Geology of the Moon

Smithsonian Institution Senior Scientist Tom Watters talks aboutthe Moon’s recent geological activity.

False-color image of the Moon taken by the Galileo orbiter show-ing geological features. NASA photo

The geology of theMoon (sometimes called selenology,although the latter term can refer more generally to “lunarscience”) is quite different from that of Earth. The Moonlacks a significant atmosphere, which eliminates erosiondue to weather; it does not have any form of plate tecton-ics, it has a lower gravity, and because of its small size, itcooled more rapidly. The complex geomorphology of thelunar surface has been formed by a combination of pro-cesses, especially impact cratering and volcanism. TheMoon is a differentiated body, with a crust, mantle, andcore.Geological studies of the Moon are based on a combi-

The same image using different color filters

nation of Earth-based telescope observations, measure-ments from orbiting spacecraft, lunar samples, and geo-physical data. A few locations were sampled directlyduring the Apollo missions in the late 1960s and early1970s, which returned approximately 380 kilograms (838lb) of lunar rock and soil to Earth, as well as severalmissions of the Soviet Luna programme. The Moon isthe only extraterrestrial body for which we have sampleswith a known geologic context. A handful of lunar mete-orites have been recognized on Earth, though their sourcecraters on the Moon are unknown. A substantial portionof the lunar surface has not been explored, and a numberof geological questions remain unanswered.

1 Elemental composition

Elements known to be present on the lunar surfaceinclude, among others, oxygen (O), silicon (Si), iron(Fe), magnesium (Mg), calcium (Ca), aluminium (Al),manganese (Mn) and titanium (Ti). Among the moreabundant are oxygen, iron and silicon. The oxygen con-tent is estimated at 45% (by weight). Carbon (C) andnitrogen (N) appear to be present only in trace quantitiesfrom deposition by solar wind.Neutron spectrometry data from the Lunar Prospector in-dicate the presence of hydrogen (H) concentrated at thepoles.[1]

1

2 3 GEOLOGIC HISTORY

CompositionOf Lunar SoilRelativeconcentration,%

0

10

20

30

40

Oxygen Silicon Iron Calcium Aluminum Magnesium Other

Relative concentration of various elements on the lunarsurface (in weight %)

HydrogenTitanium

MagnesiumPotassium

SodiumCalcium

IronAluminum

SiliconOxygen

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Concentration of Elements on Lunar Highlands, Lunar Lowlands, and Earth

Concentration (%)

Earth

Lunar Lowland

Lunar Highland

Relative Concentration (in weight %) of Various Ele-ments on Lunar Highlands, Lunar Lowlands, and Earth

2 Formation

Visible face of the Moon

Main article: Origin of the Moon

For a long period of time, the fundamental question re-garding the history of the Moon was of its origin. Earlyhypotheses included fission from Earth, capture, and co-accretion. Today, the giant impact hypothesis is widelyaccepted by the scientific community.[2]

3 Geologic history

Cliffs in the lunar crust indicate the moon shrank globally in thegeologically recent past and is still shrinking today.

The geological history of the Moon has been defined intosix major epochs, called the lunar geologic timescale.Starting about 4.5 billion years ago,[3] the newly formedMoon was in a molten state and was orbiting much closerto Earth resulting in tidal forces.[4] These tidal forces de-formed the molten body into an ellipsoid, with the majoraxis pointed towards Earth.The first important event in the geologic evolution of theMoon was the crystallization of the near global magmaocean. It is not known with certainty what its depth was,but several studies imply a depth of about 500 km orgreater. The first minerals to form in this ocean werethe iron and magnesium silicates olivine and pyroxene.Because these minerals were denser than the molten ma-terial around them, they sank. After crystallization wasabout 75% complete, less dense anorthositic plagioclasefeldspar crystallized and floated, forming an anorthositiccrust about 50 km in thickness. The majority of themagma ocean crystallized quickly (within about 100 mil-lion years or less), though the final remaining KREEP-rich magmas, which are highly enriched in incompati-ble and heat-producing elements, could have remainedpartially molten for several hundred million (or perhaps1 billion) years. It appears that the final KREEP-richmagmas of the magma ocean eventually became concen-trated within the region of Oceanus Procellarum and theImbrium basin, a unique geologic province that is nowknown as the Procellarum KREEP Terrane.Quickly after the lunar crust formed, or even as it wasforming, different types of magmas that would give riseto the Mg-suite norites and troctolites[5] began to form,although the exact depths at which this occurred arenot known precisely. Recent theories suggest that Mg-suite plutonism was largely confined to the region of theProcellarum KREEP Terrane, and that these magmas aregenetically related to KREEP in some manner, thoughtheir origin is still highly debated in the scientific com-munity. The oldest of the Mg-suite rocks have crystal-lization ages of about 3.85 Ga. However, the last largeimpact that could have excavated deep into the crust (theImbrium basin) also occurred at 3.85 Ga before present.

