Lunar Crust Research Articles

A long-lived magma ocean on a young Moon.

A giant impact onto Earth led to the formation of the Moon, resulted in a lunar magma ocean (LMO), and initiated the last event of core segregation on Earth. However, the timing and temporal link of these events remain uncertain. Here, we demonstrate that the low thermal conductivity of the lunar crust combined with heat extraction by partial melting of deep cumulates undergoing convection results in an LMO solidification time scale of 150 to 200 million years. Combining this result with a crystallization model of the LMO and with the ages and isotopic compositions of lunar samples indicates that the Moon formed 4.425 ± 0.025 billion years ago. This age is in remarkable agreement with the U-Pb age of Earth, demonstrating that the U-Pb age dates the final segregation of Earth's core.

Variations of porosity in intermediate-sized lunar impact basins

The Moon is cratered with hundreds of well-preserved impact basins, and shared bombardment history with Earth makes lunar basins a good proxy for the processes affecting early Earth. Basin-forming impact events alter the bulk density and porosity of the targeted crust, and measurement of these quantities offers a means by which we can investigate the mechanisms of basin formation. In this work, we calculate the bulk density of the crust in the vicinity of 50 basins with diameters of 170–360 km—a size range that has been neglected by previous studies—with the goal of understanding the deformational mechanisms present during impact events. Our bulk density estimates are derived from gravity data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission and topography data from the Lunar Orbiter Laser Altimeter (LOLA) using a new technique similar to the Nettleton method in terrestrial gravimetry. By making assumptions about crustal compositions across the Moon, we can furthermore estimate porosity in and around the basins. Low porosity is observed in the interior of 76% of the basins studied, and this trend becomes more apparent when basins in the Feldspathic Highlands Terrane are observed alone. Overall, impact events appear to decrease porosity inside basins and increase porosity outside basins, suggesting the prevalence of impact melting within the basins and fracturing and ejecta deposition in the exteriors. Our results are seemingly inconsistent with previous studies that measured high porosity values inside basins from residual Bouguer anomalies. However, these disparate results may be reconciled if impact melting processes are confined to the uppermost kilometers of the lunar crust and are underlain by heavy impact-induced fracturing.

Determination of Global Density Inhomogeneities and Stresses Inside the Moon

The possible distribution of density and stress anomalies in the interior of the Moon is determined. For this purpose, we have developed a new solution of the incorrect inverse gravimetry problem developed by the authors, tested for the Earth and Mars. The idea of this solution is that the isostatic compensation depths are uniquely determined for the harmonics of the expansion of the relief heights in spherical functions based only on cosmic data on the gravitational field and the relief of the planet. We have shown that possible isostatic relief compensation is carried out in the depth range of 0–530 km. The most probable compensation depths in this range were determined and the distribution of compensation masses at these depths was found. For the remaining harmonics, two other compensation options were chosen: (1) compensation is carried out at two levels, the first of which is the mean depth of the crust of 2.7 km, corresponding to a layer of 0–10 km; the possible depths for the second level were determined from an analysis of the results obtained for isostatically compensated harmonics; (2) uncompensated harmonics of the relief lead to vertical stresses in the Moon’s lithosphere reaching the compressive strength (16 MPa) and tensile strength (15 MPa).

Electrical Resistivity Measurements of Fe‐Si With Implications for the Early Lunar Dynamo

AbstractThe effect of impurities on the electrical resistivity of Fe at core conditions is controversial due to the challenges of measuring transport properties of Fe alloys under high‐pressure and high‐temperature conditions. In this study we describe an innovative technique to make wires of Fe‐Si samples initially in powder form for measuring electrical resistivity. The electrical resistivity of Fe‐2, Fe‐8.5, and Fe‐17 wt%Si was measured at 3, 4, and 5 GPa at temperatures into the liquid state and compared to results of Fe‐4.5 wt%Si and Fe from prior studies. Isothermal electrical resistivity increases linearly with increasing Si content. Yet the effect of Si content on the magnitude of the electrical resistivity compared to that of Fe diminished as temperature increased at all experimental pressures. This implies the contribution to the electrical resistivity due to Si impurities is dependent on temperature, in disagreement with Matthiessen's rule. Thermal conductivity of Fe‐Si alloys calculated using the Wiedemann‐Franz law indicates a nonnegligible influence of the Si content on the thermal conductive properties of Fe‐Si alloys. The results are used to calculate the adiabatic heat flux of an Fe‐Si lunar core and date the end of the high magnetic field intensity era of the lunar dynamo to be in the range 3.32–3.80 Ga.

