  The chemical composition of a planetary body reflects its starting conditions
modified by numerous processes during its formation and geological evolution.
Measurements by X-ray, gamma-ray, and neutron spectrometers on the MESSENGER
spacecraft revealed Mercury's surface to have surprisingly high abundances of
the moderately volatile elements sodium, sulfur, potassium, chlorine, and
thorium, and a low abundance of iron. This composition rules out some formation
models for which high temperatures are expected to have strongly depleted
volatiles and indicates that Mercury formed under conditions much more reducing
than the other rocky planets of our Solar System. Through geochemical modeling
and petrologic experiments, the planet's mantle and core compositions can be
estimated from the surface composition and geophysical constraints. The bulk
silicate composition of Mercury is likely similar to that of enstatite or
metal-rich chondrite meteorites, and the planet's unusually large core is most
likely Si rich, implying that in bulk Mercury is enriched in Fe and Si (and
possibly S) relative to the other inner planets. The compositional data for
Mercury acquired by MESSENGER will be crucial for quantitatively testing future
models of the formation of Mercury and the Solar System as a whole, as well as
for constraining the geological evolution of the innermost planet.
