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.
The Moon has been volcanically active throughout much of its history, with the first volcanic eruptions having occurred about 4.2 billion years ago. Volcanism was most intense between 3.8 and 3 billion years ago, during which time much of the lunar lava plains were created. This activity was originally thought to have petered out about 1 billion years ago, but more recent evidence suggests that smaller-scale volcanism may have occurred in the last 50 million years. Today, the Moon has no active volcanoes even though a significant amount of magma may persist under the lunar surface.
In 1610, Italian astronomer Galileo Galilei misinterpreted the lunar lava plains as seas while observing the Moon through an early telescope. Galilei therefore dubbed them maria after the Latin word for "seas". The bowl-shaped depressions distributed throughout the lunar landscape were first suggested to be volcanoes in 1665 by British chemist Robert Hooke. Their volcanic origin was bolstered by their similarity to the Phlegraean Fields craters in Italy, although much larger. French astronomer Pierre Puiseux proposed that the Moon's craters were collapsed volcanic domes that had vented all their gases. Pierre-Simon Laplace, another French astronomer, proposed in the 18th century that meteorites were volcanic projectiles ejected from lunar craters during major eruptions. [1] British astronomer William Herschel, in one of his early papers, claimed to have seen three volcanoes on the Moon in the late 1700s, which later turned out to be earthshine. [2]
The origin of lunar craters remained controversial throughout the first half of the 20th century, with volcano supporters arguing that bright rays fanning out of some craters were streaks of volcanic ash similar to those found at Mount Aso in Japan. Astronomers also reported flashes of light and red clouds over the Alphonsus and Aristarchus craters. [1] Evidence collected during the Apollo program (1961–1972) and from uncrewed spacecraft of the same period proved conclusively that meteoric impact, or impact by asteroids for larger craters, was the origin of almost all lunar craters, and by implication, most craters on other bodies as well.
After impact cratering, volcanism is the most dominant process that has modified the lunar crust. Much of this modification has been preserved due to the lack of plate tectonics on the Moon, such that the lunar surface has changed insignificantly throughout the Moon's geological history. Lunar volcanism has mostly been confined to the near side of the Moon where basaltic lava plains are the dominant volcanic feature. [3] In contrast, positive topographic features such as domes, cones and shields represent only a tiny fraction of the lunar volcanic record. Volcanoes and lava plains have been found on both sides of the Moon. [3] [4]
The lunar maria are large basaltic plains that cover more than 15% of the Moon's surface. They are the most obvious volcanic features on the Moon, appearing as dark topographic features when seen with the naked eye. Many tend to cover the floors of large impact basins and are therefore typically circular in outline, with some smaller maria filling the bottoms of impact craters. [5] The major lunar maria range in size from more than 200 km (120 mi) to about 1,400 km (870 mi) and are outclassed only by the larger Oceanus Procellarum, which has a diameter of roughly 2,590 km (1,610 mi). [5] [6] [7] They typically range in thickness from about 500 to 1,500 m (1,600 to 4,900 ft), with individual lava flows ranging from 10 to 20 m (33 to 66 ft) thick. This suggests that each mare is the product of several overlapping eruptive events. [5]
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 billion years, whereas the youngest ages determined from crater counting are about 1.2 billion years. [8] [9] Nevertheless, the majority of mare basalts appear to have erupted between about 3 and 3.5 billion years ago. [10] The few basaltic eruptions that occurred on the far side of the Moon are old, whereas the youngest flows are found within Oceanus Procellarum on the near side. 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.
