MESSENGER's view of Mercury during its first flyby
Because of Mercury's very slow rotation, only one hemisphere of Mercury is Sunlit during any Mercury flyby. This image shows what areas of Mercury were Sunlit during MESSENGER's first flyby of Mercury in January, 2008. Both images were taken with the wide-angle camera. The right-hand image of a crescent Mercury is the inbound view, with the Sun reaching around to the dusk terminator at 275° east longitude. The left-hand image of a gibbous Mercury is MESSENGER's outbound view, with the dawn terminator at 95° east. Topography on Mercury's surface is only visible near the dawn and dusk terminators, where Sunlight falls at a slating angle across Mercury's surface, casting long shadows from cliffs. Away from the terminator, topography is lost, but more variations in brightness and darkness (or "albedo") of the surface are revealed.