Size, mass, density
The Moon has a mass of approximately 7.35 x 10^25 grams and features a nearly spherical shape with a radius of 1,737.4 kilometers Its total volume is about 21.99 x 10^9 cubic kilometers, resulting in a density of 3.343 grams per cubic centimeter, which is characteristic of dense rocks like eclogite and peridotite.
Table 1.1.The Moon as compared with the Earth.
Density of the Earth 5.517 g/cm 3
Density of the Moon 3.343 g/cm 3
Radius of the Moon 1,737.4 km
Surface area of the Moon 37:9610 6 km 2
Moon vs Earth weight ratio 1/81.3
The Moon's mass is 80 times less than that of Earth, making it the heaviest satellite in the solar system relative to its planet In contrast, Ganymede, the largest satellite in the solar system, has a mass that is 12,200 times lighter than its host planet, Jupiter.
The Moon's density is significantly lower than that of the Earth, with values of 3.34 g/cm³ for the Moon compared to 4.54 g/cm³ for the Earth This discrepancy is partly due to the Earth's massive size and the high pressure that compresses its internal matter Consequently, the Moon contains much less iron than the Earth, which has a heavy core constituting 32% of its total mass Given the Moon's average density, it is inferred that it possesses a small metallic core, aligning with constraints imposed by its moment of inertia.
Moment of inertia
The moment of inertia is a measure of resistance of a body to change in its rotation.The lunar solid moment of inertiaIsolidis determined by use of accurate Lunar Laser
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The distance measurements from terrestrial observatories to the Moon's laser reflectors are crucial for assessing its moment of inertia Recent findings indicate a dimensionless moment of inertia value of Isolid = MR² D0.3930 ± 0.0003, where M represents the mass and R denotes the radius of the Moon.
A homogeneous solid sphere has a moment of inertia represented as Isolid = MR², with a value of 0.393 indicating an increase in density with depth However, the exact nature of this density increase remains indeterminate Calculations reveal that based on the observed moment of inertia and density values, the Moon is unlikely to possess a metallic core exceeding 5% of its total mass (Hood, 1986).
The Earth possesses a dimensionless moment of inertia of 0.330, attributed to its substantial density concentration in the central region, which is primarily composed of a large metallic core.
Accurate Lunar Laser Ranging has revealed the first evidence of a fluid lunar core, as demonstrated by Williams et al (2001) The Moon's rotation and orientation variations are influenced by several factors, particularly the dissipation caused by the relative motion at the boundary between the fluid core and the solid mantle.
Orbital motion
The Moon orbits the Earth at an average distance of 384,400 kilometers, with both celestial bodies rotating around their common barycenter located 4,670 kilometers from the Earth's center The eccentricity of the Moon's orbit varies between 0.0435 and 0.0715, averaging at 0.0555.
The sidereal month, lasting 27.32166 Earth days, is the time it takes for the Moon to complete one orbit around the Earth In contrast, the synodic month, which lasts 29.53059 Earth days, represents the period between similar lunar phases, taking into account the Earth's orbit around the Sun.
The average orbital travel speed of the Moon is 1.023 km/sec.
The Moon rotates in prograde motion, that is, in the same direction as the Earth rotates around the Sun.
The Moon’s rotation has been synchronously locked by the Earth-Moon tidal in- teraction Therefore the same hemisphere of the Moon (near side) always faces the Earth.
The prograde rotation of planets requires clarification, as they are thought to have gained most of their angular momentum from solid planetesimals According to the Keplerian motion observed in a disk, the portion of a planet nearest to the Sun should exhibit the highest speed, indicating a tendency towards retrograde rotation.
It follows from the above that purely solid-state accumulation of planets by means of their growth via accumulation of planetesimals would lead to retrograde rotation of the planets.
In the solar system, six of the eight planets exhibit prograde rotation, spinning on their axes in the same direction as their orbit around the Sun Notably, Venus rotates slowly in a retrograde direction, while Uranus has a unique axial tilt, with its rotation axis and satellite system nearly aligned with the ecliptic plane.
In contrast, viewed as a fluid a Keplerian disk has positive vorticity, suggesting that planets should rotate in the prograde direction.
Instabilities in the gas and dust protoplanetary nebula surrounding the Sun lead to the formation of vortices, causing the resulting aggregations to rotate in a prograde direction As these aggregations collide, their momentum combines, ultimately resulting in the formation of planets that also rotate prograde.
It follows thataccumulation of planets must, one way or another, include the accumulation phase of the gas and dust aggregations.
The angular momentum of a planet can be modified by collisions with solid bodies and is finally determined by these two accumulation mechanisms.
Obliquities and inclinations
Obliquity refers to the angle between the rotational and orbital angular momentum vectors The Earth's equatorial plane is tilted at an angle of 23.4 degrees relative to the ecliptic plane, which is its orbital plane In contrast, the Moon's equatorial plane is tilted at a much smaller angle of only 1.5 degrees Additionally, the inclination of the lunar orbit to the ecliptic plane is approximately 1.5 degrees.
There is a notable angular difference between Earth's equatorial plane and the plane of the lunar orbit This difference does not pose a limitation on theories regarding the Moon's origin, as it is possible that the current inclination of the lunar orbit relative to Earth's axial tilt was established early on due to an impact from a relatively small planetesimal striking either Earth or the Moon at a high latitude.
Figure 1.1.Obliquities in the Earth-Moon system:˛1) obliquity of the Earth’s axis;˛2) incli- nation of lunar orbit;˛3) obliquity of the lunar axis.
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The observed inclinations of the rotation axes of the Earth and the Moon may result from impacts by relatively small bodies, with masses not exceeding 10^-2 to 10^-3 of the planetary body mass, as noted by Safronov (1969).
The obliquity of a planet can change due to variations in the orientation of its spin axis or orbit pole According to Lissauer et al (2000), planetary rotation may have been influenced by random orientations during the solar system's formation Additionally, Laskar and Robutel (1993) note that the obliquities have evolved through chaotic variations linked to spin-orbit resonances As a result, the obliquities and inclinations of the Earth and Moon's axes cannot be used as reliable indicators for determining the origins of planets.
Angular momentum
The Earth-Moon system possesses an angular momentum of 3.45 x 10^41 rad g cm²/s, which is derived from the combined rotational angular momentum of the Earth and the orbital angular momentum of the Moon.
The Earth-Moon system exhibits an unusually high rotational angular momentum, the highest among terrestrial planets This phenomenon is interpreted as evidence of a significant mega-impact event that contributed additional angular momentum to both the proto-Earth and the impacting body.
The total angular momentum of the Earth and the Moon aligns with the typical relationships observed among various cosmic bodies, from small asteroids to stars, all exhibiting similar rotation periods near their rotational instability.
