Geology of the Moon
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "
The Moon is the only extraterrestrial body for which we have samples with a known geologic context. A handful of lunar meteorites have been recognized on Earth, though their source craters on the Moon are unknown. A substantial portion of the lunar surface has not been explored, and a number of geological questions remain unanswered.
Elemental composition
Elements known to be present on the lunar surface include, among others, oxygen (O), silicon (Si), iron (Fe), magnesium (Mg), calcium (Ca), aluminium (Al), manganese (Mn) and titanium (Ti). Among the more abundant are oxygen, iron and silicon. The oxygen content is estimated at 45% (by weight). Carbon (C) and nitrogen (N) appear to be present only in trace quantities from deposition by solar wind.
Compound | Formula | Composition | |
---|---|---|---|
Maria | Highlands | ||
silica | SiO2 | 45.4% | 45.5% |
alumina | Al2O3 | 14.9% | 24.0% |
lime | CaO | 11.8% | 15.9% |
iron(II) oxide | FeO | 14.1% | 5.9% |
magnesia | MgO | 9.2% | 7.5% |
titanium dioxide | TiO2 | 3.9% | 0.6% |
sodium oxide | Na2O | 0.6% | 0.6% |
99.9% | 100.0% |
Formation
For a long period of time, the fundamental question regarding the history of the Moon was of its origin. Early hypotheses included fission from Earth, capture, and co-accretion. Today, the giant-impact hypothesis is widely accepted by the scientific community.[15]
Geologic history
The geological history of the Moon has been defined into six major epochs, called the
The first important event in the geologic evolution of the Moon was the
Quickly after the lunar crust formed, or even as it was forming, different types of magmas that would give rise to the
Analysis of the samples from the Moon seems to show that a lot of the Moon's impact basins formed in a short amount of time between about 4 and 3.85 Ga ago. This hypothesis is referred to as the lunar cataclysm or
The
Impacts by
After resumption of Lunar exploration in the 1990s, it was discovered there are scarps across the globe that are caused by the contraction due to cooling of the Moon.[21]
Strata and epochs
At the top of the Moon’s stratigraphy is the Copernican unit consisting of craters with a ray system. Below this is the Eratosthenian unit, defined by craters with established impact crater morphology, but lacking the ray system of the Copernican. These two units are present in smaller spots on the lunar surface. Further down the stratigraphy are the Mare units (previously known as the Procellarian unit), and the Imbrian unit which is related to ejecta and tectonics from the Imbrium basin. The bottom of the lunar stratigraphy is the pre-Nectarian unit, which consists of old crater plains.[22]
Lunar landscape
The lunar landscape is characterized by
Highlands
The most distinctive aspect of the Moon is the contrast between its bright and dark zones. Lighter surfaces are the lunar highlands, which receive the name of terrae (singular terra, from the
Maria
The major products of
The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting. The oldest radiometric ages are about 4.2 Ga (billion years), and ages of most of the youngest maria lavas have been determined from crater counting to be about 1 Ga. Due to better resolution of more recent imagery, about 70 small areas called irregular mare patches (each area only a few hundred meters or a few kilometers across) have been found in the maria that crater counting suggests were sites of volcanic activity in the geologically much more recent past (less than 50 million years).[6] Volumetrically, most of the mare formed between about 3 and 3.5 Ga before present. The youngest lavas erupted within Oceanus Procellarum, whereas some of the oldest appear to be located on the farside. The maria are clearly younger than the surrounding highlands given their lower density of impact craters.
A large portion of maria erupted within, or flowed into, the low-lying impact basins on the lunar nearside. However, it is unlikely that a causal relationship exists between the impact event and mare volcanism because the impact basins are much older (by about 500 million years) than the mare fill. Furthermore,
Another type of deposit associated with the maria, although it also covers the highland areas, are the "dark mantle" deposits. These deposits cannot be seen with the naked eye, but they can be seen in images taken from telescopes or orbiting spacecraft. Before the Apollo missions, scientists predicted that they were deposits produced by
Many of the lunar basalts contain small holes called vesicles, which were formed by gas bubbles exsolving from the magma at the vacuum conditions encountered at the surface. It is not known with certainty which gases escaped these rocks, but carbon monoxide is one candidate.
