Mascon Basins Research Articles

Lunar Farside South Pole‐Aitken Basin Interior: Evidence for More Extensive Central Cryptomaria in the South Pole‐Aitken Compositional Anomaly (SPACA)

AbstractIn the central area of the South Pole‐Aitken (SPA) basin, an intermediate albedo, mafic compositional anomaly (SPA Compositional Anomaly, SPACA) has been documented by previous studies, but its origin remains uncertain. We conducted an investigation of stratigraphic units defined based on morphology and composition and their relative ages, and placed these in the context of basin topography and the observed sequence of geological events, all helping to distinguish between SPACA origins from: (a) SPA impact melt, (b) volcanism induced by the SPA event and (c) lunar cryptomaria. We conclude that SPACA represents extensive traditional cryptomare deposits overlying the SPA impact melt. We interpret the basin center to be filled with cryptomare deposits at least one km thick (>1 × 105 km3 in volume) with ages not younger than Early Imbrian. We attribute the relatively high albedo of SPACA to lateral mixing of ejecta from nearby highlands craters and basins, and conclude that the cryptomaria basalts are likely to be very similar to basalts on the nearside. Our findings imply a 0.5%–1.8% increase in the total volume of global lunar mare and cryptomare deposits. These results show that mare volcanism was common only in areas of thinnest crust on the lunar farside, a factor important in understanding lunar nearside‐farside asymmetries. Despite this significant increase in total cryptomare volume in the SPA basin center, SPA remains underfilled relative to nearside mascon basins. Return of mare basalts from the SPA region by Chang’E‐6 will help determine potential mantle source region differences and petrogenetic pathways.

The Ancient Lithospheric Parameters of Impact Basins on the Far Side of the Moon Based on the Mantle and Mare Basalt Load Model

AbstractResearch on the lithospheric elastic thickness (Te) of lunar mascon basins can provide a deeper understanding of heterogeneous thermal activity. In this study, we focused on four basins (Moscoviense, Freundlich‐Sharonov, Hertzsprung, and Apollo) located on the far side of the Moon. These basins exhibit a significant variation in admittance and correlation spectra, making it challenging to develop precise fitted models. Based on the global lunar crustal thickness model and mare basalt thickness, we developed a mantle and mare basalt load model to estimate the Te. This small Te (10.1 km) suggests a double impact process or extreme thermal activity caused by volcanism during the formation of the Moscoviense. For the typical highland basins, that is, Freundlich‐Sharonov and Hertzsprung, the Te (∼20 km) corresponds to a minimum heat flux of 34 mW m−2. Considering the additional energy introduced by the impact, this heat flux can be considered as the upper limit for the heat flux of the highland itself during the formation of a mascon basin. For the Apollo basin within the South Pole‐Aitken (SPA) terrane, the Te is 30.7 km. The crust beneath the SPA region is thinned, allowing a greater contribution of stiffer mantle material to the lithosphere, perhaps explaining the higher effective Te. Furthermore, the disparity between the highland and SPA terranes can give rise to their distinct basin formation processes. Impact basins formed within the SPA terrane may have experienced a faster cooling process compared to those formed on the highland terrane following the impact event, leading to a higher Te.

Thrust Faults Bound an Elevated Mantle Plug Beneath Several Lunar Basins

AbstractThe lunar maria are large expanses of basalt that infill antecedent impact basins and show evidence for postemplacement deformation. Landforms within many of these basins suggest a period of compressive tectonics, although their formation mechanism has yet to be established. Previous work for Mare Crisium demonstrated that basin‐circumferential wrinkle ridges, which typically demarcate the inner edge of an annulus of elevated terrain, are the result of deep‐seated thrust faults that preferentially form along the boundary of an elevated superisostatic portion of mantle and a thick, subisostatic collar of crustal material. Here, we show that a similar fault architecture exists for several other mascon‐bearing basins, including Maria Serenitatis, Nectaris, Moscoviense, and, to a lesser degree, Humorum and Imbrium. These deeply penetrating basin‐circumferential thrust faults, as for Mare Crisium, form a (partial) outward‐dipping ring‐fault system that bounds the elevated mantle plug beneath each basin as a geometric consequence of mascon evolution. If this geometric arrangement is unique to the Moon, then some characteristic(s) of lunar mascon evolution enables the formation of such mascon‐bounding faults. Despite the ubiquitous nature of mascon‐bound thrust ring faults at several lunar basins, whether such structures exist at mascon basins on other terrestrial worlds remains an open question.