Figure 1.2.Angular momentum density as a function of mass for the solar system bodies and stars of some classes for comparison (after Ringwood, 1979).
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The maximum rate at which a spherical body can retain loosely bound material at its equator is determined by equating centrifugal force with gravitational force This relationship, based on the body's density, establishes a minimum rotational period necessary for stability (Lissauer et al., 2000).
TminD s 3 G whereGis the gravitational constant.
It is seen from the equation above that minimum rotation period (maximum rotation rate) does not depend on the size of the body.
The magnitudes of the rotational periods on Fig 1.2 vary within one order while variations in mass span 14 orders.
Small celestial bodies, such as asteroids, can rotate at speeds exceeding the threshold set by Tmin, yet they generally adhere to the overall relationship and spin close to this limit In contrast, the giant planets in our solar system rotate at a rate that is merely half of the rotational stability limit, a characteristic also observed in the Earth-Moon system.
If the orbital angular momentum of the Moon were added to the Earth’s spin, it would rotate at approximately half the rate required for breakup.
The sun itself and the terrestrial planets Mercury, Venus, Mars (and the Earth with- out the Moon) rotate at less than one-tenth breakup.
The observed regularity is remarkably universal, applicable to a diverse array of celestial bodies It aligns with the physical principles governing these bodies, particularly the correlation between angular momentum values that contribute to their rotational instability (Ringwood, 1979).
The specific angular momentum of the Earth-Moon system serves as a baseline, suggesting that the lower angular momentum values observed in other inner solar system planets are abnormal This discrepancy may be attributed to a historical partial loss of angular momentum experienced by these planets.
Orbital evolution
At present, the orbital angular momentum of the Moon is about 5 times larger than the spin angular momentum of the Earth Significant orbital evolution can be attributed to tides.
Tidal interactions between the Earth and the Moon result in an exchange of angular momentum, causing the Earth's rotation to slow down while the Moon's orbit expands Historically, the Moon was significantly closer to the Earth, and these interactions have led to its gradual distancing over time (Burns, 1977).
The angular differences between orbital elements of the Earth and the Moon can have occurred during the orbital evolution (Wood, 1986; Boss and Peale, 1986). www.Ebook777.com
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Libration points
Libration points, also known as Lagrangian points, are specific locations in the gravitational field of the Earth, Moon, and Sun where the net forces acting on an object are nearly zero There are five such points, with three of them located along the straight line connecting the centers of the Earth and the Moon, referred to as collinear libration points.
Lagrangian points L1, L2, and L3 are stable locations in the gravitational field between the Earth and the Moon, where a small object can remain without experiencing any displacing forces Unlike L1, where the gravitational pull from the Earth balances the Moon's attraction, satellites orbiting closer to the Earth must move faster than the Moon due to Kepler’s laws, causing their distance from the Moon to fluctuate However, at L1, an object can maintain a stable orbit with an angular speed that matches that of the Moon, allowing all three bodies to align perfectly during their rotations.
A similar type of balance exists at other Lagrangian points.
Moreover, the position of such objects at pointsL4andL5is stable, i.e., if any external factors (collisions, etc.) shift the object from its position at the libration point,
Figure 1.3.Libration points in the Earth-Moon system.L1,L2,L3– collinear libration points;
Libration points L4 and L5 act as gravitational traps where matter can accumulate, although this concentration is minimal, with notable debris found in Jupiter's stable triangular Lagrangian points known as Trojans The gravitational characteristics of these libration points have contributed to the mega-impact hypothesis, which suggests a genetic link between Earth and the Moon, as evidenced by their similar isotopic compositions, particularly in oxygen However, the dynamic mega-impact model posits that the Moon would primarily consist of material from the impactor, which would not share a cosmic relationship with Earth This contradiction could be resolved if the Earth and the impactor formed in close proximity, such as within a libration point Nonetheless, this scenario is deemed unlikely, as the mass of the impactor would need to be comparable to that of Earth, while stable accumulation in libration points typically involves significantly smaller bodies Additionally, any collision resulting from such an accumulation would not generate the necessary impact velocity to support the mega-impact theory for the Moon's origin.
The history of the study of the Moon
The scientific study of the Moon began with the invention of the telescope by Hans Lippershey in the early 17th century In 1610, Galileo Galilei enhanced the telescope and conducted systematic observations, leading to significant discoveries such as lunar craters, highlands, and lowlands, as well as the identification of dark spots as extensive lowland areas Galileo's findings also included the discovery of Jupiter's four satellites, highlighting that the Moon is not unique and that the presence of satellites orbiting planets is a common characteristic of the solar system.
By the mid-17th century, Italian astronomer Giovanni Battista Riccioli had named over 200 lunar formations, including the stunning crater "Copernicus." This naming was particularly bold given the historical context; just a few years earlier, in 1600, Giordano Bruno was executed for his beliefs, and in 1630, the Inquisition compelled Galileo to renounce the heliocentric theory.
By the late 20th century, a detailed photographic map of the Moon was created, revealing over 40,000 features Researchers determined that the Moon's radius is a quarter of Earth's, and its mass is one eightieth that of Earth Additionally, the Moon's density, moment of inertia, and orbital parameters have been accurately measured.
The era of lunar exploration began in 1959 with the launch of the Soviet spacecraft Luna-1, which, despite missing the Moon by 6,000 km, paved the way for future missions That same year, Luna-2 successfully impacted the lunar surface, marking the first man-made object to reach the Moon In 1960, Luna-3 captured the first images of the Moon's far side, previously unseen by humans The milestone continued with Luna-9's soft landing and transmission of a TV panorama of the lunar landscape Prior to this, the lunar surface's solidity was uncertain The Apollo-8 mission later orbited the Moon, allowing astronauts to view Earth from lunar distance The pinnacle of lunar exploration was achieved on July 20, 1969, when Apollo-11 landed the first humans on the Moon, followed by five additional successful American landings Astronauts conducted vital measurements and collected over 300 kilograms of lunar samples for scientific analysis on Earth.
The Soviet spacecraft Luna-16, Luna-20, and Luna-24 accomplished three successful automatic sample return missions, collectively bringing back less than 300 grams of lunar material Despite the small quantity, these missions represented significant engineering and scientific achievements in space exploration.
