The 10 μm feature of aggregates in comets

Thermal emission form cometary dust grains has been examined using model calculations of aggregates with fractal structure consisting of two components, namely silicate (olivine) and absorbing mateial (magnetite). Calculations of the thermal emission have been performed by using the discrete dipole approximation for small aggregates, and Mie theory with the Maxwell-Garnett rule for large aggregates. We have used the size distribution of the aggregates converted from the mass distribution of grains detected in comet P/Halley (Mazets et al., Nature 321, 276, 1986), taking into account the fractal aggregate model for cometary grains. It is found that the spectral feature at about 10 μm wavelength is mainly controlled by a fractal dimension D of the aggregates, wheras the temperature of the aggregates strongly depends on a mixing volume ratio M of magnetite to a whole volume, i.e. M = (magnetite (olivine - magnetite)). When the cometary dust grains consist of the fractal aggregates with D ≅ 2 and M = 0.15, a remarkable twin-peaked structure in the 10 μm silicate band apears even when large aggregates contribute to the infrared spectral feature. We can also expect a weak twin peak even when D ≅ 3. Furthermore, such grain model can predict a reasonable temperature of about 390 K at a Sun-comet distance of 0.79 AU, which gives a good fit to that reported in comet P/Halley (Campins and Ryan, Astrophys, J., 341, 1059, 1989). We conclude that the model calculations support an interpretation of the observational data in terms of porous particles consisting of different materials.