Mare Domes in Mare Tranquillitatis: Identification, Characterization, and Implications for Their Origin

AbstractMare domes, small shield volcanoes typically <∼30 km diameter, are part of the spectrum of lunar volcanic features that characterize extrusive basalt deposits. We used new spacecraft data to document these in Mare Tranquillitatis, among the oldest maria and the site commonly interpreted as an ancient degraded non‐mascon impact basin. We found 283 known and suspected mare domes, with the majority (n = 229) concentrated on a broad, ∼450 km circular topographic rise in eastern Mare Tranquillitatis. The domes (median diameter 5.6 km, height 68 m, volume 0.7 km3) contain summit pits (74%; median diameter 0.8 km), and exhibit minor compositional variability between domes and surrounding flows, suggesting that domes both supply and are embayed by these flows. Based on their characteristics and associations, we interpret the small shield volcanoes to have been built from individual low‐volume (<∼10–100 km3), low volatile content, short duration, cooling‐limited eruptions. The ∼450 km broad volcanic rise is ∼920 m high (volume ∼1.6 × 105 km3) and is interpreted to be built from multiple occurrences of small shield eruptions, a shield plains volcanism style. This implies a shallow mantle source region capable of supplying distributed dike‐emplacement and eruption events over an area of 1.75 × 105 km2 early in mare volcanism history (∼3.7 Ga). The difference between Mare Tranquillitatis and younger mare‐filled mascon basins is attributed to the more ancient thermal state and crustal structure of the viscously relaxed Tranquillitatis basin, and a shallower broad magma source region present in earlier lunar thermal history.

3‐D Density Structure of the Lunar Mascon Basins Revealed by a High‐Efficient Gravity Inversion of the GRAIL Data

AbstractThe lunar mascon basins are characterized by high gravity anomalies and thin crust. Study of the density structure beneath the mascon basins can help to understand the origin of these gravity anomalies and infer their formation and evolution. We propose an efficient forward gravity method based on the 3‐D Gauss‐Legendre quadrature (GLQ) and Fast Fourier Transforms combined with the adaptive discretization strategy to ensure high accuracy. The numerical example demonstrates that computational efficiency is increased by about three orders of magnitude compared to the traditional 3D GLQ method. We employ this forward method in a 3‐D inversion of the gravity data derived from the lunar gravity model GL1500E. The inverted results show prominent high‐density structures (namely, mascons) representing significant mantle uplift and thinned crust beneath most impact basins. Marked low‐density rings surround the mascons. Most of the corresponding low‐density anomalies extend from the near‐surface to the Moho, indicating a thick, low‐density crust. Our density model is consistent with the formation processes of mascons that an impact causes collapse of the transient crater and then the shocked mantle drives mantle flow. The low‐density rings surrounding the mascons may stem from the crust thickening following an impact and extensive fracturing of the crustal column.

