Radiation pressure forces of fluffy porous grains

Based on Mie theory and Maxwell-Garnett effective medium theory (MG-Mie), we have examined a ratio β of the radiation pressure forces to the gravity on porous aggregates with fractal structures, which consist of absorbing (graphite or magnetite) or dielectric (silicate) materials. A comparison of β by MG-Mie theory with those computed by the discreate dipole approximation (DDA) has shown that when the dust aggregates have rather compact structures and/or the constituent particles are absorbing materials, the values of β by DDA theory are reproduced reasonably by MG-Mie theory. Furthermore, it is found that (i) the higher the porosity, the smaller the value of β becomes near the maximum of β at a characteristic radius of the aggregate of about 0.1 μm. (ii) For very porous aggregates, β does not depend on size in contrast to a remarkable size dependence of β for compact spheres. (iii) It would be expected from the MG-Mie theory that regardless of the chemical composition of constituents, β of porous grains approaches a constant value, which is close to that for the individual constituent particle, as the porosity increases. Consequently, when the radiation pressure forces are significant in comparison with the gravity, the dynamical behaviour of large fluffy particles with high porosity in the interplanetary space seems to be similar to that of the small compact particles which make up their internal structure.