Mare Volcanism Research Articles

Geology of the Chang'e-5 landing site: Constraints on the sources of samples returned from a young nearside mare

Extraterrestrial materials collected by sample return exploration missions have advanced our understanding of planetary formation and evolution in a profound manner. This is achieved through sophisticated laboratory sample analyses with very high precision and accuracy that are difficult to obtained by orbital or surface landing/roving explorations. Moreover, the well-known geographical location and geological context of returned samples enable the sample measurements to be fully interpreted and mapped into the entire planetary context (in contrast to meteorites where the geological context is missing). Planetary scientists have been endeavoring to collect and returned samples from extraterrestrial bodies, despite the significant costs involved.In December 2020, China's Chang'e-5 mission successfully landed in and returned lunar samples from the northeastern Oceanus Procellarum in the western lunar nearside maria. The mare basalt at the landing site has been dated to be ~1–2 Ga through crater population studies, much younger than any basalts in the present lunar sample collections and among the youngest maria on the global Moon (e.g., Hiesinger et al., 2011). The Chang'e-5 sample will provide an unprecedented opportunity to characterize the late-stage lunar volcanism.To fully explore the scientific potential of the Chang'e-5 returned samples and assist the on-going sample analyses and interpretations, we present an in-depth investigation of the geology of the specific Chang'e-5 landing site and the nearby vicinity using high-resolution remote sensing data sets. We characterize the detailed geological context, topography, morphology, morphometry, chemical and mineralogical composition of the local landing area. These geological characterizations lead us to identify a sequence of four geological events, including mare volcanism, tectonism, impact cratering and ejecta deposition, that have potentially affected and been recorded in the returned samples. Compositional and geochronological measurements of the Chang'e-5 samples would reconstruct a detailed regional geological evolution model of the landing area. Furthermore, most of the inferred ages of these geological events lie in the age gap of the Apollo/Luna samples. Precise determination of these ages through radiogenic isotope analyses would be invaluable in narrowing down the uncertainties of the lunar cratering chronology function and improve our understanding of the geological history of the entire Moon.

Ina Lunar Irregular Mare Patch Mission Concepts: Distinguishing between Ancient and Modern Volcanism Models

Abstract The Ina irregular mare patch, an ∼2 × 3 km summit depression on an ancient ∼22 km diameter shield volcano, displays two very enigmatic units: (1) dozens of dark convex-upward mounds and (2) a very rough, optically immature floor unit with very sharp morphologic contacts between the two. Controversy surrounds the age interpretation of Ina; superposed impact crater size–frequency distributions (CSFDs) suggest an age of ∼33 Ma, consistent with the presence of sharp contacts between the units and indicating that mare volcanism continues to today. Models of the terminal stages of volcano summit pit crater activity suggest an age coincident with the building of the shield, ∼3.5 Ga; these models interpret the CSFD age and sharp contacts to be due to an extremely porous lava lake floor and extrusion and solidification of magmatic foams. We present robotic–human exploration mission concepts designed to resolve this critical issue for lunar thermal evolution.

Studying the Global Spatial Randomness of Impact Craters on Mercury, Venus, and the Moon With Geodesic Neighborhood Relationships

AbstractImpact crater records on planetary surfaces are often analyzed for their spatial randomness. Generalized approaches such as the mean second closest neighbor distance (M2CND) and standard deviation of adjacent area (SDAA) are available via a software tool but do not take the influence of the planetary curvature into account in the current implementation. As a result, the measurements are affected by map distortion effects and can lead to wrong interpretations. This is particularly critical for investigations of global data sets as the level of distortion typically increases with increasing distance from the map projection center. Therefore, we present geodesic solutions to the M2CND and SDAA statistics that can be implemented in future software tools. We apply the improved methods to conduct spatial randomness analyses on global crater data sets on Mercury, Venus, and the Moon and compare the results to known crater population variations and surface evolution scenarios. On Mercury, we find that the emplacement of smooth plain deposits strongly contributed to a global clustering of craters and that a random distribution of Mercury's basins is not rejected. On Venus, the randomness analyses show that craters are largely randomly distributed across all sizes but where local nonrandom distributions due to lower crater densities in regions of recent volcanic activity may appear. On the Moon, the global clustering of craters is more pronounced than on Mercury due to mare volcanism and the Orientale impact event. Furthermore, a random distribution of lunar basins is not rejected.

