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Lunar nearside with major maria and craters labeled

A global albedo map of the Moon obtained from the Clementine missionThe dark regions are the lunar maria, whereas the lighter regions are the highlands. The image is a cylindrical projection, with longitude increasing left to right from -180 E to 180 E and latitude decreasing from top to bottom from 90 N to 90 S. The center of the image corresponds to the mean sub-Earth point, 0 N and 0 E.

The Lunar maria (singular: mare, two syllables) 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 compositions, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the near-side visible from Earth. The few maria on the far-side are much smaller, residing mostly in very large craters where only a small amount of flooding occurred. 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 latter three are smaller than maria, but have the same nature and characteristics.

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^[1], whereas the youngest ages determined from crater counting are about 1.2 Ga (1 Ga = 1 billion years old)^[2]. 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.

1. Since many mare basalts fill low-lying impact basins, it was once thought that the impact event itself somehow caused the volcanic eruption. Given that mare volcanism typically occurred about 500 million years after the impact, a causal relationship is unlikely.
2. It is sometimes suggested that the gravity field of the Earth might preferentially allow eruptions to occur on the near side, but not far side. However, in a reference frame rotating with the Moon, the centrifugal acceleration is exactly equal and opposite to the gravitational acceleration of the Earth. There is thus no net force directed towards the Earth. The Earth tides do act to deform the shape of the Moon, but this shape is one of an elongated ellipsoid with high points at both the sub- and anti-Earth points. As an analogy, one should remember that there are two high tides per day on Earth, and not one.
3. Since mare basaltic magmas are more dense than upper crustal anorthositic materials, basaltic eruptions might be favored at locations of low elevation where the crust is thin. However, the farside South Pole-Aitken basin contains the lowest elevations of the Moon and is yet only modestly filled by basaltic lavas. In addition, the crustal thickness beneath this basin is predicted to be much smaller than beneath Oceanus Procellarum. While crustal thickness might modulate the quantity of basaltic lavas that ultimately reach the surface, crustal thickness by itself can not be the sole factor controlling the distribution of mare basalts.^[3]
4. It is commonly suggested that there is some form of link between the synchronous rotation of the Moon about the Earth, and the mare basalts. However, gravitational torques that result in tidal despinning only arise from the moments of inertia of the body (these are directly relatable to the spherical harmonic degree-2 terms of the gravity field), and the mare basalts hardly contribute to this (see also Tidal locking). (Hemispheric structures correspond to spherical harmonic degree-1, and do not contribute to the moments of inertia.) Furthermore, tidal despinning is predicted to have occurred quickly (on the order to 10s of millions of years), whereas the majority of mare basalts erupted about 1 billion years later.

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.^[4][5][6] 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^[7].

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.%. Other geochemical subdivisions are based on the abundance of aluminium and potassium.

• Moon
• Geology of the Moon
• List of maria on the Moon
• Moon rocks
• Selenography

• Lunar and Planetary Institute: Exploring the Moon
• Lunar and Planetary Institute: Lunar Atlases
• Ralph Aeschliman Planetary Cartography and Graphics: Lunar Maps

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