Schematic showing the difference between the single-seed model and the many-seeds model for the internal structure of icy pebbles in the outer disk. In the single-seed model, the water ice layer (white) on the silicate core (red) becomes smaller at the evaporation front, and after complete evaporation of the ice, only one single silicate core remains. Since we take ζ = 0.5, the silicate core contains half the mass of the original pebble: m_core = m_ice = 0.5m_p,start. In the many-seeds model, pebbles in the outer disk consist of many micron-sized silicate grains (red) “glued” together by water ice (white). As the ice evaporates, silicate grains are released as well. After complete evaporation of the ice, many micron-sized silicate grains remain. Again, with ζ = 0.5, the total mass in silicate grains in a pebble in the outer disk is half of the total pebble mass, but this mass is now divided among many silicate grains that each have mass m_sil. See text for more details.