Table 2.1.History of lunar exploration
2 Jan 1959 Luna 1 FIRST lunar flyby, magnetic field
12 Sept 1959 Luna 2 FIRST hard landing, magnetic field
20 Apr 1960 Luna 3 FIRST photos of lunar far side
26 Jan 1962 Ranger 3 Missed the Moon by 36,793 km
23 Apr 1962 Ranger 4 Crashed on the lunar far side
18 Oct 1962 Ranger 5 Missed the Moon by 724 km
2 Apr 1963 Luna 4 Missed the Moon by 8,500 km
30 Jan 1964 Ranger 6 Hard landing, television failed
29 July 1964 Ranger 7 Hard landing, close-up TV
17 Feb 1965 Ranger 8 Hard landing, close-up TV
21 Mar 1965 Ranger 9 Hard landing, close-up TV
9 May 1965 Luna 5 Crashed on the Moon
8 June 1965 Luna 6 Missed the Moon by 161,000 km
18 July 1965 Zond 3 Photographed lunar far side
4 Oct 1965 Luna 7 Crashed on the Moon
3 Dec 1965 Luna 8 Crashed on the Moon
31 Jan 1966 Luna 9 FIRST soft landing, TV panorama
31 Mar 1966 Luna 10 FIRST lunar.satellite, gamma-spectra, magnetic and gravity measurements
30 May 1966 Surveyor 1 On-surface TV, soil- mechanics
10 Aug 1966 Lunar Orb 1 TV imaging, radiation, micrometeorites
22 Oct 1966 Luna 12 TV imaging from orbit
6 Nov 1 966 Lunar Orb 2 TV imaging, radiation, micrometeorites
21 Dec 1966 Luna 13 On-surface TV, soil mechanics
5 Feb 1967 Lunar Orb 3 TV imaging, radiation, micrometeorites
17 Apr 1967 Surveyor 3 On-surface TV, soil-mechanics
4 May 1967 Lunar Orb 4 TV imaging, radiation, micrometeorites
19 July 1967 Explorer 35 Fields and particles
1 Aug 1967 Lunar Orb 5 TV imaging, radiation, micrometeorites
8 Sept 1967 Surveyor 5 On-surface TV, first chemistry data
7 Nov 1967 Surveyor 6 On-surface TV, chemistry
7 Jan 1968 Surveyor 7 On-surface TV, chemistry
7 Apr 1968 Luna 14 Gamma spectra, magnetic measurements
14 Sep 1968 Zond 5 FIRST lunar flyby and Earth return, returned bi- ological objects and photos
10 Nov 1968 Zond 6 Lunar flyby and Earth return, returned bio- logicalobjects and photos
21 Dec 1968 Apollo 8 FIRST humans to orbit the Moon
18 May 1969 Apollo 10 FIRST docking in lunar orbit
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13 July 1969 Luna 15 Failed robot sampler
16 July 1969 Apollo 11 FIRST humans on the Moon (20 July)
8 Aug 1969 Zond 7 Lunar flyby and Earth return, returned biological objects, photos
14 Nov 1969 Apollo 12 Human landing, Oceanus Procellarum
11 Apr 1970 Apollo 13 Aborted human landing
12 Sept 1970 Luna 16 FIRST robot sample return, Mare Fecunditatis
20 Oct 1970 Zond 8 Lunar flyby and Earth return, returned photos, landing in the Indian Ocean
10 Nov 1970 Luna 17 FIRST robotic rover Lunokhod 1, Mare Imbrium
31 Jan 1971 Apollo 14 Human landing, Fra Mauro
26 July 1971 Apollo 15 Human landing, Hadley-Apennine
2 Sept 1971 Luna 18 Failed robot sampler
28 Sept 1971 Luna 19 Orbiter, lunar gravity, TV, micrometeorites
14 Feb 1972 Luna 20 Robot sample return, Apollonius
16 Apr 1972 Apollo 16 Human landing, Descartes
7 Dec 1972 Apollo 17 Human landing, FIRST geologist on the Moon,
8 Jan 1973 Luna 21 Lunokhod 2, Le Monier
29 May 1974 Luna 22 Orbiter, lunar gravity, TV, micrometeorites
28 Oct 1974 Luna 23 Failed robot sampler
9 Aug 1976 Luna 24 Robot sample return, Mare Crisium
The Soviet missions successfully demonstrated fully automatic sampling and sample return, enhancing the Apollo collection by retrieving samples from various sites through robotic Luna missions.
The Luna-17 mission marked the debut of an automatic rover, with Lunokhod-1 (1970) and Lunokhod-2 (1973) successfully traversing several kilometers across the Moon's surface through remote control from Earth.
The last mission of that unprecedented lunar race was by Luna-24, which returned samples from Mare Crisium in August 1976.
The recent era of lunar exploration began with the Clementine mission in 1994 and has since included significant missions such as Lunar Prospector, Japan's SELENE (Kaguya), India's Chandrayaan, China's Chang'E, and NASA's Lunar Reconnaissance Orbiter These missions have yielded crucial insights into the Moon's water presence, detailed gravitational mapping, lunar topography, and elemental distribution Historically, the Moon was viewed merely as a celestial body, but recent studies over the past two decades have transformed its understanding into that of a complex geological entity.
The Moon as a geological body
Lunar gravity
A gravitational field is characterized by gravitational potential, with equal potential values creating equipotential planes For a solid ball, these planes are spherical; however, if the mass distribution is uneven, the equipotential planes become distorted due to variations in mass.
The modern approach to studying gravitational fields involves monitoring gravitational perturbations in artificial satellite orbits The lunar gravitational field was first examined by the Soviet satellite Luna-10 in 1966 This research was further advanced by a series of US satellites, including Lunar Orbital missions LO-I through LO-V, which were deployed in various orbits These missions produced a relatively accurate gravity map of the Moon, revealing significant positive gravity anomalies in large circular maria basins characterized by low topography, known as mascons.
The vertical acceleration at the lunar surface, depicted in Figure 3.1, illustrates the LP 165P gravity field as analyzed by Konopliv et al (2001) This representation contrasts the near side of the Moon on the left with the far side on the right, using milligal units and contour lines spaced every 100 milligals to highlight variations in gravitational acceleration.
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The Clementine, Lunar Prospector, and Selene missions significantly enhanced our understanding of lunar gravity Notably, the laser altimetry data from the Clementine mission revealed the Moon's global shape for the first time (Smith et al., 1997).
Before the Lunar Prospector (LP) mission, all identified mascons were located on the Moon's near side and linked to extensive maria-filled impact basins However, research by Neumann et al (1996) revealed the discovery of new mascons in impact basins that show little to no signs of maria fill.
Mascons, or mass concentrations, are key features of the lunar gravity field, formed by a combination of mantle plugs and mare fill in basins, as reviewed by Konopliv et al (1998) Each mascon anomaly exhibits a significant density contribution from the mare, with densities of 3.3 g/cm³ compared to the crust's 2.9 g/cm³, as demonstrated for Serenitatis by Phillips et al (1972) These anomalies feature sharp shoulders and gravity plateaus, surrounded by negative gravity anomalies (Konopliv et al., 2001) The formation of mascons may be linked to giant intrusions beneath the mare and surface volcanism (McGovern, 2012) The Japanese Selene (Kaguya) mission produced the most accurate gravity map of the lunar far side using its two sub-satellites, revealing that, unlike the near side, only a few maria basins exist on the far side (Namiki et al., 2009).