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. [11] [12] [13] 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. [14]
Latin Name | English Name | Lat. | Long. | Diameter |
---|---|---|---|---|
Mare Australe [4] | Southern Sea [15] | 47.77° S [15] | 91.99° E [15] | 996.84 km (619.41 mi) [15] |
Mare Cognitum [4] | Sea that has become known [16] | 10.53° S [16] | 22.31° W [16] | 350.01 km (217.49 mi) [16] |
Mare Crisium [4] | Sea of Crises [17] | 16.18° N [17] | 59.1° E [17] | 555.92 km (345.43 mi) [17] |
Mare Fecunditatis [4] | Sea of Fecundity [18] | 7.83° S [18] | 53.67° E [18] | 840.35 km (522.17 mi) [18] |
Mare Frigoris [4] | Sea of Cold [19] | 57.59° N [19] | 0.01° E [19] | 1,446.41 km (898.76 mi) [19] |
Mare Humboldtianum [4] | Sea of Alexander von Humboldt [20] | 56.92° N [20] | 81.54° E [20] | 230.78 km (143.40 mi) [20] |
Mare Humorum [4] | Sea of Moisture [21] | 24.48° S [21] | 38.57° W [21] | 419.67 km (260.77 mi) [21] |
Mare Imbrium [4] | Sea of Showers [22] | 34.72° N [22] | 14.91° W [22] | 1,145.53 km (711.80 mi) [22] |
Mare Ingenii [4] | Sea of Cleverness [23] | 33.25° S [23] | 164.83° E [23] | 282.2 km (175.4 mi) [23] |
Mare Marginis [4] | Sea of the Edge [24] | 12.7° N [24] | 86.52° E [24] | 357.63 km (222.22 mi) [24] |
Mare Moscoviense [4] | Sea of Muscovy [25] | 27.28° N [25] | 148.12° E [25] | 275.57 km (171.23 mi) [25] |
Mare Nectaris [4] | Sea of Nectar [26] | 15.19° S [26] | 34.6° E [26] | 339.39 km (210.89 mi) [26] |
Mare Nubium [4] | Sea of Clouds [27] | 20.59° S [27] | 17.29° W [27] | 714.5 km (444.0 mi) [27] |
Mare Orientale [4] | Eastern Sea [28] | 19.87° S [28] | 94.67° W [28] | 294.16 km (182.78 mi) [28] |
Mare Serenitatis [4] | Sea of Serenity [29] | 27.29° N [29] | 18.36° E [29] | 674.28 km (418.98 mi) [29] |
Mare Smythii [4] | Smyth's Sea [30] | 1.71° N [30] | 87.05° E [30] | 373.97 km (232.37 mi) [30] |
Mare Tranquillitatis [4] | Sea of Tranquility [31] | 8.35° N [31] | 30.83° E [31] | 875.75 km (544.17 mi) [31] |
Mare Vaporum [4] | Sea of Vapors [32] | 13.2° N [32] | 4.09° E [32] | 242.46 km (150.66 mi) [32] |
A number of domes and cones are present on the Moon, but such features likely formed differently than those on Earth. [33] Because gravity on the Moon is only one sixth of that on Earth, lunar volcanism is capable of throwing ejecta much further, leaving little to pile up near the vent. [4] [33] Instead of a volcanic cone, such lunar eruptions should form a broad, thin layer around the vent. On Earth, lava domes form from very viscous, pasty lavas. Basaltic lavas are more liquid and tend to form broad, flat lava flows. On the Moon, most of the domes and cones appear to be made of basalts. As a result, they are unlikely to have formed like Earth domes from thick, non-basaltic lavas. Instead, the lunar domes and cones may mark places where the erupted basalts were just barely molten. [33]
Lunar domes are seldom found in isolation. Instead, they more commonly form in groups throughout the lunar lava plains. [34] A prominent example are the Marius Hills, one of the largest volcanic complexes on the Moon. [34] [35] They consist of several cones and domes that occupy the summit of a broad topographic swell, which may be the lunar equivalent of a shield volcano. [34] The complex rises 100 to 200 m (330 to 660 ft) from the surrounding plains and forms a 35,000 km2 (14,000 sq mi) lava plateau. A total of 59 cones and 262 domes ranging in diameter from 2 to 25 km (1.2 to 15.5 mi) have been identified. [35]
Mons Rümker is a smaller complex similar in appearance to the Marius Hills. [34] It comprises a plateau with an area of roughly 2,000 km2 (770 sq mi) and rises 200 to 1,300 m (660 to 4,270 ft) above the surrounding surface. Three main basalt units ranging in age from 3.51 to 3.71 billion years have been identified at Mons Rümker, although the youngest volcanic features may be steep-sided domes on the plateau surface as they show indications of having been active until the Eratosthenian. More than 20 domes overlie the plateau and are the most prominent volcanic landforms of Mons Rümker. [36]
