The lunar maria /ˈmɑːriə/ (singular: mare /ˈmɑːreɪ/) are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", 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 names of maria refer to sea features (Humorum, Imbrium, Insularum, Nubium, Spumans, Undarum, Vaporum, Procellarum, Frigoris), sea attributes (Australe, Orientale, Cognitum, Marginis), or states of mind (Crisium, Ingenii, Serenitatis, Tranquillitatis). Mare Humboldtianum and Mare Smythii were established before the final nomenclature, that of states of mind, was accepted, and do not follow this pattern. When Mare Moscoviense was discovered by the Luna 3, and the name was proposed by the Soviet Union, it was only accepted by the International Astronomical Union with the justification that Moscow is the state of mind.
The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting. The radiometric ages range from about 3.16 to 4.2 Ga, whereas the youngest ages determined from crater counting are about 1.2 Ga (1 Ga = 1 billion years old). Nevertheless, the majority of mare basalts appear to have erupted between about 3 and 3.5 Ga. The few basaltic eruptions that occurred on the far side are old, whereas the youngest flows are found within Oceanus Procellarum on the nearside. While many of the basalts either erupted within, or flowed into, low-lying impact basins, the largest expanse of volcanic units, Oceanus Procellarum, does not correspond to any known impact basin.
There are many common misconceptions concerning the spatial distribution of mare basalts.
The reason that the mare basalts are predominantly located on the near-side hemisphere of the Moon is still being debated by the scientific community. Based on data obtained from the Lunar Prospector mission, it appears that a large proportion of the Moon's inventory of heat producing elements (in the form of KREEP) is located within the regions of Oceanus Procellarum and the Imbrium basin, a unique geochemical province now referred to as the Procellarum KREEP Terrane. While the enhancement in heat production within the Procellarum KREEP Terrane is most certainly related to the longevity and intensity of volcanism found there, the mechanism by which KREEP became concentrated within this region is not agreed upon.
Using terrestrial classification schemes, all mare basalts are classified as tholeiitic, but specific subclassifications have been invented to further describe the population of lunar basalts. Mare basalts are generally grouped into three series based on their major element chemistry: high-Ti basalts, low-Ti basalts, and very-low-Ti (VLT) basalts. While these groups were once thought to be distinct based on the Apollo samples, global remote sensing data from the Clementine mission now shows that there is a continuum of titanium concentrations between these end members, and that the high-titanium concentrations are the least abundant. TiO2 abundances can reach up to 15 wt.% for mare basalts, whereas most terrestrial basalts have abundances much less than 4 wt.%. A special group of lunar basalts is the KREEP basalts, which are abnormally rich in potassium (K), rare earth elements (REE), and phosphorus (P). A major difference between terrestrial and lunar basalts is the near-total absence of water in any form in the lunar basalts. Lunar basalts do not contain hydrogen-bearing minerals like the amphiboles and phyllosilicates that are common in terrestrial basalts due to alteration or metamorphism.
Borealis Planitia is a large basin on Mercury with a smooth floor, thought to be similar to a lunar mare. It is centered at 73.4° N, 79.5° W. The name is Latin for "Northern Plain".Borel (crater)
Borel is a tiny lunar impact crater located in the southeast part of Mare Serenitatis. It was named after French mathematician Émile Borel. To the northeast is the crater Le Monnier and to the southeast is the crater Abetti. Borel was previously identified as Le Monnier C.
This is a roughly circular, cup-shaped formation with inner floors that slope down to the midpoint of the crater. The interior has a higher albedo than the surrounding dark lunar mare.C. Herschel (crater)
C. Herschel is a small lunar impact crater that lies on the western part of Mare Imbrium. It is named after British astronomer Caroline Herschel. It is a circular, bowl-shaped formation that has not undergone significant erosion. The interior floor has the same low albedo as the surrounding lunar mare. To the south-southwest is the similar crater Heis. C. Herschel lies on a wrinkle ridge of the lunar mare named the Dorsum Heim.Carmichael (crater)
Carmichael is a lunar impact crater that is located along the eastern edge of the Sinus Amoris, in the northeastern quadrant of the Moon's near side. Its diameter is 20 km. It was named after American psychologist Leonard Carmichael. It lies within a couple of crater diameters south-southwest of the smaller crater Hill. Further to the east-northeast is the prominent crater Macrobius. Carmichael was designated Macrobius A before being given its current name by the IAU.
