List of plains on the Moon
The Moon has numerous named plains on its surface made of dark-appearing basaltic rocks. The plains are visibly prominent on the near side of the Moon because of their contrasting appearance from the surrounding lunar highlands, which are made up of anorthosite, an igneous rock containing significant concentrations of highly reflective plagioclase feldspar minerals. As with other topographical features on the Moon, the official nomenclature for lunar plains uses a Latin naming system. Due to their darker appearance, differing elevation from the highlands, and visual prominence, noted ancient Greek astronomers such as Plutarch believed the plains were water; subsequent astronomers such as Galileo Galilei and Michael van Langren built on this belief, using Latin words for water bodies in classifying them. The Latin classifications for lunar plains are oceanus, mare, lacus, palus, and sinus. The largest plain is Oceanus Procellarum, covering much of the western half of the Moon's near side and the only plain to be classified as an oceanus. Most other prominent plains on the near side are the lunar maria, while the Moon's lacūs, sinūs, and paludes are generally smaller and less visually prominent.
The classification of and other designations for lunar geographic features are determined by the International Astronomical Union —the official international organization for the standardization of astronomy.
Geology
In its early stages of formation, the Moon's surface was largely molten, with its deep ocean of magma persisting for roughly tens of millions to hundreds of millions of years. After the cooling of the magma at the end of this period, minerals began to crystallize and differentiate, with heavier minerals sinking into the Moon's inner layers while the lighter, highly-reflective mineral of plagioclase feldspar rose to the surface and fused with other light, mafic-type crystals to form anorthosite rocks. The anorthosite accumulated to become an early stage of the Moon's outer crust. This early outer crust later became heavily bombarded by immense numbers of meteoroids and other solar system debris around 4 billion years ago, with substantial parts of the surface being blasted out to create craters and large basins. Millions of years after these massive impact basins formed, lava from the interior flowed up to the surface through cracks in the crust and filled the basins. The lava cooled into the dark, volcanic rock basalt, creating the lunar plains of today.Currently, basaltic rock, which is also fine-grained and rich in iron, magnesium, and plagioclase feldspar, makes up most of the lunar plains' composition. As with the distribution of the lunar plains, basalt is also not uniformly distributed over the surface of the Moon, with approximately 26% of the Moon's near side and only 2% of its far side composed of basalt.
Nomenclature
The modern system of Latin-based lunar nomenclature began in 1651, when Italian astronomer and Catholic priest Giovanni Battista Riccioli published Almagestum Novum, an encyclopedic work in which he and fellow Italian priest and scientist Francesco Maria Grimaldi created a detailed map of the lunar near side's surface. In the book, Riccioli introduced a system differentiating lunar topography into three categories, with dark plains comprising one category and given Latin labels for bodies of water together with poetic, abstract names. Riccioli's system was used significantly in Europe in the succeeding centuries and became the basis for the current global standard in 1935, when the IAU adopted much of Riccioli's names and conventions. Since the current IAU nomenclature rules were affirmed in 1973, new names for lunar plains have been restricted to Latin terms that describe weather and other abstract concepts. Currently, there are 5 lunar plains' names referring to sea features, 5 indicating sea attributes, and 5 expressing states of mind.Of plains inscribed prior to 1973 by the IAU, there are four that carry non-abstract names: Mare Humboldtianum and Mare Smythii, named for German natural historian Alexander von Humboldt and British astronomer William Henry Smyth, respectively; Mare Moscoviense, named for Muscovy, a historical predecessor of Russia; and Sinus Lunicus, named for the Soviet space probe Luna 2, which landed there in 1959. Because Mare Humboldtianum and Mare Smythii were established before the final nomenclature, they were maintained and not required to follow the name conventions. In the case of Mare Moscoviense, which was discovered by the Luna 3 and whose name was proposed by the Soviet Union, it was only accepted by the IAU with the justification that Moscow is a 'state of mind'.