Experimental investigation of the effect of nickel on the electrical resistivity of Fe-Ni and Fe-Ni-S alloys under pressure

AbstractElectrical resistivity experiments were conducted on three alloys in the iron-rich side of the Fe-Ni(-S) system (Fe-5 wt% Ni, Fe-10 wt% Ni, Fe-10 wt% Ni-5 wt% S) at 4.5 and 8 GPa and up to 1900 K using the multi-anvil apparatus and the 4-electrode technique. For all samples, increasing temperature increases resistivity. At a specified temperature, Fe-Ni(-S) alloys are more resistive than Fe by a factor of about 3. Fe-Ni alloys containing 5 and 10 wt% Ni present comparable electrical resistivity values. The resistivity of Fe-Ni(-S) alloys is comparable to the one of Fe = 5 wt% S at 4.5 GPa and is about three times higher than the resistivity of Fe = 5 wt% S at 8 GPa, due to a different pressure dependence of electrical resistivity between Fe-Ni and Fe-S alloys. Based on these electrical results and experimentally determined thermal conductivity values from the literature, lower and upper bounds of thermal conductivity were calculated. For all Ni-bearing alloys, thermal conductivity estimates range between ~12 and 20 W/(m⋅K) over the considered pressure and temperature ranges. Adiabatic heat fluxes were computed for both Ganymede's core and the Lunar core, and heat flux values suggest a significant dependence to both core composition and the adiabatic temperature. Comparison with previous thermochemical models of the cores of Ganymede and the Moon suggests that some studies may have overestimated the thermal conductivity and hence, the heat flux along the adiabat in these planetary cores.

Evidence for diverse lunar melt compositions and mixing of the pre-3.9 Ga crust from zircon chemistry

Lunar samples collected during Apollo missions are typically impact-related breccias or regolith that contain amalgamations of rocks and minerals with various origins (e.g., products of igneous differentiation, mantle melting, and/or impact events). The largest intact pre-Nectarian (∼≥3.92 Ga) fragments of igneous rock contained within the breccia and regolith rarely exceed 1 cm in size, and they often show evidence for impact recrystallization. This widespread mixing of disparate materials makes unraveling the magmatic history of pre-Nectarian period fraught with challenges. To address this issue, we combine U-Pb geochronology of Apollo 14 zircons (207Pb–206Pb ages from 3.93 to 4.36 Ga) with zircon trace element chemistry and thermodynamic models. Zircon crystallization temperatures are calculated with Ti-in-zircon thermometry after presenting new titania and silica activity models for lunar melts. We also present rare earth element (REE), P, actinide, and Mg + Fe + Al concentrations. While REE patterns and P yield little information about the parent melt origins of these out-of-context grains, U and Th concentrations are highly variable among pre-4.2 Ga zircons when compared to younger grains. Thus, the distribution of heat-producing radioactive elements in melt sources pervading the early lunar crust was heterogenous. Melt composition variation is confirmed by zircon Al concentrations and thermodynamic modeling that reveal at least two dominant magma signatures in the pre-4.0 Ga zircon population. One inferred magma type has a high alumina activity. This magma likely assimilated anorthosite-rich rocks (e.g., Feldspathic Highlands Terrane; FHT), though impact-generated melts of an alumina-rich target rock is a viable alternative. The other magma signature bears more similarities to KREEP basalts from the Procellarum KREEP Terrane (PKT), reflecting lower apparent alumina activities. Melt diversity seems to disappear after 4.0 Ga, with zircon recording magma compositions that largely fall in-between the two main groups found for pre-4.0 Ga samples. We interpret <4 Ga zircons to have formed from a mixture of KREEP basalts and FHT-like rocks, consistent with the upper ∼15 km of the crust being thoroughly mixed and re-melted by basin-forming impacts during the pre-Nectarian period.