The samples of pyroclastic glasses are of green, yellow, and red tints. The difference in color indicates the concentration of titanium that the rock has, with the green particles having the lowest concentrations (about 1%), and red particles having the highest concentrations (up to 14%, much more than the basalts with the highest concentrations).
Rilles
Domes
A variety of shield volcanoes can be found in selected locations on the lunar surface, such as on Mons Rümker. These are thought to be formed by relatively viscous, possibly silica-rich lava, erupting from localized vents. The resulting lunar domes are wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the midpoint. They are typically 8–12 km in diameter, but can be up to 20 km across. Some of the domes contain a small pit at their peak.
Wrinkle ridges
Wrinkle ridges are features created by compressive tectonic forces within the maria. These features represent buckling of the surface and form long ridges across parts of the maria. Some of these ridges may outline buried craters or other features beneath the maria. A prime example of such an outlined feature is the crater Letronne.
Grabens
Impact craters
The origin of the Moon's craters as impact features became widely accepted only in the 1960s. This realization allowed the impact history of the Moon to be gradually worked out by means of the geologic principle of superposition. That is, if a crater (or its ejecta) overlaid another, it must be the younger. The amount of erosion experienced by a crater was another clue to its age, though this is more subjective. Adopting this approach in the late 1950s, Gene Shoemaker took the systematic study of the Moon away from the astronomers and placed it firmly in the hands of the lunar geologists.[23]
Impact cratering is the most notable geological process on the Moon. The craters are formed when a solid body, such as an asteroid or comet, collides with the surface at a high velocity (mean impact velocities for the Moon are about 17 km per second). The kinetic energy of the impact creates a compression shock wave that radiates away from the point of entry. This is succeeded by a rarefaction wave, which is responsible for propelling most of the ejecta out of the crater. Finally there is a hydrodynamic rebound of the floor that can create a central peak.
These craters appear in a continuum of diameters across the surface of the Moon, ranging in size from tiny pits to the immense
The most recent impacts are distinguished by well-defined features, including a sharp-edged rim. Small craters tend to form a bowl shape, whereas larger impacts can have a central peak with flat floors. Larger craters generally display slumping features along the inner walls that can form terraces and ledges. The largest impact basins, the multiring basins, can even have secondary concentric rings of raised material.
The impact process excavates high albedo materials that initially gives the crater, ejecta, and ray system a bright appearance. The process of space weathering gradually decreases the albedo of this material such that the rays fade with time. Gradually the crater and its ejecta undergo impact erosion from micrometeorites and smaller impacts. This erosional process softens and rounds the features of the crater. The crater can also be covered in ejecta from other impacts, which can submerge features and even bury the central peak.
The ejecta from large impacts can include large blocks of material that reimpact the surface to form secondary impact craters. These craters are sometimes formed in clearly discernible radial patterns, and generally have shallower depths than primary craters of the same size. In some cases an entire line of these blocks can impact to form a valley. These are distinguished from catena, or crater chains, which are linear strings of craters that are formed when the impact body breaks up prior to impact.
Generally speaking, a lunar crater is roughly circular in form. Laboratory experiments at NASA's Ames Research Center have demonstrated that even very low-angle impacts tend to produce circular craters, and that elliptical craters start forming at impact angles below five degrees. However, a low angle impact can produce a central peak that is offset from the midpoint of the crater. Additionally, the ejecta from oblique impacts show distinctive patterns at different impact angles: asymmetry starting around 60˚ and a wedge-shaped "zone of avoidance" free of ejecta in the direction the projectile came from starting around 45˚.[24]
Dark-halo craters are formed when an impact excavates lower albedo material from beneath the surface, then deposits this darker ejecta around the main crater. This can occur when an area of darker basaltic material, such as that found on the maria, is later covered by lighter ejecta derived from more distant impacts in the highlands. This covering conceals the darker material below, which is later excavated by subsequent craters.
The largest impacts produced melt sheets of molten rock that covered portions of the surface that could be as thick as a kilometer. Examples of such impact melt can be seen in the northeastern part of the Mare Orientale impact basin.
