Today in Science there were a number of articles reporting on new data from NASA’s MESSENGER mission to Mercury, which aims to investigate the surface, environment and geochemistry of Mercury, amongst other properties. One of these articles, by William B. McKinnon, suggests that the internal structure of Mercury may be different to the other terrestrial planets (Venus, Mars, and Earth) (Science 336, pg 162). McKinnon proposes that the Mercury may have a solid ‘anticrust’ of iron sulfide at a depth of 300-400km, overlain by a solid mantle and a thin volcanic crust. This anticrust also has the effect of isolating Mercury’s proportionally large molten core from the geologically active crust and mantle.
A good explanation of the structure of the terrestrial planets, such as Earth can be found at Wikipedia.
More information on the MESSENGER mission can be found at the NASA website, images from this mission even appeared in a recent episode of The Big Bang Theory.