A Comet Model. III. The Zodiacal Light.

The Poynting-Robertson effect is shown to require about 1 ton/sec of small particles to maintain the zodiacal cloud, irrespective of particle shape, density, or dimension within the range of 10-Ul .0cm. The zodiacal cloud is assumed to be of the nature deduced by van de llulst and Allen from their studies of the Fraunhofer corona and the zodiacal light. The probable cometary contribution to the zodiacal cloud is here considered on the basis of the icy-comet model. Some 30 tons/sec of meteoritic material contributed continuously in typical comet orbits are mostly lost by the action of the following physical forces or processes as the particles spiral inward toward the sun by the Poynting-Robertson effect: (a) interstellar wind, (b) Jupiter's random perturbations, (c) the Jupiter perturbational barrier, and (d) collisional destruction. Of these, b and d are found to be the most important. The final calculated contribution is about the required amount. Collisions among the particles appear to be largely responsible for the cutoff in zodiacal particle size above about 0.03 cm, as found by van de Hulst. Corpuscular radiation from the sun will simulate the Poynting-Robertson effect but will simultaneously tend to destroy the particles. No allowance for this effect is included in the calculations because of uncertainties in the numerical quantities involved. Corpuscular radiation, however, if sufficiently powerful, may exceed the Poynting-Robertson effect in importance and may also demand a larger source of material for the zodiacal cloud. If so, the corpuscular radiation will also increase the critical cutoff dimension.