Regolith
The surface of the Moon has been subject to billions of years of collisions with both small and large asteroidal and cometary materials. Over time, these impact processes have pulverized and "gardened" the surface materials, forming a fine-grained layer termed regolith. The thickness of the lunar regolith varies between 2 meters (6.6 ft) beneath the younger maria, to up to 20 meters (66 ft) beneath the oldest surfaces of the lunar highlands. The regolith is predominantly composed of materials found in the region, but also contains traces of materials ejected by distant impact craters. The term mega-regolith is often used to describe the heavily fractured bedrock directly beneath the near-surface regolith layer.
The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts. In most of the lunar regolith, half of the particles are made of mineral fragments fused by the glassy particles; these objects are called agglutinates. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in
The lunar regolith is very important because it also stores information about the history of the Sun. The atoms that compose the solar wind – mostly hydrogen, helium, neon, carbon and nitrogen – hit the lunar surface and insert themselves into the mineral grains. Upon analyzing the composition of the regolith, particularly its isotopic composition, it is possible to determine if the activity of the Sun has changed with time. The gases of the solar wind could be useful for future lunar bases, because oxygen, hydrogen (water), carbon and nitrogen are not only essential to sustain life, but are also potentially very useful in the production of fuel. The composition of the lunar regolith can also be used to infer its source origin.
Lunar lava tubes
Lunar lava tubes form a potentially important location for constructing a future lunar base, which may be used for local exploration and development, or as a human outpost to serve exploration beyond the Moon. A lunar lava cave potential has long been suggested and discussed in literature and thesis.[25] Any intact lava tube on the Moon could serve as a shelter from the severe environment of the lunar surface, with its frequent meteorite impacts, high-energy ultraviolet radiation and energetic particles, and extreme diurnal temperature variations.[26][27][28] Following the launch of the Lunar Reconnaissance Orbiter, many lunar lava tubes have been imaged.[29] These lunar pits are found in several locations across the Moon, including Marius Hills, Mare Ingenii and Mare Tranquillitatis.
Lunar magma ocean
The first rocks brought back by Apollo 11 were basalts. Although the mission landed on Mare Tranquillitatis, a few millimetric fragments of rocks coming from the highlands were picked up. These are composed mainly of plagioclase feldspar; some fragments were composed exclusively of anorthite. The identification of these mineral fragments led to the bold hypothesis that a large portion of the Moon was once molten, and that the crust formed by fractional crystallization of this magma ocean.
A natural outcome of the hypothetical giant-impact event is that the materials that re-accreted to form the Moon must have been hot. Current models predict that a large portion of the Moon would have been molten shortly after the Moon formed, with estimates for the depth of this magma ocean ranging from about 500 km to complete melting. Crystallization of this magma ocean would have given rise to a differentiated body with a compositionally distinct crust and mantle and accounts for the major suites of lunar rocks.
As crystallization of the lunar magma ocean proceeded, minerals such as olivine and pyroxene would have precipitated and sank to form the lunar mantle. After crystallization was about three-quarters complete, anorthositic plagioclase would have begun to crystallize, and because of its low density, float, forming an anorthositic crust. Importantly, elements that are incompatible (i.e., those that partition preferentially into the liquid phase) would have been progressively concentrated into the magma as crystallization progressed, forming a KREEP-rich magma that initially should have been sandwiched between the crust and mantle. Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials. Additionally, zircon analysis of Apollo 14 samples suggests the lunar crust differentiated 4.51±0.01 billion years ago.[30]
Lunar rocks
Surface materials
The
The maria are composed predominantly of basalt, whereas the highland regions are iron-poor and composed primarily of anorthosite, a rock composed primarily of calcium-rich plagioclase feldspar. Another significant component of the crust are the igneous Mg-suite rocks, such as the troctolites, norites, and KREEP-basalts. These rocks are thought to be related to the petrogenesis of KREEP.
Composite rocks on the lunar surface often appear in the form of breccias. Of these, the subcategories are called fragmental, granulitic, and impact-melt breccias, depending on how they were formed. The mafic impact melt breccias, which are typified by the low-K Fra Mauro composition, have a higher proportion of iron and magnesium than typical upper crust anorthositic rocks, as well as higher abundances of KREEP.
