Lunar Crust Research Articles

Potassium map from Chang'E-2 constraints the impact of Crisium and Orientale basin on the Moon.

KREEP materials were thought to be last crystallized at the lunar crust and mantle boundary. Impact cratering and volcanism are mainly responsible for their distributions on the lunar surface. Therefore, observation of global KREEP materials and investigation of distributions in the areas of large basins are of critical importance to understand the geologic history of the Moon. Here we report the new global potassium distribution on the Moon detected by Chang'E-2 Gamma-ray Spectrometer. We found that our new measurements are in general agreement with previous observation. A new finding and an important difference is that relatively higher K abundances in the Mare Crisium and Mare Orientale than their surrounding rims were detected for the first time. In light of our observations in these two areas, we propose that Crisium and Orientale basin-forming impact events may have penetrated to the lower crust and excavate the deeper materials to the lunar surface.

On the possibility of lunar core phase detection using new seismometers for soft-landers in future lunar missions

Information on the lunar central core; size, current state and composition; are key parameters to understand the origin and evolution of the Moon. Recent studies have indicated that possible seismic energies of core-reflected phases can be identified from past Apollo seismic data, and core sizes are determined, but we have still uncertainties to establish the lunar core parameters. We, therefore, plan to detect seismic phases that pass through the interior of the core and/or those reflected from the core–mantle boundary to ensure the parameters using new seismometers for future lunar soft-landing missions such as SELENE-2 and Farside Explorer projects.As the new seismometers, we can apply two types of sensors already developed; they are the Very Broad Band (VBB) seismometer and Short Period (SP) seismometer. We first demonstrate through waveform simulations that the new seismometers are able to record the lunar seismic events with S/N much better than Apollo seismometers. Then, expected detection numbers of core-phases on the entire lunar surface for the two types of seismometers are evaluated for two models of seismic moment distributions of deep moonquakes using the recent interior model (VPREMOON).The evaluation indicates that the VBB has performance to detect reflected S phases (ScS) from the core–mantle boundary mainly on the lunar near-side, and the P phases (PKP) passing through the interior of the core on some areas of the lunar far-side. Then, the SP can also detect PKP phases as first arrival seismic phase on limited regions on the lunar far-side. If appropriate positions of the seismic stations are selected, core-phases can be detected, allowing us to constrain the origin and evolution of the Moon with future lunar soft-landing missions.

Multiple waves and the lunar core

The model of the internal structure of the Moon with "open" deep faults and weak contacts between sides for many lunar regions explains the existence of a block structure (1-3). A high seismic Qfactor of lunar structures and significant impact energy (~10 19 erg) produced by the fall of a large meteorite led to formation of multiple waves on the major internal lunar boundaries. The known statistical methods have allowed us to determine the vibration periods for these waves and, hence, to define the internal lunar struc� ture, including the central zone (core). Let us consider the manifestation of a block struc� ture in different portions of an impact seismogram and the influence of the block structure on wave propaga� tion in more detail. We also discovered some seismic effects that had previously been observed on the Earth. The peculiarities of lunar geological structures and waveforms of seismograms became the prerequisites and evidence for the existence of a high modulation

The lunar moho and the internal structure of the Moon: A geophysical perspective

Extraterrestrial seismology saw its advent with the deployment of seismometers during the Apollo missions that were undertaken from July 1969 to December 1972. The Apollo lunar seismic data constitute a unique resource being the only seismic data set which can be used to infer the interior structure of a planetary body besides the Earth. On-going analysis and interpretation of the seismic data continues to provide constraints that help refine lunar origin and evolution. In addition to this, lateral variations in crustal thickness (~ 0–80 km) are being mapped out at increasing resolution from gravity and topography data that have and continue to be collected with a series of recent lunar orbiter missions. Many of these also carry onboard multi-spectral imaging equipment that is able to map out major-element concentration and surface mineralogy to high precision. These results coupled with improved laboratory-based petrological studies of lunar samples provide important constraints on models for lunar magma ocean evolution, which ultimately determines internal structure. Whereas existing constraints on initial depth of melting and differentiation from quantitative modeling suggested only partial Moon involvement (< 500 km depth), more recent models tend to favor a completely molten Moon, although the former cannot be ruled out sensu stricto. Recent geophysical analysis coupled with thermodynamical computations of phase equilibria and physical properties of mantle minerals suggest that the Earth and Moon are compositionally distinct. Continued analysis of ground-based laser ranging data and recent discovery of possible core reflected phases in the Apollo lunar seismic data strengthens the case for a small dense lunar core with a radius of < 400 km corresponding to 1–3% of lunar mass.