3.1 Strata and epochs 3

Exploring Shorty Crater during the Apollo 17 mission to theMoon. This was the only Apollo mission to include a geologist(Harrison Schmitt). NASA photo

Thus, it seems probable that Mg-suite plutonic activitycontinued for a much longer time, and that younger plu-tonic rocks exist deep below the surface.Analysis of the lunar samples seems to imply that a signif-icant percentage of the lunar impact basins formed withina very short period of time between about 4 and 3.85 Gaago. This hypothesis is referred to as the lunar cataclysmor late heavy bombardment. However, it is now recog-nized that ejecta from the Imbrium impact basin (one ofthe youngest large impact basins on the Moon) should befound at all of the Apollo landing sites. It is thus possi-ble that ages for some impact basins (in particular MareNectaris) could have been mistakenly assigned the sameage as Imbrium.The lunar maria represent ancient flood basaltic erup-tions. In comparison to terrestrial lavas, these containhigher iron abundances, have low viscosities, and somecontain highly elevated abundances of the titanium-richmineral ilmenite. The majority of basaltic eruptions oc-curred between about 3 and 3.5 Ga ago, though somemare samples have ages as old as 4.2 Ga, and the youngest(based on the method of crater counting) are believed tohave erupted only 1 billion years ago. Along with marevolcanism came pyroclastic eruptions, which launchedmolten basaltic materials hundreds of kilometres awayfrom the volcano. A large portion of the mare formed, orflowed into, the low elevations associated with the near-side impact basins. However, Oceanus Procellarum doesnot correspond to any known impact structure, and thelowest elevations of the Moon within the farside SouthPole-Aitken basin are onlymodestly covered bymare (seelunar mare for a more detailed discussion).

Moon – Oceanus Procellarum (“Ocean of Storms”)

Ancient rift valleys – rectangular structure (visible –topography – GRAIL gravity gradients) (October 1,2014).

Ancient rift valleys – context.

Ancient rift valleys – closeup (artist’s concept).

Impacts by meteorites and comets are the only abrupt ge-ologic force acting on the Moon today, though the varia-tion of Earth tides on the scale of the Lunar anomalisticmonth causes small variations in stresses.[6] Some of themost important craters used in lunar stratigraphy formedin this recent epoch. For example, the crater Copernicus,which has a depth of 3.76 km and a radius of 93 km, is be-lieved to have formed about 900million years ago (thoughthis is debatable). The Apollo 17 mission landed in anarea in which the material coming from the crater Tychomight have been sampled. The study of these rocks seemto indicate that this crater could have formed 100 millionyears ago, though this is debatable as well. The surfacehas also experienced space weathering due to high energyparticles, solar wind implantation, and micrometeoriteimpacts. This process causes the ray systems associatedwith young craters to darken until it matches the albedoof the surrounding surface. However, if the compositionof the ray is different from the underlying crustal materi-als (as might occur when a “highland” ray is emplaced onthe mare), the ray could be visible for much longer times.After resumption of Lunar exploration in the 1990s, itwas discovered there are scarps across the globe that arecaused by the contraction due to cooling of the Moon.[7]

3.1 Strata and epochs

Main article: Lunar geologic timescale

4 4 LUNAR LANDSCAPE

On the top of the lunar stratigraphical sequence rayed im-pact craters can be found. Such youngest craters belong tothe Copernican unit. Below it can be found craters with-out the ray system, but with rather well developed impactcrater morphology. This is the Eratosthenian unit. Thetwo younger stratigraphical units can be found in cratersized spots on the Moon. Below them two extendingstrata can be found: mare units (earlier defined as Pro-cellarian unit) and the Imbrium basin related ejecta andtectonic units (Imbrian units). Another impact basin re-lated unit is the Nectarian unit, defined around the Nec-tarian Basin. At the bottom of the lunar stratigraphicalsequence the pre-Nectarian unit of old crater plains canbe found. The stratigraphy of Mercury is very similar tothe lunar case.