Carmichael is generally circular, with a small floor at the middle of the sloping interior walls. There is a low rise of scree along the southeast inner wall. The crater is free of notable impacts along the rim or the interior, although a tiny craterlet is situated in the lunar mare just outside the rim to the south-southwest.Caventou (crater)
Caventou is a tiny lunar impact crater located in the western part of the Mare Imbrium. It is a circular, cup-shaped formation surrounded by the lunar mare. It was named after French chemist Joseph B. Caventou in 1976. Prior to that, it had the designation La Hire D, being associated with the mountain Mons La Hire to the southeast.Clausius (crater)
Clausius is a lunar impact crater that is located in the southwest part of the Moon, in the small lunar mare designated Lacus Excellentiae. It is completely enclosed by mare material, although the tiny satellite crater Clausius A lies just to the north. The rim of Clausius is low and sharp, with a slightly oval shape that is longer in the north-south direction. The interior floor has been flooded by basaltic lava, and appears level and featureless with a darker surface that matches the mare surface that surrounds the crater exterior.Courtney (crater)
Courtney is a tiny lunar impact crater on the Mare Imbrium, a lunar mare in the northwest quadrant of the Moon. It lies about two crater diameters to the northwest of Euler, in an otherwise isolated stretch of the mare. The dark surface in this region is marked by Euler's ray material. The name is an English male given name; it does not refer to a specific person.Crozier (crater)
Crozier is a lunar impact crater that is located on the southwest edge of Mare Fecunditatis, a lunar mare in the eastern part of the Moon's near side. It lies to the east-northeast of the prominent crater Colombo, and southeast of the small crater Bellot.
The narrow rim of this crater forms a distorted enclosure that has outward bulges along the northwest, southwest, and southeastern sides. The interior floor has been resurfaced and nearly filled by basaltic lava, producing a level surface with a low albedo that matches the dark hue of the nearby lunar mare. Nearly adjacent to the outer rim are the similar flooded craters Crozier D to the east and Crozier M to the southeast.Darney (crater)
Darney is a small lunar impact crater that is located on the region of the Moon where the Mare Nubium joins the Oceanus Procellarum. It was named after French astronomer Maurice Darney. To the south is the lava-flooded crater Lubiniezky. The southern rim of Darney is attached to a series of low ridges that extend to the southwest.
This is a bowl-shaped formation with a small interior floor at the midpoint of the sloping inner walls. The crater has a relatively high albedo compared to the surrounding dark lunar mare, and the crater is at the focus of a small ray system extending for 110 kilometers.Daubrée (crater)
Daubrée is a lunar impact crater that is located to the southwest of the Mare Serenitatis, just to the west-southwest of the crater Menelaus in the Montes Haemus range. The small lunar mare Lacus Hiemalis lies along the southwest rim of Daubrée. The crater was named after French geologist Gabriel A. Daubrée. It was previously designated Menelaus S.
This is a horseshoe-shaped formation with the rim open to the northwest. The interior has been flooded by basaltic lava, leaving a level, featureless floor. The rim has a low cut through the southern end, and the eastern rim is attached to low ridges belonging to the Montes Haemus.Dechen (crater)
Dechen is a small, bowl-shaped crater that is located in the northwest part of the Oceanus Procellarum, near the northwest limb of the Moon. The rim of the crater projects slightly above the surrounding lunar mare, and the interior is symmetrical and nearly featureless. It lies to the northeast of the crater Harding, but is otherwise relatively isolated.Dunthorne (crater)
Dunthorne is a small lunar impact crater that is located to the northwest of the small lunar mare called Palus Epidemiarum, in the southwest part of the Moon's near side. It was named after British astronomer Richard Dunthorne. It lies to the southwest of the crater Campanus, east of Vitello. Due south is Ramsden.
