  Mercury's crust has a complex structure resulting from a billion years of
volcanism. The surface variations in chemical composition have been identified
from orbit by the spacecraft MESSENGER. Combining these measurements with
laboratory experiments on partial melting, we estimate which variations in
surface density and degree of mantle melting are required to produce surface
rocks. If the surface density is representative of the deep crustal density,
more than one half of crustal thickness variations in the northern hemisphere
are explained by lateral variations in mantle melting. The crust is thin below
the magnesium-poor Northern Volcanic Plains whereas the thickest crust is found
in the magnesium-rich region located at mid-northern latitudes in the Western
Hemisphere. The magnesium-rich region is thus not due to an early impact but
rather to extensive mantle melting. The thickness-melting relation has also
been observed for the oceanic crust on Earth and might be a common feature of
terrestrial planets.