4 Lunar landscape

The lunar landscape is characterized by impact craters,their ejecta, a few volcanoes, hills, lava flows and depres-sions filled by magma.

A photograph of full moon taken from Earth

4.1 The highlands

The most distinctive aspect of the Moon is the contrastbetween its bright and dark zones. Lighter surfaces arethe lunar highlands, which receive the name of terrae(singular terra, from the Latin for Earth), and the darkerplains are called maria (singular mare, from the Latin forsea), after Johannes Kepler who introduced the name inthe 17th century. The highlands are anorthositic in com-position, whereas the maria are basaltic. The maria oftencoincide with the “lowlands,” but it is important to notethat the lowlands (such as within the South Pole-Aitkenbasin) are not always covered bymaria. The highlands are

older than the visible maria, and hence are more heavilycratered.

4.2 The maria

Main article: Lunar mare

The major products of volcanic processes on the Moonare evident to Earth-bound observers in the form of thelunar maria. These are large flows of basaltic lava thatcorrespond to low-albedo surfaces covering nearly a thirdof the near side. Only a few percent of the farside hasbeen affected by mare volcanism. Even before the Apollomissions confirmed it, most scientists already thought thatthe maria are lava-filled plains, because they have lavaflow patterns and collapses attributed to lava tubes.The ages of the mare basalts have been determined bothby direct radiometric dating and by the technique ofcrater counting. The oldest radiometric ages are about 4.2Ga, whereas the youngest ages determined from cratercounting are about 1 Ga (1 Ga = 1 billion years). Vol-umetrically, most of the mare formed between about 3and 3.5 Ga before present. The youngest lavas eruptedwithin Oceanus Procellarum, whereas some of the oldestappear to be located on the farside. The maria are clearlyyounger than the surrounding highlands given their lowerdensity of impact craters.A large portion of maria erupted within, or flowed into,the low-lying impact basins on the lunar nearside. Never-theless, it is unlikely that a causal relationship exists be-tween the impact event and mare volcanism because theimpact basins aremuch older (by about 500million years)than the mare fill. Furthermore, Oceanus Procellarum,which is the largest expanse of mare volcanism on theMoon, does not correspond to any known impact basin.It is commonly suggested that the reason the mare onlyerupted on the nearside is that the nearside crust is thinnerthan the farside. Although variations in the crustal thick-ness might act to modulate the amount of magma thatultimately reaches the surface, this hypothesis does notexplain why the farside South Pole-Aitken basin, whosecrust is thinner than Oceanus Procellarum, was onlymod-estly filled by volcanic products.Another type of deposit associated with the maria, al-though it also covers the highland areas, are the “darkmantle” deposits. These deposits cannot be seen with thenaked eye, but they can be seen in images taken from tele-scopes or orbiting spacecraft. Before theApollomissions,scientists believed that they were deposits produced bypyroclastic eruptions. Some deposits appear to be associ-ated with dark elongated ash cones, reinforcing the ideaof pyroclasts. The existence of pyroclastic eruptions waslater confirmed by the discovery of glass spherules similarto those found in pyroclastic eruptions here on Earth.Many of the lunar basalts contain small holes called

4.3 Impact craters 5

vesicles, which were formed by gas bubbles exsolvingfrom the magma at the vacuum conditions encounteredat the surface. It is not known with certainty which gasesescaped these rocks, but carbon monoxide is one candi-date.The samples of pyroclastic glasses are of green, yellow,and red tints. The difference in color indicates the con-centration of titanium that the rock has, with the greenparticles having the lowest concentrations (about 1%),and red particles having the highest concentrations (up to14%, much more than the basalts with the highest con-centrations).

