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Mare Imbrium (Latin imbrium, the " Sea of Showers" or " Sea of Rains") is a vast lunar mare within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the collision of a Protoplanet during the Late Heavy Bombardment. Basaltic lava later flooded the giant Impact crater to form the flat volcanic plain seen today. The basin's age has been estimated using uranium–lead dating methods to approximately 3.9 billion years ago, and the diameter of the impactor has been estimated to be 250 ± 25 km.P. H. Schultz & D.A. Crawford, Origin and implications of non-radial Imbrium Sculpture on the Moon, Nature 535, 391–394 (2016) The Moon's maria (plural of Lunar mare) have fewer features than other areas of the Moon because molten lava pooled in the craters and formed a relatively smooth surface. Mare Imbrium is not as flat as it would have originally been when it first formed as a result of later events that have altered its surface. Mare Imbrium was the location of the first robotic Lunar rover deployed on an extraterrestrial body.
The Imbrium Basin is surrounded by three concentric rings of mountains, uplifted by the colossal impact event that excavated it. The outermost ring of mountains has a diameter of 1300 km and is divided into several different ranges; the Montes Carpatus to the south, the Montes Apenninus to the southeast, and the Montes Caucasus to the east. At their highest, the outer ring of mountains rise more than 5 km above the surface of Mare Imbrium. The Moon's Highs and Lows The ring mountains are not as well developed to the north and west, and it appears they were simply not raised as high in these regions by the Imbrium impact. The middle ring of mountains forms the Montes Alpes and Montes Archimedes. The innermost ring, with a diameter of 650 km, is defined by Montes Recti, Montes Teneriffe, and possibly Montes Spitzbergen. Much of this ring is submerged under the mare's basalt, resulting in only isolated peaks remaining in some areas, such as Mons Pico and Mons La Hire.
Numerous estimates of the depth of the mare material have been made using various methods. These include analyses of gravity, seismic, and radar data; studies of craters partially filled with basalt and those that have completely penetrated the mare; and comparisons of lunar basins filled with mare deposits to unfilled basins. These studies have yielded depth estimates ranging from 2 km to 5 km in the central part of the mare. It is thought that the original crater left by the Imbrium impact was as much as 100 km deep, but that the floor of the basin bounced back upwards immediately afterwards.
Surrounding the Imbrium Basin is a region blanketed by ejecta from the impact, extending roughly 800 km outward. Also encircling the basin is a pattern of radial grooves called the "Imbrium Sculpture", which have been interpreted as furrows cut in the Moon's surface by large projectiles blasted out of the basin at low angles, causing them to skim across the lunar surface ploughing out these features. The sculpture pattern was first identified by Grove Karl Gilbert in 1893.Gilbert, Grove Karl. The Moon's face, a study of the origin of its features. Washington, Philosophical Society of Washington, 1893. Furthermore, a Moon-wide pattern of faults which run both radial to and concentric to the Imbrium basin were thought to have been formed by the Imbrium impact; the event literally shattered the Moon's entire lithosphere. At the region of the Moon's surface exactly opposite Imbrium Basin, there is a region of chaotic terrain (the crater Van de Graaff) which is thought to have been formed when the seismic waves of the impact were focused there after travelling through the Moon's interior. Mare Imbrium is about wide.
A mass concentration (mascon), or gravitational high, was identified in the center of Mare Imbrium from Doppler tracking of the five Lunar Orbiter spacecraft in 1968. The Imbrium mascon is the largest on the Moon. It was confirmed and mapped at higher resolution with later orbiters such as Lunar Prospector and GRAIL.
The earliest known name for the mare may be "The Shrine of Hecate"; Plutarch records that the Ancient Greeks gave this name to the largest of the "hollows and deeps" on the Moon, believing it to be the place where the souls of the deceased were tormented. Ewen A. Whitaker argues that this likely refers to Mare Imbrium, "the largest regular-shaped dark area unbroken by bright patches" that can be seen with the naked eye.Ewen A. Whitaker, Mapping and Naming the Moon (Cambridge University Press, 1999), p. 7
Around 1600, William Gilbert made a map of the Moon that names Mare Imbrium "Regio Magna Orientalis" (the Large Eastern Region). Michael van Langren's 1645 map named it "Mare Austriacum" (the Austrian Sea).Ewen A. Whitaker, Mapping and Naming the Moon (Cambridge University Press, 1999), pp. 15, 41.
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