Interior Laboratory Research Articles

Lunar Low‐Titanium Magmatism During Ancient Expansion Inferred From Ejecta Originating From Linear Gravity Anomalies

AbstractLinear gravity anomalies (LGAs) on the Moon have been interpreted as ancient magmatic intrusions formed during the lunar expansion. The composition of such ancient subsurface intrusions may offer hints for the lunar thermodynamic state in the initial stage of lunar history. To pose a first compositional constraint on magmatism related to lunar expansion, this study analyzed the spectrum and gravity around craters on LGAs, such as Rowland, Roche, and Edison craters. Using reflectance spectra around the craters, we first surveyed non‐mare basaltic exposures. To test the LGA excavation scenario as a possible origin of the discovered exposures, we then compared the Gravity Recovery and Interior Laboratory data and post‐cratering gravity simulation with the iSALE shock physics code. Our spectral analysis reveals no basaltic exposure around the Rowland crater. Further, the observed termination of LGA at the crater rim contradicts the gravity simulation, which assumes that LGA predates the Rowland crater. These results suggest that LGA formation might postdate the Rowland formation and that lunar expansion lasted even after the Nectarian age. On the other hand, we found that both Roche and Edison craters possess basaltic exposures in their peripheries. Because the gravity reduction inside Roche crater can be reproduced in our simulation, the discovered basaltic exposures are possibly LGA materials ejected from these craters. The composition of those exposures shows that the LGA intrusions at the two locations are composed of low‐titanium magma, indicating that ancient magma during the expansion did not contain ilmenite‐rich melt, perhaps resulting from the low‐ilmenite content of the ancient upper mantle.

A Low‐Viscosity Lower Lunar Mantle Implied by Measured Monthly and Yearly Tides

AbstractThe Moon's frequency‐dependent tidal response, expressed as temporal variations in its gravity field through the Love number and as dissipation through the quality factor , provides information about its interior structure. Lunar laser ranging has provided measurements for , but so far no frequency‐dependent values for have been determined. We provide the first spacecraft measurements of and at two frequencies, monthly and yearly, from an analysis of Gravity Recovery and Interior Laboratory and Lunar Reconnaissance Orbiter radio tracking data. Interior modeling indicates that these values can be matched only with a low‐viscosity zone at the base of the lunar mantle, even when using complex rheological laws to model the mantle's response. The existence of this zone has profound implications for the Moon's thermal state and evolution.

Boundaries identification of geological structure in lunar Oceanus Procellarum region using full gravity gradient tensor methodology

Boundary recognition visually delineates the horizontal extent of subsurface anomalies, providing a foundational basis for interpreting gravity data. Compared to gravity observations, gravity gradient data offers the benefits of multiple components and the enhancement of shortwave information in graphical representations. In our study, we investigated the delineation of geological structure boundaries in the lunar Oceanus Procellarum region using gravity gradient techniques. First, based on the observations from the Gravity Recovery and Interior Laboratory (GRAIL) mission, we obtained high-precision synthetic full tensor values of the gravity gradient as our primary research data. Moreover, to reduce curvature errors during large-scale terrain forward modeling, we applied the spherical prism of tesseroid to eliminate the effects of near-surface topography. Grounded on the Bouguer anomalies of gravity gradient, four distinct boundary recognition methods have been employed to explore the geological structure of the lunar Oceanus Procellarum region. It includes the Theta map method, the directional Theta map method, the combination of total horizontal derivative and the modulus of full tensor gravity gradient, and the improved edge detection method based on the Theta map method.The common feature of these methods is the utilization of the multi-component data combination from gravity gradient tensors, which can enhance the accuracy of edge detection. From the investigation results presented in the paper, we have found the following: 1) The improved edge detection method, based on the Theta map principle, enables better identification of the center of subsurface anomaly bodies during boundary recognition, utilizing a consistent single color in graphic displays. 2) According to the results of boundary recognition, numerous strip anomalies exist in the Oceanus Procellarum area that show less correlation with topographic relief. One possible explanation for these graphical anomalies of gravity gradient is that they serve as channels for the upwelling of mantle material during lunar evolution.

