Thermal gradients in micrometeoroids during atmospheric entry

Melted rims found on micrometeorites recovered from Antarctic ice indicate that micrometeoroids as small as 50 pm in diameter can maintain temperature differences of at least 600 K between their surfaces and cores. We present the results of finite element simulations of the thermal evolution of micrometeoroids during entry heating that indicate that large thermal gradients cannot arise simply as a result of the non-steady state heating of particles. The generation of thermal gradients resulting in melted rims may occur in fine-grained micrometeorites due to energy losses at the melt-core boundary due to the endothermic decomposition of volatile-bearing phases. However, the occurrence of melted rims on many coarse-grained particles that lack such low-temperature phases suggests this is not the primary cause of the temperature differences. Large mass losses due to vaporisation and energy losses due to fusion may therefore be involved in the generation of melted rims. The presence of thermal gradients in micrometeoroids during atmospheric entry increases the likelihood that low-temperature primary phases such as abiotic carbonaceous compounds will survive atmospheric entry heating.