This crater is roughly circular and bowl-shaped, with an interior that has a higher albedo than the surrounding terrain. It lies in a region that has a number of rille systems, with the Rimae Hippalus to the northwest, and the Rimae Ramsden to the south and east.Eckert (crater)
Eckert is a tiny, isolated lunar impact crater in the northern part of the Mare Crisium (a circular region of relatively dark, flat material on the surface of the Moon). This crater forms a circular pit in the dark surface of the surrounding lunar mare. Just to the west is a wrinkle ridge in the mare surface, a feature that is prominent only under oblique lighting from the Sun. The nearest craters of note are Peirce to the west-northwest, and Picard to the southwest. Both of these craters lie in the Mare Crisium basin.
The crater is named for American astronomer Wallace John Eckert, and name was approved by the IAU in 1973.Elger (crater)
Elger is a lunar impact crater that lies along the southern edge of Palus Epidemiarum, the Marsh of Epidemics, in the southwest part of the Moon's near side. To the northeast is the flooded crater Capuanus, and farther to the northwest is Ramsden.
The crater is named after British astronomer Thomas Gwyn Elger.The rim of this formation is rough and somewhat eroded formation, with a break and an outward bulge along the northern end, while a ridge intrudes into the southern rim. The interior has been resurfaced by lava, although the albedo of the floor is not quite as low as the lunar mare surface to the north.Epimenides (crater)
Epimenides is a lunar impact crater that is located in the southwestern part of the Moon's near side, just to the east of the oddly shaped crater Hainzel. Just to the north and northeast is Lacus Timoris, a small lunar mare. The crater is 27 kilometers in diameter and 2,000 meters deep. It may be from the Pre-Nectarian period, 4.55 to 3.92 billion years ago.The outer rim of this crater is roughly circular, but uneven due to the irregular terrain in which it is located. The southern edge is distended where a smaller formation has overlapped the side. The interior floor is relatively level and featureless. About five kilometers southeast from the crater's southern edge is the satellite crater Epimenides S, which is almost the same size as the main crater and is nearly circular, except for a craterlet making an indentation in its eastern side.The crater is named for the 6th-century BC Cretan poet and prophet Epimenides.Fabbroni (crater)
Fabbroni is a small lunar impact crater that lies along the northern edge of the Mare Tranquillitatis, at the eastern edge of the gap where the lunar mare joins Mare Serenitatis to the north. To the southeast is the crater Vitruvius.
Fabbroni is named after the Italian economist, chemist and naturalist Giovanni Fabbroni. It was previously designated Vitruvius E.
This is a circular crater with a conical interior formed by the inner walls gradually sloping down to the tiny floor at the midpoint. The northern rim of this crater lies along the southeastern flanks of the peak Mons Argaeus.Fedorov (crater)
Fedorov is a lunar geologic feature ("crater" in IAU nomenclature) located in the western Mare Imbrium. It was named after Russian rocket scientist A. P. Fyodorov. It lies east-northeast of the crater Diophantus, and southeast of Delisle. About 20 kilometers to the south-southeast is the slightly larger formation of Artsimovich.
This feature is slightly elongated and oddly shaped, with a ridge on the northern side. This ridge is about as large around the base as Federov crater, and rises about 0.8 km above the surrounding lunar mare.Feuillée (crater)
Feuillée is a small lunar impact crater in the eastern part of the Mare Imbrium. It was named after French natural scientist Louis Feuillée. It lies less than a half crater diameter to the northwest of Beer, and the two formations form a nearly matched pair. To the west is the small but prominent crater Timocharis.
Like Beer, Feuillée is a circular, bowl-shaped formation with a small interior floor at the midpoint of the sloping inner walls. This sharp-edged crater is not notably worn or eroded, and lacks any distinguishing features. It does, however, lie across a wrinkle ridge in the surface of the lunar mare, a feature that is best observed under oblique lighting conditions when the crater is near the terminator.
The crater name is incorrectly spelled Feuillet on some lunar charts.Freud (crater)
Freud is a tiny lunar impact crater that lies on a plateau within the Oceanus Procellarum, in the northwest part of the Moon's near side. It is located a few kilometers to the west of the Vallis Schröteri, a large, sinuous valley that begins to the north of the crater Herodotus, then meanders north, then northwest, and finally southwest, until it reaches the edge of the lunar mare.