Composition of the maria
The main characteristics of the
Internal structure
The current model of the interior of the Moon was derived using seismometers left behind during the crewed Apollo program missions, as well as investigations of the Moon's gravity field and rotation.
The mass of the Moon is sufficient to eliminate any voids within the interior, so it is estimated to be composed of solid rock throughout. Its low bulk density (~3346 kg m−3) indicates a low metal abundance. Mass and moment of inertia constraints indicate that the Moon likely has an iron core that is less than about 450 km in radius. Studies of the Moon's physical librations (small perturbations to its rotation) furthermore indicate that the core is still molten. Most planetary bodies and moons have iron cores that are about half the size of the body. The Moon is thus anomalous in having a core whose size is only about one quarter of its radius.
The crust of the Moon is on average about 50 km thick (though this is uncertain by about ±15 km). It is estimated that the far-side crust is on average thicker than the near side by about 15 km.[33] Seismology has constrained the thickness of the crust only near the Apollo 12 and Apollo 14 landing sites. Although the initial Apollo-era analyses suggested a crustal thickness of about 60 km at this site, recent reanalyses of this data suggest that it is thinner, somewhere between about 30 and 45 km.
Magnetic field
Compared with that of Earth, the Moon has only a very weak external magnetic field. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this hypothesis. Alternatively, it is possible that on airless bodies such as the Moon, transient magnetic fields could be generated during impact processes. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the largest impact basins. Although the Moon does not have a dipolar magnetic field like Earth's, some of the returned rocks do have strong magnetizations. Furthermore, measurements from orbit show that some portions of the lunar surface are associated with strong magnetic fields.
See also
References
- Cited references
- ISBN 9780521813068.
- ^ NASA 1994, p. 91.
- ^ NASA 1994, p. 93.
- ^ a b NASA 1994, p. 13.
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- ^ a b c Imster, Eleanor (12 October 2014). "Active moon volcanos in geologically recent times". earthsky.org. EarthSky. Retrieved 25 January 2023.
- ^ NASA 1994, p. 10.
- ^ "Lunar Rocks and Soils from Apollo Missions". NASA. Retrieved 21 November 2022.
- ^ Ivankov, A. "Luna 16". National Space Science Data Center Catalog. NASA. Retrieved 13 October 2018.
The drill was deployed and penetrated to a depth of 35 cm before encountering hard rock or large fragments of rock. The column of regolith in the drill tube was then transferred to the soil sample container... the hermetically sealed soil sample container, lifted off from the Moon carrying 101 grams of collected material
- ^ Ivankov, A. "Luna 20". National Space Science Data Center Catalog. NASA. Retrieved 13 October 2018.
Luna 20 was launched from the lunar surface on 22 February 1972 carrying 30 grams of collected lunar samples in a sealed capsule
- ^ Ivankov, A. "Luna 24". National Space Science Data Center Catalog. NASA. Retrieved 13 October 2018.
the mission successfully collected 170.1 grams of lunar samples and deposited them into a collection capsule
- ^ "China's Chang'e-5 retrieves 1,731 grams of moon samples". Xinhua News Agency. 19 December 2020. Archived from the original on 20 December 2020. Retrieved 19 December 2020.
- ISBN 978-0080182742.
- ^ S. Maurice. "Distribution of hydrogen at the surface of the moon" (PDF).
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- ^ Stevens, Tim (November 9, 2011). "Ancient lunar dynamo may explain magnetized moon rocks". Regents of the University of California. Retrieved August 13, 2012.
- ^ "Apollo 17 troctolite 76535". NASA/Johnson Space Center photograph S73-19456. Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM). Retrieved 2006-11-21.
- ^ Eric Hand (12 October 2014). "Recent volcanic eruptions on the moon". science.org. Retrieved 3 February 2023.
- ^ Yu. V. Barkin, J. M. Ferrándiz and Juan F. Navarro, 'Terrestrial tidal variations in the selenopotential coefficients,' Astronomical and Astrophysical Transactions, Volume 24, Number 3 / June 2005, pp. 215 - 236.) [1][permanent dead link]
- ^ "NASA's LRO Reveals 'Incredible Shrinking Moon'". Lunar Reconnaissance Orbiter. NASA. Retrieved 21 August 2010.
- ^ "Geologic History of the Moon". ser.sese.asu.edu. Retrieved 2024-01-19.