The Gruithuisen Domes in northwestern Mare Imbrium consist of two volcanic edifices: Mons Gruithuisen Gamma to the north and Mons Gruithuisen Delta to the south. [37] They are situated on the rim of an impact crater and differ in color from the surrounding rocks. The domes may mark a rare instance of non-basaltic volcanism on the Moon. [38] Mons Hansteen, a roughly triangular-shaped dome on the southern margin of Oceanus Procellerum, is another example of a rare non-basaltic lunar volcano. It consists of high-silica material that was erupted roughly 3.5 to 3.7 billion years ago from vents along northeast, northwest and southwest-trending fractures. [39]
The Compton–Belkovich Volcanic Complex (CBVC) is a 25 km (16 mi) wide and 35 km (22 mi) long non-mare feature on the far side of the Moon. It differs from other lunar volcanic features due to its evolved lithology, regional tectonic setting, its location being near the north pole, far from the Procellarum KREEP Terrane and its recent association with endogenic water. In the middle of the CBVC lies an irregular-shaped depression bounded by fault scarps that is believed to be a caldera. Just to the west is a roughly 10 km (6.2 mi) wide and 18 km (11 mi) long feature called West Dome. A volcanic cone-like feature, called East Dome, lies near the eastern caldera margin. It has a more or less north–south trend, measuring 12 km (7.5 mi) long and 7 km (4.3 mi) wide. [40] Just north of the caldera is a feature called Little Dome, 500 m (1,600 ft) in diameter. Further north is an elongated dome, oriented north–south, called Middle Dome. It is 2.5 km (1.6 mi) long and 0.6 km (0.37 mi) wide. Both Little Dome and Middle Dome have boulders on top that may be volcanic blocks. [41] Big Dome, also known as North Dome, is further to the north at the edge of the CBVC. [40] [41] It is 2.5 km (1.6 mi) in diameter with a depression in the top. [41] Small-crater size frequency distribution has given inconclusive results for the timing of CBVC volcanism, with ages ranging from less than 1 billion years to greater than 3 billion years. [42]
Although lava tubes have long been known to exist on Earth, it has only been relatively recently that they have been confirmed to also exist on the Moon. Their existence is sometimes revealed by the presence of a "skylight", a place in which the roof of the tube has collapsed, leaving a circular hole that can be observed by lunar orbiters. [43] [44] An area displaying a lava tube is the Marius Hills region. [45] In 2008, an opening to a lava tube in this area may have been discovered by the Japanese Kaguya spacecraft. [46] The skylight was photographed in more detail in 2011 by NASA's Lunar Reconnaissance Orbiter, showing both the 65-meter-wide pit and the floor of the pit about 36 m (118 ft) below. [44] [47] In 2023–2024, radar imaging of the Mare Tranquillitatis pit crater from NASA's Lunar Reconnaissance Orbiter was analyzed and determined to have been formed by the collapse of a lava tube that resulted in the formation of a cave conduit at least tens of meters long, proving the existence of lunar caves. [48] There may also be lava tubes in the Mare Serenitatis. [49] [50]
Lunar lava tubes may potentially serve as enclosures for human habitats. [46] [49] [51] Tunnels larger than 300 m (980 ft) in diameter may exist, lying under 40 m (130 ft) or more of basalt, with a stable temperature of −20 °C (−4 °F). [52] These natural tunnels provide protection from cosmic radiation, solar radiation, meteorites, micrometeorites, and ejecta from impacts. They are insulated from the extreme temperature variations on the lunar surface and could provide a stable environment for inhabitants. [53]
Near the edges of the lunar mare are dark layers of material that cover many thousands of square kilometers. They contain many small spheres of orange and black glass that probably formed from small drops of lava that cooled very quickly. Such droplets are believed to be ejecta from lava fountain eruptions that were larger than those on Earth. [54] The largest known deposits occur at Taurus–Littrow, Sinus Aestuum, Sulpicius Gallus, Rima Bode, Mare Vaporum, Mare Humorum and the Aristarchus plateau in the central near side of the Moon. [55]
Many smaller pyroclastic deposits measure only a few kilometers in diameter and are almost always located near the mare or in large impact crater floors, although several also lie along clear fault lines. [54] They were likely produced by small volcanic explosions since most contain a small elongated or irregular-shaped central pit or crater. [54] [56] Examples are preserved along the crater floor edge of Alphonsus, an impact crater on the eastern edge of Mare Nubium. [56]