Why the lunar South Pole-Aitken Basin is not a mascon

Lunar mascon basins exist for only a range of observed diameters. We modeled the full formation of South-Pole Aitken basin using a sequential two-code (hydrocode and Finite Element Model) approach to understand why this range does not extend to the largest lunar basin. Similar to previous work, we found that the best-fit hydrocode had impact parameters of a 170km in diameter dunite projectile striking at 10km/s, with a pre-impact lunar thermal gradient of 50K/km (until a depth of 28 km at which an adiabat is reached), and pre-impact crustal thickness value of 40 km. Unlike previous work, we matched the crustal distribution of the inner basin by utilizing a weaker melt rheology in our model. The crust in the models with a weaker melt rheology flowed inward after crater collapse to cover the basin center; therefore, not requiring an additional step of melt differentiation. Given the large diameter of South-Pole Aitken and the high geothermal gradient at impact, the steady state configuration after relaxation and thermal cooling was driven by isostatic forces without a significant contribution from lithospheric rigidity. Without lithospheric rigidity, South-Pole Aitken relaxed to a slightly negative free-air gravity signature, not forming a mascon, in agreement with GRAIL observations.

The origin of neotectonics on the lunar nearside

Abstract New observations of wrinkle ridges on the nearside maria of the Moon display signs of ongoing ridge modification. In association with the wrinkle ridges, we observed an absence of superposed craters, narrow (<30 m) lobate scarps and graben, and thermal anomalies related to exposures of meter-size blocks. Many of these active wrinkle ridge systems are well beyond the influence of mascon basins and unrelated to any global tectonic pattern. Nevertheless, they spatially correlate with ancient deep-seated dike intrusions on the lunar nearside revealed by gravity data analysis. We propose that this active nearside tectonic system (ANTS) reflects ongoing reactivation of an ancient system related to offset antipodal effects from the South Pole–Aitken basin.

Wrinkle ridges on Mercury and the Moon within and outside of mascons

Found on all terrestrial planets, wrinkle ridges are anticlines formed by thrust faulting and folding resulting from crustal shortening. The MErcury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft's orbital phase returned high resolution images and topographic data of the previously unimaged northern high latitudes of Mercury where there are large expanses of smooth plains deformed by wrinkle ridges. Concurrently, the Lunar Reconnaissance Orbiter (LRO) is obtaining high resolution images and topographic data covering lunar mare wrinkle ridges. These data allow quantitative comparison of the scale of wrinkle ridges in smooth plains volcanic units on Mercury with mare wrinkle ridges. We evaluate the topographic relief of 300 wrinkle ridges within and outside of mascon basins on the Moon and Mercury. Measured wrinkle ridges range from ~112 to 776 m in relief with a mean of ~350 m (median = ~340 m, n = 150) on Mercury and from ~47 to 678 m in relief with a mean of ~198 m (median = ~168 m, n = 150) on the Moon. Wrinkle ridges on Mercury thus are approximately twice as large in mean relief compared to their counterparts on the Moon. The larger scale of Mercury's wrinkle ridges suggests that their formation can be attributed, in part, to global contraction. As global contraction on the Moon is estimated to be an order of magnitude smaller than on Mercury, the smaller scale of lunar wrinkle ridges suggests they most likely form primarily by load induced subsidence of the mare basalt. Wrinkle ridges located in lunar mascon basins and in the Caloris mascon on Mercury are not statistically significantly different in relief than ridges in non-mascon regions, suggesting comparable levels of contractional strain. The fact that mascon basins do not host wrinkle ridges with greater structural relief relative to non-mascon units may indicate the critical role lithospheric thickness plays in controlling subsidence and contraction of thick volcanic sequences on the Moon and Mercury.

Efficient 3‐D Large‐Scale Forward Modeling and Inversion of Gravitational Fields in Spherical Coordinates With Application to Lunar Mascons