Asymmetry of the lunar shape
The Moon exhibits distinct topographical features on its near side compared to its far side, with significant differences in crust thickness; specifically, the far side has a thicker crust than the near side (Smith et al., 1997).
The Moon exhibits a significant geological contrast characterized by a dichotomy in terrain and elevation, with most maria found on the near side and a predominance of highlands on the far side.
In addition, the center of mass of the Moon is displaced to the near side by 2 km relative to the center of figure.
The observed asymmetry of the Moon is thought to result from an internal mass redistribution, with a dense Fe- and Ti-rich layer migrating towards the Earth-Moon line, potentially explaining the contrasting thermal and volcanic histories of the near and far sides This topographical difference is attributed to their distinct early thermal conditions; during the intense bombardment period 3.8–4 billion years ago, the near side remained hot, allowing its interior material to deform under impacts and create mascons, while the far side cooled and became firmer, resulting in less distortion.
M Jutzi and E Asphang (2011) suggested that the Moon's observed asymmetry may result from late accretion involving a second satellite orbiting Earth This hypothetical companion, estimated to have a diameter of approximately 1,300 km, collided with the Moon at a speed of 2 km/s.
However, Paul Warren (Warren, 2012) has shown that this model runs into serious chemical and physical difficulties.
Magnetic field
The Moon lacks its own dipole magnetic field, yet it features localized magnetic anomalies that span areas from dozens to hundreds of square kilometers Notably, some of these anomalies, observed on the Moon's far side by the Lunar Prospector spacecraft, seem to correspond with young, circular antipodal maria visible on the near side The origins of these local magnetic anomalies remain a topic of debate, with theories suggesting they may be linked to piezoelectric effects resulting from impacts by meteorites and comets.
Samples of the oldest rocks from the Apollo missions indicate that the Moon may have possessed an internal magnetic field between 3.6 and 3.9 billion years ago Runcorn (1996) suggests that this weak magnetic field, with an intensity of 4 microtesla, experienced a significant increase to 100 microtesla during the interval of 4.1 to 3.9 billion years ago However, Weiczorek et al (2006) contest that the lunar core dynamo could not generate such a strong magnetic field at the surface Research on an ilmenite basalt sample dated at 3.6 billion years reveals that its residual magnetization is linked to an external magnetic field rather than impacts, supporting the idea of an active lunar dynamo from 4.2 to 3.6 billion years ago In contrast, studies of younger rocks (3.2 to 3.3 billion years) show no evidence of paleomagnetism, indicating that the lunar dynamo ceased around 3.3 billion years ago.
4 Ga (Lillis et al., 2008; Arkani-Hamed, 2012).
Topography
Lunar topography is characterized by two main features: the lunar continents and the maria The continents, which are highlands, exhibit a contrasting mountainous relief and appear as light regions on the lunar surface In contrast, the maria are low-lying areas that are identifiable as dark spots on the lunar disk.
The continents occupy over 80 % of the lunar surface, mostly on the far side of the Moon, where they account for more than 95 % of the surface.
The Moon's highest point, Leibniz mountain, reaches an elevation of approximately 10,755 meters, while its lowest point lies about 9,060 meters deep in the South Pole-Aitken Basin, both located on the far side of the Moon.
Continental rocks consist of magmatic series, in which ferroan anorthosites prevail.They have originated from early melting of the lunar crust, and are the oldest rocks.
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Figure 3.2.From atlas Antonia Rüke (2004).
Throughout its history, the Moon has experienced numerous meteorite impacts, including those from large celestial bodies These impacts have resulted in the formation of breccias, which are composed of fragmented materials cemented together by impact melts, extending to depths of 20–25 km.
The maria cover approximately 20% of the Moon's surface, with nearly 90% found on the Moon's near side These vast plains are the remnants of massive craters filled with basalt lava, showcasing a landscape marked by craters, volcanic formations, furrows, and evidence of ancient lava flows.
The lunar surface is predominantly marked by craters, which range in size from massive basins to tiny formations Notably, the South Pole Aitken (SPA) basin stands out as the Moon's largest impact structure.
2400 km in diameter and about 8 km deep SPA is also probably the oldest structure.
It has a severely eroded surface and abundant superimposed craters.
Mare Imbrium, with a diameter of about 1,200 km, is an example of a gigantic crater in combination with the formation of maria basins.
As craters grow larger, their structure changes from a simpler cup-like type to more complex reliefs (Fig 3.3).
Craters with diameters ranging from 10 to 15 km typically exhibit a cup-like shape characterized by a level bottom and steep slopes of 30 to 40 degrees, with a depth to diameter ratio of 0.2 to 0.25 In contrast, larger craters, measuring 25 to 40 km in diameter, feature a flatter bottom, a central hill that can rise up to 1.5 km, and concentric circular elevations, resulting in a depth to diameter ratio of 0.1 to 0.15 A notable example of a crater with a central hill is Rửmer, situated between Mare Serenitatis and Mare Crisium.
Among craters with complex structure and high hills are such young and well- preserved craters as Tycho (85 km) and Copernicus (93 km).
Figure 3.3.Transition from simple structure craters to more complex ones:
1 Isidor, a simple cup-like crater, diameter 15 km (coordinates:4; 2 ı S, 34; 1 ı E), photo taken by Apollo-16 (NASA);
2 Bessel, a flat bottom crater, diameter 17 km (21; 8 ı N,17; 9 ı E), photo taken by Apollo-15 (NASA);
3 Rửmer, a crater with central hill, diameter 39 km (25; 4 ı S,36; 4 ı E), photo taken by the Lunar Orbiter-4 station (NASA);
4 Crater Tycho, diameter 85 km (43 ı S,11 ı W), photo taken by the Lunar Orbiter-5 station (NASA);
5 Crater Copernicus, diameter 93 km (10 ı S,20 ı W), photo taken by the Lunar Orbiter-4 station (NASA);
6 Multi-ring basin in Mare Orientale, diameter 900 km (20 ı S,95 ı W), photo taken by the Lunar Orbiter-4 station (NASA) (from Legostaev and Lopota, 2011, by permission).
What are referred to as lunar mountain systems are usually the flank walls of large craters, and sometimes combined walls of superimposed impact structures.
Mare Orientale is a prominent multi-ring crater characterized by four concentric edge walls, measuring an outer diameter of 900 kilometers This geological formation is part of the Cordilleras system, while its inner circular structures, with diameters of 620 kilometers and 480 kilometers, are referred to as the Rocky Mountains.
Lunar mountain systems known as the Carpathians, the Caucasus, the Apennines and the Alps form parts of the circular wall embracing the gigantic impact structure called Mare Imbrium.