Long-lived volcanism expressed through mare infilling, domes and IMPs in the Arago region of the Moon

Mare Tranquillitatis corresponds to the deposit of successive Early to Late Imbrian basaltic units filling the Tranquillitatis basin on the Moon. The present study focuses on the western half of the mare, in the vicinity of the Arago crater (6.16°N, 21.42°E). High resolution datasets from recent remote sensing missions were used to reconstruct the geologic history of the area, which includes a variety of geological features such as: 8 extrusive domes, numerous wrinkle ridges, a sinuous rille and 37 Irregular Mare Patches (IMPs). We performed crater counting to date the domes emplacement and estimated the domes lava rheologic properties (plastic viscosities, lava effusion rates, and durations of effusion) using their geometric characteristics. As a result we classify the Arago domes into three groups: E1-type domes (Arago 1 and 8), H1-type domes (Arago 4 to 7), and B-type domes (Arago 2 and 3) respectively emplaced ​~ ​3.7 ​Ga, ~3.4 ​Ga, and ~2.8 ​Ga ago. IMPs are observed in the younger mare unit and on the top of the Arago 6 dome; they likely correspond to a late stage of waning mare volcanism in the area. Both IMPs and domes have a composition similar to the surrounding mare in the VNIR spectral domain, consistent with mafic materials. The exceptionally long-lived volcanism and its diversity recorded in the Arago region may be related to both a Th-rich anomaly reported nearby and to the large-scale magma center responsible for the Lamont positive Bouguer anomaly. In addition, volcanic features of the Arago region are superimposed on a Ti-rich mare unit visited by the Apollo 11 crew 175 ​km to the south at Tranquility Base. The geological and historical richness of this region makes it a compelling site for future science and/or In Situ Resources Utilization (ISRU) driven missions to the Moon.

Estimation of the elastic thickness over ancient Mare Moscoviense

The Moscoviense basin is an atypical lunar impact basin with concentric rings of positive and negative gravity anomalies. This basin can provide insights into the inhomogeneous thermal activities across the farside of the Moon. Based on an updated spherical harmonic thin elastic-shell loading model, we used localized admittance analyses to estimate the elastic thickness as well as other associated selenophysical parameters for the Moscoviense basin. The high precision gravity and topography data employed in our estimation were collected by the Gravity Recovery and Interior Laboratory and the Lunar Orbiter Laser Altimeter missions. Our results indicate that the crust-mantle interface is mainly compensated by the prefilling depth rather than the observed surface topography. The results constrained within two standard deviations yielded a small load ratio (∼0.168), a best-fit crustal thickness of 36.2 km, and an optimized crustal density of 3159.5 kg m−3. Such large density approaches the density of olivine-rich mantle materials, implying that the excavation of the Mare Moscoviense occurred during a basin-forming impact. The inversed elastic thickness at Mare Moscoviense was around 18 km, lower than the previous results (∼60 km) found over Mare basins on the lunar nearside. These results indicate that extreme thermal activity existed during the Moscoviense basin-forming period such as reheating mechanisms from a double-impact process and mare volcanism.

Mineral chemistry and in situ U[sbnd]Pb geochronology of the mare basalt Northwest Africa 10597: Implications for low-Ti mare volcanism around 3.0 Ga

