
The lunar maria ( ; mare ) are large, dark,
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
ic plains on
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, formed by
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
flowing into ancient impact basins. They are less
reflective
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The ...
than the "highlands" as a result of their iron-rich composition, and hence appear dark to the
naked eye
Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...
. 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 (ocean), as well as features with the names ('lake'), ('marsh'), and ('bay'). The last three are smaller than maria, but have the same nature and characteristics.
The names of maria refer to sea features (
Mare Humorum,
Mare Imbrium
Mare Imbrium (Latin ''imbrium'', the "Sea of Showers" or "Sea of Rains") is a vast lunar mare, 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 ...
,
Mare Insularum,
Mare Nubium,
Mare Spumans
Mare Spumans (Latin ''spūmāns'', the "foaming sea") is a lunar mare located just south of Mare Undarum on the lunar near side. It is one of the many elevated lakes contained in the Crisium basin, surrounding Mare Crisium. The surrounding basi ...
,
Mare Undarum,
Mare Vaporum
Mare Vaporum (Latin ''vapōrum'', the "sea of vapors") is a lunar mare located between the southwest rim of Mare Serenitatis and the southeast rim of Mare Imbrium. It was named by Giovanni Battista Riccioli in 1651.''Autostar Suite Astronomer ...
,
Oceanus Procellarum
Oceanus Procellarum ( ; from ) 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 ("s ...
,
Mare Frigoris
Mare Frigoris (Latin ''frīgōris'', the "Sea of Cold") is a lunar mare in the far north of the Moon. It is located in the outer rings of the Procellarum basin, just north of Mare Imbrium, and stretches east to north of Mare Serenitatis. It is j ...
), sea attributes (
Mare Australe,
Mare Orientale
Mare Orientale (Latin ''orientāle'', the "eastern sea") is a lunar mare. It is located on the western border of the near side and far side of the Moon, and is difficult to see from an Earthbound perspective. Images from spacecraft have revea ...
,
Mare Cognitum
Mare Cognitum (Latin ''cognitum'', the "Sea that has Become Known") is a lunar mare located in a basin or large Impact crater, crater which sits in the second ring of Oceanus Procellarum. To the northwest of the mare is the Montes Riphaeus mounta ...
,
Mare Marginis
Mare Marginis (Latin for 'Sea of the Edge'); ) is a lunar mare that lies on the very edge of the lunar nearside. The selenographic coordinates of this feature are 13.3° N, 86.1° E, and the diameter is 358 km.
This mare differs from most ...
), or states of mind (
Mare Crisium
Mare Crisium (Latin ''crisium'', the "Sea of Crises") is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. Mare Crisium is a basin of Nectarian age. It was formed by the flooding of basaltic lava that fill ...
,
Mare Ingenii,
Mare Serenitatis
Mare Serenitatis (Latin ''serēnitātis'', the "Sea of Serenity") is a lunar mare located to the east of Mare Imbrium on the Moon. Its diameter is .
Geology
Mare Serenitatis is located within the Serenitatis basin, which is of the Nectarian ...
,
Mare Tranquillitatis
Mare Tranquillitatis (Latin for Sea of Tranquillity or Sea of Tranquility) is a lunar mare that sits within the Tranquillitatis basin on the Moon. It contains Tranquility Base, the first location on another celestial body to be visited by huma ...
).
Mare Humboldtianum
Mare Humboldtianum (Latin ''humboldtiānum'', the "Sea of Alexander von Humboldt") is a lunar mare located just to the east of Mare Frigoris, in the center of Humboldtianum basin. It is located along the northeastern limb of the Moon, and cont ...
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
Luna 3, or E-2A No.1 (), was a Soviet spacecraft launched in 1959 as part of the Luna programme. It was the first mission to photograph the far side of the Moon and the third Soviet space probe to be sent to the neighborhood of the Moon. The hi ...
, and the name was proposed by the Soviet Union, it was only accepted by the
International Astronomical Union
The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
with the justification that Moscow is a state of mind.
Ages
The ages of the mare basalts have been determined both by direct
radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...
and by the technique of
crater counting
Crater counting is a method for estimating the age of a planet's surface based upon the assumptions that when a piece of planetary surface is new, then it has no impact craters; impact craters accumulate after that at a rate that is assumed known. ...
