THE CONTRIBUTION OF COMETARY DUST TO THE ZODIACAL CLOUD

Any theory of the origin of the particles that supply the zodiacal cloud must account for two key, well-established observations. These are: (1) the observed plane of symmetry of the cloud; and (2) the observed shape of the cloud, that is, the observed variation of the flux in a given waveband with ecliptic latitude for a given elongation angle. The dynamics of small asteroidal particles appears to account for the first observation (Dermott et al., Chaos, Resonance and Collective Phenomena the Solar System, pp. 333-347. Kluwer, Dordrecht, 1992). However, asteroidal particles that spiral towards the Sun under the action of drag forces without significant disintegration due to dust-dust collisions do not, on their own, provide an explanation for the observed shape of the cloud, because asteroidal dust models provide insufficient flux tat the Earth's ecliptic poles (Dermott et al., Chaos, Resonance and Collective Phenomena in the Solar System, pp. 333-347. Kluwer, Dordrecht, 1992). In an attempt to account for this major discrepancy, the dynamics of cometary particles is investigated. The orbital evolution of 9 mu m diameter dust particles that originate from Comet Encke is described. The orbits of 5000 particles are integrated numerically in order to determine their spatial distribution in 1983. All planetary perturbations (except those due to Mercury and Pluto), radiation pressure, Poynting-Robertson drag, ,and solar wind drag are included in the calculation. The SIMUL code (Dermott et al., Comets to Cosmology, pp. 3-18. Springer, Berlin, 1988) is used to calculate the shapes of several model zodiacal clouds consisting of a range of combinations of cometary and asteroidal particles. By comparing the model results with IRAS observations in the 25 mu waveband, it is shown that the observed shape of the zodiacal cloud can be accounted for by a combination of about 1/4 to 1/3 asteroidal dust and about 3/4 to 2/3 cometary dust. This result is consistent with other work on the structure of the IRAS dust bands and the Earth's resonant ring (Dermott et al., Nature 369, 719-723, 1994; Asteroids, Comets and Meteors, 1993, pp. 127-142. Kluwer, Dordrecht, 1994). It is also consistent with the conclusions of other workers that the cloud must be heterogeneous (Levasseur-Regourd et al., Icarus 86, 264-272, 1990; Origin and Evolution of Interplanetary Dust, pp. 131-138. Kluwer, Japan, 1991).