As noted in Chap. 2, a generic terrestrial planet has a dense metallic core at its centre, surrounded by a less-dense silicate mantle that is overlain by a silicate crust of slightly lower density and slightly different composition. Prior to MESSENGER, the approximate size of Mercury’s core had been inferred from the planet’s density (though core size now appears to have been underestimated), and the presence of the core’s liquid zone had been established from the magnetic field and the amount of libration (Sect. 2.5.4). However the thickness of the crust was unknown, and there were no data that could be used to infer the thickness of Mercury’s outer mechanical layer, the lithosphere.