The Earth, Venus, Mars, and some extrasolar terrestrial planets 1 have a mass and radius that is consistent with a mass fraction of about 30% metallic core and 70% silicate mantle 2 . At the inner frontier of the solar system, Mercury has a completely different composition, with a mass fraction of about 70% metallic core and 30% silicate mantle 3 . Several formation or evolution scenarios are proposed to explain this metal-rich composition, such as a † 95% credible upper limit * Assuming a zero albedo. * * Assuming a zero albedo and a tidally synchronised rotation.