Packing fraction of pebbles as a function of the dust-to-ice ratio. The four panels correspond to different initial filling factors for the pebbles: a) of φ₀ = 0.1; b) φ₀ = 0.2; c) φ₀ = 0.3; and d) φ₀ = 0.4. Each panel shows the results for the four different cloud masses as solid curves: very low-mass (blue); low-mass (yellow); intermediate-mass (green); and high-mass (red). In panels a) and b), the curves for the very low- and low-mass clouds are outside the plotting range, because the packing fraction becomes unphysically high (φ_P ≳ 1). In panel d), the green, blue, and yellow curves are overlapping because pebbles are barely compressed in these cases and thus have the same filling factor (see Fig. 4). The shaded area (violet) indicates the range of φ_P given by the random loose packing (RLP, φ_P = 0.55) and the random close packing (RCP, φ_P = 0.64). The error bars show the uncertainty of φ_P, based on the uncertainty of the volume-filling factor of the pebbles. Initial conditions, i.e. dust-to-ice ratio and cloud mass, which are consistent with a cometary bulk density of ρ_c = 0.5 g cm^-3, result in a value for φ_P within the shaded area. Very low- and low mass clouds require initially compact pebbles (φ₀ = 0.4) and dust-to-ice ratios 3 ≲ ξ ≲ 7, whereas intermediate- and high-mass clouds need 3 ≲ ξ ≲ 9, regardless of initial filling factor.