4.2.1 Rilles

Rilles on the Moon sometimes resulted from the forma-tion of localized lava channels. These generally fall intothree categories, consisting of sinuous, arcuate, or linearshapes. By following thesemeandering rilles back to theirsource, they often lead to an old volcanic vent. One of themost notable sinuous rilles is the Vallis Schröteri feature,located in the Aristarchus plateau along the eastern edgeof Oceanus Procellarum. An example of a sinuous rilleexists at the Apollo 15 landing site, Rima Hadley, locatedon the rim of the Imbrium Basin. Based on observationsfrom the mission, it is generally believed that this rille wasformed by volcanic processes, a topic long debated beforethe mission took place.

4.2.2 Domes

A variety of shield volcanoes can be found in selected lo-cations on the lunar surface, such as on Mons Rümker.These are believed to be formed by relatively viscous,possibly silica-rich lava, erupting from localized vents.The resulting lunar domes are wide, rounded, circular fea-tures with a gentle slope rising in elevation a few hundredmeters to the midpoint. They are typically 8–12 km indiameter, but can be up to 20 km across. Some of thedomes contain a small pit at their peak.

4.2.3 Wrinkle ridges

Wrinkle ridges are features created by compressive tec-tonic forces within the maria. These features representbuckling of the surface and form long ridges across partsof the maria. Some of these ridges may outline buriedcraters or other features beneath the maria. A prime ex-ample of such an outlined feature is the crater Letronne.

4.2.4 Grabens

Grabens are tectonic features that form under extensionstresses. Structurally, they are composed of two normalfaults, with a down-dropped block between them. Most

grabens are found within the lunar maria near the edgesof large impact basins.

4.3 Impact craters

Mare Imbrium and the crater Copernicus. NASA photo.

The origin of the Moon’s craters as impact features be-came widely accepted only in the 1940s. This realiza-tion allowed the impact history of the Moon to be grad-ually worked out by means of the geologic principle ofsuperposition. That is, if a crater (or its ejecta) overlaidanother, it must be the younger. The amount of erosionexperienced by a crater was another clue to its age, thoughthis is more subjective. Adopting this approach in the late1950s, Gene Shoemaker took the systematic study of theMoon away from the astronomers and placed it firmly inthe hands of the lunar geologists.Impact cratering is the most notable geological processon the Moon. The craters are formed when a solid body,such as an asteroid or comet, collides with the surfaceat a high velocity (mean impact velocities for the Moonare about 17 km per second). The kinetic energy of theimpact creates a compression shock wave that radiatesaway from the point of entry. This is succeeded by ararefaction wave, which is responsible for propelling mostof the ejecta out of the crater. Finally there is a hydrody-namic rebound of the floor that can create a central peak.These craters appear in a continuum of diameters acrossthe surface of the Moon, ranging in size from tiny pitsto the immense South Pole–Aitken Basin with a diam-eter of nearly 2,500 km and a depth of 13 km. In avery general sense, the lunar history of impact crateringfollows a trend of decreasing crater size with time. Inparticular, the largest impact basins were formed duringthe early periods, and these were successively overlaid bysmaller craters. The size frequency distribution (SFD) ofcrater diameters on a given surface (that is, the numberof craters as a function of diameter) approximately fol-lows a power law with increasing number of craters with

6 4 LUNAR LANDSCAPE

decreasing crater size. The vertical position of this curvecan be used to estimate the age of the surface.

The lunar crater King displays the characteristic features of alarge impact formation, with a raised rim, slumped edges, ter-raced inner walls, a relatively flat floor with some hills, and acentral ridge. The Y-shaped central ridge is unusually complexin form. NASA photo.

The most recent impacts are distinguished by well-defined features, including a sharp-edged rim. Smallcraters tend to form a bowl shape, whereas larger im-pacts can have a central peak with flat floors. Largercraters generally display slumping features along the in-ner walls that can form terraces and ledges. The largestimpact basins, the multiring basins, can even have sec-ondary concentric rings of raised material.The impact process excavates high albedo materials thatinitially gives the crater, ejecta, and ray system a brightappearance. The process of space weathering graduallydecreases the albedo of this material such that the raysfade with time. Gradually the crater and its ejecta un-dergo impact erosion from micrometeorites and smallerimpacts. This erosional process softens and rounds thefeatures of the crater. The crater can also be covered inejecta from other impacts, which can submerge featuresand even bury the central peak.The ejecta from large impacts can include larges blocksof material that reimpact the surface to form secondaryimpact craters. These craters are sometimes formedin clearly discernible radial patterns, and generally haveshallower depths than primary craters of the same size.In some cases an entire line of these blocks can impactto form a valley. These are distinguished from catena, orcrater chains, which are linear strings of craters that areformed when the impact body breaks up prior to impact.Generally speaking, a lunar crater is roughly circular inform. Laboratory experiments at NASA’s Ames Re-search Center have demonstrated that even very low-angle impacts tend to produce circular craters, and that