Extending about 7 km (4.3 mi) east-southeast from the CBVC is a highly reflective area that may be a pyroclastic flow deposit. Its reflectivity is stronger in the 7.1 to 7 μm (0.00028 to 0.00028 in) range, indicating that quartz or alkali feldspar is the major constituent. [41] Explosive remains also appear scattered to the east for about 300 km (190 mi), covering an area of 70,000 km2 (27,000 sq mi). The large extent of this pyroclastic deposit is due to the Moon's low gravity, such that a giant explosive eruption from the CBVC was able to spread debris over an area much greater than would be possible on Earth. [57]
These are long, narrow depressions in the lunar surface that resemble channels. Their precise formation remains to be determined, but they were likely formed by different processes. For instance, sinuous rilles meander in a curved path like a mature river and are thought to represent lava channels or the remains of collapsed lava tubes. [58] They normally extend from small pit structures that are believed to have been volcanic vents. [58] [59] Schroter's Valley between Mare Imbrium and Oceanus Procellarum is the largest sinuous rille. [59] Another prominent example is Rima Hadley, which formed nearly 3.3 billion years ago. [59] [60]
Arcuate rilles have a smooth curve and are found on the edges of the dark lunar maria. They are believed to have formed when the lava flows that created a mare cooled, contracted and sank. [61] These are found all over the Moon; prominent examples can be seen near the southwestern border of Mare Tranquillitatis and on the western southeastern border of Mare Humorum. [62]
Analysis of Moon magma samples retrieved by the Apollo missions indicate that volcanism on the Moon produced a relatively thick lunar atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. It has been theorized, in fact, that this ancient atmosphere could have supported life, though no evidence of life has been found. [63] The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. [64]
Partial melting of the lunar mantle and the emplacement of Oceanus Procellarum flood basalts may have caused axial tilting of the Moon 3 billion years ago, during which time the lunar poles shifted 125 mi (201 km) to their modern positions. This polar wander is inferred from polar hydrogen deposits that are antipodal and displaced equally from each pole along opposite longitudes. [65]
In 2014, NASA announced "widespread evidence of young lunar volcanism" at 70 irregular mare patches identified by the Lunar Reconnaissance Orbiter, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously estimated, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. [66] [67] [68] [69] Just prior to this, evidence has been presented for 2–10 million years younger basaltic volcanism inside the crater Lowell, [70] [71] located in the transition zone between the near and far sides of the Moon. An initially hotter mantle and/or local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities also on the far side in the Orientale basin. [72] [73] There are currently no active volcanoes on the Moon, although moonquake data published in 2012 suggest that there is a substantial amount of magma under the lunar surface. The lack of active volcanism on the Moon may be due to the magma being too dense to rise to the surface. [74]
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 Australe is a lunar mare located in the southeastern hemisphere of the Moon. It is 997 kilometers in diameter, overlapping the near and far sides of the Moon. Smooth, dark volcanic basalt lines the bottom of the mare. The Australe basin was formed in the Pre-Nectarian epoch, while the mare material inside formed in the Upper Imbrian epoch. The basin was almost completely destroyed by impacts prior to the appearance of the mare.
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.
Mare Marginis ; MAR-jin-iss) 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.
Mare Nectaris is a small lunar mare or sea located south of Mare Tranquillitatis southwest of Mare Fecunditatis, on the near side of the Moon. Montes Pyrenaeus borders the mare to the east and Sinus Asperitatis fuses to its northwestern edge. It is 84,000 square kilometers in size.