Global Distribution and Volume of Cryptomare and Visible Mare on the Moon From Gravity and Dark Halo Craters

AbstractThe present‐day distribution of mare basalts on the Moon is an important constraint on the timing, duration, and flux of volcanism on the Moon. In this work, we find the global distribution of visible mares and cryptomares using the effective density (ρeff) of the Moon, which is sensitive to the vertical density distribution of the crust. We compute ρeff using the Gravity Recovery and Interior Laboratory (GRAIL) data and the Lunar Orbiter Laser Altimeter (LOLA) topography data. We apply this ρeff approach to the search of cryptomare for the first time, and we use a higher resolution grid and larger search area to constrain visible mare thicknesses compared to previous work. We use a Bayesian approach to find the distribution of mare thicknesses and density gradients of the underlying crust that is consistent with the localized ρeff. Assuming a bulk density of the mare basalts of 3,460 kg/m3, we find a mean visible mare thickness of km which is consistent with previous work. We find a candidate cryptomare volume of km3 representing more than half of the total mare volume on Moon. This volume is consistent with the higher end of the previously proposed values. Finally, we search for dark halo craters (DHC) globally to obtain geologic evidence of cryptomares and find 258 craters. Our results are consistent with extensive ancient volcanism, which explains the previously identified discrepancy between the early high volcanic flux predicted by thermal models of the Moon and the dearth of ancient volcanic deposits currently exposed.

Detection of Subsurface Density Structures of the Aristarchus Plateau by Gravity Inversion

AbstractThe Aristarchus plateau, located at the center of Oceanus Procellarum, exhibits one of the most complex volcanic features on the Moon. To understand the subsurface three‐dimensional density distribution under the Aristarchus plateau, we performed gravity inversion using high‐resolution gravity data obtained from the Gravity Recovery and Interior Laboratory mission. Our inversion results indicate the presence of a strong lateral density differentiation, with some positive and negative density anomalies possibly exhibiting a correlation with the volcanic features observed on the surface. A linear high‐density anomaly near the Cobra Head magma source and an elliptical high‐density anomaly both exactly match the surface basaltic exposures observable in the remote sensing data. We executed the density separation to extract low‐density anomalies of the whole plateau and removed most of the density artifacts. The low‐density anomalies display elevated terrain evidence of multiple “semi‐ring” structures, suggesting the location of buried remnants of crater rims. The Aristarchus crater has a central low‐density anomaly in the exact size and shape of the later‐formed impact crater. This anomaly is consistent with the high crater porosity produced by extensive impact‐generated fracturing and dilatant bulking, though the observed gravity and density anomalies are greater in magnitude than expected for this process. The remote sensing data expose high‐silica material in the crater rim, implying that the young crater excavated an underlying layer containing both plagioclase and Si‐rich materials, and indicating that the local uplift of feldspathic and/or silicic materials also contributes to the high amplitude of the low‐density anomaly in the Aristarchus crater.

The Missing Craters and Basin Rings Beneath the Lunar Maria

AbstractEvidence for a population of craters buried beneath the nearside lunar maria has been found in the gravity data returned from the Gravity Recovery and Interior Laboratory mission. Although the total population of buried and visible craters within maria is comparable to the crater population in non‐mare regions at large diameters, a deficit was observed for craters less than ∼90 km in diameter. This deficit is surprising because the data can resolve craters down to 10 km in diameter. Similarly, the Imbrium basin only has a partially exposed ring system, with individual ring widths of up to ∼100 km, but where those rings should be buried beneath the mare surface, we find the gravitational signature mostly non‐existent. In this study, we test a series of mechanisms and scenarios that may explain the observed deficits in the buried crater populations by comparing localized Bouguer gravity power spectra and recovered crater size‐frequency distributions from models of a simulated volcanically flooded cratered surface to the observed data. Our results indicate that the observed crater deficit and missing rings of Imbrium are best explained by a smoothing of the pre‐mare surface. We represent this smoothing as a diffusional process, as might occur with thermomechanical erosion during the earliest stages of the mare eruptions. The removal of the missing craters and Imbrium rings was a massive and unprecedented event that sheds light on the early evolution of the mare region, possibly supporting high temperature voluminous floods of lava early during mare formation.

