The maps of Mercury on this site are based on data from the single Mariner 10 mission which made three flyby encounters with Mercury in 1974 and 1975.
 

Mission	Encounter dates	Type of encounter	Significant discoveries	Closest approach	Best resolution
Mariner 10	29 March 1974	flyby	first detailed views highly cratered surface  Caloris impact basin	704 km	120 m
Mariner 10	21 September 1974	flyby		47,000 km	1 km
Mariner 10	16 March 1975	flyby	south polar area imaged	327 km	?