Mare Imbrium

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A map showing the location of Mare Imbrium

Mare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare (mahr'-ay) filling a basin on Earth's Moon. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the moon long ago. The moon's maria (mahr'-ee-ah) (plural of 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 was originally because later events have altered its surface.

With a diameter of 1123 km it is second only to Oceanus Procellarum in size among the maria, and it is the largest mare associated with an impact basin. Apollo 15 landed in the southwestern region of Mare Imbrium, near the Apennine Mountains.

Detail map of Imbrium's features

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 southwest, and the Montes Caucasus to the east. 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 the mountainous regions near the craters Archimedes and Plato. The innermost ring, with a diameter of 600 km, has been largely buried under the mare's basalt leaving only low hills protruding through the mare plains and mare ridges forming a roughly circular pattern.

The outer ring of mountains rise roughly 7 km above the surface of Mare Imbrium. The mare material is thought to be about 5 km deep, giving the Imbrium Basin a total depth of 12 km; it is thought that the original crater left by the Imbrium impact was as much as 100 km deep, but the floor of the basin bounced back upward immediately afterward.

Oblique view of Mare Imbrium looking south towards Copernicus crater. NASA photo.

Surrounding the Imbrium Basin is a region blanketed by ejecta from the impact, extending roughly 800 km outward. Also encircling the Imbrium 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. 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 which is thought to have been formed when the seismic waves of the impact were focused there after travelling through the Moon's interior.