. The radiometric ages range from about 3.16 to 4.2 billion years old (Ga), whereas the youngest ages determined from crater counting are about 1.2 Ga. Updated measurements of samples collected by the Chang’e-5 mission show that some lunar basalts could be as young as 2.03 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
Oceanus Procellarum ( ; from ) 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 ("s ...
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.
Distribution of basalts
There are many common misconceptions concerning the spatial distribution of mare basalts.
#Since many mare basalts fill low-lying impact basins, it was once assumed that the impact event itself somehow caused the volcanic eruption. Note: current data in fact may not preclude this, although the timing and length of mare volcanism in a number of basins cast some doubt on it. Initial mare volcanism generally seems to have begun within 100 million years of basin formation. Although these authors felt that 100 million years was sufficiently long that a correlation between impact and volcanism seemed unlikely, there are problems with this argument. The authors also point out that the oldest and deepest basalts in each basin are likely buried and inaccessible, leading to a sampling bias.
#It is sometimes suggested that the gravity field of the Earth might preferentially allow eruptions to occur on the
near side, but not on the
far side. However, in a reference frame rotating with the Moon, the
centrifugal acceleration
Centrifugal force is a fictitious force in Newtonian mechanics (also called an "inertial" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It appears to be directed radially away from the axi ...
the Moon is experiencing 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 that of an elongated ellipsoid with high points at both the sub- and anti-Earth points. As an analogy, there are two high tides per day on Earth, and not one.
#Since mare basaltic magmas are denser than upper crustal
anorthositic materials, basaltic eruptions might be favored at locations of low elevation where the crust is thin. However, the
far side South Pole–Aitken basin contains the lowest elevations of the Moon and yet is only sparingly filled by basaltic lavas. In addition, the crustal thickness beneath this basin is predicted to be much smaller than beneath
Oceanus Procellarum
Oceanus Procellarum ( ; from ) 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 ("s ...
. While the thickness of the crust might modulate the quantity of basaltic lavas that ultimately reach the surface, crustal thickness by itself cannot be the sole factor controlling the distribution of mare basalts.
#It is commonly suggested that there is some form of link between the
synchronous rotation
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
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
In mathematics and Outline of physical science, physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields. The tabl ...
degree-2 terms of the gravity field), and the mare basalts hardly contribute to this (see also
tidal locking
Tidal locking between a pair of co-orbiting astronomical body, astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where ...
). (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 (in the order of thousands of years), whereas the majority of mare basalts erupted about one 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
''Lunar Prospector'' was a spacecraft that orbited the Moon for 19 months in 1998-99. From a low polar orbit, it mapped surface composition including lunar hydrogen deposits, measured magnetic and gravity fields, and studied lunar outgassing e ...
mission, it appears that a large proportion of the Moon's inventory of heat producing elements (in the form of
KREEP
KREEP, an acronym built from the letters K (the atomic symbol for potassium), REE (rare-earth elements) and P (for phosphorus), is a geochemical component of some lunar impact breccia and basaltic rocks. Its most significant feature is somewhat e ...
) is located within the regions of
Oceanus Procellarum
Oceanus Procellarum ( ; from ) 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 ("s ...
and the
Imbrium basin
Mare Imbrium (Latin ''imbrium'', the "Sea of Showers" or "Sea of Rains") 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 pro ...
, 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.
Chemical composition
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. TiO
2 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
Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
(K),
rare-earth element
The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set o ...
s (REE), and
phosphorus
Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
(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
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, the crystalline forms of silica (silicon dio ...
that are common in terrestrial basalts due to alteration or metamorphism.
See also
References
Further reading
* Paul D. Spudis, ''The Once and Future Moon'', Smithsonian Institution Press, 1996, .
*
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External links
Google MoonLunar and Planetary Institute: Exploring the MoonLunar and Planetary Institute: Lunar Atlases
*
ttp://www.psrd.hawaii.edu/Archive/Archive-Moon.html Moon articles in Planetary Science Research Discoveriesbr>
Cryptomare formations
{{DEFAULTSORT:Lunar Mare