Another important component of the relief is crater ejecta A clearly visible system of crater ejecta rays from Copernicus extends almost 2,000 km.
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Figure 3.4.Stone fields A fragment of the video taken by Lunokhod-2 The cross dimension of the largest stone is1m.
The lunar surface is scattered with ejecta from both large and small craters, resulting in stones that range in size from centimeters to several meters Notably, larger stones tend to accumulate around the walls of these craters.
Stones measuring from 20 cm to 2 m are typical of the lunar relief Normally there are 2–4 stones per m 2 of the lunar surface (Fig 3.4).
Precise global topographic data on the Moon with about 5 m accuracy was obtained by a laser altimeter on board the Kaguya spacecraft (Araki et al., 2009).
Lunar soil, or regolith, is a dust-like, grainy layer that varies in granulometric composition from 100 meters to a few millimeters Its lithological makeup is influenced by the regional rock mix and materials brought in by impacts Regolith forms under the effects of space particle radiation, solar winds, and micro-meteorite impacts.
The regolith surface is dotted with multiple cavities that form a typical honeycomb structure (Fig 3.5).
Figure 3.5.Honeycomb structure of the lunar regolith surface A fragment of the video taken by the Luna-9 station (from Legostaev and Lopota, 2011, by permission).
Micro-meteorite impacts cause the fragmentation and fusion of rock particles, leading to the formation of agglutinates—aggregates of particles bonded by impact melts The concentration of agglutinates in the soil increases with prolonged exposure Additionally, protons from solar winds facilitate the reduction of FeO, resulting in the emergence of nano-sized zero-valency metallic iron particles in the regolith The maturity of the regolith is assessed by the quantity of agglutinates and the concentration of these iron nanoparticles.
The rate of accumulation of regolith is very low It takes several hundred million years to accumulate a detectable two meter thick regolith layer (Sharpton and Head,1982).
Lunar Rocks
Highland rocks
The rocks forming the continental areas of the Moon account for95% of the volume of the upper lunar crust.
They include a number of petrological series (Fig 3.6).
Figure 3.6.Classification scheme of lunar rocks (from Weizorek et al., 2006) 1 Anorthosite,
2 Noritic (Gabbroic) Anorthosite, 3 Troctolitic Anorthosite, 4 Anorthositic Notite (Gabbro),
5 Anorthositic Troctolite, 6 Norite (Gabbro), 7 Olivine Norite (Gabbro), 8 Troctolite, 9 Pyrox- enite, 10 Peridolite, 11 Dunite.
The lunar crust is mainly made up of rocks rich in calcium plagioclase and iron oxide, specifically ferroan anorthosites The iron oxide (FeO) content ranges from 1% in pure anorthosites to 15% in ferroan norites, with an average value indicating significant variability in composition.
4 % Concentration of TiO 2 is low ( 0:5% by weight) Thanks to the prevailing plagioclase, the rocks feature a high percentage of Al 2 O 3 (> 24%).
The average depth of formation for these lunar rocks is around 25 km, linked to the emergence of light plagioclase crystals during the crystallization of the Moon's ancient magmatic ocean These rocks represent the earliest solidified materials on the lunar surface, with ferroan anorthosites being recognized as the oldest lunar rocks.
Section 3.5 Lunar Rocks 21 estimated age of 4.44–4.45 billion years Samples of this suite prevail at the landing sites of the spacecrafts Apollo-16 and Luna-20.
Continental rocks encompass not only anorthosites but also magnesian magmatic rocks known as the Mg-suite, which includes key types such as troctolite, spinel troctolite, norite, gabbronorite, anorthosite norites, and ultramafic rocks The dominant mineral found in these rocks is plagioclase, primarily anorthite, along with various samples containing orthopyroxenes like enstatite and bronzite, clinopyroxenes such as diopside, augite, and pigeonite, as well as olivine.
Figure 3.7.Magnesium factor mg#DMg=.MgCFe/and anorthite content in plagioclase in the rocks of: I-magnesian suite and II-ferroan anorthosites (Ariskin, 2007).
The MgO content in rocks ranges from 45% by weight in dunites to 7% in anorthosite norites, while Al2O3 varies from less than 2% in dunites to 29% in troctolite anorthosites, and TiO2 remains below 0.5% These rocks date back to 4.1–4.5 billion years and are believed to have formed in the lower crust at depths of 30–50 km They were intruded into a ferroan anorthosite substrate and later exposed during the creation of large impact basins Notably, the ages of the magnesian suite rocks overlap with those of the ferroan anorthosites, suggesting they likely originated around the same time.
A number of samples of the alkaline suite were collected at the landing places of Apollo-12, -14 and -15 (alkaline suite).
Alkaline suite rocks are characterized by typical minerals such as plagioclase, clinopyroxene, orthopyroxene, potassium feldspar, quartz, apatite, merrillite, ilmenite, chromspinel, fayalite, zircon, baddeleyite, troilite, and a metallic phase of Fe-Ni These rocks exhibit a notable richness in potassium (0.3–0.5% by weight) and sodium (1.25–1.6%), which is significant given the Moon's overall scarcity of alkaline elements Additionally, iron content in these rocks ranges from 0.4% by weight in alkaline anorthosites to 17% by weight in alkaline norites, highlighting their diverse mineral composition.
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Chapter 3 discusses the Moon as a geological body, highlighting that its composition varies significantly, with weight percentages of certain elements ranging from less than 0.5% to 5% The concentration of Thorium (Th) varies from 5 g/t to 12 g/t, with a maximum of 40 g/t Quartz monzodiorites on the Moon are characterized by 65-75% SiO2, less than 10% FeO, 3-8% K2O, and 1-2% TiO2, and they also contain elevated levels of Rare Earth Elements (REE).
Zr, Hf, Rb, Cs, Nb, Ta, Th and U.
The age of the alkaline suite rocks is estimated at 3.8–4.3 billion years Formation depth of the rocks is estimated at up to 2 km, i.e., the upper lunar crust.
In addition to the main petrological suites, the Moon's continental region features various other rocks, primarily consisting of brecciated materials resulting from the impact processing of primary magmatic rocks This includes non-maria basalts and other lesser volumes of rock types.
Maria rocks
Maria rocks cover approximately 30% of the lunar surface, yet they constitute a minor part of the lunar crust by volume The basalt layer's average thickness is around 400 meters, with some areas reaching up to 1,000 meters The total volume of extruded mare basalts is estimated at 10^7 km³, accounting for only 0.1% to 0.5% of the crust's overall volume.