AbstractAn efficient forward modeling algorithm for calculation of gravitational fields in spherical coordinates is developed for 3‐D large‐scale gravity inversion problems. The 3‐D Gauss‐Legendre quadrature (GLQ) is used to calculate the gravitational fields of mass distributions discretized into tesseroids. Equivalence relations in the kernel matrix of the forward modeling are exploited to decrease storage and computation time. The numerical tests demonstrate that the computation time of the proposed algorithm is reduced by approximately 2 orders of magnitude, and the memory requirement is reduced by N′λ times compared with the traditional GLQ method, where N′λ is the number of the model elements in the longitudinal direction. These significant improvements in computational efficiency and storage make it possible to calculate and store the dense Jacobian matrix in 3‐D large‐scale gravity inversions. The equivalence relations can be applied to the Taylor series method or combined with the adaptive discretization to ensure high accuracy. To further illustrate the capability of the algorithm, we present a regional synthetic example. The inverted results show density distributions consistent with the actual model. The computation took about 6.3 hr and 0.88 GB of memory compared with about a dozen days and 245.86 GB for the traditional 3‐D GLQ method. Finally, the proposed algorithm is applied to the gravity field derived from the latest lunar gravity model GL1500E. Three‐dimensional density distributions of the Imbrium and Serenitatis basins are obtained, and high‐density bodies are found at the depths 10–60 km, likely indicating a significant uplift of the high‐density mantle beneath the two mascon basins.

Deep-seated thrust faults bound the Mare Crisium lunar mascon

Mare Crisium is composed of a set of volcanic deposits situated in an impact basin on the Moon's near side. The topography of the mare is dominated by an annulus of elevated topography, the inner edge of which is delineated by basin-concentric wrinkle ridges. From a combination of remotely sensed image and topographic data and numerical modeling, we show that the thrust faults that underlie these ridges penetrate up to 20 km in depth, considerably below the base of the mare deposits themselves. Thrust faults of this scale have not heretofore been recognized on the Moon. Mare Crisium sits above a region of uplifted mantle, which contributes to a mass excess beneath the basin, and we demonstrate by comparison with free-air gravity anomaly and derived crustal thickness data for Crisium that the thrust faults structurally bound this elevated mantle material. By means of finite-element models of stresses induced by lithospheric loading within the basin, we argue that the deep-seated thrusts may have been localized by the boundary between the superisostatic mantle material and a sub-isostatic collar of thickened crust that resulted from basin formation and modification shortly after impact. Importantly, numerous other mare-filled mascon basins on the Moon share the same topographic and tectonic characteristics as Crisium, suggesting that they, too, are underlain by deep-seated thrust faults that formed in a similar manner.

The formation of lunar mascon basins from impact to contemporary form

AbstractPositive free‐air gravity anomalies associated with large lunar impact basins represent a superisostatic mass concentration or “mascon.” High‐resolution lunar gravity data from the Gravity Recovery and Interior Laboratory spacecraft reveal that these mascons are part of a bulls‐eye pattern in which the central positive anomaly is surrounded by an annulus of negative anomalies, which in turn is surrounded by an outer annulus of positive anomalies. To understand the origin of this gravity pattern, we modeled numerically the entire evolution of basin formation from impact to contemporary form. With a hydrocode, we simulated impact excavation and collapse and show that during the major basin‐forming era, the preimpact crust and mantle were sufficiently weak to enable a crustal cap to flow back over and cover the mantle exposed by the impact within hours. With hydrocode results as initial conditions, we simulated subsequent cooling and viscoelastic relaxation of topography using a finite element model, focusing on the mare‐free Freundlich‐Sharonov and mare‐infilled Humorum basins. By constraining these models with measured free‐air and Bouguer gravity anomalies as well as surface topography, we show that lunar basins evolve by isostatic adjustment from an initially subisostatic state following the collapse stage. The key to the development of a superisostatic inner basin center is its mechanical coupling to the outer basin that rises in response to subisostatic stresses, enabling the inner basin to rise above isostatic equilibrium. Our calculations relate basin size to impactor diameter and velocity, and they constrain the preimpact lunar thermal structure, crustal thickness, viscoelastic rheology, and, for the Humorum basin, the thickness of its postimpact mare fill.

