THE SIZE, MASS AND EVOLUTION OF THE SOLAR-SYSTEM DUST CLOUD

It is suggested that the form of the doughnut-shaped Solar System dust cloud can be deduced from cometary mass, decay and orbital statistics . In saying this, we stress the fact that most of the mass of this clo ud is in the form of meteoroids with individual masses in the range 10 (-6) to 10(2) g, and in this mass range the contribution from sources such as asteroid breakup and interstellar dust capture is less than Io per cent of the total. The cloud mass is estimated to be (3.0 +/- 1.0 ) x 10(20) g. At the present time, Earth picks up about 2.2 X 10(11) g of interplanetary material each year, the cometary component of this dust having a spatial density of about 1.0 x 10-(21) g cm(-3) at the E arth's orbit. Today's mass influx to the cloud is found to be dominate d by the decay of comets Pens-Brooks, Halley, Olbers and Swift-Tuttle. These are all 'intermediate', i.e. Halley-type comets. This paper con siders the mechanisms responsible for removing dust from the cloud, an d concludes that it is not in equilibrium, being more massive at the p resent time than is to be expected from the decay of the comets that c an be seen during the present epoch.