A Past Lunar Dynamo Thermally Driven by the Precession of Its Inner Core

AbstractThe Cassini state equilibrium associated with the precession of the Moon predicts that the mantle, fluid core, and solid inner core precess at different angles. We present estimates of the dissipation from viscous friction associated with the differential precession at the core‐mantle boundary (CMB), Qcmb, and at the inner core boundary (ICB), Qicb, as a function of the evolving lunar orbit. We focus on the latter and show that, provided the inner core was larger than 100 km, Qicb may have been as high as 1010–1011 W for most of the lunar history for a broad range of core density models. This is larger than the power required to maintain the fluid core in an adiabatic state; therefore, the heat released by the differential precession at the ICB can drive a past lunar dynamo by thermal convection. This dynamo can outlive the dynamo from precession at the CMB and may have shut off only relatively recently. Estimates of the magnetic field strength at the lunar surface are of the order of a few μT, compatible with the lunar paleomagnetic intensities recorded after 3 Ga. We further show that it is possible that a transition of the Cassini state associated with the inner core may have occurred as a result of the evolution of the lunar orbit. The heat flux associated with Qicb can be of the order of a few mW m−2, which should slow down inner core growth and be included in thermal evolution models of the lunar core.

Bulk composition of regolith fines on lunar crater floors: Initial investigation by LRO/Mini-RF

Identifying polarimetric radar signatures of ice in smooth regolith fines on the floors of permanently shadowed lunar craters is hindered by uncertainties in their dielectric properties. We address this deficiency through polarimetric radar analysis of surface backscatter to derive the dielectric constant (εr′) of smooth, rock-free regolith fines covering brecciated crater floors observed by Mini-RF, which offer ideal locations for unambiguous retrieval of surface εr′ from linear polarimetric scattering models and CPR analysis for volatile identification. Specifically, we select fines covering crater fills in north polar and equatorial regions to constrain the range of variability of εr′ as a function of latitude and crater diameter, where we hypothesize that the latter is indicative of the excavation depth of these fines. Our observations suggest that there is measurable variability in the dielectric properties of fines on lunar crater floors as a function of crater size and potentially with impact excavation depth, suggesting that small craters <5-km in diameter have εr′ ranging from 2.3-to-3, and large ones >5-km have higher values of εr′ that range from 3-to-3.8. We find that the most plausible explanation for the observed variability of εr′ of regolith fines on crater floors is mineralogical differences, suggesting an increase in metal abundance in the original excavated substrate with depth, i.e., in the uppermost kilometer of the lunar crust. Finally, we suggest that regolith fines on the floors of permanently shadowed craters <5 km in diameter are optimal targets for the unambiguous detection of water-ice enrichment using S-band radar observations.

Evidence of extensive lunar crust formation in impact melt sheets 4,330 Myr ago

Accurately constraining the formation and evolution of the lunar magnesian suite is key to understanding the earliest periods of magmatic crustal building that followed accretion and primordial differentiation of the Moon. However, the origin and evolution of these unique rocks is highly debated. Here, we report on the microstructural characterization of a large (~250-μm) baddeleyite (monoclinic-ZrO2) grain in Apollo troctolite 76535 that preserves quantifiable crystallographic relationships indicative of reversion from a precursor cubic-ZrO2 phase. This observation places important constraints on the formation temperature of the grain (>2,300 °C), which endogenic processes alone fail to reconcile. We conclude that the troctolite crystallized directly from a large, differentiated impact melt sheet 4,328 ± 8 Myr ago. These results suggest that impact bombardment would have played a critical role in the evolution of the earliest planetary crusts. A zirconium-based crystal (baddeleyite) found embedded in a sample brought to Earth by Apollo 17 provides evidence of large-scale impact bombardment of the Moon about 4.33 Gyr ago, when the baddeleyite grain was formed. This result points to the importance of impacts in the early evolution of planetary crusts.