elliptical craters start forming at impact angles below fivedegrees. However, a low angle impact can produce a cen-tral peak that is offset from the midpoint of the crater.Additionally, the ejecta from oblique impacts show dis-tinctive patterns at different impact angles: asymmetrystarting around 60˚ and a wedge-shaped “zone of avoid-ance” free of ejecta in the direction the projectile camefrom starting around 45˚.[8]

Dark-halo craters are formed when an impact excavateslower albedo material from beneath the surface, then de-posits this darker ejecta around the main crater. This canoccur when an area of darker basaltic material, such asthat found on the maria, is later covered by lighter ejectaderived from more distant impacts in the highlands. Thiscovering conceals the darker material below, which islater excavated by subsequent craters.The largest impacts produced melt sheets of molten rockthat covered portions of the surface that could be as thickas a kilometer. Examples of such impact melt can beseen in the northeastern part of the Mare Orientale im-pact basin.

4.4 Regolith

See also: Lunar soil

The surface of the Moon has been subject to billions ofyears of collisions with both small and large asteroidaland cometary materials. Over time, these impact pro-cesses have pulverized and “gardened” the surface mate-rials, forming a fine grained layer termed “regolith”. Thethickness of the regolith varies between 2 meters beneaththe younger maria, to up to 20 meters beneath the oldestsurfaces of the lunar highlands. The regolith is predom-inantly composed of materials found in the region, butalso contains traces of materials ejected by distant im-pact craters. The term “mega-regolith” is often used todescribe the heavily fractured bedrock directly beneaththe near-surface regolith layer.The regolith contains rocks, fragments of minerals fromthe original bedrock, and glassy particles formed dur-ing the impacts. In most of the lunar regolith, half ofthe particles are made of mineral fragments fused bythe glassy particles; these objects are called agglutinates.The chemical composition of the regolith varies accord-ing to its location; the regolith in the highlands is rich inaluminium and silica, just as the rocks in those regions.The regolith in the maria is rich in iron and magnesiumand is silica-poor, as are the basaltic rocks from which itis formed.The lunar regolith is very important because it also storesinformation about the history of the Sun. The atoms thatcompose the solar wind – mostly helium, neon, carbonand nitrogen – hit the lunar surface and insert themselvesinto the mineral grains. Upon analyzing the composi-

7

tion of the regolith, particularly its isotopic composition,it is possible to determine if the activity of the Sun haschanged with time. The gases of the solar wind could beuseful for future lunar bases, because oxygen, hydrogen(water), carbon and nitrogen are not only essential to sus-tain life, but are also potentially very useful in the pro-duction of fuel. The composition of the lunar regolithcan also be used to infer its source origin.

4.5 Lunar lava tubes

Lunar lava tubes form a potentially important location forconstructing a future lunar base, which may be used forlocal exploration and development, or as a human outpostto serve exploration beyond the Moon. A lunar lava cavepotential has long been suggested and discussed in litera-ture and thesis.[9] Any intact lava tube on the Moon couldserve as a shelter from the severe environment of the lunarsurface, with its frequent meteorite impacts, high-energyultraviolet radiation and energetic particles, and extremediurnal temperature variations.[10][11][12] Following thelaunch of the Lunar Reconnaissance Orbiter, many lu-nar lava tubes have been imaged.[13] These lunar pits arefound in several locations across the Moon, includingMarius Hills, Mare Ingenii and Mare Tranquillitatis.

