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Like a target ring bull's-eye, the lunar mare Mare Orientale (the "eastern sea") is one of the most striking large scale lunar features. Located on the extreme western edge of the lunar nearside, this impact basin is unfortunately difficult to see from an Earthbound perspective.
Material from this basin was not sampled by the Apollo program, so the basin's precise age is not known. However, it is the freshest impact basin on the Moon and is believed to be slightly younger than the Imbrium Basin, which formed about 3.85 billion years ago. The surrounding basin material is of the Lower Imbrian epoch, with the mare material being of the Upper Imbrian epoch.
The mare is about 600 miles across and was formed by the impact of an asteroid sized object. Unlike most other basins on the Moon, Orientale is relatively unflooded by mare basalts, exposing much of the basin structure to view; the central portion of Mare Orientale is covered by a thin layer of mare basalt probably less than 1 kilometer deep, much less than in other nearside mare basins. The collision caused ripples in the lunar crust resulting in the three concentric circular features. The innermost ring of this vast, multi-ringed crater are the Inner Rook Mountains, the middle ring are the Outer Rook Mountains, and the outermost ring are the Cordillera Mountains, some 900 km in diameter. Basin ejecta begins just outside the Cordillera Mountains and extends up to 500 kilometers beyond the base of the mountains. This ejecta has a rough, hummocky texture and contains linear patterns that point back at the center of Orientale.
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