The predominant type of rocks found in the lunar maria are maria basalts, which are categorized based on their titanium, aluminum, and potassium content High-titanium basalts (TiO2 > 8% by weight) were collected during the Apollo-11 and Apollo-17 missions, while Apollo-12 and Apollo-15 returned samples containing low-titanium and low-aluminum basalts (TiO2 2–6% by weight, Al2O3 < 12% by weight) Additionally, basalts with low titanium but higher aluminum content (TiO2 3–6% by weight, Al2O3 12–15% by weight) were obtained from the Luna-16 mission Finally, very low titanium basalts (TiO2 < 1% by weight) were found in the samples returned by Luna-24.
There are several types of basalts identified based on their potassium and titanium content: low potassium, low titanium basalts with approximately 0.1% K2O; high potassium, high titanium basalts with around 0.3% K2O; and extremely high potassium basalts containing 0.9% K by weight The mineral composition of these groups varies, particularly in the amounts of Ti-bearing ilmenite and feldspar, which contribute significantly to the aluminum and alkali content (Longhi, 2006).
Maria basalts are believed to have formed from the partial melting of the Moon's interior at depths reaching 400 km, suggesting that their composition reflects the characteristics of the lunar mantle.
Vented basalt lavas filled the impact cavities of the maria less than 4 billion years ago, when intensive bombardment was over.
Within the maria formations, one comes across a pyroclastic material characterized by green and orange glass balls They are deemed to have resulted from sprays of lava www.Ebook777.com
Section 3.6 Lunar chronology 23 fountains Such orange and green glass is found in the sample collections of Apollo-15 and Apollo-17.
KREEP basalts are a unique type of rock characterized by their high concentrations of Potassium (K), Rare Earth Elements (REE), and Phosphorus (P) These basalts typically contain 13–16 wt% Al2O3 and 9–15% FeO, along with trace elements that are 100–150 times more abundant than those found in chondritic materials (Weiczorik et al., 2006) Unlike maria basalts, KREEP basalts feature iron olivine (fayalite) instead of magnesian olivine (forsterite) and have a significantly higher content of plagioclase The pyroxene present in KREEP basalts includes pigeonite and augite (clinopyroxenes).
Samples of the KREEP-basalts were collected by the Apollo-15 mission near theApennine area, around Mare Imbrium and near Crater Aristillus.
Lunar chronology
Lunar chronology relies on two primary methods: the absolute dating of samples collected by Apollo and Luna missions and the statistical analysis of impact craters on the Moon's surface.
The larger the crater is, the slower it degrades The average life of a 1 km crater is
5 million years, of a 100 km crater 250 Ma (million years), and of a 300 m crater 1.3 Ga (billion years) (Basilevsky, 1976).
Older locations exhibit a higher density of craters per unit area By analyzing the absolute ages of samples collected from Apollo spacecraft landing sites alongside the observed crater density in those regions, scientists have established a scale for measuring age based on crater statistics.
The early history of the Moon is categorized into three distinct periods based on impact crater frequency and size: the Pre-Nectarian period (older than 3.92 billion years ago), the Nectarian period (3.92–3.85 billion years ago), and the Early Imbrian period (3.85–3.8 billion years ago) Key lunar samples have been dated to major impact events, including Nectaris at 3.9 billion years, Crisium at 3.895 billion years, Serenitatis at 3.893 billion years, and Imbrium at 3.85 billion years Additionally, crater-counting methods reveal the ages of various lunar basalts, such as Autolycus at 2.1 billion years, Aristillus at 1.29 billion years, and Copernicus at 0.8 billion years.
The age of the Moon is closely linked to our understanding of its origin, which dates back to the formation of the solar system approximately 4.568 billion years ago According to Hf-W data, the Moon's core segregation could not have happened earlier than 50 million years after the solar system's inception (Kliene et al., 2009).
The formation of the Moon is estimated to have occurred between 4.51 and 4.52 billion years ago if the segregation of its core happened simultaneously with its accumulation However, if the core segregation occurred after the Moon was fully formed, its formation date could be earlier The oldest lunar rocks identified, particularly ferroan anorthosites, are highlighted in Table 3.2, indicating they are the most ancient materials found on the Moon's surface.
24 Chapter 3 The Moon as a geological body
Table 3.2.Age estimates for the oldest lunar rocks.
After solar system formation (Ma) Ferroanorthosite 60,025
206 Pb/ 207 Pb Recalculated data of Hanan and Tilton (1987)
Sm-Nd Sm-Nd Sm-Nd Rb-Sr U–Pb
Tera et al., 1973 Alibert et al., 1994 Borg et al., 1999
Nyquist and Shih, 1992 Shih et al., 1993 Carlson and Lugmair, 1988 Carlson and Lugmair, 1981 Nyquist et al., 1981 Edmunson et al., 2008
The geological history of the Moon began with a magma ocean approximately 500 km deep, which underwent differentiation as it cooled The solidification of plagioclase, due to its lower density, led to the formation of the early anorthositic crust Meanwhile, the mafic cumulates are believed to be the source of the mare basalts Additionally, the residual liquid enriched with large ion lithophilic elements (KREEP) could later be re-mobilized by magmatic cumulates, resulting in basalts containing a KREEP component.
Mare basalt volcanism took place during the great bombardment, filling large impact structures with basalt lava that dates back to between 3.6 and 3.9 billion years The Apollo-17 landing site features low titanium Al-bearing basalts estimated to be 3.9 billion years old, while the extremely low titanium basalts at the Luna-24 site are around 3.3 billion years old At the Apollo-11 site, high titanium basalts are dated between 3.5 and 3.8 billion years, with high potassium, high titanium basalts at approximately 3.55 billion years Additionally, low titanium basalts near the Apollo-12 and Apollo-15 sites range from 3.08 to 3.37 billion years old.
The average age of the KREEP model is approximately 4.42 billion years, as noted by Nyquist and Shih in 1992 Research by Snyder et al in 2000 indicates that the ages of ferroan anorthosites and Mg-suite rocks overlap, suggesting that both types of magmas were being generated on the Moon around 4.4 billion years ago.
Section 3.7 Internal structure and temperature 25
The Moon differentiation was completed by 4.49 Ga (80million years after the solar system formed) assuming a chondritic uniform reservoir bulk composition for the Moon (Edmunson et al., 2008).
Along with the most ancient ferroan anorthosites there are relatively young ones. For example, sample 62,236 (Apollo-16) yields an age of4:29˙0:03Ga (Borg et al.,
1999) This is inconsistent with the concept of a short-living magma ocean.
The complete solidification of the lunar magma ocean, which extends to depths of 500–1,000 km, can occur in just 10 million years Notably, the initial 80% solidification, allowing plagioclase to start floating, takes less than 1,000 years (Elkins-Tanton et al., 2011) However, the ages of ferroan anorthosites vary significantly.
Lunar crust magmatism and formation persisted for 200 million years, potentially driven by tidal heating from interactions with the early Earth, which may have sustained magma ocean solidification (Meyer et al., 2010) Additionally, early dynamical heating could have been on par with internal radiogenic heating (Weiczorik et al., 2006) This tidal energy during the Moon's formative period might have facilitated convection and dynamo activity if the core was metallic (Weiczorik et al., 2006).