Geology and petrology of enormous volumes of impact melt on the Moon: A case study of the Orientale basin impact melt sea

Lunar basin-forming impacts produce enormous volumes (>105km3) of impact melt. All known basin-forming impacts combined may produce ∼108km3 of impact melt, ∼1/20th the volume of the lunar crust. Despite their volumetric importance, the geology and petrology of massive deposits of impact melt on the Moon have been little studied, in part because most basin impact melt deposits are old and have been obscured or buried by subsequent impact cratering and mare infill. We investigate the geology and model the petrology of fresh massive impact melt deposits in the relatively young 930km diameter Orientale basin. Models of impact melt production combined with geologic analyses based on new LOLA topographic data suggest that most of the impact melt (∼2/3) produced by the Orientale-forming impact occurs in a ∼15km thick impact melt sheet (better described as an impact melt sea) ∼350km in diameter with a volume of ∼106km3. We anticipate that the Orientale melt sea has undergone large-scale igneous differentiation, since terrestrial impact melt sheets (such as Manicouagan, Sudbury, and Morokweng) less than a tenth of the thickness and a hundredth of the volume of the Orientale melt sea have differentiated. We develop a model for the cumulate stratigraphy of the solidified Orientale impact melt sea. A modeled cumulate stratigraphy (occurring below a quench crust and anorthositic fallback breccia) with an ∼8km thick layer of norite overlying a ∼4km layer of pyroxenite and a basal ∼2km thick layer of dunite produced by equilibrium crystallization of a homogenized melt sea, consistent with vigorous convection in that melt sea, is supported by remotely-sensed norite excavated by the central peak of Maunder crater from ∼4km depth. Generally, we predict that very large basin-forming impacts, including the South Pole-Aitken (SPA) basin-forming impact, produce melt seas with a cumulate stratigraphy similar to that of the Orientale melt sea. Impact melt differentiation may explain apparently anomalous lithologies excavated in the SPA basin interior. We note that impact melt differentiates are slow-cooled (the Orientale melt sea took on the order of 105years to solidify) and, if meteoritic siderophiles fractionate into metal or sulfide layers, may not be siderophile-enriched; therefore, impact melt differentiates may pass for pristine highland plutonic rocks in the lunar sample suite. These predictions can be tested with current and future mission data.

The Evolution of the Lunar Highlands Crust: A Complicated History

More than 30 years after the first Lunar Highlands Crust conference, lunar petrology and remote sensing experts from around the world gathered to discuss and debate the formation and evolution of the lunar highlands crust. Huge strides in orbital remote sensing have enabled researchers to put the samples gathered during the Apollo missions into a larger, global context, yet many of the original, key questions remain. What was the extent and fate of the lunar magma ocean (LMO)? What is the nature of the lower lunar crust? Do lunar sample ages still suggest that the inner solar system was subject to an increase in impact flux around 3.9 billion years ago? At the heart of these questions is the desire to understand not only the formation and evolution of the Moon but of Earth, the terrestrial planets, and the inner solar system as a whole.

Experimental petrology of the Apollo 15 group A green glasses: Melting primordial lunar mantle and magma ocean cumulate assimilation

