In the 1970s, the Mariner 10 spacecraft flew by Mercury three times, imaging nearly all of one hemisphere (45% of the planet). Due to the geometry of the flybys, a large swath of terrain in the northern portion of that hemisphere was never seen (left image), until October 6, 2008, when the MESSENGER spacecraft flew by the planet for the second time, revealing this ancient cratered terrain (right image).
Filling in this gap allows scientists to use both Mariner 10 and MESSENGER data to better characterize the various processes that shaped the surface of Mercury over time. (View images taken by Mariner 10.)