Geological Chronicle
Mare Marginis (Latin for 'Sea of the Edge') is an irregular lunar mare spanning 358 kilometers across the eastern limb of the Moon at 13.3° N, 86.1° E. Unlike circular maria that occupy deep impact basins, Marginis fills a low-lying section of lunar highlands where volcanic lavas reached the surface through crustal fractures. Because of its location on the lunar limb, observers on Earth can see Marginis only during favorable lunar libration.
The volcanic plains of Mare Marginis are shallow compared to central nearside maria. The basalt layer measures only 300 to 500 meters (1,000 to 1,700 feet) thick, leaving circular and elongated ghost outlines of buried impact craters visible across the surface. Surrounding crater floors also sit below the local highland elevation, marking where subsurface magma rose near the crust. Major craters bordering the mare include Al-Biruni to the north, Ibn Yunus to the southeast, Goddard and Goddard C to the west, and Neper to the south.
Mare Marginis contains bright, high-albedo lunar swirls resembling the Reiner Gamma marking in Oceanus Procellarum. These sinuous ribbons correspond to local crustal magnetic anomalies. The mare sits directly at the antipode of the Mare Orientale impact basin on the lunar far side. Seismic energy focused through the Moon during the Orientale impact may have fractured the crust and contributed to magnetic remnant structures here. The resulting magnetic field deflects the solar wind, shielding the surface regolith from space weathering and keeping the reflective swirls bright.
Orbital spacecraft have mapped the structure and surface patterns of Mare Marginis from lunar orbit. Lunar Orbiter 4 captured oblique photographs of the western mare in May 1967, and Apollo 16 photographed its flooded depressions in April 1972. In December 1972, the mapping camera aboard the Apollo 17 Command Module America took detailed panoramic images from an altitude of 124 kilometers, recording the swirls around Goddard alongside isolated basalt patches near the crater Cannon.