Phase equilibrium experiments have been performed to place constraints on the melting processes in the lunar interior using the Apollo 15 group A ultramafic green glass, the most abundant pyroclastic glass composition found in the polymict regolith breccia (15426) near Hadley Rille at station 7. Experiments on two compositional end members of the 15A green glass trend yield experiments saturated with olivine and orthopyroxene on the liquidus at 1520°C and 2.1GPa. Experiments were also performed on high-Al and -Ca analog compositions and on mixtures of the green glass compositions and natural minerals (olivine, orthopyroxene and garnet) in order to obtain experimental liquids multiply saturated with olivine, orthopyroxene, clinopyroxene and garnet. Three liquid compositions in equilibrium with a garnet-lherzolite residual phase assemblage were produced at 2.35GPa, 1440 and 1420°C and at 2.6GPa, 1460°C.Observed compositional variations in the 15A glasses rule out any simple partial melting process that would involve a olivine+orthopyroxene bearing source region. The results of these experiments are used to test petrogenetic models for the origin of the 15A green glasses. Synergistic evidence from major and trace element geochemistry, geophysical data on the lunar interior structure and results from models of lunar magma ocean differentiation is used to develop a model for the origin of the 15A ultramafic glasses. We propose that a mixing process involving a primary magma, derived by melting primordial lunar mantle, and remelted late-stage magma ocean cumulates led to the origin of the 15A glass. The model uses the 15A green glass multiple saturation point at 1520°C and 2.1GPa to constrain the depth of a cumulate remelting process. Decompression partial melting of undifferentiated lunar mantle, produced by adiabatic upwelling of the deep, hot interior of the moon, produced melts that infiltrated and selectively melted late stage magma ocean cumulates that had sunk to the base of a lunar magma ocean. The compositional variations observed in the 15A glasses are consistent with this model.

Sound velocity measurements in liquid Fe–S at high pressure: Implications for Earth's and lunar cores

The P-wave velocity (VP) of liquid Fe57S43 was measured up to 5.4GPa using an ultrasonic method combined with the synchrotron X-ray technique. The VP of liquid Fe57S43 showed little change with temperature, but increased almost linearly from 3105±11m/s to 3845±9m/s with increasing pressure from 2.4 to 5.4GPa. This can be approximated by VP [m/s]=2664+205.4×P, where P is the pressure in GPa. The VP of liquid Fe57S43 at 2.4–5.4GPa was significantly lower than that of pure liquid Fe. However, the pressure dependence of VP of the liquid Fe57S43 was markedly higher than that of pure liquid Fe. The marked difference in the pressure dependences of VP between pure liquid Fe and liquid Fe57S43 may cause VP crossover at around 7GPa. As a result, the VP of liquid Fe57S43 would become higher than that of pure liquid Fe at pressures higher than 7GPa. Thus, S decreases VP at low pressures such as those of the lunar outer core, but would increase it at the high pressures of the Earth's outer core. Assuming the lunar core consists of a liquid Fe–FeS outer core and a solid Fe inner core, the expected VP in the lunar outer core ranges from 3756 to 4230m/s.

New map of lunar gravity gives insight into Moon's early history

A new map of the Moon's gravity field, created by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, provides a detailed view of the Moon's many impact craters, evidence for ancient volcanic structures, and new estimates of the thickness of the lunar crust. Scientists announced the results from the first phase of the GRAIL mission at the AGU Fall Meeting; the results are also reported in three papers in the journal Science. GRAIL, launched in September 2011, consists of two satellites that send signals to each other. When a region of stronger gravity pulls on one satellite more than the other, the distance between the two changes. By measuring changes in the distance between the two satellites, researchers can create a map of the Moon's gravity field. The same method is used by the twin satellites of NASA's Gravity Recovery and Climate Experiment mission to measure Earth's gravity field.

Spectral and photogeologic mapping of Schrödinger Basin and implications for post-South Pole-Aitken impact deep subsurface stratigraphy

Schrödinger Basin provides a window into the stratigraphy of the lunar crust adjacent to the South Pole-Aitken Basin region that we have probed with Lunar Reconnaissance Orbiter, Moon Mineralogy Mapper (M3), and crater-scaling relationships. The composition of materials that make up the basin wall, impact melt, and peak ring provide a cross-section of the lunar crust, which reveals products of the lunar magma ocean, subsequent magmatism, and reworking of those components into a megaregolith. Large hectometer- to kilometer-size areas of anorthite-rich material (anorthosite), low-Ca pyroxene material (a noritic unit), and olivine-rich material (troctolite or dunite) are exposed, with a few areas of intermediate composition. The Schrödinger impact excavated ∼20km into an orthopyroxene+plagioclase (noritic) lunar crust, which is exposed in the basin walls, rim, and proximal ejecta, and dominates the composition of materials that make up the basin floor. Substantially later in lunar history, two spatially and chronologically isolated volcanic eruptions occurred on the basin floor. Two large craters east of Schrödinger excavated a compositionally gabbroic subsurface unit that was not tapped by Schrödinger. This indicates a compositional crustal facies change, which may be from SPA ejecta, but could reflect heterogeneity in the original lunar crust.

