Geological Chronicle
Mare Anguis (Latin for 'Serpent Sea') is a narrow volcanic mare ribbon stretching 146 kilometers across the northeastern outer ramparts of the Crisium basin at 22.6° N, 67.7° E. The dark basalt deposit sits along a concentric structural ring of the impact basin, perched at an elevation roughly 800 meters above the floor of Mare Crisium. Channels cut down through the highland terrain between Mare Anguis and Mare Crisium, preserving conduits where lava once drained toward the lower basin interior.
The underlying structural trough originated during the Nectarian period impact that excavated the Crisium multi-ring basin. Subsequent volcanic eruptions during the Early Imbrian epoch flooded this elevated fracture shelf with dark iron-rich basalts. Along its northwestern margin sits the 45-kilometer impact crater Eimmart, while the seven-kilometer impact crater Eimmart A punctures the eastern edge of the mare floor. Impact excavation at Eimmart A penetrated the basalt layer, spreading a high-albedo blanket of crushed highland material across the dark surface.
Spectroscopic observations show that the ejecta of Eimmart A contains a mixture of olivine and low-calcium pyroxene, confirming that the impact pierced through mare basalt into underlying plutonic rocks. Planetary scientists have investigated Eimmart A as a potential ejection origin for ALHA81005, the first lunar meteorite recognized on Earth after its discovery in Antarctica in 1982. Orbital imagery from Apollo 17 in December 1972 documented the sinuous morphology of the perched mare and its highland boundaries.