The dominant consequences of impacts are observed in every lunar scene. At the largest scale are the ancient basins, which extend hundreds of kilometres across. A beautiful example is Orientale Basin, or Mare Orientale, whose mountain walls can just be seen from Earth near the Moon's limb (the apparent edge of the lunar disk) when the lunar libration is favourable. Orientale Basin appears to be the youngest large impact basin on the Moon. Its multiring ramparts are characteristic of the largest basins; they are accented by the partial lava flooding of low regions between the rings. A multiringed basin typically resembles a bull's-eye and may cover an area of many thousands of square kilometres. The outer rings of the basins are clifflike scarps that face inward. The rings probably were formed as part of the crater-forming process during impact, although some hypotheses suggest that they were formed, or were enhanced, by postimpact collapse. Transitional structures between bowl-shaped craters and mul-tiringed basins include craters with central peaks and larger craters with central rings of peaks.
Orientale's name arises from lunar-mapping conventions. During the great age of telescopic observation in the 17th-19th centuries, portrayals of the Moon usually showed south at the top because the telescopes inverted the image. East and west referred to those directions in the sky—i.e., the Moon moves eastward and so its leading limb was east, and the portion of the basin that could be seen from Earth was accordingly called Mare Orientale. For mapping purposes lunar coordinates were taken to originate near the centre of the near-side face, at the intersection of the equator and a meridian defined by the mean librations. A small crater, Mosting A, was agreed upon as the reference point. With the Moon considered as a world, rather than just a disk moving across the sky, east and west are interchanged. Thus, Orientale, despite its name, is located at west lunar longitudes.
Smaller impact features, ranging in diameter from tens of kilometres to microscopic size, are described by the term crater. The relative ages of lunar craters are indicated by their form and structural features. Young craters have rugged profiles and are surrounded by hummocky blankets of debris, called ejecta, and long light-coloured rays made by expelled material hitting the lunar surface. Older craters have rounded and subdued profiles, the result of continued bombardment.
A crater's form and structure also yield information about the impact process. When a body strikes a much larger one at speeds of many kilometres per second, the available kinetic energy is enough to completely melt, even partly vaporize, the impacting body along with a small portion of its target material. On impact, a melt sheet is thrown out, along with quantities of rubble, to form the ejecta blanket around the contact site. Meanwhile, a shock travels into the subsurface, shattering mineral structures and leaving a telltale signature in the rocks. The initial cup-shaped cavity is unstable and, depending on its size, evolves in different ways. A typical end result is the great crater Aristarchus, with slumping terraces in its walls and a central peak. Aristarchus is about 40 km (25 miles) in diameter and 4 km (2.5 miles) deep.
The region around Aristarchus shows a number of peculiar lunar features, some of which have origins not yet well explained. The Aristarchus impact occurred on an elevated, old-looking surface surrounded by lavas of the northern part of the mare known as Oceanus Procellarum. These lava flows inundated the older crater Prinz, whose rim is now only partly visible. At one point on the rim, an apparently volcanic event produced a crater; subsequently, a long, winding channel, called a sinuous rille, emerged to flow across the mare. Other sinuous rilles are found nearby, including the largest one on the Moon, discovered by the German astronomer Johann Schröter in 1787. Named in his honour, Schröter's Valley is a deep, winding channel, hundreds of kilometres long, with a smaller inner channel that meanders just as slow rivers do on Earth. The end of this "river" simply tapers away to nothing and disappears on the mare plains. In some way that remains to be accounted for, hundreds of cubic kilometres of fluid and excavated mare material vanished.
The results of seismic and heat-flow measurements suggest that any volcanic activity that persists on the Moon is slight by comparison with that of Earth. Over the years reliable observers have reported seeing transient events of a possibly volcanic nature, and some spectroscopic evidence for them exists. In the late 1980s a cloud of sodium and potassium atoms was observed around the Moon, but it was not necessarily the result of volcanic emissions. It is possible that interactions of the lunar surface and the solar wind produced the cloud. In any case, the question of whether the Moon is volcanically active remains open.
Telescopic observers beginning in the 19th century applied the term rille (German for "furrow") to several types of trenchlike lunar features. Rilles measure about 1-5 km (0.6-3 miles) wide and as much as several hundred kilometres long. They are divided into two main types, straight rilles and sinuous rilles, which seem to have different origins. Those of the first variety are flat floored and relatively straight; they are occasionally associated with crater chains and are sometimes arranged in an echelon pattern. Some of these structures are thought to be grabens, or elongated blocks of crust that have collapsed between parallel faults. Other straight rilles, some of which have branches—for example, Rima Hyginus (the word rima [from Latin, "fissure"] is often used for the rilles) and the rilles on the floor of the great crater Alphonsus—appear to be tension cracks in regions where subsurface gases have driven eruptions of dark material resulting in rimless vent craters.
Sinuous rilles resemble winding river valleys on Earth. They are thought to be similar to flow channels created by lava flows on Earth, but the shape of these lunar valleys is more meandering, perhaps because ancient lunar lavas were much less viscous than those now known on Earth. In 1971 Apollo 15 astronauts explored the sinuous Hadley Rille and found a V-shaped valley filled with rubble from walls that appeared to contain exposed rock layers laid down by successive lava flows. Their observations, however, did not clarify the feature's origin. Though the Moon shows both tension and compression features (low wrinkle ridges, usually near mare margins, may result from compression), it gives no evidence of having experienced the large, lateral motions of plate tectonics marked by faults in Earth's crust.
Among the most enigmatic features of the lunar surface are several light, swirling patterns with no associated topography. A prime example is Reiner Gamma, located in the southeastern portion of Oceanus Procellarum. Whereas other relatively bright features exist—e.g., crater rays—they are explained as consequences of the impact process. Features such as Reiner Gamma have no clear explanation. Some scientists have suggested that they are the marks of comet impacts, in which the impacting body was large in size but had so little density as to produce no crater. Reiner Gamma is also unusual in that it coincides with a large magnetic anomaly (region of magnetic irregularity) in the crust.
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