Craters on the morning terminator...

Caption NASA originale:"As Mariner 10 passed by Mercury on its second encounter with the planet on September 21, 1974, this picture (FDS 166850) of a large circular (350 Km - or 220 mile diameter) basin was obtained near the morning terminator. The basin appears to have been flooded with the plain material and then subsequently cratered by numerous large events. Filling of the basin, presumably by lava flows analogous to those of the lunar maria, partially inundated small craters which had formed along the basin rim (lower left) and in some places overflowed the basin rim and spilled onto the surrounding terrain (top)".