Geological Chronicle
Mare Insularum, translated from Latin as the 'Sea of Islands', is a lunar mare situated on the central near side of the Moon directly south of western Mare Imbrium. Centered at selenographic coordinates of 7.5° N and 30.9° W, the dark basaltic plain extends across a 512-kilometer diameter. Geological mapping places the underlying basin foundation in the Lower Imbrian epoch, while subsequent basaltic lava flows filled the depression during the Upper Imbrian epoch. The smooth volcanic surface merges southwestward into Oceanus Procellarum and extends northeastward toward Sinus Aestuum.
The plain is bounded by two large impact craters. On its eastern border lies Copernicus, a 93-kilometer-wide crater formed during the Copernican period, while the crater Kepler marks its western edge. Bright ray systems radiating from both Copernicus and Kepler cross the dark basaltic surface of the mare. Along the southeastern perimeter lies the degraded crater Fra Mauro and the extensive Fra Mauro formation, a hummocky geological blanket composed of ejecta thrown out by the giant impact that excavated the Imbrium basin.
The terrain surrounding Mare Insularum provided landing sites for two Apollo lunar expeditions. In November 1969, Apollo 12 executed a pinpoint landing south of the mare in northern Oceanus Procellarum within walking distance of the robotic probe Surveyor 3, returning 34.35 kilograms of lunar samples. In February 1971, Apollo 14 landed in the adjacent Fra Mauro highlands, where astronauts Alan Shepard and Edgar Mitchell sampled Imbrium ejecta near the rim of Cone crater, bringing back 42.28 kilograms of rocks and soil. The name Mare Insularum was proposed by lunar geologist Don E. Wilhelms and officially approved by the International Astronomical Union in 1976.