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- ^ Marius Hills Pit Offers Potential Location for Lunar Base; March 25, 2010; NASA
- ^ Moon hole might be suitable for colony; January 1, 2010; CNN-Tech
- ^ Scientists eye moon colonies - in the holes on the lunar surface Archived 2010-01-07 at the Wayback Machine; By Rich O'Malley; January 4th 2010; DAILY NEWS, NY
- ^ New Views of Lunar Pits; September 14, 2010; NASA
- ^ Barboni et al. "Early formation of the Moon 4.51 billion years ago." Science Advances. Vol 3. Issue 1. January, 2017. https://doi.org/10.1126/sciadv.1602365
- LCCN 00061677. NASA SP-2000-4029. Retrieved August 1, 2013.
- ^ "Craters Expose the Moon's Insides". Space.com. 5 July 2010. Retrieved 2015-12-23.
- doi:10.2138/rmg.2006.60.3.)
{{cite journal}}
: CS1 maint: numeric names: authors list (link
- Scientific references
- Don Wilhelms, Geologic History of the Moon, U.S. Geological Survey.
- To a Rocky Moon: A Geologist's History of Lunar Exploration, by D.E. Wilhelms. University of Arizona Press, Tucson (1993).
- New views of the Moon, B. L. Jolliff, M. A. Wieczorek, C. K. Shearer and C. R. Neal (editors), Rev. Mineral. Geochem., 60, Min. Soc. Amer., Chantilly, Virginia, 721 pp., 2006.
- The Lunar Sourcebook: A User's Guide to the Moon, by G.H. Heiken, D.T. Vaniman, B.M. French, et al. Cambridge University Press, New York (1991). ISBN 0-521-33444-6.
- Origin of the Moon, edited by W.K. Hartmann, R.J. Phillips, G. J. Taylor, ISBN 0-942862-03-1.
- Canup, R.; Righter, K., eds. (2000). Origin of the Earth and Moon. University of Arizona Press, Tucson. ISBN 0-8165-2073-9.
- General references
- ISBN 1-56098-847-9.
- Dana Mackenzie, The Big Splat, or How Our Moon Came to Be, ISBN 0-471-15057-6.
- Charles Frankel, Volcanoes of the Solar System, Cambridge University Press, ISBN 0-521-47201-6.
- G. Jeffrey Taylor (November 22, 2005). "Gamma Rays, Meteorites, Lunar Samples, and the Composition of the Moon". Planetary Science Research Discoveries.
- Linda Martel (September 28, 2004). "Lunar Crater Rays Point to a New Lunar Time Scale". Planetary Science Research Discoveries.
- Marc Norman (April 21, 2004). "The Oldest Moon Rocks". Planetary Science Research Discoveries.
- G. Jeffrey Taylor (November 28, 2003). "Hafnium, Tungsten, and the Differentiation of the Moon and Mars". Planetary Science Research Discoveries.
- G. Jeffrey Taylor (December 31, 1998). "Origin of the Earth and Moon". Planetary Science Research Discoveries.
- "Exploring the Moon: A Teacher's Guide with Activities for Earth and Space Sciences". NASA. 1994. p. 91.
External links
- Apollo over the Moon: A View from Orbit, edited by Harold Masursky, G. W. Colton, and Farouk El-baz, NASA SP-362.
- Eric Douglass, Geologic Processes on the Moon
- Lunar Sample Information (JSC)
- The Apollo Lunar Surface Journal (NASA)
- Lunar and Planetary Institute: Exploring the Moon
- Clementine Lunar Image Browser
- Ralph Aeschliman Planetary Cartography and Graphics: Lunar Maps Archived 2004-02-06 at the Wayback Machine
- Lunar Gravity, Topography and Crustal Thickness Archive Archived 2015-02-13 at the Wayback Machine
- Lunar and Planetary Institute: Lunar Atlas and Photography Collection
- Moon Rocks through the Microscope Retrieved 22 August 2007
- Moon articles in Planetary Science Research Discoveries
- Another Hit to Hoax:Traces of Man on Lunar Surface
- Visible and Terrain Map of the Moon
- Video (04:56) – The Moon in 4K (NASA, April 2018) on YouTube