Geochemistry and Petrogenesis of Northwest Africa 10401: A New Type of the Mg‐Suite Rocks

AbstractThe petrogenetic models of the lunar crust are built on the returned Apollo and Luna samples collected from limited parts of the lunar nearside that are chemically unusual (i.e., material rich in K, Rare Earth Elements, and P [KREEP]) and not representative of the entire lunar lithologic suite. The lunar Mg‐suite is part of this sample collection and ubiquitously has geochemical characteristics indicating the involvement of KREEP in their petrogenesis and seemed to be linked to the Procellarum KREEP Terrain (PKT). However, it is unclear if KREEP is necessary for Mg‐suite magmatism or whether Mg‐suite magmatism was a global event that occurred without significant KREEP contribution, and thus, Mg‐suite rocks outside of the PKT region may exist without containing a significant KREEP signature. Here, we investigate lunar meteorite Northwest Africa (NWA) 10401, an anorthositic troctolitic breccia with a granulitic texture. NWA 10401 shares many characteristics of Apollo Mg‐suite rocks: both its bulk rock composition and alumina content, as well as its mineralogy and mineral chemistry, are more consistent with typical Apollo Mg‐suite rocks, rather than ferroan anorthosites. In addition, olivine‐spinel equilibria calculations indicate that NWA 10401 is consistent with being derived from a common parent to the Apollo Mg‐suite troctolites. However, despite these many shared characteristics, NWA 10401 is strongly depleted in REE, starkly separating it from the typical Apollo Mg‐suite of the PKT. This indicates that NWA 10401 (and pairs) could represent a Mg‐suite component outside the PKT, and thus, KREEP‐poor Mg‐suite magmatism may have been a global phenomenon on the Moon.

Rheology of a KREEP analog magma: Experimental results applied to dike ascent through the lunar crust

Among the Apollo returned samples were basaltic rocks belonging to the KREEP suite, which are thought to have been erupted ~4.1–3.9 ​Ga via dike propagation from the residual magmatic layer residing below the lunar crust as the last part of the solidifying lunar magma ocean. The emplacement dynamics of KREEP magma reaching the lunar surface are generally poorly understood, in part because no rheological data are available to model the conditions of KREEP ascent in detail, even ~50 years after KREEP basalt discovery. This study provides the first rheological data for KREEP magmas, as a function of temperature and crystal content, from the liquidus temperature (~1205 ​°C) to ~16% crystals by volume at ~1177 ​°C. The lava’s rheological behavior becomes non-Newtonian at just a few degrees of cooling below the liquidus and is well represented by a shear-thinning power-law equation. We applied these data to a buoyancy-driven dike ascent model, suggesting that if KREEP magma ascended through dikes comparable in size to those that fed the lunar mare, it most likely erupted below its liquidus in the range of 1180 ​°C to 1160 ​°C, with effective viscosities in the range 103–107 ​Pas and traversing the lunar crust on timescales of minutes to hours.

Potassium isotopic composition of the Moon

The Moon is depleted in water and other volatiles compared to Earth and the bulk solar composition. Such depletion of volatile elements and the stable isotope fractionations of these elements can be used to better understand the origin and early differentiation history of the Moon. In this study, we focus on the moderately volatile element, potassium, and we report the K elemental abundances and isotopic compositions (δ41K relative to NIST SRM 3141a) for nineteen Apollo lunar rocks and lunar meteorites (twenty-two subsamples), spanning all major geochemical and petrologic types of lunar materials. The K isotopic compositions of low-Ti and high-Ti basalts are indistinguishable, providing a lunar basalt average δ41K of –0.07 ± 0.09‰ (2SD), which we also consider to be the best estimate of the lunar mantle and the bulk silicate Moon. The significant enrichment of K in its heavier isotopes in the bulk silicate Moon, compared with the bulk silicate Earth (δ41K = –0.48 ± 0.03‰), is consistent with previous analyses of K isotopes and other moderately volatile elements (e.g., Cl, Cu, Zn, Ga, and Rb). We also report analyses of K isotopes for lunar nonmare samples, which show large variations of K isotopic ratios compared to lunar basalts. We interpret this large K isotopic fractionation as the result of late-stage magma ocean degassing during urKREEP formation, which is also coupled with Cl isotope fractionation. Degassing of urKREEP likely triggered redistribution of K isotopes in the Moon, enriching the urKREEP reservoir in heavy K isotopes while implanting the light K isotopic signatures onto the lunar surface. This scenario suggests a heterogeneous distribution of K isotopes in the Moon as a consequence of its magmatic evolution.

