The compositions of meteorites and the morphologies of asteroid surfaces provide strong evidence that partial melting and differentiation were widespread among the planetesimals of the early Solar system. It has been increasingly accepted in recent years that planets were formed from differentiated planetesimals and that the differentiation of the proto-planets was facilitated by planetesimals being pre-differentiated. With respect to the degree of differentiation meteorites can be divided into two classes: chondrites that originated from primitive, undifferentiated parent bodies and achondrites, iron meteorites and stony and stony-iron meteorites that originated from bodies that were apparently fractionated into (at least) a silicate mantle and a metallic core. A large variety in the degree of differentiation has been identified: metal separated partially or completely from silicates and silicates fractionated from each other, causing the composition of rock to deviate moderately to strongly from a primitive chondritic composition. Also, the surfaces of asteroids show large variations. The most massive of these, Ceres and Vesta, are most complementary. Ceres has a very primitive surface characterized by water-bearing minerals. Vesta – on the contrary – has a dry, basaltic composition indicating that the body is differentiated and has been resurfaced by basaltic lava flows. Vesta has most probably an iron rich core and once had a magma ocean just like the Moon. Differentiation of asteroids and planetesimals (assuming that the former are surviving examples of the latter) must have occurred within the first few million years of the solar system judging from the ages of meteorites and the surfaces of asteroids. Further evidence for rapid iron-silicate differentiation, i.e., core formation, comes from 182Hf-182W concentration variations in iron meteorites and for basalt formation from the concentrations of 26Al-26Mg and 53Mn-53Cr in eucrites and angrites. Recently, it has been suggested that some of the chondritic meteorites, i.e., CV meteorites, are samples of the unprocessed crust of an internally differentiated early planetesimal. The asteroid belt may still contain these kinds of objects. The idea of an undifferentiated crust above a differentiated mantle and an iron-rich core is supported by thermal evolution models.
Figure 1: Possible internal structures of the asteroid 21 Lutetia assuming different present-day porosity (0.04, 0.05, 0.06, 0.08, 0.13 and 0.25, respectively for panels a-f). The colours indicate the iron core (red), the silicate mantle (green), the partially molten, but undifferentiated layer (or having some smallscale phase separation, light blue), the compacted layer (greenish blue) and the porous layer (grey). Figure from: (Neumann et al., 2013).