Geological Chronicle
Mare Smythii (Latin for 'Smyth\'s Sea') is a 374-kilometer-wide lunar mare situated on the equator along the easternmost limb of the Moon's near side at 1.3° N, 87.5° E. Named after nineteenth-century British astronomer William Henry Smyth, it is one of only two lunar maria named for individual people alongside Mare Humboldtianum. The underlying circular basin was excavated to a depth of 4 kilometers during the Pre-Nectarian epoch, making it one of the older major impact structures on the lunar surface. Because of its position on the eastern limb, Smythii is subject to lunar libration, bringing its dark volcanic plains into view from Earth under favorable viewing geometry.
Subsequent volcanism filled low-lying depressions within the impact structure with layers of low-alumina basalt. Geologists identify an older stratum of Upper Imbrian basalt overlain by younger Eratosthenian basalt flows. Lava moved from the southeast and west toward the northeast, settling into the lowest sections of the basin floor. Smooth mare basalt occupies the northeastern basin and a smaller section of the west-central floor, while the surrounding terrain is hummocky and has a higher albedo. The basin floor contains wrinkle ridges, volcanic domes, rilles, crater chains, and submerged ghost craters.
Mare Smythii is notable for an abundance of floor-fractured craters, produced when subsurface magma intruded beneath crater floors and forced the overlying rock upward. The crater Haldane on the basin floor displays complex interior fracturing resulting from this magmatic uplift. Geophysical mapping by NASA's GRAIL spacecraft and Apollo mission observations confirmed a prominent mass concentration (mascon) centered over the basin, where high-density basaltic rock and an uplifted lunar mantle create a strong local positive gravity anomaly.
Surrounding impact structures define the margins of the Smythii basin. To the north lies the crater Neper, which forms part of the southern rim of Mare Marginis. The dark basalt-filled crater Kästner marks the southern boundary of the mare, while Schubert and Schubert B sit off the northwestern edge. Southwest of the basin lies Bandfield crater, recognized as a prominent lunar cold spot. Apollo 10 and Apollo 16 captured detailed orbital photographs of Smythii's flooded depressions, documenting its volcanic and structural history.