Dust mantle of comet 9P/Tempel 1: dynamical constraints on physical properties

The trajectories of dust particles ejected from a comet are affected by solar radiation pressure as a function of their ratios of radiation pressure cross section to mass. Therefore, a study of the orbital evolution of the particles caused by the radiation pressure reveals the physical properties of dust on the surface of the comet nucleus. In the course of NASA's Deep Impact mission, the ejecta plume evolved under the influence of the radiation pressure. From the evolution and shape of the plume, we have succeeded in obtaining β ≈ 0.4, where β is the ratio of the radiation pressure to the solar gravity. Taking β ≈ 0.4 into account as well as the observational constraints of a high color temperature and a small silicate-feature strength, dust particles ejected from the surface of comet 9P/Tempel 1 are likely to be compact dust aggregates of sizes ≈ 20 μm (mass ~ 10^-8 g). This is comparable to the major dust on the surface of comet 1P/Halley (~10 μm) inferred from in-situ measurements and theoretical considerations. Since such dust aggregates with β ≈ 0.4 must have survived jets due to ice sublimation on the surface, the temperature of ice in the nucleus must be kept below 145 K, which is much lower than the equilibrium temperature determined by solar irradiation and thermal emission. These facts indicate that 9P/Tempel 1 has a dust mantle composed of 20 μm-sized dust aggregates with low thermal conductivities ~1 erg cm^-1 K^-1 s^-1.