The current state of understanding of the lunar interior is the sum of nearly four decades of work and a range of exploration programs spanning that same time period. Missions of the 1960s including the Rangers, Surveyors, and Lunar Orbiters, as well as Earth-based telescopic studies, laid the groundwork for the Apollo program and provided a basic understanding of the surface, its stratigraphy, and chronology. Through a combination of remote sensing, surface exploration, and sample return, the Apollo missions provided a general picture of the lunar interior and spawned the concept of the lunar magma ocean. In particular, the discovery of anorthite clasts in the returned samples led to the view that a large portion of the Moon was initially molten, and that crystallization of this magma ocean gave rise to mafic cumulates that make up the mantle, and plagioclase flotation cumulates that make up the crust (Smith et al. 1970; Wood et al. 1970). This model is now generally accepted and is the framework that unifies our knowledge of the structure and composition of the Moon. The intention of this chapter is to review the major advances that have been made over the past decade regarding the constitution of the Moon’s interior. Much of this new knowledge is a direct result of data acquired from the successful Clementine and Lunar Prospector missions, as well as the analysis of new lunar meteorites. As will be seen, results from these studies have led to many fundamental amendments to the magma ocean model.

Much of what we know from sample analyses has been previously summarized elsewhere, and only their most important aspects will be discussed in this chapter. The reader is referred to the relevant chapters in the books Basaltic Volcanism on the Terrestrial Planets (Basaltic Volcanism Study Project 1981), The …

The Constitution and Structure of the Lunar Interior

X-ray computed tomography (CT) is a technique that allows non-destructive imaging and quantification of internal features of objects. It was originally developed as a medical imaging technique, but it is now also becoming widely used for the study of materials in engineering and the geosciences. X-ray CT reveals differences in density and atomic composition and can therefore be used for the study of porosity, the relative distribution of contrasting solid phases and the penetration of injected solutions. As a non-destructive technique, it is ideally suited for monitoring of processes, such as the movement of solutions and the behaviour of materials under compression. Because large numbers of parallel two-dimensional cross-sections can be obtained, three-dimensional representations of selected features can be created. In this book, various applications of X-ray CT in the geosciences are illustrated by papers covering a wide range of disciplines, including petrology, soil science, petroleum geology, geomechanics and sedimentology.

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Applications of X-ray computed tomography in the geosciences

[1] Gamma ray spectroscopy data acquired by Lunar Prospector are used to determine global maps of the elemental composition of the lunar surface. Maps of the abundance of major oxides, MgO, Al2O3, SiO2, CaO, TiO2, and FeO, and trace incompatible elements, K and Th, are presented along with their geochemical interpretation. Linear spectral mixing is used to model the observed gamma ray spectrum for each map pixel. The spectral shape for each elemental constituent is determined by a Monte Carlo radiation transport calculation. Linearization of the mixing model is accomplished by scaling the spectral shapes with lunar surface parameters determined by neutron spectroscopy, including the number density of neutrons slowing down within the surface and the effective atomic mass of the surface materials. The association of the highlands with the feldspathic lunar meteorites is used to calibrate the mixing model and to determine backgrounds. A linear least squares approach is used to unmix measured spectra to determine the composition of each map pixel. The present analysis uses new gamma ray production cross sections for neutron interactions, resulting in improved accuracy compared to results previously submitted to the Planetary Data System. Systematic variations in lunar composition determined by the spectral unmixing analysis are compared with the lunar soil sample and meteorite collections. Significant results include improved accuracy for the abundance of Th and K in the highlands; identification of large regions, including western Procellarum, that are not well represented by the sample collection; and the association of relatively high concentrations of Mg with KREEP-rich regions on the lunar nearside, which may have implications for the concept of an early magma ocean.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JE002656

Elemental composition of the lunar surface: Analysis of gamma ray spectroscopy data from Lunar Prospector

The surface of the Moon is a critical boundary that shapes our understanding of the Moon as a whole. All geologic mapping and remote sensing techniques utilize only the outermost portion of the Moon. Before leaving the Moon for study in our laboratories, all lunar samples that have been studied existed at or very near the surface. With the exception of the deeply probing geophysical techniques, our understanding of the interior of the Moon is derived from surficial, but not superficial, information, coupled with boundary of the lunar crust, it is the lower boundary layer of the tenuous lunar atmosphere and constitutes both a source and a sink for atmospheric gases. The surface is also where the Moon interacts with the space environment, causing changes in the physical nature of lunar materials, and provides a laboratory for the study of processes that occur on all airless bodies.

The data obtained remotely by the Galileo, Clementine, and Lunar Prospector missions, as well as data derived from lunar meteorites, have resulted in major changes to our understanding of global distributions of chemistry and rocks. This chapter summarizes the current understanding of this critical interface, the surface of the Moon, in its role as the lower boundary of the lunar atmosphere, the upper boundary of the crust, and the window through which we view, through remote sensing, the composition of the crust and the history of the Moon. In this post-Lunar Prospector time, the view of the Moon has changed, lending new perspectives to lunar samples and lunar processes. But the New View will likely remain in flux as we continue to digest the results from these recent space missions and move forward to a new era of lunar exploration.

