Parameters producing best fits for the four observations and for different fractal dimensions D_f between 2.0 and 2.5. (a) Values of N obtained for the best fits of the spectral opacity (Fig. 7) as a function of D_f. The constraints on N retrieved from the fit of the phase functions from DISR analysis are shown with black dots and dashed curves (reported from the graph (b)). (b) Relative least-square values for comparisons between phase functions derived with DISR (Tomasko et al. 2008) and phase functions computed for aggregates of various fractal dimensions and number of grains.