Note:  Mariner 10 (1974-75) imaged only one hemisphere of Mercury.


1st flyby (January 2008)                                                                    2nd flyby (October 2008)

By October 2008, Messenger had flown past Mercury 2 times on its way to being the first spacecraft to orbit the planet by 2011.
A large part of the previously un-imaged surface has already been revealed.

                                        Mariner 10 image                                                                                            Messenger image
Smooth bottomed craters, flood basalts (lava) filled after impacts.  Other evidence for internal tectonic forces acting after the initial high impact rate subsided include many high cliffs formed by contraction of the planet as it cooled.  Note the long scarp (cliff face) cutting through the right of the Messenger image.  (Images are not the same scale.)