Geological Chronicle
Mare Nubium, translated from Latin as the 'Sea of Clouds', is an ancient volcanic plain located on the southern near side of the Moon, southeast of Oceanus Procellarum. Centered at 21.3° S and 16.6° W, the depression occupies a 715-kilometer-wide basin that formed during the Pre-Nectarian period over 3.9 billion years ago. Subsequent eruptions of basaltic lava during the Early Imbrian and Eratosthenian periods flooded the eroded impact structure, leaving a dark, relatively smooth expanse.
The mare contains notable tectonic and volcanic landmarks. In its southeastern region lies Rupes Recta, commonly known as the Straight Wall. This normal fault extends for 110 kilometers, with a width of 2 to 3 kilometers and a vertical drop of 240 to 300 meters. When the sun illuminates the fault at an oblique angle around the eighth day of the lunar cycle, the escarpment casts a wide, distinct shadow across the surrounding floor. Other structures bordering the plain include the crater Pitatus on the southern margin and the flooded crater Guericke to the north.
On the western side of the basin sits Bullialdus, an impact crater of Eratosthenian age spanning 61 kilometers in diameter. Bullialdus has terraced walls, a circular rim with slightly polygonal sections, and a central mountain complex that rises over 1 kilometer above the crater floor. Spectroscopic observations indicate that the central peaks consist of gabbroic norite originating from depths between 6.0 and 9.2 kilometers, exposing crustal rock beneath the basalt cover.
Early lunar cartographers described the area with several different titles. William Gilbert included it in his Continens Meridionalis around 1600, and Michael van Langren named it Mare Borbonicum on his 1645 chart. In 1651, Italian astronomer Giovanni Riccioli introduced the name Mare Nubium in his lunar map, a designation later adopted by the International Astronomical Union. In modern exploration, NASA's Ranger 7 spacecraft transmitted 4,308 close-range photographs of the lunar surface in July 1964 before impacting just northwest of the mare, an area later named Mare Cognitum.