Geochemical data from MESSENGER have revealed details of Mercury’s surface composition, showing that it differs from the other rocky planets in the inner solar system. For example, the planet’s surface is enriched in S and C, and depleted in Fe, indicating that Mercury formed under much more reducing conditions than other planets. The surface is also enriched in Mg and depleted in Al and Ca. Observed elemental heterogeneities and percent levels of graphite suggest that Mercury underwent a magma ocean phase early in its history. These findings have important implications for understanding Mercury’s origin and evolution.