Unbrecciated mare basalts are rare in the lunar meteorite collection. Found in 2015, Northwest Africa (NWA) 10597 is a medium-grained low-Ti mare basalt with a subophitic texture. The meteorite consists mostly of mm-sized pyroxene and plagioclase, with minor olivine, spinel, ilmenite, phosphates, silica, and trace Zr-rich minerals, such as baddeleyite, zirconolite and tranquillityite. A portion of plagioclase and silica has been transformed to their high-pressure polymorphs due to shock metamorphism. NWA 10597 has a low TiO2 content (2.9 wt%) but is relatively enriched in rare earth elements (REE) (Laaverage = 65 × CI) with an overall unfractionated pattern except for a negative Eu anomaly. Calculated REE concentrations of parent melts in equilibrium with Mg-rich pyroxene and Ca-rich plagioclase suggest no significant assimilation of REE-rich melts after the onset of pyroxene crystallization. In situ UPb analyses of baddeleyite and apatite reveal a mutually consistent age of ~3.0 Ga, which is also in excellent agreement with that of low-Ti mare basalts NWA 4734 and LaPaz Icefield (LAP) 02205 dated with other independent techniques. The concordance suggests no significant thermal disturbance in the UPb isotopic system of NWA 10597 although it was heavily shocked. NWA 10597 closely resembles NWA 4734 in terms of petrographic texture, mineral chemistry and geochronology, indicating a pairing relationship.

Gravity constraints on the age and formation of the Moon's Reiner Gamma magnetic anomaly

The Moon once possessed a dynamo that produced numerous magnetic anomalies in the lunar lithosphere. Determining the ages and magnetization strengths of these anomalies would constrain the timing and strength of the dynamo. Unfortunately, most anomalies are not associated with dated geologic features. Here we report that the northeastern portion of the Reiner Gamma magnetic anomaly overlies a relative-negative Bouguer gravity anomaly that we interpret to be a buried impact crater (consistent with Evans et al., 2016). Reiner Gamma is associated with sinuous optical markings known as swirls, which are likely correlated with the underlying magnetization. These markings provide a higher resolution map of the magnetization than possible with existing field data. We present two arguments that suggest the crater overprinted by the Reiner Gamma swirl formed in a pre-existing flood basalt layer. Hence, we constrain the Reiner Gamma source body's age to be bounded by the approximate duration of mare volcanism, ~3.3 Ga to ~3.9 Ga. This range includes the putative high-field epoch of the Moon's dynamo (~3.56–3.9 Ga), which could explain Reiner Gamma's high magnetization. Finally, we calculate the potential range of positions of the rim of the buried crater, and find that the main body of Reiner Gamma is likely outside the buried rim. This finding may be useful for constraining the origin of the Reiner Gamma magnetic source bodies.

MTE Features of Apollo Basin and Its Significance in Understanding the SPA Basin

Apollo basin is located within the large South Pole–Aitken (SPA) basin. The study on Apollo basin will provide interesting information about the basic geologic issues about the lunar farside. In this paper, the normalized brightness $(T_{B})$ temperature $(nT_{B})$ maps and the $T_{B}$ difference $(dT_{B})$ maps are generated with the Chang'E-2 microwave sounder data to study the microwave thermal emission features of Apollo basin. The results are as follows. First, the mare volcanism in Apollo basin is re-understood according to the $nT_{B}$ performances at noon, and they should be originated from the southern part of the Apollo basin and strongly altered by the later impact ejecta. Second, the $nT_{B}$ maps indicate that there exists a special material from Dryden crater to Chaffee crater, whose thickness is more than 31 cm but less than 76.9 cm. Third, the similar $dT_{B}$ performances at 3.0 GHz indicate the homogeneous regolith thermophysical parameters of Apollo basin in the lateral direction. Fourth, the $dT_{B}$ maps and the discovered cold $T_{B}$ anomaly indicate the homogeneity of the SPA basin at least in the microwave thermophysical parameters. Our study also shows that the scientific study about the lunar surface is not sufficient only by visible data.