The distribution of Mg-spinel across the Moon and constraints on crustal origin

A robust assessment is made of the distribution and (spatially resolved) geologic context for the newly identified rock type on the Moon, a Mg-spinel-bearing anorthosite (pink-spinel anorthosite, PSA). Essential criteria for confirmed detection of Mg-spinel using spectroscopic techniques are presented and these criteria are applied to recent data from the Moon Mineralogy Mapper. Altogether, 23 regions containing confirmed exposures of the new Mg-spinel rock type are identified. All exposures are in highly feldspathic terrain and are small-a few hundred meters-but distinct and verifiable, most resulting from multiple measurements. Each confirmed detection is classified according to geologic context along with other lithologies identified in the same locale. Confirmed locations include areas along the inner rings of four mascon basins, knobs within central peaks of a few craters, and dispersed exposures within the terraced walls of several large craters. Unexpected detections of Mg-spinel are also found at a few areas of hypothesized non-mare volcanism. The small Mg-spinel exposures are shown to be global in distribution, but generally associated with areas of thin crust. Confirmation of Mg-spinel exposures as part of the inner ring of four mascon basins indicates this PSA rock type is principally of lower crust origin and predates the basin-forming era.

External calibration for laser altimetry mission in Chang’E-1

When calculating the lunar digital elevation model by using LAM data of ChangE-1 lunar orbiter, the radical error of measurement in lunar mass center fixed coordinate is one kind of the key error sources. For LAM, two stages of oscillators, with one from satellite bus system and one from LAM itself, supplied time synchronization, timing and frequency standard of lunar altimetry. The independent unstable oscillations of them created the relevant errors to LAM. We analyzed the time slips and drifts in data time tags, estimated the possible systematic bias of reference frequency of LAM oscillator. Then we calibrated the effects of them on LAM data. The budgets of these three kinds of error sources are 0.7-1.2 m, ~10 m and ~145 m, respectively. Using the calibrated LAM data, a new 360 orders and degrees lunar topography model, CLTM-s03, has been obtained. By combining the new DEM model of the moon with CEGM02 lunar gravity model, a couple of middle scale type II lunar mascon basins, Fitzgerald-Jackson and Amundsen-Ganswindt, have been newly identified.

On the origin of mascon basins on the Moon (and beyond)

AbstractMascon basins on the Moon are large craters that display significant positive free‐air and Bouguer gravity anomalies. An important question is why is not every large crater a mascon, as less than half have been previously determined to be. We detrend the free‐air, topographic, and Bouguer gravity anomalies and find that most large basins (28 of 41) display mascon characteristics (e.g., strong positive Bouguer anomalies narrower than the surface rims). Negative free‐air gravity annuli surrounding the central highs generally are absent in the Bouguer gravity, implicating surface topography. We propose that beneath a forming large basin, the relatively narrow transient crater drives mantle uplift, while upward and inward collapse forms the surface topography. Furthermore, the nonmascon basins are all ancient and heavily degraded, indicating a postimpact evolutionary process. Our results suggest that mascon formation is the standard for large impacts on the Moon and by extension on other terrestrial planets.

The Chang’E-1 orbiter plays a distinctive role in China’s first successful selenodetic lunar mission

The first Chinese lunar orbiter Chang’E-1 is a successful mission with many fruitful results obtained in various disciplines. The scientific data acquired by the Chang’E-1 payloads can benefit studies of the lunar origin and evolution, as well as other relevant research areas, after careful validation of the data. Among the new results, the Chang’E-1 selenodetic products are continually uncovering characteristics of the lunar surface, undersurface and inner structure. Successful lunar orbiters such as the Clementine, Lunar Prospector, KAGUYA/SELENE, Chang’E-1, Lunar Reconnaissance Orbiter and GRAIL have been revealing, with increasing clarity, global selenodetic characteristics with state-of-the-art fine resolution and high precision. In particular, the Chang’E-1 plays an important distinctive role in selenodetic exploration through enhancing lunar topography and gravity models. The gravity model has been successfully improved with a factor of two after applying the Chang’E-1 long-wavelength tracking data. Using the new models, some medium-scale lunar surface characteristics such as basins and volcanoes have been identified. Furthermore, the old mascon basins of Bouguer, gravity anomaly and craters have been discovered with the Chang’E-1 selenodetic data.

