Geological Chronicle
Mare Humboldtianum (Latin for 'Sea of Alexander von Humboldt') is a 273-kilometer-wide lunar mare located within a larger 650-kilometer multi-ring impact basin on the Moon's northeastern limb at 56.8° N, 81.5° E. The structure extends across the boundary between the near side and the far side, lying just east of Mare Frigoris. Because of its peripheral position, Earth-based observers view the mare at a steep angle, and lunar libration occasionally moves the entire plain out of view beyond the lunar horizon.
The Humboldtianum basin formed during the Nectarian epoch when an asteroid impact excavated the multi-ring structure and raised mountainous rim massifs visible under oblique lighting. During the subsequent Upper Imbrian epoch, between 3.4 and 3.7 billion years ago, volcanic eruptions flooded the basin's central topographic depression with low-titanium mare basalts. Southeast of the dark mare plains, a rugged region of lighter hills occupies part of the inner basin floor.
Impact craters intersect the perimeter of Mare Humboldtianum. The large walled plain Bel'kovich intrudes upon the northwestern portion of the mare floor, and the flooded crater Bel'kovich A overlays both the southwestern rim of Bel'kovich and part of the basaltic plain. Gravitational measurements from the Lunar Prospector spacecraft identified a positive gravity anomaly, or mascon, centered over the Humboldtianum basin, caused by dense basalt and mantle uplift.
German astronomer Johann Heinrich von Mädler named the mare in the nineteenth century in honor of naturalist and explorer Alexander von Humboldt. Mädler selected the name because the formation sits at the border between the known near side and the uncharted far side of the Moon. Mare Humboldtianum and Mare Smythii are the only two lunar maria named for individual people rather than atmospheric conditions or states of mind.