5 The lunar magma ocean

The first rocks brought back by Apollo 11 were basalts.Although the mission landed on Mare Tranquillitatis, afewmillimetric fragments of rocks coming from the high-lands were picked up. These are composed mainly ofplagioclase feldspar; some fragments were composed ex-clusively of anorthositic plagioclase. The identification ofthese mineral fragments led to the bold hypothesis that alarge portion of the Moon was once molten, and that thecrust formed by fractional crystallization of this magmaocean.A natural outcome of the giant impact event is that thematerials that reaccreted to form the Moon must havebeen hot. Current models predict that a large portion ofthe Moon would have been molten shortly after the Moonformed, with estimates for the depth of this magma oceanranging from about 500 km to complete melting. Crys-tallization of this magma ocean would have given rise to adifferentiated body with a compositionally distinct crustand mantle and accounts for the major suites of lunarrocks.As crystallization of the lunar magma ocean proceeded,minerals such as olivine and pyroxene would have precip-itated and sank to form the lunar mantle. After crystal-lization was about three-quarters complete, anorthositicplagioclase would have begun to crystallize, and becauseof its low density, float, forming an anorthositic crust. Im-portantly, elements that are incompatible (i.e., those that

partition preferentially into the liquid phase) would havebeen progressively concentrated into the magma as crys-tallization progressed, forming a KREEP-rich magmathat initially should have been sandwiched between thecrust and mantle. Evidence for this scenario comes fromthe highly anorthositic composition of the lunar highlandcrust, as well as the existence of KREEP-rich materials.

Formation of the anorthosite crust

6 Lunar rocks

Main article: Moon rock

6.1 Surface materials

Olivine basalt collected by Apollo 15

The Apollo program brought back 380.05 kilograms(837.87 lb) of lunar surface material,[14] most of whichis stored at the Lunar Receiving Laboratory in Houston,Texas, and the unmanned Soviet Luna programme re-turned 326 grams (11.5 oz) of lunar material. These

8 8 GALLERY

rocks have proved to be invaluable in deciphering the ge-ologic evolution of the Moon. Lunar rocks are in largepart made of the same common rock forming miner-als as found on Earth, such as olivine, pyroxene, andplagioclase feldspar (anorthosite). Plagioclase feldspar ismostly found in the lunar crust, whereas pyroxene andolivine are typically seen in the lunar mantle.[15] Themin-eral ilmenite is highly abundant in somemare basalts, anda new mineral named armalcolite (named for Armstrong,Aldrin, and Collins, the three members of the Apollo 11crew) was first discovered in the lunar samples.The maria are composed predominantly of basalt,whereas the highland regions are iron-poor and com-posed primarily of anorthosite, a rock composed pri-marily of calcium-rich plagioclase feldspar. Anothersignificant component of the crust are the igneous Mg-suite rocks, such as the troctolites, norites, and KREEP-basalts. These rocks are believed to be genetically relatedto the petrogenesis of KREEP.Composite rocks on the lunar surface often appear in theform of breccias. Of these, the subcategories are calledfragmental, granulitic, and impact-melt breccias, depend-ing on how they were formed. The mafic impact meltbreccias, which are typified by the low-K FraMauro com-position, have a higher proportion of iron and magnesiumthan typical upper crust anorthositic rocks, as well ashigher abundances of KREEP.

6.2 Composition of the maria

The main characteristics of the basaltic rocks with re-spect to the rocks of the lunar highlands is that the basaltscontain higher abundances of olivine and pyroxene, andless plagioclase. They are more rich in iron than terres-trial basalts, and also have lower viscosities. Some ofthem have high abundances of a ferro-titanic oxide calledilmenite. Because the first sampling of rocks contained ahigh content of ilmenite and other related minerals, theyreceived the name of “high titanium” basalts. The Apollo12 mission returned to Earth with basalts of lower tita-nium concentrations, and these were dubbed “low tita-nium” basalts. Subsequent missions, including the Sovietunmanned probes, returned with basalts with even lowerconcentrations, now called “very low titanium” basalts.The Clementine space probe returned data showing thatthe mare basalts have a continuum in titanium concentra-tions, with the highest concentration rocks being the leastabundant.