Geological characterization of the Chang'e-4 landing area on the lunar farside

On January 3, 2019, China's Chang'e-4 probe, accomplishes the first-ever soft landing on Moon's farside in the Von Kármán crater floor mare volcanism plain (177.59°E, 45.46°S) within the South Pole-Aitken (SPA) basin. Its surface in-situ measurements would provide the first “ground truth” for lunar farside volcanism and SPA materials. We present a detailed geologic context, topographical, morphological, quantitative geochemical and mineralogical characterization of a 20 × 20 km area centered at the Chang'e-4 landing site, using the latest multi-source high-resolution orbital data sets. The landing area is very flat at a baseline of 30 m, with 90% of surface having slope <5°, and deficient of surface boulder exposures. The landing area mare is superposed by abundant sub-kilometer, block-poor circular impact craters and secondary craters in chains and clusters. Mare basalts at the landing site is dominated by low-titanium basalts, and appear to be less mafic and iron-depleted than all typical kinds of retuned lunar basalt samples and may represent a new kind of mare basalt. The landing site mare is heavily sculptured by a set of linear parallel ridged and furrowed textures formed by ballistic ejecta from the Finsen crater, which are characterized by a more feldspathic composition, while with elevated orthopyroxene abundance, and may represent compositional properties of the primitive SPA basin floor materials. We propose the Finsen ejecta and mare basalt outcrops at the landing area as prioritized targets for Chang'e-4 surface geological and spectroscopic explorations. Follow-on mission data interpretations should consider the mixing effect of local mare basalts and distant feldspathic materials.

Retrieving volume FeO and TiO2 abundances of lunar regolith with CE-2 CELMS data using BPNN method**A contributed paper of the International Symposium on Lunar and Planetary Science (ISLPS) on 2018 June 12–15 at Macau University of Science and Technology.

The volume FeO and TiO2 abundances (FTAs) of lunar regolith can be more important for understanding the geological evolution of the Moon compared to the optical and gamma-ray results. In this paper, the volume FTAs are retrieved with microwave sounder (CELMS) data from the Chang’E-2 satellite using the back propagation neural network (BPNN) method. Firstly, a three-layered BPNN network with five-dimensional input is constructed by taking nonlinearity into account. Then, the brightness temperature (TB) and surface slope are set as the inputs and the volume FTAs are set as the outputs of the BPNN network. Thereafter, the BPNN network is trained with the corresponding parameters collected from Apollo, Luna, and Surveyor missions. Finally, the volume FTAs are retrieved with the trained BPNN network using the four-channel TB derived from the CELMS data and the surface slope estimated from Lunar Orbiter Laser Altimeter (LOLA) data. The rationality of the retrieved FTAs is verified by comparing with the Clementine UV-VIS results and Lunar Prospector (LP) GRS results. The retrieved volume FTAs enable us to re-evaluate the geological features of the lunar surface. Several important results are as follows. Firstly, very-low-Ti (<1.5 wt.%) basalts are the most spatially abundant, and the surfaces with TiO2 > 5 wt.% constitute less than 10% of the maria. Also, two linear relationships occur between the FeO abundance (FA) and the TiO2 abundance before and after the threshold, 16 wt.% for FA. Secondly, a new perspective on mare volcanism is derived with the volume FTAs in several important mare basins, although this conclusion should be verified with more sources of data. Thirdly, FTAs in the lunar regolith change with depth to the uppermost surface, and the change is complex over the lunar surface. Finally, the distribution of volume FTAs hints that the highlands crust is probably homogeneous, at least in terms of the microwave thermophysical parameters.

Diversity of basaltic lunar volcanism associated with buried impact structures: Implications for intrusive and extrusive events

Relatively denser basalt infilling and the upward displacement of the crust–mantle interface are thought to be contributing factors for the quasi-circular mass anomalies for buried impact craters in the lunar maria. Imagery and gravity observations from the Lunar Reconnaissance Orbiter (LRO) and dual Gravity Recovery and Interior Laboratory (GRAIL) missions have identified 10 partially or fully buried impact structures where diversity of observable basaltic mare volcanism exists. With a detailed investigation of the characteristics of associated volcanic landforms, we describe their spatial distribution relationship with respect to the subsurface tectonic structure of complex impact craters and propose possible models for the igneous processes which may take advantage of crater-related zones of weakness and enable magmas to reach the surface. We conclude that the lunar crust, having been fractured and reworked extensively by cratering, facilitates substance and energy exchange between different lunar systems, an effect modulated by tectonic activities both at global and regional scales. In addition, we propose that the intrusion-caused contribution to gravity anomalies should be considered in future studies, although this is commonly obscured by other physical factors such as mantle uplift and basalt load.