Late delivery of chondritic hydrogen into the lunar mantle: Insights from mare basalts

Recent analytical advances have enabled first successful in-situ detection of water (measured as OH) in lunar volcanic glasses, and, melt inclusions and minerals from mare basalts. These in-situ measurements in lunar materials, coupled with observations made by orbiting spacecraft missions have challenged the traditional view of the Moon as an anhydrous body. By synthesizing and modeling of previously published data on OH contents and H isotope compositions of apatite from mare basalts, we demonstrate that a model of hydrogen delivery into the lunar interior by late accretion of chondritic materials adequately accounts for the measured “water” content and its hydrogen isotopic composition in mare basalts. In our proposed model, “water” in the lunar interior was mostly constituted by hydrogen, delivered by the late accretion of chondrite-type materials. Our model is also consistent with previously proposed models to account for other geochemical characteristics of the lunar samples.

Native silicon and iron silicides in the Dhofar 280 lunar meteorite

The Dhofar 280 lunar highland meteorite is the first one in which native silicon was identified in association with iron silicides. This association is surrounded by silicate material enriched in Si, Na, K, and S and occurs within an impact-melt matrix. Compared to the meteorite matrix, the objects with native Si and the silicate material around them show high Al-normalized concentrations of volatile elements and/or elements with low sensitivity to oxygen but are not any significantly enriched in refractory lithophile elements. Some lithophile elements (V, U, Sm, Eu, and Yb) seem to be contained in reduced forms, and this predetermines REE proportions atypical of lunar rocks and a very low Th/U ratio. The admixture of siderophile elements (Ni, Co, Ge, and Sb) suggests that the Si-bearing objects were contaminated with meteorite material and were produced by the impact reworking of lunar rocks. The high concentrations of volatile elements suggest that the genesis of these objects could be related to the condensation of silicate vapor generated during meteorite impacts. The reduction of silicon and other elements could take place in an impact vapor cloud, with the subsequent condensation of these elements together with volatile components. On the other hand, condensates of silicate vapor could be reduced by impact reworking of impact breccias. Impact-induced vaporization and condensation seem not to play any significant role in forming the composition of the lunar crust, but the contents of the products of such processes can be locally relatively high. The greatest amounts of silicate vapor were generated during significant impact events. For example, more than 70% of the total mass of lunar material evaporated in the course of impact events should have resulted from the collision of the Moon with a cosmic body that produced the Moon’s largest South Pole-Aitken basin.

Constraint on the lunar core size from electromagnetic sounding based on magnetic field observations by an orbiting satellite

We have attempted to constrain the lunar core size from electrical conductivity sounding by using magnetic field data from the Kaguya (KG) and Lunar Prospector (LP) satellites. As suggested by previous studies, the signature of induction in the core can sometimes be detected by the satellites as an internal dipole field when the Moon enters from the magnetosheath to the tail lobe of the Earth’s magnetosphere. Since the magnetic anomaly field is up to about 2nT at the orbital altitude (∼100km), we removed the anomaly field from the observed magnetic field on the basis of the magnetic anomaly analysis. A spherical harmonic analysis tuned for the satellite observations was applied for separation of the internal and external fields, and the internal to external dipole ratio was used to estimate the size of the core. We estimated the effect of lunar mantle induction due to the external field fluctuation in the tail lobe region for two typical conductivity models of the lunar interior. Simulation results show that mantle induction fields seriously disturb the core signal. However, data selection made with reference to statistical distribution of the dipole ratio is effective to minimize the influence of the mantle conductivity on the core size estimates. The internal to external dipole ratio obtained for the selected period of KG observation was 0.0023±0.0019. The mean value corresponded to a lunar core size of 290km, and the upper bound of the lunar core size was estimated to be about 400km with a 95% confidence limit. On the other hand, it was not possible to select a time interval that was suitable for successful application of the method adopted in the present work to the LP dataset. Larger external magnetic field variation during the period of LP observation caused this, although the stronger magnetic field at the LP observation period was expected to be suitable for the core size estimation. The larger external field variation during the period of LP observation was a consequence of moderately active solar conditions in 1998 relative to the deep solar minimum conditions in 2008.

