A numerical model for the atmospheric entry of hydrated, phyllosilicate-rich micrometeorites

Abstract

Numerical modelling is crucial for understanding micrometeorite atmospheric entry, yet most existing models treat cosmic dust grains as chemically inert, anhydrous particles. In contrast, empirical studies of micrometeorites recovered on Earth reveal that hydrated, phyllosilicate-bearing particles, similar to the fine-grained matrix of carbonaceous chondrites, dominate the cosmic dust flux at size fractions above ∼100 μm. The thermal decomposition of phyllosilicates is expected to play a significant but currently unmodelled role in reducing peak temperatures during entry, thereby increasing the likelihood of their survival to the Earth's surface. To address this, we developed the first numerical model simulating the thermal response of phyllosilicate-dominated micrometeorites during atmospheric entry. Building on the Love and Brownlee (1991, Icarus, 89:26-43) model, we incorporate both sub-solidus decomposition and supra-solidus evaporation processes, as constrained by thermogravimetric analysis data from heating experiments on cronstedtite and saponite, reflecting the main phyllosilicate species found in CM, CR and CI chondrites. Three factors are crucial in determining the decomposition behaviour of phyllosilicate-dominated micrometeorites during entry: (1) grain density (d), (2) enthalpy of dehydration (Q), and (3) the volatile budget ($\zeta$_max). Of these, density is the most influential. The sub-solidus loss of water helps reduce peak temperatures in phyllosilicate micrometeorites, but the effect, compared to anhydrous olivine, is modest (<35 °C for cronstedtite and <100 °C for saponite, on average). Furthermore, on average, saponite experiences peak temperatures that are 76 °C lower than those of cronstedtite. This is 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, creating thermal histories similar to olivine-dominated micrometeorites. Since CI chondrites contain saponite, CI-like micrometeorites are more likely to survive entry without melting compared to CM-like micrometeorites under the same conditions. Finally, our results suggest that hydrated micrometeorites >50 μm are 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 accounts for the rarity of hydrated fine-grained micrometeorites that contain intact crystalline phyllosilicates, as reported from petrographic studies of unmelted cosmic dust.