7 Internal structure of the Moon

Main article: Internal structure of the Moon

The current model of the interior of the Moon was de-rived using seismometers left behind during the manned

Apollo program missions, as well as investigations of theMoon’s gravity field and rotation.The mass of the Moon is sufficient to eliminate any voidswithin the interior, so it is believed to be composed ofsolid rock throughout. Its low bulk density (~3346 kgm−3) indicates a lowmetal abundance. Mass and momentof inertia constraints indicate that the Moon likely has aniron core that is less than about 450 km in radius. Stud-ies of the Moon’s physical librations (small perturbationsto its rotation) furthermore indicate that the core is stillmolten. Most planetary bodies andmoons have iron coresthat are about half the size of the body. The Moon is thusanomalous in having a core whose size is only about onequarter 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 widelybelieved that the far-side crust is on average thicker thanthe near side by about 15 km.[16] Seismology has con-strained the thickness of the crust only near the Apollo12 and 14 landing sites. Although the initial Apollo-eraanalyses suggested a crustal thickness of about 60 km atthis site, recent reanalyses of this data suggest that it isthinner, somewhere between about 30 and 45 km.Compared to that of Earth, the Moon has only a veryweak external magnetic field. Other major differencesare that the Moon does not currently have a dipolar mag-netic field (as would be generated by a geodynamo in itscore), and the magnetizations that are present are almostentirely crustal in origin. One hypothesis holds that thecrustal magnetizations were acquired early in lunar his-tory when a geodynamo was still operating. The smallsize of the lunar core, however, is a potential obstacle tothis hypothesis. Alternatively, it is possible that on airlessbodies such as the Moon, transient magnetic fields couldbe generated during impact processes. In support of this,it has been noted that the largest crustal magnetizationsappear to be located near the antipodes of the largest im-pact basins. Although the Moon does not have a dipolarmagnetic field like Earth’s, some of the returned rocks dohave strong magnetizations. Furthermore, measurementsfrom orbit show that some portions of the lunar surfaceare associated with strong magnetic fields.

8 Gallery

• Moon photos

• Lunar near side

• Lunar far side

• Lunar north pole

• Lunar south pole

9

9 See also• Lunar geologic timescale

• Selenography

• Transient lunar phenomenon

10 References

Cited references

[1] S. Maurice. “DISTRIBUTION OF HYDROGEN ATTHE SURFACE OF THE MOON” (PDF).

[2] Lang, Kenneth (2011). The Cambridge Guide to the SolarSystem (2 ed.). New York: Cambridge University Press.p. 199. ISBN 978-0-521-19857-8.

[3] Kleine, T.; Palme, H.; Mezger, K.; Halliday, A.N.(2005). “Hf–W Chronometry of Lunar Metals and theAge and Early Differentiation of the Moon”. Science310 (5754): 1671–1674. Bibcode:2005Sci...310.1671K.doi:10.1126/science.1118842. PMID 16308422.

[4] Stevens, Tim (November 9, 2011). “Ancient lunar dy-namo may explain magnetized moon rocks”. Regents ofthe University of California. Retrieved August 13, 2012.

[5] “Apollo 17 troctolite 76535”. NASA/Johnson Space Cen-ter photograph S73-19456. Curation and Analysis Plan-ning Team for Extraterrestrial Materials (CAPTEM). Re-trieved 2006-11-21.

[6] Yu. V. Barkin, J. M. Ferrándiz and Juan F. Navarro, 'Ter-restrial tidal variations in the selenopotential coefficients,'Astronomical and Astrophysical Transactions, Volume 24,Number 3 / June 2005, pp. 215 - 236.)

[7] “NASA’s LRO Reveals 'Incredible Shrinking Moon'".Lunar Reconnaissance Orbiter. NASA. Retrieved 21 Au-gust 2010.

[8] Proceedings of the Ninth Lunar and Planetary Conference.1978. Bibcode:1978LPSC....9.3843G. Missing or empty|title= (help)

[9] Coombs, Cassandra R.; Hawke, B. Ray (September1992). “A search for intact lava tubes on the Moon:Possible lunar base habitats”. The Second Confer-ence on Lunar Bases and Space Activities of the 21stCentury (NASA. Johnson Space Center) 1: 219–229.Bibcode:1992lbsa.conf..219C.