New Insights Into Lithology Distribution Across the Moon

AbstractLithology distribution across the Moon is pivotal for understanding lunar evolution. However, so far, the distribution of lunar rock suites is still uncertain; as a result, many related core issues on lunar evolution have long been in dispute. This work reports on a new lithology distribution map across the Moon and discusses some critical issues of the lunar evolutionary process. The oxide abundances derived from Chang'E‐1 Interference Imaging Spectrometer imagery and the Th contents inferred by Lunar Prospector Gamma Ray and Neutron Spectrometer data are employed to generate the lithological map by using the decision tree C5.0 algorithm. The following conclusions are inferred from this new map. (1) Magnesian suite is widely distributed across the Feldspathic Highlands Terrane and in the periphery of the South Pole‐Aitken Terrane. Thus, the viewpoints have been validated that the early magnesian magmatism may be a global phenomenon and that KREEP basalt is not necessary for the petrogenesis of Mg‐rich rocks. Moreover, the regions of Dryden, Chaffee S, Theophilus, and Moscoviense are confirmed as magnesian suite exposures. (2) The observation that the Feldspathic Highlands Terrane has a higher Mg/(Mg + Fe) value than the maria is related to the flood of the Mg suite across the Feldspathic Highlands Terrane. (3) The specific exposed locations of the alkali suite across the Moon have long been unsolved, and this work discloses that the alkali suite prevails in the outskirts of the Procellarum KREEP Terrane, the center of the South Pole‐Aitken Terrane, and some isolated locales. (4) Focusing on distinguishing mare basalts from other mafic rocks, seven geochemical indices are proposed to determine the potential exposures of mare basalts across the Moon. (5) In Mare Insularum, a series of KREEP volcanisms may have occurred and lasted longer than the mare volcanism.

Lunar mare TiO2 abundances estimated from UV/Vis reflectance

The visible (400–700 nm) and near-infrared (700–2800 nm) reflectance of the lunar regolith is dominantly controlled by variations in the abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g., ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced rims on soil grains, and Fe-metal). The same materials control reflectance into the near-UV (250–400 nm) with varying degrees of importance. A key difference is that while ilmenite is spectrally neutral in the visible to near-infrared, it exhibits a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC) filters were specifically designed to take advantage of this spectral feature to enable more accurate mapping of ilmenite within mare soils than previously possible. Using the reflectance measured at 321 and 415 nm during 62 months of repeated near-global WAC observations, first we found a linear correlation between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of each sample return site. We then used the coefficients from the linear regression and the near-global WAC multispectral mosaic to derive a new TiO2 map. The average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 values were found in Mare Tranquillitatis (∼12.6 wt%) and Oceanus Procellarum (∼11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the low-TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium) exhibit very low TiO2 values (<2 wt%). We find that the Clementine visible to near-infrared based TiO2 maps (Lucey et al., 2000) have systematically higher values relative to the WAC estimates. The Lunar Prospector Gamma-Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map, although there are local offsets possibly due to the different depth sensitivities and large pixel scale of the GRS relative to the WAC. We find a wide variation of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga), whereas only medium to high TiO2 values (average = 6.8 wt%, minimum = 4.5 wt%) are found for younger mare units (<2.6 Ga).