Unsupervised machine learning with petrological database ApolloBasaltDB reveals complexity in lunar basalt major element oxide and mineral distribution patterns

Diversity of lunar basalt characteristics is partly a consequence of lunar mantle heterogeneity. Although the cumulate mantle overturn hypothesis is the current standard model invoked to explain mantle asymmetries of unknown length scale in both compositional and geometrical space, successful petrological modeling of this mixing event requires a specific set of parameters not currently agreed upon. In contrast, surface basalt patterns may yield clues to both localized and nearside lunar interior structure.Using two multidimensional data analysis approaches – principal component analysis (PCA) and K-means cluster analysis (KCA) – we report the patterns produced from basalt characteristics over changing spatial scales, from intra-site to inter-site to nearside. The data are sourced from a newly developed, self-contained database of lunar basalt characteristics (ApolloBasaltDB), which includes major element oxides, mineral modes, ages, and textures for petrological and statistical modeling. Through the simultaneous considerations of multiple basalt characteristics contained in the database, we find that terrestrial-based basalt classifications cannot adequately describe the complex and overlapping distribution patterns of major element oxides and mineral modes that define multiple distinct basalt groupings over multidimensional space. These patterns provide opportunities for alternative lunar basalt classification schemes. Our analyses suggest that Al2O3 volumetric content is more diverse inside the Procellarum KREEP Terrane rift boundary versus content for older Apollo samples in close proximity to the eastern arm of the same rift boundary. Northernmost basalt samples show increased pyroxene diversity. Easternmost sites suggest anti-correlations in modal ilmenite and plagioclase, based on major element oxide PCA biplots, while nearside analyses of either major element oxides or mineral modes similarly suggest plagioclase (and Al2O3) diversity comes at the expense of ilmenite (and TiO2) diversity. There is evidence to suggest that approximate mineral content can be extracted from major element oxide data based on correlative patterns between major element oxide PCA biplots and mineral mode PCA biplots. These patterns have implications for remote sensing missions in that onboard data manipulation may provide lithologic basalt vectors of interest.

Crustal Porosity of Lunar Impact Basins

AbstractLateral variations in bulk density and porosity of the upper lunar highland crust are mapped using a high‐resolution Gravity Recovery and Interior Laboratory (GRAIL) gravity field model and Lunar Reconnaissance Orbiter (LRO) derived topography. With a higher spatial resolution gravity model than previous studies, we focus on individual impact basins with diameters greater than 200 km. The bulk density of the upper few kilometers of the lunar crust is estimated by minimizing the correlation between the topography and Bouguer gravity at short wavelengths that are unaffected by lithospheric flexure. Porosity is then derived using estimates of the grain density obtained from remote sensing data of the surface composition. The near surface crust in proximity to many large basins is found to exhibit distinct radial porosity signatures. Low porosities are found in the basin centers within the peak ring, whereas high porosities are identified near and just exterior to the main rim. The larger basins exhibit a more pronounced porosity signature than the smaller basins. Though the number of basins investigated in this study is limited, younger basins appear to be associated with the largest amplitude variations in porosity. For basins with increasing age the magnitude of the porosity variations decreases.

The timing of lunar solidification and mantle overturn recorded in ferroan anorthosite 62237