[10] Marius Hills Pit Offers Potential Location for Lunar Base;March 25, 2010; NASA

[11] Moon hole might be suitable for colony; January 1, 2010;CNN-Tech

[12] Scientists eye moon colonies - in the holes on the lunarsurface; By Rich O'Malley; January 4th 2010; DAILYNEWS, NY

[13] New Views of Lunar Pits; September 14, 2010; NASA

[14] Orloff, Richard W. (September 2004) [First published2000]. “Extravehicular Activity”. Apollo by the Numbers:A Statistical Reference. NASA History Division, Office ofPolicy and Plans. The NASAHistory Series (Washington,D.C.: NASA). ISBN 0-16-050631-X. LCCN 00061677.NASA SP-2000-4029. Retrieved August 1, 2013.

[15] http://www.space.com/scienceastronomy/moon-mantle-exposed-craters-100705.html

[16] Mark Wieczorek and 15 coauthors, M. A. (2006). “Theconstitution and structure of the lunar interior”. Re-views in Mineralogy and Geochemistry 60 (1): 221–364.doi:10.2138/rmg.2006.60.3.

Scientific references

• Don Wilhelms, Geologic History of the Moon, U.S.Geological Survey.

• To a Rocky Moon: A Geologist’s History of LunarExploration, by D.E. Wilhelms. University of Ari-zona Press, Tucson (1993).

• New views of the Moon, B. L. Jolliff, M. A. Wiec-zorek, C. K. Shearer and C. R. Neal (editors), Rev.Mineral. Geochem., 60, Min. Soc. Amer., Chan-tilly, Virginia, 721 pp., 2006.

• The Lunar Sourcebook: A User’s Guide to the Moon,by G.H. Heiken, D.T. Vaniman y B.M. French, etal. 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.

• R. Canup and K. Righter, editors (2000). Origin ofthe Earth and Moon. University of Arizona Press,Tucson. p. 555 pp. ISBN 0-8165-2073-9.

General references

• Paul D. Spudis, The Once and Future Moon, 1998,Smithsonian Books, ISBN 1-56098-847-9.

• Dana Mackenzie, The Big Splat, or How Our MoonCame to Be, 2003, JohnWiley & Sons, ISBN 0-471-15057-6.

• Charles Frankel, Volcanoes of the Solar System,Cambridge University Press, 1996, ISBN 0-521-47201-6.

• G. Jeffrey Taylor (November 22, 2005). “GammaRays, Meteorites, Lunar Samples, and the Compo-sition of the Moon”. Planetary Science ResearchDiscoveries.

• Linda Martel (September 28, 2004). “Lunar CraterRays Point to a New Lunar Time Scale”. PlanetaryScience Research Discoveries.

10 11 EXTERNAL LINKS

• Marc Norman (April 21, 2004). “The Oldest MoonRocks”. Planetary Science Research Discoveries.

• G. Jeffrey Taylor (November 28, 2003). “Hafnium,Tungsten, and the Differentiation of the Moon andMars”. Planetary Science Research Discoveries.

• G. Jeffrey Taylor (December 31, 1998). “Origin ofthe Earth and Moon”. Planetary Science ResearchDiscoveries.

11 External links• Apollo over the Moon: A View from Orbit, edited byHaroldMasursky, 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 andGraphics: Lunar Maps

• Lunar Gravity, Topography and Crustal ThicknessArchive

• Lunar and Planetary Institute: Lunar Atlas and Pho-tography Collection

• Moon Rocks through the Microscope Retrieved 22August 2007

• Moon articles in Planetary Science Research Dis-coveries

• Another Hit to Hoax:Traces of Man on Lunar Sur-face

11

12 Text and image sources, contributors, and licenses

12.1 Text• Geology of the Moon Source: https://en.wikipedia.org/wiki/Geology_of_the_Moon?oldid=690466729 Contributors: AxelBoldt, Bryan

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12.2 Images• File:14-236-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg Source: https://upload.wikimedia.org/

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12 12 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

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• File:NASA_Spacecraft_Reveals_Recent_Geological_Activity_on_the_Moon.ogv Source: https://upload.wikimedia.org/wikipedia/commons/9/9f/NASA_Spacecraft_Reveals_Recent_Geological_Activity_on_the_Moon.ogv License: Public domain Contributors:Goddard Multimedia Original artist: NASA/Goddard Space Flight Center

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