Scaliger Crater Region on the Moon: Implications for the Australe North Basin and magmatism in the region

Australe North (35.5°S, 96°E) is a pre-Nectarian impact basin north of Mare Australe, first identified using the GRAIL data. It does not show clear topographic signatures typical of any large impact structure on the Moon. However, results from GRAIL (Gravity Recovery and Interior Laboratory) mission suggested the presence of a ∼ 880 km wide basin whose boundary does not coincide with the earlier identified Australe Basin. In this study, we investigate the Scaliger Crater region to understand the controls that the proposed Australe North Basin played on the local geology in the region. The location of the Scaliger Crater at the intersection of the rims of two pre-Nectarian Basins, Milne and the proposed Australe North provides a unique geological setting to reconstruct the region's geological history. We provide geological evidence substantiating the existence of the rim of the Australe North Basin, as has been proposed using gravity data. Based upon the spectral signatures and morphometry, we suggest the presence of a mafic pluton and/or lower crustal/mantle rocks in the region and provide constraints on its emplacement timescale. For the first time, Late-stage volcanism has been reported inside the Bowditch Crater and Lacus Solitudinis, emplaced at ∼1.7 Ga and ∼ 2.3 Ga, respectively, suggesting prolonged volcanism at the eastern lunar nearside-farside boundary inside an obscured pre-Nectarian basin.

Recovering crustal thickness of the Moon from an extended Vening Meinesz-Moritz inversion

Crustal thickness (Moho surface relief) is closely related to geotectonic, which is the predominant research topic in lunar geophysics. For lunar crustal depth inversion, to address the problem of downward continuation in traditional spectral domain methodologies, we adopt a solution from gravity disturbance (geophysical gravity anomaly). It is available to investigate crustal thickness at arbitrary surface outside the geoid. Using gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission and terrain information recovered from the Lunar Orbiter Laser Altimeter (LOLA) mission, global crustal depth of the Moon is recovered by the extended Vening Meinesz-Moritz method. The first application result of this technique on the Moon is validated by four types of released models in the form of spherical harmonic expansion with 310°. Moreover, from the statistics, our lunar Moho solution demonstrates that the average crustal thickness is approximately 41.5 km. The maximum value of 83.1 km and the minimum value of 0.8 km are located in the area of high mountains and Moscoviense mascon, respectively.

Precise mapping of the Moon with the Clementine Ultraviolet/Visible Camera

The Ultraviolet/Visible (UVVIS) camera on the Clementine spacecraft provided a global, multispectral view of the Moon. Scientists commonly use individual observations and derived products (optical maturity, mineral abundance, etc.) over 25 years later, addressing questions concerning the composition and relative age of surface features. However, since the mission concluded, our knowledge of lunar topography and the locations of features on the surface have improved with results from the Lunar Reconnaissance Orbiter (LRO) and Gravity Recovery and Interior Laboratory (GRAIL) missions. Before this work, cross-mission comparisons were impaired by spatial offsets between the derived products, which are as large as 2.5 km in some regions. Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) images, acquired under similar lighting conditions, were used as a cartographic reference. We used image-based feature-matching algorithms to automatically derive control points to improve the positional accuracy of each UVVIS observation with the LROC WAC basemap. From ground control points, we calculate a precise camera model (focal length, optical distortion, etc.) for the UVVIS camera and update the pointing for each UVVIS image. Using the updated geometric information and projecting the UVVIS image to the LOLA global shape model, we map the five-band multispectral UVVIS mosaic, the optical maturity map, and FeO and TiO2 abundance maps. We also analyze pitch observations of the polar regions to investigate the influence phase angle has on the derived optical maturity. The new images are registered to the GRAIL-based LRO geodetic framework within a WAC pixel (Ground Sampling Distance ∼75 m; average UVVIS sigma0 = 0.084), creating a foundational geospatial data product that does not require any manual interpretation or nonlinear warping of map products to align with the current lunar reference frame.

Spherical Planting Inversion of GRAIL Data

In large-scale potential field data inversion, constructing the kernel matrix is a time-consuming problem with large memory requirements. Therefore, a spherical planting inversion of Gravity Recovery and Interior Laboratory (GRAIL) data is proposed using the L1-norm in conjunction with tesseroids. Spherical planting inversion, however, is strongly dependent on the correct seeds’ density contrast, location, and number; otherwise, it can cause mutual intrusion of anomalous sources produced by different seeds. Hence, a weighting function was introduced to limit the influence area of the seeds for yielding robust solutions; moreover, it is challenging to set customized parameters for each seed, especially for the large number of seeds used or complex gravity anomalies data. Hence, we employed the “shape-of-anomaly” data-misfit function in conjunction with a new seed weighting function to improve the spherical planting inversion. The proposed seed weighting function is constructed based on the covariance matrix for given gravity data and can avoid manually setting customized parameters for each seed. The results of synthetic tests and field data show that spherical planting inversion requires less computer memory than traditional inversion. Furthermore, the proposed seed weighting function can effectively limit the seed influence area. The result of spherical planting inversion indicates that the crustal thickness of Mare Crisium is about 0 km because the Crisium impact may have removed all crust from parts of the basin.

