Pebble compression for pure ice and silica pebbles. Volume-weighted mean filling factor at the end of collapse as a function of initial filling factor: a) silica pebbles; b) ice pebbles. The four cases of a very low-mass (blue), a low-mass (yellow), an intermediate-mass (green), and high-mass (red) cometesimal are shown. Along the black dotted line, the final filling factor equals the initial filling factor. The error bar indicate the standard deviation of the pebble ensemble. Silica pebbles are significantly compressed for M> 2.6 × 10²⁰ g. The maximum filling factor ice pebbles can obtain by compression is considerably lower than for silica, φ ~ 0.23 compared to φ ~ 0.43.