Sumita Jayaraman

Orbital Evolution of Interplanetary Dust

The two most important dynamical features of the zodiacal cloud are: (i) t he dust bands associated with t he major Hirayama asteroid families, and (ii) the circumsolar ring of dust particles in resonant lock with th e Eart h. Oth er important dynamical features include the offset of th e center of symmetry of th e cloud from the Sun, the radial gradient of the ecliptic polar brightness at th e Earth, and th e warp of th e cloud. The dust bands provide th e st rongest evidence th at a substantial and possibly dominant fraction of the cloud originate s from aster oids. However, the characteristic diameter of these asteroidal particles is probably several hundred microns and the migration of th ese large particles towards th e inner Solar System due to Poynting Robert son light drag and their slow passage through secular resonances at the inner edge of the asteroid belt result s in large increases in th eir eccent ricities and inclinations. Because of these orbital changes, the dividing line between asteroidal and comet ary type orbits in the inner Solar System is probably not sharp, and it may be difficult to distinguish clearly between ast eroidal and cometary particles on dynamical grounds alone.

Origin of the ten degree Solar System dust bands

The Solar System dust bands discovered by IRAS are toroidal distributions of dust particles with common proper inclinations. It is impossible for particles with high eccentricity (approximately 0.2 or greater) to maintain a near constant proper inclination as they precess, and therefore the dust bands must be composed of material having a low eccentricity, pointing to an asteroidal origin. The mechanism of dust band production could involve either a continual comminution of material associated with the major Hirayama asteroid families, the equilibrium model (Dermott et al. (1984) Nature 312, 505–509) or random disruptions in the asteroid belt of small, single asteroids (Sykes and Greenberg (1986) Icarus 65, 51–69). The IRAS observations of the zodiacal cloud from which the dust band profiles are isolated have excellent resolution, and the manner in which these profiles change around the sky should allow the origin of the bands, their radial extent, the size-frequency distribution of the material and the optical properties of the dust itself to be determined. The equilibrium model of the dust bands suggests Eos as the parent of the 10° band pair. Results from detailed numerical modeling of the 10° band pair are presented. It is demonstrated that a model composed of dust particles having mean semimajor axis, proper eccentricity and proper inclination equal to those of the Eos family member asteroids, but with a dispersion in proper inclination of 2.5°, produces a convincing match with observations. Indeed, it is impossible to reproduce the observed profiles of the 10° band pair without imposing such a dispersion on the dust band material. Since the dust band profiles are matched very well with Eos, Themis and Koronis type material alone, the result is taken as strong evidence in favor of the equilibrium model. The effects of planetary perturbations are included by imposing the appropriate forced elements on the dust particle orbits (these forced elements vary with heliocentric distance). A subsequent model in which material is allowed to populate the inner solar system by a Poynting-Robertson drag distribution is also constructed. A dispersion in proper inclination of 3.5° provides the best match with observations, but close examination of the model profiles reveals that they are slightly broader than the observed profiles. If the variation of the number density of asteroidal material with heliocentric distance r is given by an expression of the form 1rτ then these results indicate that γ < 1 compared with γ = 1 expected for a simple Poynting-Robertson drag distribution. This implies that asteroidal material is lost from the system as it spirals in towards the Sun, owing to interparticle collisions.