Lunar ephemeris at sub microarcsecond accuracy (LESMA) leads to sub-millimeter positional accuracy of the moon

The most accurate LLR (lunar laser ranging) initiative, named APOLLO (apache point observatory lunar laser-ranging operation) demonstrated millimeter-range positional accuracy in 2009, thus improving LLR by one order-of-magnitude. Since, LLR is a foundational technique in studying gravity, Murphy (principal investigator of APOLLO) stated in 2009, that with this millimeter-range accuracy, the simulation model has been found to be the limiting-factor in extracting the theoretical science results, and hence, we should: (1) develop the science case and expand our ability to model LLR for a new regime of high precision, (2) develop the theoretical tools for honing the science case for submillimeter LLR, and (3) explore which model/code is worth putting our efforts into. (4) Since millimeter-quality data are a recent development, the model effort lags. (5) Finally, we will code-in new physics so that we may simulate sensitivities. In connection with simulation model/code, Murphy stated in 2013, that among the four available LLR simulation models: JPL (jet propulsion laboratory), CfA (the Harvard-Smithsonian center for astrophysics), LU (leibniz University, Hannover, Germany), and IMCCE (Institut de Mecanique celeste et de calcul des Ephemerides, France), the JPL model currently produces weighted RMS (root-mean-square) residuals at ∼18 mm, which is about half of the other models; so, clearly a gap exists from millimeter ranging-precision of APOLLO. Hence, the CfA, LU, and IMCCE are engaged, since 2013, in a stepwise comparative streamlining effort to identify the model-differences, errors, and shortcomings. All the four available LLR simulation models can be classified as GR (general relativity)-astronomers model; they are basically similar. Professor Douglas Currie of the University of Maryland, College Park, NASA Lunar Science Institute, stated in a Conference presentation, in 2012, that Ground stations, that is, the lunar observatories, have improved by a factor of 200, but the agreement between observations and fitted theory has plateaued at ∼2 cm over the past two decades. However, no substantial progress on improving the fit has been reported in the published literature, till date. Based on about a quarter-century of experience in doing high-precision numerical simulation of celestial orbits, the authors have developed LESMA (lunar Ephemeris at sub Microarcsecond accuracy) utilizing the methodology of evolved general relativity (EGR) that has incorporated the following two concepts: (1) Relativistic time for integration and (2) methodology of conservation of magnitude of the angular momentum, MΦ , for Φ-rotation (in addition to the θ-rotation that leads to the rosetting ellipse) of the orbital plane. Incorporation of the two above-mentioned concepts has led to three orders-of-magnitude accuracy-improvement of the computed (1) precession (compared to JPL's DE405) of Lunar orbit, as verified using three independent methods and (2) radial position (compared to JPL's DE430/431) of the Moon. LESMA will enable scientists to make efficient use of research-funds from NASA, etc., for production of new science results from APOLLO. LESMA will also be useful for getting better science results (than Folkner reported {in 2014} submeter accurate Position of the Moon) from the GRAIL (gravity recovery and Interior laboratory) mission (costing 500 million USD), by spending a little more for revisiting the computations, utilizing LESMA data.

Widespread impact-generated porosity in early planetary crusts

NASA’s Gravity Recovery and Interior Laboratory (GRAIL) spacecraft revealed the crust of the Moon is highly porous, with ~4% porosity at 20 km deep. The deep lying porosity discovered by GRAIL has been difficult to explain, with most current models only able to explain high porosity near the lunar surface (first few kilometers) or inside complex craters. Using hydrocode routines we simulated fracturing and generation of porosity by large impacts in lunar, martian, and Earth crust. Our simulations indicate impacts that produce 100–1000 km scale basins alone are capable of producing all observed porosity within the lunar crust. Simulations under the higher surface gravity of Mars and Earth suggest basin forming impacts can be a primary source of porosity and fracturing of ancient planetary crusts. Thus, we show that impacts could have supported widespread crustal fluid circulation, with important implications for subsurface habitable environments on early Earth and Mars.