Despite the freshness of our perspective, this is an important moment to capture, …

Understanding the Lunar Surface and Space-Moon Interactions

We investigated the transfer of meteorites from Mars to Earth with a combined mineralogical and numerical approach We used quantitative shock pressure barometry and thermodynamic calculations of post-shock temperatures to constrain the pressure/temperature conditions for the ejection of Martian meteorites The results show that shock pressures allowing the ejection of Martian meteorites range from 5 to 55 GPa, with corresponding post-shock temperature elevations of 10 to about 1000 °C With respect to shock pressures and post-shock temperatures, an ejection of potentially viable organisms in Martian surface rocks seems possible A calculation of the cooling time in space for the most highly shocked Martian meteorite Allan Hills (ALH) 77005 was performed and yielded a best-fit for a post-shock temperature of 1000 °C and a meteoroid size of 04 to 06 m The final burial depths of the sub-volcanic to volcanic Martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for Martian meteorites during the impact of a small projectile (200 m), as defined by numerical modeling (Artemieva and Ivanov 2004) A comparison of shock pressures and ejection and terrestrial ages indicates that, on average, highly shocked fragments reach Earth-crossing orbits faster than weakly shocked fragments If climatic changes on Mars have a significant influence on the atmospheric pressure, they could account for the increase of recorded ejection events of Martian meteorites in the last 5 Ma

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.2005.tb00409.x

Ejection of Martian meteorites

The lunar meteorite Sayh al Uhaymir 169 consists of an impact melt breccia extremely enriched with potassium, rare earth elements, and phosphorus [thorium, 32.7 parts per million (ppm); uranium, 8.6 ppm; potassium oxide, 0.54 weight percent], and adherent regolith. The isotope systematics of the meteorite record four lunar impact events at 3909 ± 13 million years ago (Ma), ∼2800 Ma, ∼200 Ma, and <0.34 Ma, and collision with Earth sometime after 9.7 ± 1.3 thousand years ago. With these data, we can link the impact-melt breccia to Imbrium and pinpoint the source region of the meteorite to the Lalande impact crater.

Pinpointing the Source of a Lunar Meteorite: Implications for the Evolution of the Moon

Fifty-four fragments of ordinary chondrites from 50 finds representing all searched areas in central Oman and all weathering stages were selected to compare the physical, chemical, and mineralogical effect of terrestrial weathering with 14C terrestrial ages. 14C ages range from 2.0 to >49 kyr with a median value of 17.9 kyr. The peak of the age range, which is between 10-20 kyr, falls in an arid climate period. A comparison of the chemical composition of Omani chondrites with literature data for unweathered H and L chondrites demonstrates a strong enrichment in Sr and Ba, and depletion in S during weathering. Water contents in H chondrites increase with terrestrial age, whereas L chondrites show a rapid initial increase followed by nearly constant water content. Correlating Sr, Ba, and H2O with age indicates two absorption trends: i) an initial alteration within the first 20 kyr dominated by H2O uptake, mainly reflecting Fe-Ni metal alteration, and ii) a second Baand Sr-dominated stage correlated with slower and less systematic weathering of troilite that starts after H2O reaches ~2 wt%. Sulfur released from troilite partly combines with Ba and Sr to form sulfate minerals. Other parameters correlated with 14C age are degree of weathering, color of powdered meteorites, and the Ni/Fe ratio. Chemical analyses of 145 soils show a high degree of homogeneity over the entire interior Oman Desert, indicating large-scale mixing by wind. Soil samples collected from beneath meteorite finds typically are enriched in Ni and Co, confirming mobilization from the meteorites. High Cr and Ni concentrations in reference soil samples, which decrease from NE to SW, are due to detrital material from ultramafic rocks of the Oman Mountains.

Weathering of meteorites from Oman: Correlation of chemical and mineralogical weathering proxies with 14C terrestrial ages and the influence of soil chemistry