The crystallization of denser phases, such as ferrous silicates and ilmenite, during the final stages of magmosphere formation may have triggered gravitational instability This instability likely caused an overturn, resulting in the influx of primitive magnesian magmas into the upper layers of the crust (Hess and Parmentier, 1995; Elkins-Tanton et al., 2002).
Internal structure and temperature
Research on the elastic properties of the Moon, alongside studies of its gravity and magnetic fields, has led to a reconstruction of the Moon's internal structure Additionally, the analysis of surface rock samples, which reflect the composition of the upper lunar layer, contributes valuable insights into this understanding.
Current understanding of the Moon's internal structure relies on data collected from four seismic stations established during the Apollo missions, which recorded lunar seismic activity over an eight-year period from July 1969 to September 1977.
The tectonic activity of the Moon is incomparably lower than that of the Earth The annual output of seismic energy on the Moon is around10 10 J, compared to around
10 18 J on the Earth (Goins et al., 1981).
Most seismic events take place within the depth range from 50 to 220 km (Khan et al., 2000) Another active interval is deep focus moon-quakes in the 950–1,000 km depth range.
The Moon exhibits distinct stratification, with scholars differing in their methods of interpreting seismograms to define the strata boundaries The crust's estimated thickness varies between 30 to 70 km, averaging around 45 km Additionally, the lower boundary of the upper mantle is consistently identified at a depth of 500 km, often accompanied by a transitional zone known as the middle mantle.
Chapter 3 discusses the Moon as a geological entity, highlighting its identification between the upper and lower mantle at depths ranging from 500 to 750 km It notes the absence of seismic data from the Apollo missions for regions deeper than 1,100 km.
Recent studies indicate a consensus among scholars regarding the velocity of elastic waves in the upper lunar lithosphere, with average values of VS = 4.48 ± 0.05 km/s and VP = 7.71 ± 0.06 km/s, as reported by Goins et al (1981), Nakamura (1983), Lognonne et al (2003), and Khan and Mosegaard (2001) However, estimates for the lower mantle vary significantly among these authors For instance, Nakamura (1983) and Lognonne et al (2003) present differing estimates, as shown in Fig 3.8 Initially, the average thickness of the lunar crust was estimated at approximately 70 km, but subsequent seismic data reanalyses revealed lower values of 45 ± 5 km (Khan et al., 2000), 38 ± 8 km (Khan and Mosegaard, 2001), and 30 ± 2.5 km (Lognonne et al.).
According to Weiczorik et al (2006), the lunar crust has an estimated average thickness of 49.1 ± 6 km, with a maximum thickness of 85 km found at coordinates 199° E, 4° N Conversely, the minimum thickness approaches zero and is situated near the edge of the South Pole-Aitken basin beneath Mare.
Figure 3.8.Lunar seismic P-and-S-wave velocities (straight line – author Nakamura, 1983,dotted line – author Lognnone et al., 2003).
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Section 3.7 Internal structure and temperature 27
Moscoviense, located at coordinates 151° W and 36° S, is believed to have formed due to a significant impact that caused mantle uplift Recent research by Fei et al (2012) proposed an average lunar crustal thickness of 42.2 km, but we adopt a figure of 45 km for the average thickness of the lunar crust.
The lunar crust consists of two distinct layers: an upper anorthositic layer and a lower noritic layer, each approximately 25 km thick This crust accounts for 8-10% of the Moon's total volume, with an average density of 2.9 g/cm³.
The lunar mantle remains unrepresented by direct samples; however, its upper mantle composition can be inferred from the analysis of mare basalts and volcanic glasses Additionally, seismic data analysis provides further verification of its structure and composition.
The significant rise in velocities beyond 500 km suggests a transition in the mantle's chemical composition, shifting from a differentiated upper layer to a primordial lower mantle (Nakamura, 1983; Hood, 1986).
In contrast to the Nakamura model, the P-wave velocity in the Khan et al (2000) model remains relatively constant or experiences a slight increase with depth, ranging from 8.1 to 11.5 km/s Below 500 km, the velocity rises significantly, reaching up to 10.1 to 15 km/s, and at approximately 620 km, it drops back to 8.1 km/s Subsequently, the velocity gradually increases again, peaking at 11.0 to 12 km/s at a depth of 950 km Notably, the highest P-wave velocity is observed during the deepest lunar moon-quakes.
Elastic properties of rocks are influenced by their composition and temperature The upper mantle exhibits a high seismic inverse dissipation factor (Q) ranging from 4,000 to 7,000, indicating an unusually prolonged seismic signal (Lognonne, 2005) These elevated Q-values suggest the presence of hard rock with low porosity and minimal volatile content Additionally, the upper mantle's temperature limit can be estimated through mascon anisostasy, as noted by Lambeck and Pullan (1980), indicating that temperatures must remain below a certain threshold.
800 ı C, to keep mascons stable during almost 4 Ga In the interval 500–600 km the
The Q-factor decreases to approximately 1,500, indicating temperatures significantly below the solidus, as noted by Toksửz et al (1974) Despite this, the majority of deep moon-quakes occurring at depths of 850–950 km suggest the presence of solid rock necessary for stress accumulation (Hood and Zuber, 2000) At depths exceeding 1,000 km, both S-wave velocity and Q-values drop to around 100, indicating temperatures nearing the solidus L Hood and M Zuber (2000) propose a corresponding temperature profile.
750 ı C at 300 km depth, 1; 200 ı C at 800 km depth, and 1; 400 ı C at 1,100 km depth.
In the upper crust, plagioclases make up approximately 82%, corresponding to 27-29% of Al2O3, while in the lower crust, their percentage ranges from 71-75%, with Al2O3 content between 18-20% (Tompkins and Pieters, 1999).
Research indicates that the pyrolytic composition is inadequate for accurately representing the chemical makeup of the upper mantle, as it forecasts implausibly high temperatures at those depths More reliable results can be achieved through alternative methods.
28 Chapter 3 The Moon as a geological body pyroxenite composition with low Al and Ca contents:400–500 ı C at 100 km, 600–
Iron content
The Moon has significantly less iron than Earth and likely lacks an iron core, as indicated by their density differences of 4.45 g/cm³ for Earth and 3.3 g/cm³ for the Moon Geophysical data reveals that Earth's core is primarily composed of iron and nickel, with up to 10% of lighter elements such as hydrogen, carbon, oxygen, silicon, and sulfur.
Total iron content of the Earth is around 32.5 %, whereas total iron content of the Moon is estimated at less than 15 % (Fig 4.1).
Figure 4.1.Comparative iron content in the Earth and the Moon.
