Basins are the largest features of the Moon, and near-side basins can easily be seen with the unaided eye from Earth, appearing as relatively dark circles on the surface. Basins are different from craters mainly with respect to their size. Basins are lunar impact excavations with diameters of at least 300 km, while craters are smaller than 300 km. This is the size at which central crater peaks are observed to change into central rings.
Three important basins on the Moon are the Orientale Basin, the Imbrium Basin, and the South Pole-Aitken Basin. The Orientale Basin was not filled with lava, so its interior structure is visible (Figure 1.4). Four well-defined rings can be seen, the outermost having a diameter of 930 km. Within this ring is another, 620 km in diameter, whose peaks are more jagged and stand higher than those of the outer ring. Within these two rings is a third ring with a diameter of 480 km, and within that, the central ring with a diameter of 320 km.
The Imbrium Basin, which is easily seen on the lunar near side, is filled with lava and thus appears as a dark circle on the face of the Moon (Figure 1.5). Most of the large lunar basins are very old, indicating that in the distant past, impacts of very large objects were much more common than they are today. In fact, the basins of the Moon may come from a time very early in solar system history, shortly after the planets and satellites first formed by the accretion of planetesimals.7 It is surprising that the planet was not broken completely apart by some of these impacts, considering the size of the basins that were created.
6 Breccias (from the Latin word for "broken") are complex assemblages of rock fragments cemented together by heat, pressure, or both.
7 Planetesimal: bodies from millimeter size up to hundreds of kilometers in diameter that formed from the ring of dust that encircled the early Sun. Most of these accreted to form the planets, and the rest were eventually ejected from the solar system.
The South Pole-Aitken Basin is a large impact crater with a diameter of 2,500 kilometers and a maximum depth of 12 to 13 kilometers. It extends from the south polar region to the Aitken Crater on the far side of the Moon, and is the largest known impact crater in the solar system. The extent of the basin was not known until it was photographed by the Lunar Orbiter Satellite. Further imaging and data of the basin were obtained by the Clementine and Galileo missions. The impact event that produced the basin may have exposed the upper mantle of the Moon, but confirmation of that possibility awaits surface analysis.
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