Mare Smythii is a lunar mare located along the equator on the easternmost edge of the Moon's near side, named for the 19th-century British astronomer William Henry Smyth. It is one of only two lunar maria that are named after people, the other being Mare Humboldtianum.
Mare Undarum is a shallow, irregular lunar mare located just north of Mare Spumans on the lunar near side, between the crater Firmicus and the eastern limb. It lies within a trough between the third and fourth raised rings formed by the impact that created the Mare Crisium. The selenographic coordinates of this mare are 7.5° N, 68.7° E. It has a maximum diameter of 245 km.
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.
Marius is a lunar impact crater located on the Oceanus Procellarum. The surface to the west and north of this crater contains a large number of lunar domes spread across an area over a hundred kilometers in diameter that may be of volcanic origin dubbed the Marius Hills. These domes, if volcanic, may have been formed by magma that is fairly more viscous than the volcanic material that formed the basaltic lunar maria. The nearest named crater feature is Reiner to the southwest. Kepler is located to the east-southeast, and rays from that formation reach the rim of Marius.
Mairan is a lunar impact crater that is located on a highland peninsula between Oceanus Procellarum to the west and Mare Imbrium to the east. To the north-northeast is the comparably sized crater Sharp. Northwest of Mairan is the heavily eroded Louville.
Billy is a lunar impact crater that is located at the southern fringes of the Oceanus Procellarum, in the western hemisphere of the Moon. It was named after French mathematician Jacques de Billy. It lies to the southeast of the similar-sized crater Hansteen, and west-southwest of the flooded Letronne.
Mons Gruithuisen Gamma (γ) is a lunar dome that lies to the north of the crater Gruithuisen at the western edge of the Mare Imbrium.
Lacus Mortis is a hexagonal-shaped plain of basaltic lava flows in the northeastern part of the Moon's near face. It was formed as a floor-fractured crater during the pre-Imbrian epoch, then flooded during the late Imbrian period. This feature lies just to the south of the elongated Mare Frigoris, being separated by a slender arm of rugged ground and linked at the eastern extreme. To the south is the Lacus Somniorum, separated from this mare by the joined craters Plana and Mason, and a strip of uneven surface.
Palus Putredinis is a small lunar mare in the basin of Mare Imbrium. It stretches from the crater Archimedes southeast toward the rugged Montes Apenninus range located on the southeastern edge of Mare Imbrium. This region is a nearly level, lava-flooded plain bounded by the crater Autolycus and nearby highlands to the northeast and the foothills of the Montes Archimedes to the southwest. The selenographic coordinates are 27.4° N, 0.0° E, and it lies within a diameter of 180 kilometers (110 mi).
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.
A cinder cone is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions or lava fountains from a single, typically cylindrical, vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as either cinders, clinkers, or scoria around the vent to form a cone that often is symmetrical; with slopes between 30 and 40°; and a nearly circular ground plan. Most cinder cones have a bowl-shaped crater at the summit.
The Marius Hills are a set of volcanic domes located in Oceanus Procellarum on Earth's Moon. The domes are thought to have formed from lavas more viscous than those that formed lunar mares. These domes average approximately 200–500 m (660–1,640 ft) in height. The Marius Hills take their name from the nearby 41 km (25 mi) diameter crater Marius. These hills represent the highest concentration of volcanic features on the Moon.
Hesperia Planum is a broad lava plain in the southern highlands of the planet Mars. The plain is notable for its moderate number of impact craters and abundant wrinkle ridges. It is also the location of the ancient volcano Tyrrhena Mons. The Hesperian time period on Mars is named after Hesperia Planum.
The Compton–Belkovich Thorium Anomaly is a volcanic complex on the far side of the Moon. It was found by a gamma-ray spectrometer in 1998 and is an area of concentrated thorium, a 'fertile' element. Lunar rock samples from the Apollo missions reveal that most lunar volcanism occurred around 3 to 4 billion years ago, but this feature could have formed as recently as 1 billion years ago due to the unknown history of the Moon's far side.