Plausible Detection of Feasible Cave Networks Beneath Impact Melt Pits on the Moon Using the Grail Mission

In the future, when humans build their bases on terrestrial planets and their moons, caves will be the safest place for inhabitation. Large holes, believed to be cave entrances, have been discovered on the Moon, along with small features called “impact melt pits.” In the Gravity Recovery and Interior Laboratory (GRAIL) gravity model, which is expressed in spherical harmonics (SH), it is difficult to express the gravity anomaly created by a small empty space below the surface. Nevertheless, we propose that a cave network, akin to an anthill, exists under the impact melt pits discovered on the Moon. This is because we think it is natural to apply a network created by Earth’s small caves to the Moon. We obtained accurate Bouguer gravity measurements by calculating regional crustal density using localized admittance of the study area and detected weak gravity (mass deficit) information. By increasing the degrees and order of SH at regular intervals, we estimated the change in gravity at a specific position at high degrees and order, thereby extracting shallow depth information. To validate our method, we compared our results with those of existing studies that analyzed the previously known Marius Hills Hole (MHH) area. The analysis of seven regions in our study area revealed a mass deficit in some impact melt pits in four lunar regions (Copernicus, King, Stevinus, and Tycho). We propose that there is a cave network in this region, indicated by the gravitation reduction in the impact melt pits region. Our results can be useful for the selection of landing sites for future in situ explorations of lunar caves.

Degree‐Depth Relation for Planetary Gravity Field Model Based on Wavelength

AbstractIn this paper we investigate the relation between the spherical harmonic (SH) degree of gravity field model and the corresponding depth of the source body. By investigating the gravity of a buried point mass, we find that a relationship can be found between the horizontal extent of the signal (which we assume to be indicative of the wavelength) and the depth of the source. We introduce a threshold to define the gravity anomaly wavelength of an isolated mass body. The region where the gravity anomaly of an isolated mass body is larger than the threshold defines its gravity anomaly wavelength. For an isolated point mass with fixed total mass, the wavelength is a function of the depth. The ratio between the maximum wavelength and the depth at which the maximum wavelength occurs (effective resolvable depth) is invariant provided that the mass and threshold are determined. When this maximum wavelength is larger than the minimum resolvable wavelength of n‐degree gravity field, this isolated mass body is considered as a reliable density feature. Combining the relation between maximum wavelength and effective resolvable depth of the isolated mass body and the relation between minimum resolvable wavelength and SH degree of gravity field, we derived the degree‐depth relation under the limiting resolvable condition: . This relation was validated with Gravity Recovery and Interior Laboratory gravity field data at lunar Orientale basin region, and provided insights for the planetary gravity field analysis.

Combined use of band shape algorithm, linear spectral un-mixing on Clementine & Moon Mineralogy Mapper data for identifying the imprints of magmatic differentiation – A study around Aristarchus Plateau

In this study, we have implemented a band shape algorithm (BSA) to identify the surface distribution of different minerals on parts of the Aristarchus Plateau using selected bands of Clementine and hyperspectral Moon Mineralogy Mapper (M3) data. BSA mineral maps derived using spectral bands of Clementine and M3 sensors could determine broadly similar surface distribution of minerals in the study area. Here we show that M3 based BSA mineral map could detect additional mineralogical details in selected places, especially around Vallis Schroteri, Aristarchus etc. areas. Therefore, we implemented linear spectral un-mixing (LSU) method on hyperspectral bands of M3 data using the reference spectra of Reflectance Experiment Laboratory (RELAB) to estimate the relative abundance of different minerals in above-mentioned areas. Based on the relative abundance of minerals in the LSU map, we could infer the presence of different rocks such as dunite/troctolite, norite, harzburgite, olivine-pyroxenite in those sites of the study area. The relative abundance of each mineral in a particular place was used as the basis for delineating specific rock in that place of the study area. These mineralogically diverse rocks with distinct proportions of mafic mineral and plagioclase content are exposed along a lunar structure that has been delineated using Gravity Recovery and Interior Laboratory (GRAIL) data. We hypothesize that some of these rocks might have formed by a magmatic differentiation process triggered by the structure by separating magma from the crystallized rocks and allowing the magma to compositionally evolve to produce different rocks under the influence of syn-magmatic structural activity.

