31. The interplanetary dust cloud (survey paper)

Abstract

The first part of the paper reviews the present state of knowledge of the characteristics of cosmic dust in interplanetary space. Since this is derived from a variety of observational techniques, some attempt is made critically to assess the difficulties in interpretation, particularly those due to differ ences in observational selection. Attention is drawn to the doubts that recently have arisen concerning the existence of a terrestrial dust cloud. The second part describes some radio investigations of the structure of meteor streams and of the sporadic background. Systematic variations in magnitude distribution with solar longitude which are observed in both the Geminids and Perseids cannot be simply interpreted as due to selective perturbation of the smaller meteoroids. Experimental data are described which point to the existence of considerable radiant structure in the sporadic background. IntroductionMinor objects in the solar system range in size from the asteroids to dust grains of the order of sub-micron size, which are just large enough to survive the pressures due to solar electromagnetic and corpuscular radiation. The smaller particles have a limited lifetime due to braking effects, and hence we must assume a dynamic equilib rium in which matter is continuously supplied from sources usually believed to be cometary. This is not to say that over the history of the solar system, the cosmic dust cloud has had stationary characteristics; indeed modern views of planetary evolution invoke the prior existence of cosmic dust on a considerable scale. It is perhaps only from studies of material of cosmic origin in deep-sea sediments and, to a limited extent, in tracing backwards orbital parameters obtained from meteors, that we can find direct evidence for evolutionary change in the cosmic-dust distribution.The range of sizes of the particles we are considering is so wide that no single observational technique encompasses more than a limited part and each is subject to observational selection. Nor are the size distributions obtained in different ways necessarily completely independent estimates. For instance, in interpreting the zodi acal light data it is necessary to postulate the form of the particle-size distribution and this may be obtained from meteor observations, even though these latter refer to much larger particles. Although no single method of observation leads to an unambig uous result for the abundance of cosmic dust, many attempts have been made to assemble and compare the various experimental data. One such is illustrated in