Sayh al Uhaymir 094 is a 223.3 g, partially crusted, strongly to very strongly shocked melanocratic olivine-porphyric rock of the shergottite group showing a microgabbroic texture. The rock consists of pyroxene (52.0 - 58.2 vol%) - dominantly prismatic pigeonite (En60-68Fs20-27Wo7-9) associated with minor augite (En46-49Fs15-16Wo28-31) - brown (shock-oxidized) olivine (Fo65-69; 22.1 - 31%), completely isotropic interstitial plagioclase glass (maskelynite; An50-64Or0.3-0.9, 8.6 - 13.0%), chromite and titanian magnesian chromite (0.9 - 1.0%), traces of ilmenite (Ilm80-86), pyrrhotite (Fe92-100; 0.1 - 0.2%), merrillite (<< 0.1%), and pockets (4.8 - 6.7%) consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals. The average maximum dimensions of minerals are: olivine (1.5 mm), pyroxene (0.3 mm) and maskelynite (0.3 mm). Primary melt inclusions in olivine and chromite are common and account for 0.1 - 0.6% of the rock. X-ray tomography revealed that the specimen contains approximately 0.4 vol% of shock-melt associated vesicles, up to 3 mm in size, which show a preferred orientation. Fluidization of the maskelynite, melting and recrystallization of pyroxene, olivine and pyrrhotite indicate shock stage S6. Minor terrestrial weathering resulted in calcite-veining and minor oxidation of sulfides. The meteorite is interpreted as paired with SaU 005/008/051. The modal composition is similar to Dar al Gani 476/489/670/735/876, with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation. The restricted mineral composition, predominance of chromite among the oxides, and abundance of olivine indicate affinities to the lherzolitic shergottites.

Sayh al Uhaymir 094: A new martian meteorite from the Oman desert

— Sayhal Uhaymir (SaU) 094 is a 223.3 g, partially crusted, strongly to very strongly shocked melanocratic olivine-porphyric rock of the shergottite group showing a microgabbroic texture. The rock consists of pyroxene (52.0–58.2 vol%)—dominantly prismatic pigeonite (En60–68Fs20–27Wo7–9) associated with minor augite (En46–49Fs15–16Wo28–31)—brown (shock-oxidized) olivine (Fo65–69; 22.1–31%), completely isotropic interstitial plagioclase glass (maskelynite; An50–64Or0.3-0.9; 8.6–13.0%), chromite and titanian magnesian chromite (0.9-1.0%), traces of ilmenite (Ilm80–86), pyrrhotite (Fe92–100; 0.1-0.2%), merrillite (<<0.1%), and pockets (4.8-6.7%) consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals. The average maximum dimensions of minerals are: olivine (1.5 mm), pyroxene (0.3 mm) and maskelynite (0.3 mm). Primary melt inclusions in olivine and chromite are common and account for 0.1-0.6% of the rock. X-ray tomography revealed that the specimen contains ˜0.4 vol% of shock-melt associated vesicles, up to 3 mm in size, which show a preferred orientation. Fluidization of the maskelynite, melting and recrystallization of pyroxene, olivine and pyrrhotite indicate shock stage S6. Minor terrestrial weathering resulted in calcite-veining and minor oxidation of sulfides. The meteorite is interpreted as paired with SaU 005/008/051. The modal composition is similar to Dar al Gani 476/489/670/735/876, with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation. The restricted mineral composition, predominance of chromite among the oxides, and abundance of olivine indicate affinities to the lherzolitic shergottites.

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The recently discovered Jiddat al Harasis (JaH) 073 strewn field is the largest found so far in the Sultanate of Oman, covering an area of 19 6 km. The 3463 single stones collected range in weight from 52.2 kg down to <1 g (total weight 600.8 kg) and show a pronounced mass sorting. The strewn field shape can be approximated by a NW-SE-oriented ellipsoid, indicating an atmospheric entry from SE at a low angle relative to the surface. The meteorite belongs to the L6 ordinary chondrite group and shows S4 average shock grade. Smaller stones generally show a higher weathering grade resulting in a spread from W2 and W4. Enhanced weathering of the stones causing fragmentation after the fall is observed in sandy depressions. Five 14C measurements on stones of variable size and weathering grade yielded 14C from 3.8 to 49.9 dpm/kg. Three samples give a 14C/ 10Be age consistent with about 14.4 ka. For two samples the cosmogenic, trapped, and radiogenic noble gases were measured. The ratio of the 4He and 40Ar gas retention ages of 0.29 ±  0.10 and that of the 3He and 21Ne cosmic ray exposure ages of 0.36 ±  0.08 Ma indicate that JaH 073 experienced a complex exposure history and lost 4He and 40Ar due to a major collision. Fragmentation statistics indicate a single major atmospheric disruption and an originally relatively spherical shape of the object. Assuming the material collected represents the majority of fallen mass, and 90-99% of the original weight was lost by ablation, the pre-atmospheric minimum radius of the meteoroid with density 3.4 g cm^(-3) would have been at least 75 cm.

A. Al-Kathiri

Papers

Pinpointing the Source of a Lunar Meteorite: Implications for the Evolution of the Moon

Weathering of meteorites from Oman: Correlation of chemical and mineralogical weathering proxies with 14C terrestrial ages and the influence of soil chemistry

Sayh al Uhaymir 094: A new martian meteorite from the Oman desert

Sayh al Uhaymir 094: A new martian meteorite from the Oman desert

The Jiddat al Harasis 073 strewn field, Sultanate of Oman