Ferroan anorthosite suite (FAS) rocks are widely interpreted to represent primordial lunar crust. Despite their importance in pinpointing the timing of lunar crust formation, robust chronological investigations for this rock type are scarce. Here, we report the Ar-Ar, Rb-Sr, and Sm-Nd isotopic systematics for the FAS troctolitic anorthosite 62237. The Ar-Ar isotopic system has been reset by a thermal event at 3710 ± 48 Ma, and the Rb-Sr isotopic systematics has been disturbed such that a Rb-Sr isochron age cannot be determined. However, an internal isochron for the Sm-Nd isotopic system has yielded an age of 4350 ± 73 Ma (MSWD = 2.0) with an initial ϵ143NdCHUR of −0.53 ± 0.26. The mineral and whole-rock fractions of 62237 plot on the same internal isochron as FAS sample 60025. The combined datasets define an age of 4372 ± 35 Ma (MSWD = 4.0) with an initial ϵ143NdCHUR of −0.17 ± 0.22. Literature Sm-Nd data for FAS and Mg-suite whole-rocks also plot on the 60025-62237 isochron. The coherence of data from both FAS and Mg-suite rocks examined thus far suggests that both rock suites formed contemporaneously from identical, or nearly identical, sources. In addition, the concordance of FAS and Mg-suite ages suggests that primordial crust solidification either involved both magmatic suites, or that Mg-suite magmatism was contemporaneous with FAS magmatism within resolution of the Sm-Nd chronometer. The ages for FAS and Mg-suite also coincide with the formation ages of the mare basalt source regions and urKREEP. Ferroan anorthosite suite rocks and urKREEP are thought to represent primordial LMO solidification products, whereas Mg-suite and the mare basalt source regions are argued to represent mixtures of various LMO crystallization products that were formed during density-driven overturn of the LMO. The concordance of ages implies that the 4372 ± 35 Ma Sm-Nd isochron records the age of mantle overturn, and that overturn occurred during, or shortly after, solidification of the LMO.

A possible high-temperature origin of the Moon and its geochemical consequences

The formation of the Moon is thought to be the result of a giant impact between a Mercury-to-proto-Earth-sized body and the proto-Earth. However, the initial thermal state of the Moon following its accretion is not well constrained by geochemical data. Here, we provide geochemical evidence that supports a high-temperature origin of the Moon by performing high-temperature (1973–2873 K) metal-silicate partitioning experiments, simulating core formation in the newly-formed Moon. Results indicate that the observed lunar mantle depletions of Ni and Co record extreme temperatures (>2600–3700 K depending on assumptions about the composition of the lunar core) during lunar core formation. This temperature range is within range of the modeled silicate evaporation buffer in a synestia-type environment. Our results provide independent geochemical support for a giant-impact origin of the Moon and show that lunar thermal models should start with a fully molten Moon. Our results also provide quantitative constraints on the effects of high-temperature lunar differentiation on the lunar mantle geochemistry of volatile, and potentially siderophile elements Cu, Zn, Ga, Ge, Se, Sn, Cd, In, Te and Pb. At the extreme temperatures recorded by Ni and Co, many of these elements behave insufficiently siderophile to explain their depletions by core formation only, consistent with the inferred volatility-related loss of Cr, Cu, Zn, Ga and Sn during the Moon-forming event and/or subsequent magma-ocean degassing.

Mineralogical survey of the anorthositic Feldspathic Highlands Terrane crust using Moon Mineralogy Mapper data

Spectroscopic data from the Moon Mineralogy Mapper (M3) instrument are used to study the mineralogy of the central peak or peak ring of 75 craters located in the lunar anorthositic Feldspathic Highlands Terrane (FHT-a), as defined by Jolliff et al. (2000). The thickness of South-Pole Aitken (SPA) ejecta at the location of the selected craters is estimated. Crustal thickness models are used with empirical cratering equations to estimate the depth of origin of the material excavated in the studied central peaks, and its distance to the crust-mantle interface. The goal of this survey is to study the composition of the FHT-a crust, and the extent of its potential lateral and vertical heterogeneities. High-Calcium Pyroxene (HCP) and featureless spectra are mostly detected throughout the entire FHT-a, whereas the number of pure plagioclase detections is small. No relationship between the central peak composition and the distance to SPA or the depth within the SPA ejecta is observed. The SPA ejecta material cannot be spectrally distinguished from crustal material. We interpret the paucity of plagioclase spectra in the FHT-a, which contrasts with more frequent plagioclase detections in the central peaks of craters sampling the crust in younger lunar terranes using identical spectroscopic techniques Martinot et al. (2018b), as a possible effect of terrane maturation, or of mixing with mafic components that mask their signature in the visible near-infrared. Overall, the FHT-a appears homogeneous laterally. However, data hint at a pyroxene compositional change with increasing depth, from high-calcium content in the upper crust towards less calcic compositions with increasing depth, which is consistent with prior studies of the architecture of the lunar crust.