Patched Local Lunar Gravity Solutions Using GRAIL Data.

We present a method to determine local gravity fields for the Moon using Gravity Recovery and Interior Laboratory (GRAIL) data. We express gravity as gridded gravity anomalies on a sphere, and we estimate adjustments to a background global start model expressed in spherical harmonics. We processed GRAIL Ka‐band range‐rate data with a short‐arc approach, using only data over the area of interest. We determine our gravity solutions using neighbor smoothing constraints. We divided the entire Moon into 12 regions and 2 polar caps, with a resolution of 0.15°×0.15° (which is equivalent to degree and order 1199 in spherical harmonics), and determined the optimal smoothing parameter for each area by comparing localized correlations between gravity and topography for each solution set. Our selected areas share nodes with surrounding areas and they are overlapping. To mitigate boundary effects, we patch the solutions together by symmetrically omitting the boundary parts of overlapping solutions. Our new solution has been iterated, and it has improved correlations with topography when compared to a fully iterated global model. Our method requires fewer resources, and can easily handle regionally varying resolution or constraints. The smooth model describes small‐scale features clearly, and can be used in local studies of the structure of the lunar crust.

Investigating the Influences of Crustal Thickness and Temperature on the Uplift of Mantle Materials Beneath Large Impact Craters on the Moon

AbstractIn this work, we examine variations in the mantle uplift associated with large lunar impact craters and basins between major terranes. This study is based on Bouguer gravity anomalies of 100–650‐km diameter impact craters using Gravity Recovery and Interior Laboratory (GRAIL) observations and the Lunar Orbiter Laser Altimeter (LOLA) crater database. The Bouguer gravity anomalies of 324 large impact craters analyzed herein are primarily controlled by the uplifted crust‐mantle (Moho) interface in the central region of these impact craters, while postimpact mare deposits that represent ∼25%–35% of the postimpact deposit column also contribute to the gravity anomalies of mare craters. The central uplift of the Moho interface is primarily controlled by impact energy and increases to ∼ 30 km for a 650‐km diameter crater. Analyses of nonmare craters in the Feldspathic Highlands Terrane (FHT) with varied crustal thickness (Tc) reveal that the onset crater diameter (Dmin) with an uplifted Moho interface is dependent on the local Tc: Dmin ∼ 150 + 1.3Tc (in a unit of km). This equation also provides a quantification of the depth‐dependent attenuation of impact‐induced structural uplift, using the Moho uplift as a proxy for the structural uplift. The Moho uplift of large craters in the South Pole‐Aitken (SPA) Terrane is not statistically different from the FHT craters, consistent with limited overall thermal difference between these two terranes and compensational thermal effects at crater collapse and viscoelastic relaxation stages.

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.

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.

High‐Resolution Gravity Field Models from GRAIL Data and Implications for Models of the Density Structure of the Moon's Crust

AbstractWe present our latest high‐resolution lunar gravity field model of degree and order 1200 in spherical harmonics using Gravity Recovery and Interior Laboratory (GRAIL) data. In addition to a model with the standard spectral Kaula regularization constraint, we determine models by applying a constraint based on topography called rank‐minus‐one (RM1). The new models using this RM1 constraint have high correlations with topography over the entire degree range by design. The RM1 models allow the determination of apparent crustal densities at all spatial scales (called effective density) covered by the model, whereas the Kaula‐constrained model can only be used globally up to spherical harmonic degree 700. We find that the effective density spectrum has a smaller slope for the high degrees when compared to the medium degrees. We interpret this as indicative of a global average surface density, as opposed to an ever‐decreasing effective density as one approaches the surface. We use the RM1 models to derive maps of lateral and vertical density variations in the lunar crust. These models allow us to increase the resolution of this analysis compared to previous studies, by increasing the degree range over which to fit theoretical models of vertical density variations, and by decreasing the size of the spherical caps used in a localized analysis. Several regions on the Moon, such as South Pole‐Aitken and Mare Orientale, are distinct from their surroundings in terms of surface densities. The RM1 models are especially valuable in (localized) spectral studies of the structure of the lunar crust.