  The MESSENGER mission sought to discover what physical processes determined
Mercury's high metal to silicate ratio. Instead, the mission has discovered
multiple anomalous characteristics about our innermost planet. The lack of FeO
and the reduced oxidation state of Mercury's crust and mantle are more extreme
than nearly all other known materials in the solar system. In contrast,
moderately volatile elements are present in abundances comparable to the other
terrestrial planets. No single process during Mercury's formation is able to
explain all of these observations. Here, we review the current ideas for the
origin of Mercury's unique features. Gaps in understanding the innermost
regions of the solar nebula limit testing different hypotheses. Even so, all
proposed models are incomplete and need further development in order to unravel
Mercury's remaining secrets.

Earth and Planetary Astrophysics