A sawtooth-like timeline for the first billion years of lunar bombardment

We revisit the early evolution of the Moon's bombardment. Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile elements in the lunar crust and mantle. We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, but also there was no prominent and narrow impact spike ∼3.9Gy ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile, with an uptick in the impact flux near ∼4.1Gy ago. The impact flux at the beginning of this weaker cataclysm was 5–10 times higher than the immediately preceding period. The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since ∼4.1Gy ago declined slowly and adhered relatively close to classic crater chronology models (Neukum and Ivanov, 1994). Overall we expect that the sawtooth event accounted for about one-fourth of the total bombardment suffered by the Moon since its formation. Consequently, considering that ∼12–14 basins formed during the sawtooth event, we expect that the net number of basins formed on the Moon was ∼45–50. From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains has an age of 4.38–4.42Gyr. Moreover, the largest lunar basin, South Pole Aitken, is older than 4.3Gy, and therefore was not produced during the lunar cataclysm.

Estimating transient crater size using the crustal annular bulge: Insights from numerical modeling of lunar basin‐scale impacts

The transient crater is an important impact cratering concept. Its volume and diameter can be used to predict impact energy and momentum, impact melt volume, and maximum depth and volume of ejected material. Transient crater sizes are often estimated using scaling laws based on final crater rim diameters. However, crater rim estimates, especially for lunar basins, can be controversial. Here, we use numerical modeling of lunar basin‐scale impacts to produce a new, alternative method for estimating transient crater radius using the annular bulge of crust observed beneath most lunar basins. Using target thermal conditions appropriate for the lunar Imbrian and Nectarian periods, we find this relationship to be dependent on lunar crust and upper mantle temperatures. This result is potentially important when analyzing lunar basin subsurface structures inferred from the GRAIL mission.

月隕石（MIL 090034， 090036， 090070）の全岩化学組成に基づく月地殻の形成過程

月隕石（MIL 090034， 090036， 090070）の全岩化学組成に基づく月地殻の形成過程

Geology, geochemistry, and geophysics of the Moon: Status of current understanding

The Moon is key to understanding both Earth and our Solar System in terms of planetary processes and has been a witness of the Solar System history for more than 4.5Ga. Building on earlier telescopic observations, our knowledge about the Moon was transformed by the wealth of information provided by Apollo and other space missions. These demonstrated the value of the Moon for understanding the fundamental processes that drive planetary formation and evolution. The Moon was understood as an inert body with its geology mainly restricted to impact and volcanism with associated tectonics, and a relative simple composition. Unlike Earth, an absence of plate tectonics has preserved a well-defined accretion and geological evolution record. However recent missions to the Moon show that this traditional view of the lunar surface is certainly an over simplification. For example, although it has long been suspected that ice might be preserved in cold traps at the lunar poles, recent results also indicate the formation and retention of OH− and H2O outside of polar regions. These volatiles are likely to be formed as a result of hydration processes operating at the lunar surface including the production of H2O and OH by solar wind protons interacting with oxygen-rich rock surfaces produced during micrometeorite impact on lunar soil particles. Moreover, on the basis of Lunar Prospector gamma-ray data, the lunar crust and underlying mantle has been found to be divided into distinct terranes that possess unique geochemical, geophysical, and geological characteristics. The concentration of heat producing elements on the nearside hemisphere of the Moon in the Procellarum KREEP Terrane has apparently led to the nearside being more volcanically active than the farside. Recent dating of basalts has shown that lunar volcanism was active for almost 3Ga, starting at about 3.9–4.0Ga and ceasing at ∼1.2Ga. A recent re-processing of the seismic data supports the presence of a partially molten layer at the base of the mantle and shows not only the presence of a 330km liquid core, but also a small solid inner core. Today, the Moon does not have a dynamo-generated magnetic field like that of the Earth. However, remnant magnetization of the lunar crust and the paleomagnetic record of some lunar samples suggest that magnetization was acquired, possibly from an intrinsic magnetic field caused by an early lunar core dynamo. In summary, the Moon is a complex differentiated planetary object and much remains to be explored and discovered, especially regarding the origin of the Moon, the history of the Earth–Moon system, and processes that have operated in the inner Solar System over the last 4.5Ga. Returning to the Moon is therefore the critical next stepping-stone to further exploration and understanding of our planetary neighborhood.