Magma source transition of lunar mare volcanism at 2.3 Ga

AbstractMare basalts provide insights into the composition and thermal history of the lunar mantle. The ages of mare basalts suggest a first peak of magma activity at 3.2–3.8 Ga and a second peak at ~2 Ga. In this study, we reassess the correlation between the titanium contents and the eruption ages of mare basalt units using the compositional and chronological data updated by SELENE (Kaguya). Using morphological and geological criteria, we calculated the titanium content of 261 mare units across a representative area of each mare unit. In the Procellarum KREEP Terrane, where the latest eruptions are located, an increase in the mean titanium content is observed during the Eratosthenian period, as reported by previous studies. We found that the increase in the mean titanium content occurred within a relatively short period near approximately 2.3 Ga, suggesting that the magma source of the mare basalts changed at this particular age. Moreover, the high‐titanium basaltic eruptions are correlated with a second peak in volcanic activity near ~2 Ga. The high‐titanium basaltic eruptions occurring during the last volcanic activity period can be explained by the three possible scenarios (1) the ilmenite‐bearing cumulate rich layer in the core‐mantle boundary formed after the mantle overturn, (2) the basaltic material layers beneath the lunar crust formed through upwelling magmas, and (3) ilmenite‐bearing cumulate blocks remained in the upper mantle after the mantle overturn.

Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 2: Predicted emplacement processes and observations)

Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 2: Predicted emplacement processes and observations)

Late stage Imbrium volcanism on the Moon: Evidence for two source regions and implications for the thermal history of Mare Imbrium

Abstract Large open fissures or volcanic rifts can form in volcanic terrain and they are also conduits for magma ascending through the lunar crust. On the Moon, we investigated two volcanic source regions within Mare Imbrium by tracking surface morphologic features and compositional information. The Euler source region is situated at the southwest edge of the basin, while the Lambert source region lies off the south margin of the Imbrium mascon. Survey of dike surface manifestations in Euler source site suggest that dikes are the possible source of the local upper basaltic flows and the last lava phases with well developed scarps near the Euler crater, which extend northeast to the basin center. The Euler dike swarm are radial to the basin and reveal possible dike-to-conduit transition mechanism. They reveal radial subsurface fractures which may be tensional cracks preceding to the emplacement of the last stage of the mare fill. Of these, the largest dike has a more than 100 km length. The spatial arrangement of tectonic and volcanic features in Lambert source site is directly or indirectly controlled by the regional compression and extension stresses associated with flexure in response to mascon and basalt loading. In addition, compositional variation trends show a general southwest-to-northeast flooding direction of the exposed high-Ti basalts. This will have important implications for both the Imbrium basin's mare volcanism and for the thermal evolution of Mare Imbrium and the Moon.

Mini-RF and LROC observations of mare crater layering relationships

The lunar maria cover approximately 17% of the Moon's surface.Discerning discrete subsurface layers in the mare provides some constraints on thickness and volume estimates of mare volcanism. Multiple types of data and measurement techniques allow probing the subsurface and provide insights into these layers, including detailed examination of impact craters, mare pits and sinuous rilles, and radar sounders. Unfortunately, radar sounding includes many uncertainties about the material properties of the lunar surface that may influence estimates of layer depth and thickness. Because they distribute material from depth onto the surface, detailed examination of impact ejecta blankets provides a reliable way to examine deeper material using orbital instruments such as cameras, spectrometers, or imaging radars. Here, we utilize Miniature Radio Frequency (Mini-RF) data to investigate the scattering characteristics of ejecta blankets of young lunar craters. We use Circular Polarization Ratio (CPR) information from twenty-two young, fresh lunar craters to examine how the scattering behavior changes as a function of radius from the crater rim. Observations across a range of crater size and relative ages exhibit significant diversity within mare regions. Five of the examined craters exhibit profiles with no shelf of constant CPR near the crater rim. Comparing these CPR profiles with LROC imagery shows that the magnitude of the CPR may be an indication of crater degradation state; this may manifest differently at radar compared to optical wavelengths. Comparisons of radar and optical data also suggest relationships between subsurface stratigraphy and structure in the mare and the block size of the material found within the ejecta blanket. Of the examined craters, twelve have shelves of approximately constant CPR as well as discrete layers outcropping in the subsurface, and nine fall along a trend line when comparing shelf-width with thickness of subsurface layers. These observations suggest that surface CPR measurements may be used to identify near-surface layering. Here, we use ejected material to probe the subsurface, allowing observations of near-surface stratigraphy that may be otherwise hidden by layers higher from remote observations.

