Sadly, Mercury has got to be the least interesting of the planets. It lacks any moons, lacks any substantial atmosphere and seems at a glance to be a duplicate of our own moon. At this point in time there has only been one mission to study Mercury up-close, Mariner 10 in 1973, so our information is quite limited. However, there is a mission on its way to Mercury right now named Messenger as well as a European/Japanese effort name Bepi-Columbo. Perhaps those missions will enlighten us all to things unseen on Mercury.
NOTE: This “portrait” wallpaper has been updated with a more recent color image taken during the January 2008 Messenger flyby.