Mercury is the least known of the terrestrial planets. Until the recent departure of the "Messenger"Mission" (due to arrive in about 6 years) only one flyby mission (Mariner 10) provided any detailed imagery.
The fairly good quality images returned indicate Mercury is more like the Moon than the other planets, but with some interesting differences. Mercury seems to show geomorphic features that can be attributed to "weathering" and "erosion" by the outflow of solar radiation. In addition, many Mercury surface features appear similar to those of the lunar surface except that they appear to be older. The comparison raises intresting questions regarding lunar history.