The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period with its rotation period at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun's—are the main drivers of Earth's tides.
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Moon. It is the only one of the lunar maria to be called an "Oceanus" (ocean), due to its size: Oceanus Procellarum is the largest of the maria ("seas"), stretching more than 2,500 km (1,600 mi) across its north–south axis and covering roughly 4,000,000 km2 (1,500,000 sq mi), accounting for 10.5% of the total lunar surface area.
Mare Imbrium is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the collision of a proto-planet during the Late Heavy Bombardment. Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today. The basin's age has been estimated using uranium–lead dating methods to approximately 3.9 billion years ago, and the diameter of the impactor has been estimated to be 250 ± 25 km. The Moon's maria 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 would have originally been when it first formed as a result of later events that have altered its surface.
The South Pole–Aitken basin is an immense impact crater on the far side of the Moon. At roughly 2,500 km (1,600 mi) in diameter and between 6.2 and 8.2 km (3.9–5.1 mi) deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest, and deepest basin recognized on the Moon. It is estimated that it was formed approximately 4.2 to 4.3 billion years ago, during the Pre-Nectarian epoch. It was named for two features on opposite sides of the basin: the lunar South Pole at one end and the crater Aitken on the northern end. The outer rim of this basin can be seen from Earth as a huge mountain chain located on the Moon's southern limb, sometimes informally called "Leibnitz mountains".
KREEP, an acronym built from the letters K, REE and P, is a geochemical component of some lunar impact breccia and basaltic rocks. Its most significant feature is somewhat enhanced concentration of a majority of so-called "incompatible" elements and the heat-producing elements, namely radioactive uranium, thorium, and potassium.
Mare Cognitum is a lunar mare located in a basin or large crater which sits in the second ring of Oceanus Procellarum. To the northwest of the mare is the Montes Riphaeus mountain range, part of the rim of the buried crater or basin containing the mare. Previously unnamed, the mare received its name in 1964 in reference to its selection as the target for the successful impact probe Ranger 7, the first American spacecraft to return closeup images of the Moon's surface.
The Fra Mauro formation is a formation on the near side of Earth's Moon that served as the landing site for the American Apollo 14 mission in 1971. It is named after the 80-kilometer-diameter crater Fra Mauro, located within it. The formation, as well as Fra Mauro crater, take their names from a 15th-century Italian monk and mapmaker of the same name. Apollo 13 was originally scheduled to land in the Fra Mauro highlands, but was unable due to an in-flight technical failure.
The lunar geological timescale divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian, Nectarian, and Pre-Nectarian. The boundaries of this time scale are related to large impact events that have modified the lunar surface, changes in crater formation through time, and the size-frequency distribution of craters superposed on geological units. The absolute ages for these periods have been constrained by radiometric dating of samples obtained from the lunar surface. However, there is still much debate concerning the ages of certain key events, because correlating lunar regolith samples with geological units on the Moon is difficult, and most lunar radiometric ages have been highly affected by an intense history of bombardment.
A lunar meteorite is a meteorite that is known to have originated on the Moon. A meteorite hitting the Moon is normally classified as a transient lunar phenomenon.
The Discovery Program is a series of Solar System exploration missions funded by the U.S. National Aeronautics and Space Administration (NASA) through its Planetary Missions Program Office. The cost of each mission is capped at a lower level than missions from NASA's New Frontiers or Flagship Programs. As a result, Discovery missions tend to be more focused on a specific scientific goal rather than serving a general purpose.
Aristarchus is a lunar impact crater that lies in the northwest part of the Moon's near side. It is considered the brightest of the large formations on the lunar surface, with an albedo nearly double that of most lunar features. The feature is bright enough to be visible to the naked eye, and displays unusually bright features when viewed through a large telescope. It is also readily identified when most of the lunar surface is illuminated by earthshine. The crater is deeper than the Grand Canyon.
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by lava flowing into ancient impact basins. They were dubbed maria by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus ('lake'), palus ('marsh'), and sinus ('bay'). The last three are smaller than maria, but have the same nature and characteristics.
The far side of the Moon is the lunar hemisphere that always faces away from Earth, opposite to the near side, because of synchronous rotation in the Moon's orbit. Compared to the near side, the far side's terrain is rugged, with a multitude of impact craters and relatively few flat and dark lunar maria ("seas"), giving it an appearance closer to other barren places in the Solar System such as Mercury and Callisto. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. The hemisphere has sometimes been called the "Dark side of the Moon", where "dark" means "unknown" instead of "lacking sunlight" – each location on the Moon experiences two weeks of sunlight while the opposite location experiences night.
The Genesis Rock is a sample of Moon rock retrieved by Apollo 15 astronauts James Irwin and David Scott in 1971 during the second lunar EVA, at Spur crater on Earth's Moon. With a mass of c. 270 grams, it is currently stored at the Lunar Sample Laboratory Facility in Houston, Texas.
The geology of the Moon is quite different from that of Earth. The Moon lacks a true atmosphere, and the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, it has a lower gravity, and because of its small size, it cooled faster. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.
Moon rock or lunar rock is rock originating from Earth's Moon. This includes lunar material collected during the course of human exploration of the Moon, and rock that has been ejected naturally from the Moon's surface and landed on Earth as meteorites.
The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of the Moon. The LMO was likely present on the Moon from the time of the Moon's formation to tens or hundreds of millions of years after that time. The LMO was a thermodynamic consequence of the Moon's relatively rapid formation in the aftermath of a giant impact between the proto-Earth and another planetary body. As the Moon accreted from the debris from the giant impact, gravitational potential energy was converted to thermal energy. Due to the rapid accretion of the Moon, thermal energy was trapped since it did not have sufficient time to thermally radiate away energy through the lunar surface. The subsequent thermochemical evolution of the LMO explains the Moon's largely anorthositic crust, europium anomaly, and KREEP material.
MoonRise is a robotic mission concept to the south pole of the Moon. It was proposed in 2017 for NASA's New Frontiers program mission 4, but it was not selected. If funded and launched by another NASA opportunity, it would focus on the giant South Pole–Aitken basin on the far side of the Moon between the Moon's South Pole and Aitken Crater, 16° south of the Moon's equator. This basin measures nearly 2,500 kilometres (1,600 mi) in diameter and 12 kilometres (7.5 mi) in depth. This region is the oldest and deepest observable impact basin on the Moon and provides a window into the deep crust of the Moon and its history as a result. The basin is also among the largest recognized impact structures in the Solar System.
Moon Diver is a proposed lunar mission concept by NASA's Jet Propulsion Laboratory that would employ a robotic lander and a – distant coaxial – two-wheeled rover called Axel to investigate the exposed geological layers on the walls of a deep lunar pit.
Volcanism on the Moon is represented by the presence of volcanoes, pyroclastic deposits and vast lava plains on the lunar surface. The volcanoes are typically in the form of small domes and cones that form large volcanic complexes and isolated edifices. Calderas, large-scale collapse features generally formed late in a volcanic eruptive episode, are exceptionally rare on the Moon. Lunar pyroclastic deposits are the result of lava fountain eruptions from volatile-laden basaltic magmas rapidly ascending from deep mantle sources and erupting as a spray of magma, forming tiny glass beads. However, pyroclastic deposits formed by less common non-basaltic explosive eruptions are also thought to exist on the Moon. Lunar lava plains cover large swaths of the Moon's surface and consist mainly of voluminous basaltic flows. They contain a number of volcanic features related to the cooling of lava, including lava tubes, rilles and wrinkle ridges.