Lunar true polar wander inferred from polar hydrogen

The earliest dynamic and thermal history of the Moon is not well understood. The hydrogen content of deposits near the lunar poles may yield insight into this history, because these deposits (which are probably composed of water ice) survive only if they remain in permanent shadow. If the orientation of the Moon has changed, then the locations of the shadowed regions will also have changed. The polar hydrogen deposits have been mapped by orbiting neutron spectrometers, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury's poles. Here we show that polar hydrogen preserves evidence that the spin axis of the Moon has shifted: the hydrogen deposits are antipodal and displaced equally from each pole along opposite longitudes. From the direction and magnitude of the inferred reorientation, and from analysis of the moments of inertia of the Moon, we hypothesize that this change in the spin axis, known as true polar wander, was caused by a low-density thermal anomaly beneath the Procellarum region. Radiogenic heating within this region resulted in the bulk of lunar mare volcanism and altered the density structure of the Moon, changing its moments of inertia. This resulted in true polar wander consistent with the observed remnant polar hydrogen. This thermal anomaly still exists and, in part, controls the current orientation of the Moon. The Procellarum region was most geologically active early in lunar history, which implies that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. Our hypothesis provides an explanation for the antipodal distribution of lunar polar hydrogen, and connects polar volatiles to the geologic and geophysical evolution of the Moon and the bombardment history of the early Solar System.

Annealing of radiation damage in zircons from Apollo 14 impact breccia 14311: Implications for the thermal history of the breccia

AbstractImpact breccia 14311, was collected from the Apollo 14 landing site as a potential sample of the underlying Fra Mauro Formation. Published zircon U‐Pb ages of &gt;4000 Ma date the source material of the breccia and the apatite U‐Pb age of ~3940 Ma is interpreted as dating thermal resetting of the apatite U‐Pb systems. In this contribution we present new age information on the late stage thermal history of the breccia based on the annealing of radiation damage in the zircons. From Raman spectroscopic determination of the radiation damage within SIMS analytical spots on the zircons and the U and Th concentrations determined on these spots, we demonstrate that the radiation damage in the zircons has been annealed and we estimate the age of annealing at 3410 ± 80 Ma. This age is interpreted as a cooling age following heating of the breccia to above the annealing temperature of ~230 °C for stage 1 radiation damage in zircon, but below the temperature needed to reset the U‐Pb system of apatite (~500 °C). It is proposed that this thermal event was associated with the prolonged period of Mare volcanism, from 3150 to 3750 Ma, that generated massive basalt flows in the vicinity of the sample location.

Lunar cryptomaria: Physical characteristics, distribution, and implications for ancient volcanism

Cryptomaria, lunar volcanic deposits obscured by crater and basin impact ejecta, can provide important information about the thermal and volcanic history of the Moon. The timing of cryptomare deposition has implications for the duration and flux of mare basalt volcanism. In addition, knowing the distribution of cryptomaria can provide information about mantle convection and lunar magma ocean solidification. Here we use multiple datasets (e.g., M3, LOLA, LROC, Diviner) to undertake a global analysis to identify the general characteristics (e.g., topography, surface roughness, rock abundance, albedo, etc.) of lunar light plains in order to better distinguish between ancient volcanic deposits (cryptomaria) and impact basin and crater ejecta deposits. We find 20 discrete regions of cryptomaria, covering approximately 2% of the Moon, which increase the total area covered by mare volcanism to 18% of the lunar surface. Comparisons of light plains deposits indicate that the two deposit types (volcanic and impact-produced) are best distinguished by mineralogic data. On the basis of cryptomaria locations, the distribution of mare volcanism does not appear to have changed in the time prior to its exposed mare basalt distribution. There are several hypotheses explaining the distribution of mare basalts, which include the influence of crustal thickness, mantle convection patterns, asymmetric distribution of source regions, KREEP distribution, and the influence of a proposed Procellarum impact basin. The paucity of farside mare basalts means that multiple factors, such as crustal thickness variations and mantle convection, are likely to play a role in mare basalt emplacement.