Introduction

The thermal behavior of micrometeoroids determines their survival of atmospheric entry and their state of alteration and thus strongly influences the sample of the interplanetary dust population that can be collected on the Earth. Models of the atmospheric entry of micrometeoroids specifically assume that particles are thermally homogeneous during heating [1], This simplification significantly reduces the complexity of simulations and is based on a formulisation of the Biot number adapted to radiative heat loss under steady state heating and thus may not be appropriate under non-steady state transient heating by the hypervelocity collisions with air-molecules during atmospheric entry.

Micrometeorites larger than 50 (J,m collected on the Earth's surface, however, exhibit clear evidence for thermal gradients developed during entry heating [2]. Cored micrometeorites have vesicular melted rims consisting of Fe-rich olivine microphenocrysts in glassy mesostases and unmelted cores some of which retain phyllosilicates (Fig. 1). These particles suggest that temperature differences between the surface and core of the micrometeoroid can exceed 600°C [2],

Figure 2 A backscattered electron image of a coarse-grained micrometeorite with a thin melted rim.

Figure 1 A melted rim (light coloured outer layer) on an otherwise unmelted fine-grained micrometeorite.

Figure 2 A backscattered electron image of a coarse-grained micrometeorite with a thin melted rim.

The origin of large temperature gradients in micrometeoroids is problematic because only a small fraction of the incident energy flux provided by the collision of air molecules is required to heat the particle to peak temperature [1]. Low effective thermal conductivities, due to high porosity, and energy losses due to the vaporisation of low temperature phases are possible explanations for the development of large thermal gradients in small micrometeoroids. The occurrence of melted rims on compact coarse-grained micrometeorites (Fig. 2; [3]) that lack low temperature, volatile components, indicate that neither decreases in thermal conductivity or energy sinks due to devolatilisation are the primary cause of thermal heterogeneity.

On the basis of the thermal evolution of micrometeoroids predicted by 'homogeneous' particle entry heating models we have suggested that thermal gradients might be supported due to the rapid increase in the surface temperature of particles during deceleration [4]. To determine whether thermal gradients develop simply in response to non-steady state, singlepulse heating we have conducted two- and three-dimensional finite element simulations of the thermal evolution of micrometeoroids during entry heating.

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