On the origin and distribution of meteoroids

Abstract

The influence of collisional and radiative processes on the population of sporadic and shower meteoroids is examined. It is found that the observed distribution of sporadic meteoroids in the photographic and radio ranges is unstable: collisional processes would rapidly change the distribution of these particles if they are not replenished by a source. It is shown that, in order to maintain the distribution of these sporadic meteoroids in a steady-state condition, a source is needed that replaces the sporadic meteoroids destroyed by collisions. The mass distribution of the required source is calculated and found to agree with the distribution of bright radio-shower meteors, provided that the population index α of sporadic meteors (in the photographic and radio ranges) is taken to be α = 13/6. The evolution of the population in a meteor shower under the influence of collisions with sporadic meteoroids is also examined. Since small particles have shorter lifetimes (with respect to collisions) than do larger particles, the population of faint shower meteors is more strongly affected. It is found that the mass distribution of faint radio meteors in the major showers considered here agrees with the results of the analysis, provided that α = 13/6, for sporadic meteors. Taking α = 13/6 therefore provides self-consistency for analysis and observation, thereby providing new evidence favoring the cometary origin of meteors. The rate at which the population of sporadic meteoroids is losing mass is calculated and found to compare favorably with F. L. Whipple's 1967 estimate as well as his estimate of the rate at which comets are injecting meteoritic mass into the solar system. The influence of the Poynting-Robertson effect on the meteoroid mass distribution is found to be minor; collisional processes dominate the evolution of meteoroids in the faint and brighter radio range. The influence of radiation pressure may be profound. By using previous work by Harwit and geometrical optics, it is shown that the smallest masses present in the major meteoroid showers are in the range of 10−6 to 10−10 gram. This is precisely the mass range where a leveling off of the distribution of sporadic meteoroids is indicated by satellite penetration measurements; these measurements are therefore consistent with the cometary origin of sporadic meteoroids. These theoretical results may not apply to particles with masses of 10−12 gram or smaller (i.e., submicron particles, for which details of physical optics become important).