Volcanism on the Moon

In 1604, German astronomer Johannes Kepler studied the moon and mistakenly thought that the lunar spots were seas. In Dioptrice, he called them maria, the Latin word for "seas". This idea was supported by their resemblance to the craters in the Phlegraean Fields of Italy, though the lunar ones are much larger. French astronomer Pierre Puiseux later proposed that the Moon’s craters were collapsed volcanic domes that had released all their gases. In the 18th century, another French astronomer, Pierre-Simon Laplace, suggested that meteorites were volcanic projectiles thrown out during major lunar eruptions. 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. It was initially believed that meteorite impacts could not be responsible for circular craters and that they must have all been due to volcanic activity, as meteorites would create elliptic craters when impacting at an angle. However, research has shown that meteorites would have sufficient energy to cause a circular explosion upon impact, causing circular craters, unless they were to hit at an extraordinarily small angle to the horizontal. ==Features==

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. 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. Lava plains and smaller mare, such as Imbrium and Serenitatis, that sit within its ring. Left of the centerline is Procellarum proper. 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. The major lunar maria range in size from more than to about and are outclassed only by the larger Oceanus Procellarum, which has a diameter of roughly . They typically range in thickness from about , with individual lava flows ranging from thick. This suggests that each mare is the product of several overlapping eruptive events. Nevertheless, the majority of mare basalts appear to have erupted between about 3 and 3.5 billion years ago. 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. 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. Examples Volcanoes A number of domes and cones are present on the Moon, but such features likely formed differently than those on Earth. 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. A prominent example are the Marius Hills, one of the largest volcanic complexes on the Moon. 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. 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. 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. 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. The Compton–Belkovich Volcanic Complex (CBVC) is a wide and 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 wide and 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 long and wide. Just north of the caldera is a feature called Little Dome, in diameter. Further north is an elongated dome, oriented north–south, called Middle Dome. It is long and wide. Both Little Dome and Middle Dome have boulders on top that may be volcanic blocks. Big Dome, also known as North Dome, is further to the north at the edge of the CBVC. Lava tubes may be the partial collapse of a lunar lava tube 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. An area displaying a lava tube is the Marius Hills region. In 2008, an opening to a lava tube in this area may have been discovered by the Japanese Kaguya spacecraft. 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 below. 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. There may also be lava tubes in the Mare Serenitatis. Lunar lava tubes may potentially serve as enclosures for human habitats. Tunnels larger than in diameter may exist, lying under or more of basalt, with a stable temperature of . 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. Pyroclastic deposits soil discovered on the Apollo 17 mission. The orange color is due to microscopic glass beads created by volcanic processes earlier in the Moon's history. 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. 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. 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. Examples are preserved along the crater floor edge of Alphonsus, an impact crater on the eastern edge of Mare Nubium. Rilles 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. They normally extend from small pit structures that are believed to have been volcanic vents. Schroter's Valley between Mare Imbrium and Oceanus Procellarum is the largest sinuous rille. 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. 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. ==Impacts==

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. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. 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 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. ==Recent activity==

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. Just prior to this, evidence has been presented for 2–10 million years younger basaltic volcanism inside the crater Lowell, 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. 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. ==See also==