Left: differentiation sequence implemented in the model on a length scale of a few tens of microns. (a) Initial three phase system with ice particles (dark blue), rock particles (brown) and empty voids (white). (b) Partial closure of the dust pores, melting of water (light blue). (c) Complete closure of the dust pores, onset of the compaction of the melt pores. (d) Formation of a rocky core with a compacted inner part and a porous outer part. Right: locations of the local micro-structures a–d in a globally partially differentiated structure that can occur at some time during the evolution of a planetesimal. The central area consists of a rock core with a compacted inner and a porous outer layer. It is overlain by a water ocean below an undifferentiated crust. Liquid water is present in the lower crust, while the colder upper crust contains water ice.