Distinct oxygen isotope compositions of the Earth and Moon

The virtually identical oxygen isotope compositions of the Earth and Moon revealed by Apollo return samples have been a challenging constraint for lunar formation models. For a giant impact scenario to explain this observation, either the precursors to the Earth and Moon had identical oxygen isotope values or extensive homogenization of the two bodies occurred following the impact event. Here we present high-precision oxygen isotope analyses of a range of lunar lithologies and show that the Earth and Moon in fact have distinctly different oxygen isotope compositions. Oxygen isotope values of lunar samples correlate with lithology, and we propose that the differences can be explained by mixing between isotopically light vapour, generated by the impact, and the outermost portion of the early lunar magma ocean. Our data suggest that samples derived from the deep lunar mantle, which are isotopically heavy compared to Earth, have isotopic compositions that are most representative of the proto-lunar impactor ‘Theia’. Our findings imply that the distinct oxygen isotope compositions of Theia and Earth were not completely homogenized by the Moon-forming impact, thus providing quantitative evidence that Theia could have formed farther from the Sun than did Earth. High-precision measurements suggest that the Earth and Moon have distinct oxygen isotope compositions. This implies distinct oxygen isotopic compositions of the proto-Earth and its impactor that were not fully homogenized by the Moon-forming impact.

Laboratory Simulation of the Interaction of the Solar Wind with Lunar Magnetic Anomalies

The results of a laboratory experiment on the simulation of the reflection of protons over regions of the local magnetization of the lunar crust have been reported. Quadrupole and dipole sources have been used to simulate a local magnetic field. The magnetic-field-induced reflection of protons near lunar magnetic anomalies reaches 50% of the incident flow. A significant difference between the quadrupole and dipole configurations has been detected both in the structure of the mini-magnetosphere and in the reflection of protons. The data obtained are in qualitative agreement with in situ measurements by spacecrafts over lunar magnetic anomalies.

Volatiles in lunar felsite clasts: Impact-related delivery of hydrous material to an ancient dry lunar crust

In this detailed geochemical, petrological, and microstructural study of felsite clast materials contained in Apollo breccia samples 12013, 14321, and 15405, little evidence was found for relatively enriched reservoirs of endogenic lunar volatiles. NanoSIMS measurements have revealed very low volatile abundances (≤2–18 ppm hydrogen) in nominally anhydrous minerals (NAMS) plagioclase, potassic alkali feldspar, and SiO2 that make up a majority of these felsic lithologies. Yet these mineral assemblages and clast geochemistries on Earth would normally yield relatively high volatiles contents in their NAMS (∼20 to ≥80 ppm hydrogen). This difference is particularly notable in felsite 14321,1062 that exhibits extremely low volatile abundances (≤2 ppm hydrogen) and a relatively low amount of microstructural evidence for shock metamorphism given that it is a clast of the most evolved (∼74 wt.% SiO2) rock-type returned from the Moon. If taken at face value, ‘wet’ felsic magmas (∼1.2–1.7 wt.% water) are implied by the relatively high hydrogen contents of feldspar in felsite clasts in Apollo samples 12013 and 15405, but these results are likely misleading. These felsic clasts have microstructural features indicative of significantly higher shock stress than 14321,1062. These crustal lithologies likely obtained no more water from the lunar interior than the magma body producing 14321,1062. Rather, we suggest hydrogen was enriched in samples 12013 and 15405 by impact induced exchange, and/or partial assimilation of volatiles added to the surface of the Moon by a hydrated impactor (asteroid or comet) or the solar wind. Thus, the best estimate for magmatic water contents of felsic lunar magmas comes from 14321,1062 that leads to a calculated magmatic water content of ≤0.2 wt.%. This dry felsic magma has a slightly greater, but comparable water content to the ancient mafic magmas implied by the other lithologies that we have studied. Based on this and expanding evidence for a significantly dry ancient or early degassed Moon it is likely that some recent estimates (100's ppm) of the water abundances in the lunar parental magma ocean have been overestimated.