Heating and thermal transformation of micrometeoroids entering the Earth's atmosphere

We present numerical solutions for the atmospheric entry of 50,780 micrometeoroid particles between 10 μm and 1 mm in diameter, treating ablative mass loss and cooling along with gravitational and curvature effects, and using tabulated values for atmospheric density. Entry velocities ranged between 11.2 and 72 km/sec following a v^-5.394 velocity distribution, and entry angles were computed assuming a random space distribution of particles far from the Earth. Typical melted survivors were initially 1.5 to 2 times larger, with about half of all survivors larger than 70 μm being melted. At smaller diameters, the size distribution of melted particles is nearly flat, an important change from the initial size distribution slope. Little mass loss occurs in particles that do not melt. Below 70 μm, melted particles total only about 1% of the number of unmelted bodies. At sizes above 300 μm, less than 1% of the particles survive. The peak temperatures experienced by submillimeter micrometeoroids rarely exceed 1700°C. Maximum temperature and mass loss rate generally occur at altitudes between 85 and 90 km during ∼1 sec of peak heating. A typical melted particle spends ∼2 sec at temperatures above the melting point. A particle with an initial flight direction less than about 7° from the horizontal will pass through a short path of atmosphere and be lost back to interplanetary space. A major result of this work is the finding that survival of all particles in the size range 70 μm to 1 mm is limited to those with minimal entry velocity. Assuming that there is no source of low-eccentricity, low-inclination comet dust, the results of this study imply that virtually all of the >70- μm "cosmic spherules" and giant unmelted micrometeorites are asteroidal in origin.