The Moon's mantle has a higher proportion of iron compared to Earth's mantle, despite the Moon containing less iron overall While the average iron oxide (FeO) concentration in Earth's mantle is around 8%, estimates for the Moon range from 13% to 18% The lower end of this range is considered more plausible, but it still significantly exceeds the iron content found in Earth's mantle.
The Earth and the Moon differ significantly in their composition, with the Earth containing a greater overall amount of iron However, the silicate portion of the Moon has a higher concentration of iron compared to the silicate part of the Earth.
30 Chapter 4 Similarity and difference in composition of Earth and Moon
Redox state
The lunar mantle differs from Earth's mantle as it lacks any traces of Fe₂O₃, with polyvalent elements such as Cr and Ti consistently found in their lower valence states This is supported by mineralogical findings of Fe-rich olivine, cristobalite, and metal within the mesostasis of mare basalts Additionally, the presence of olivine, spinel, and metal inclusions in orange glass beads indicates that metal-melt equilibria and the chromium content of olivine and spinel suggest a pre-eruptive oxygen fugacity of approximately 1.3 log units below the iron-wüstite buffer.
Evidence suggests that the Earth's mantle has a reduced state of fO2 at or below the IW buffer, differing from the oxidized state represented by the QFM buffer found in the current upper mantle This conclusion is supported by various direct and indirect lines of research (Weiczorek et al., 2006).
The redox conditions in the mantles of the Moon and Earth differ significantly, yet this difference may be less critical than it appears Earth's substantial metallic core suggests that if we consider the balance between its silicate portion and core, Earth's overall redox state could be as reduced as that of the Moon Evidence indicates that the early Earth's atmosphere was likely strictly reduced, as demonstrated by the K/Na ratio exceeding 1, which is essential for the emergence of protein life in a hydrosphere, corresponding to a CH4/CO2 ratio greater than 1 in the primordial atmosphere.
Probable evolution of the redox condition in the Earth’s mantle has been studied in a number of works: (Kasting et al., 1993; Stevenson, 1983; Lecuyer and Ricard, 1999; Frost et al., 2004; Galimov, 2005)
Research by Galimov (1998; 2005) indicates that the gradual formation of the Earth's core over geological periods can elucidate two key phenomena: the presence of a heat source that sustains a superdiabatic temperature gradient and associated convection within the Earth's mantle, and the influx of oxygen into the mantle resulting from the disproportionation of iron valency, specifically the conversion of 3Fe₂C in the descending convective branch to DFe₀.core and 2Fe₃C in the ascending convective branch.
The initial redox conditions of the Moon and Earth are identical; however, Earth's mantle underwent gradual oxidation due to convection processes, ultimately achieving the current QFM buffer state.
The Moon has maintained its original redox state due to the absence of mechanisms that would alter it, unlike Earth, which has undergone significant changes over the past 4 billion years This preservation supports the theory that the Moon's lower mantle consists of "primitive" unmelted materials, as suggested by Wieczorek et al (2006).
Hence, it follows that the observeddifference in redox characteristics between the Moon and the Earth cannot be used as a constraint on the Moon origin.
The redox evolution model of the Earth's mantle illustrates how the disproportionation of iron contributes to core growth by transferring Fe and FeO from the mantle Additionally, excess oxygen is carried along with the backward convection flow, which includes a mix of materials originating from the core The DU-layer is believed to be a crucial zone where core and mantle materials interpenetrate, interact, and fuse together.
Volatiles
The Moon's significant iron deficit has been crucial in understanding its origin, highlighting a stark contrast with Earth, particularly in the depletion of volatile elements While all cosmic bodies exhibit a decrease in volatiles compared to their solar abundance, the Moon's composition is notably poorer in volatiles than even ordinary chondrites and is akin to achondrites, fragments of differentiated asteroid silicate shells This pronounced lack of volatile components in the Moon compared to Earth necessitates an explanation regarding the formation of the Earth-Moon system Furthermore, the Moon's volatile-poor nature reflects a broader trend observed in celestial matter located closer to the Sun within the inner solar system.
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32 Chapter 4 Similarity and difference in composition of Earth and Moon
Various authors, including Palme et al (1998) and Humayun and Cassen (2000), have estimated the concentrations of volatiles in meteorites, the Moon, and Earth Figure 4.3 illustrates the volatile depletion on the Moon, based on Ringwood's 1986 diagram, with modifications by Galimov in 2004.
Figure 4.3.Depletion of the Moon according to Ringwood (Ringwood, 1986) with modifica- tions for Si, Mg, and Fe (Galimov, 2004).
The Moon's volatile depletion is primarily influenced by the volatility of elements rather than their atomic mass, leading to significant deficiencies in potassium (K) and sodium (Na), which are also scarce on Earth, and an even greater depletion in rubidium (Rb), despite its higher atomic weight The K/U ratio is notably high in carbonaceous chondrites at 6010^-3, compared to 1110^-3 for Earth and 2.5 x 10^-3 for the Moon, while the Rb/Sr ratio is 0.3 for chondrites, 0.031 for Earth, and only 0.009 for the Moon Furthermore, the Moon exhibits extreme depletion in lead, a volatile heavy element, suggesting that evaporation processes likely occurred from smaller bodies or through hydrodynamic escape This volatile loss happened early in lunar history, as indicated by the low 87Sr/86Sr ratios in lunar samples compared to Earth, signifying a significant loss of 87Rb, the volatile radioactive precursor of 87Sr, at that time.
The Moon exhibits unusually high ratios of radiogenic lead isotopes 206 Pb, 207 Pb, and 208 Pb compared to non-radiogenic 204 Pb, with lunar rocks showing 206 Pb/204 Pb ratios between 160 and 190, in stark contrast to Earth's typical ratios of 18 to 20 (Edmunson et al., 2008).
The early catastrophic loss of lead, primarily represented by the primordial isotope 204 Pb, significantly impacted lunar history This loss was followed by the gradual accumulation of lead, which occurred as a result of the decay of radioactive isotopes of refractory elements, specifically uranium (U) and thorium (Th).
The third characteristic of volatile loss from the Moon is the absence of any isotope fractionation record Typically, evaporation is associated with isotope fractionation, as demonstrated by Wang et al (1994), who found that the evaporation of 40% melt alters the (30 Si/28 Si) ratio by 8–10 ‰ and the (26 Mg/24 Mg) ratio by 11–13 ‰, following the Raleigh distillation equation Given the significant potassium depletion observed on the Moon, the isotopic composition of potassium (41 K/39 K) is expected to have changed by over 90 ‰, according to Humayun and Clayton (1995) However, no isotopic shifts have been reported for these elements across various cosmic bodies, including the Moon.
In Chapter 7 we will discuss this phenomenon in more detail in relation to the con- sidered hypotheses of the Moon origin.