Typical heating rates for asteroidal particles (entry velocities 12 km s"1) suggested by entry heating models are -500 K s"1 [1]. The finite element simulations therefore suggest that such micrometeoroids could only support thermal gradients of -30 K (for a 500 |xm diameter particle) if these result only from non-steady state heating and that thermal gradients will quickly equilibrate at peak temperature. Core-rim temperature differences of 30 K would be sufficient to generate the melted rims observed on micrometeorites recovered from the Earth's surface, however, only those particles whose surfaces reached temperatures close to the melting point would be expected to preserve melted rims. This is contrary to the large number of fine-grained micrometeorites that have melted rims and unmelted cores. The observation that cored particles vary from those with rims a few microns in sizes to those which contain one or more small areas of unmelted fine-grained matrix suggests that melted rims are a general feature of the melting process of micrometeorites. The simulations also indicate that temperature differences of up to 600 K in particles as small as 100 (im in size do not result from non-steady state heating.

Previous steady-state calculations on the thermal evolution of phyllosilicate-bearing micrometeoroids by Flynn et al., [5] that included the contribution of the latent heat required for endothermic decomposition of water-bearing phyllosilicate minerals produce temperature discontinuities similar to those observed in micrometeorites. A dehydration/melting front thus probably exists in fine-grained micrometeorites that migrates into the particle during heating with the thermal decomposition acting as a sink for energy that maintains the lower temperature of the micrometeoroid core. Other devolatilisation and decomposition reactions such as the pyrolysis of carbonaceous materials and the breakdown of sulphide minerals may also contribute significantly to this affect and enable temperature differences of the magnitude observed in some micrometeorites to be maintained.

Melted-rims are, however, also frequently observed on coarse-grained micrometeorites that consist mainly of anhydrous silicates and glass. These particles contain no volatile-bearing minerals to maintain the temperature differences and yet particles with melted rims are abundant. The melted rims on these coarse-grained micrometeorites might arise through the melting of small amounts of fine-grained matrix material, which has a lower melting temperature, attached to the exterior of the particle. However, the observation that unmelted coarse-grained particles with fine-grained matrix are rare amongst micrometeorites is contrary to the high abundance of particles melted rims. Potentially melted rims on coarse-grained micrometeorites could be generated by temperature differences of only a few degrees since there is no means of identifying what the peak temperature the cores of these particles attained. The abundance of particles with unmelted rims is, however, not consistent with such an origin since only a small fraction of coarse-grained micrometeorites should have peak temperatures in close to the melting point of their constituent minerals.

One final possibility is that the temperature differences are in part maintained by energy losses to vaporisation at the surface of particles. If the vaporisation rate is high enough that mass losses cause significant decreases in particle size then significant energy losses could occur due to the latent heat of fusion at the melt-core boundary and the latent heat of vaporisation at the particle surface. If this process is an important factor in the development and survival of the temperature differences observed in micrometeorites then particles with melted rims have probably experienced significant mass loss and care must be taken when considering the particle-size distribution of the different micrometeorite types.

The development of melted rims on micrometeoroids during entry heating will enhance the survival of unmelted primitive extraterrestrial materials as the cores of heated particles. Micrometeoroids with relatively high geocentric velocities may therefore be more likely to be preserved to reach the Earth's surface with at least a proportion of the original nature of their refractory components intact by virtue of surface melting. Similarly low-temperature volatile materials such as abiotic hydrocarbons may also survive atmospheric entry without complete decomposition in particles with low geocentric velocities. These materials would have been a potentially important source of pre-biotic carbon on the early Earth and may have played a role in the origin of life on our planet.

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