Can tidal tomography be used to unravel the long‐wavelength structure of the lunar interior?

The Moon displays a number of hemispherically asymmetric features that may be related to long‐wavelength structure and dynamics in the lunar mantle. Here we propose to use observations of the non‐degree‐2 gravitational response of the Moon to degree‐2 tidal forcing to constrain the long‐wavelength lunar mantle structure. For a planetary body with laterally varying structure, degree‐2 tidal forces excite gravitational response at non‐degree‐2 harmonics due to mode coupling effects. Using a new numerical model, we determine that for a lunar mantle with ∼5% hemispherical variations in seismic shear wave velocityVs, the degree‐3 response could reach ∼2% of the degree‐2 response. The larger the hemispherical variations inVs, the larger the degree‐3 response. We suggest that if observations from recent lunar missions such as SELENA and GRAIL could be used to determine the non‐degree‐2 tidal response, it might be possible to place constraints on the lunar mantle structure.

Crystals stirred up: 2. Numerical insights into the formation of the earliest crust on the Moon

This is the second paper in a two‐part series examining the fluid dynamics of crystal settling and flotation in the lunar magma ocean. In the first paper, we develop a direct numerical method for resolving the hydrodynamic interactions between crystals and their feedback on the flow field in magmatic liquid. In this paper, we use this computational technique to test the leading model for the formation of the earliest crust on the Moon. The anorthositic lithology of the lunar crust is thought to have been formed by the flotation of buoyant plagioclase crystals at a time when the lunar mantle was still wholly or largely molten. This model is appealing from an observational point of view, but its fluid dynamical validity is not obvious, because (1) plagioclase probably started crystallizing very late (i.e., when the magma ocean was already 80% solidified) and (2) a significant portion of the shallow lunar crust consists of almost pure plagioclase (&gt;90 vol. %), requiring very efficient plagioclase segregation. The goal of this study is to better understand the fluid dynamical conditions that hinder or facilitate crystal settling or flotation. Our approach complements earlier studies by explicitly linking the petrological and fluid dynamical evolution and by focusing on the effect of increasing crystal fraction. We find that crystal settling was probably possible throughout the entire solidification history of the lunar magma ocean as long as crystal sizes were sufficiently large (r &gt; 1 mm) and crystal fraction sufficiently low (ϕ &lt; 13%).

Magnetization of the lunar crust

Magnetic fields measured by the satellite Lunar Prospector show large scale features resulting from remanently magnetized crust. Vector data synthesized at satellite altitude from a spherical harmonic model of the lunar crustal field, and the radial component of the magnetometer data, have been used to produce spatially continuous global magnetization models for the lunar crust. The magnetization is expressed in terms of localized basis functions, with a magnetization solution selected having the smallest root‐mean square magnetization for a given fit to the data, controlled by a damping parameter. Suites of magnetization models for layers with thicknesses between 10 and 50 km are able to reproduce much of the input data, with global misfits of less than 0.5 nT (within the uncertainties of the data), and some surface field estimates. The magnetization distributions show robust magnitudes for a range of model thicknesses and damping parameters, however the magnetization direction is unconstrained. These global models suggest that magnetized sources of the lunar crust can be represented by a 30 km thick magnetized layer. Average magnetization values in magnetized regions are 30–40 mA/m, similar to the measured magnetizations of the Apollo samples and significantly weaker than crustal magnetizations for Mars and the Earth. These are the first global magnetization models for the Moon, providing lower bounds on the magnitude of lunar crustal magnetization in the absence of multiple sample returns, and can be used to predict the crustal contribution to the lunar magnetic field at a particular location.