20. The ablation of solid meteoroids

Abstract

Ablation profiles for compact meteoroids have been obtained which take account of the effects of thermal radiation, conduction, meteoroid heat capacity and deceleration, while retaining the simplifying assumption that the onset of massive ablation occurs at a well-defined temperature.The dependence of the air density, ρa, at the onset of ablation on the initial meteoroid radius, R0, is in agreement with the results of Ceplecha and Padevět (1961). For a meteoroid of given composition, velocity and zenith angle there are four critical values for the radius, R1, R2, R3 and R4. R1 is the micrometeorite limit such that if R<R1, the ablation temperature is not reached. For R1<R<R2 the onset of ablation is delayed due to thermal radiation and occurs at air density ρa1, which is substantially independent of radius. If R2<R<R3 the delay in onset of ablation is due to the finite heat capacity of the meteoroid; it occurs at an air density ρa2, which is proportional to R0, i.e. to m01/3, where m0 is the initial mass. If R > R3 the meteoroid develops a marked thermal gradient and, if it is brittle, will fragment before ablation commences. The smallest fragments will ablate at the air density ρa1, which is shown to be close to the experimental result of Jacchia et al. (1967) for photographic meteors. If such a particle survives fragmentation, the molten droplet will fragment due to distortion if R > R4 (Lebedinec and Portnjagin, 1966); it is shown that R4 is not substantially different in magnitude from R3. Meteoroids in the size range R2<R<R3 produce underdense meteor trains and experimental measurements of echo decay as a function of echo amplitude are in agreement with the expectation that ρa2 ∝ m0n where n ~ 1/3 (Poole and Kaiser, 1967). This would support the view that the bulk of radio meteors are produced by compact particles and that the fragmentation observed with photographic meteors may be a result of thermal shock.The ablation profiles have been computed for initial meteoroid velocities between 20 and 70 km sec−1 and radii between R1 and R3; the deceleration and residual size (at the end of ablation) have also been calculated. Even quite close to the micrometeorite limit, the residual particle is small compared with the initial size. Hence, meteoroids with R > R1 will make negligible contribution to the micrometeorites reaching the Earth's surface, except, perhaps, where a significant proportion of the mass of larger meteoroids fragments into particles with radius less than R1. By neglecting deceleration, it is possible to arrive at an analytic expression for the ablation profile; this is found to be in good agreement with the computed profiles except for particles near to the micrometeorite limit.