enthalpy of decomposition and a larger volatile budget. This effect is attributed to cronstedtite’s higher density, which leads to more intense thermal processing, resulting in thermal histories that resemble those of olivine-dominated micrometeorites. Since CI chondrites contain saponite, CI-like micrometeorites are more likely to survive entry without melting relative to CM-like micrometeorites under the same conditions. Finally, our results suggest that hydrated micrometeorites ~50 µm are more likely to survive atmospheric entry without loss of water only in grazing scenarios (entry angles >80°, where entry angle is measured from zero with respect to the zenith). This explains the rarity of hydrated, fine-grained micrometeorites containing intact crystalline phyllosilicates, as observed in petrographic studies of unmelted cosmic dust.