Bouguer gravity anomaly of the Moon from CE-1 topography data: Implications for the impact basin evolution

In this study, the terrain correction for lunar free-air gravity anomaly (FAGA) is calculated in spherical coordinates based on the global topography data detected by the laser altimeter on Chang’E-1 (CE-1). The obtained lunar Bouguer gravity anomaly (BGA) reveals density irregularities of the interior mass. BGA is important in characterizing the mascon basins. According to the BGA of the Moon, the South Pole-Aitken (SPA) basin is considered the largest mascon basin on the Moon, and the feature of BGA in the basin implies the impacting direction. Further, the mascon basins seem to be classified into two types, Type Highland and Type Plain. For the mascon basins of Type Highland the dense materials mainly come from the shallow crust, which are associated with the basalt deposits. The other type, Type Plain, includes mascon basins whose major dense materials may be located deep at the lithosphere, corresponding to the uplifted mantle.

The strength of the lunar lithosphere

A self-gravitating, elastic, spherical thick shell model is used to derive the present state of the lateral variations of density and stress differences within the lunar lithosphere. The model is allowed to deform under the load of an initial surface topography and internal density distribution, such that the resulting deformed body gives rise to the observed surface topography and gravity specified by the spherical harmonics of degree up to 70. Two main models are considered, Model A and Model B, with elastic lithospheres of thickness 300 and 210 km, respectively. Model A displays density perturbations of generally less than ±200 kg/m 3 within the crustal layers, reducing rapidly to less than ±20 kg/m 3 at the base of the lithosphere. The density perturbations in Model B are similar in the crust and marginally higher at the base of the lithosphere. The major stress differences in the mantle are associated with the mascon basins and are found to reach maximums of 8–10 MPa within the lower lithosphere (150–270 km) of Model A and maximums of 12–16 MPa at 150 to 180 km depth for Model B. A moderate correlation exists between the modeled stress distributions and shallow moonquake epicenters. However, the overall results of this study imply that other remnant stresses, due to processes other than density perturbations, exist and play a critical role in the large shallow moonquakes.

The Compensation State of Intermediate Size Lunar Craters

The compensation state of 49 intermediate size (120 to 600 km diameter) lunar craters are investigated using the most recent spherical harmonic models of the lunar topography and gravity, truncated at degree n=110. The total mass anomalies per unit area (i.e., the lateral variations of the vertically integrated density perturbations per unit area) within an otherwise uniform crust of 60 km thickness are determined such that, together with the surface topography, give rise to the model gravity anomalies. Crustal thicknesses of 40 and 80 km are also considered, but the general results of this study are not significantly affected. Excess mass anomalies are obtained by subtracting from the total mass anomalies the mass anomalies that are required for the isostatic compensation of the surface topography. The excess mass anomaly of a crater denotes its particular state of compensation. Dependencies of the excess mass anomalies on crater location, size, and age are investigated, but in general few discernable trends are evident. Although the vast majority of craters indicate some compensation, no correlation exists between age or size and the state of compensation. Roughly 16% of the craters show no compensation, and in some cases have mass deficiencies most likely due to the shock fractured bedrock: the breccia lens of lower density. The crust in these regions was likely cold and rigid enough at the time of impact to rigidly support the stress caused by crater excavation. These features are seen throughout different geological periods, demonstrating that the lunar crust cooled quickly and strengthened soon after formation. A comparison of the compensation state of craters Apollo, Korolev, and Hertzsprung suggests that the thermal and mechanical properties of the crust prior to impact had an appreciable effect on the compensation, and that crustal thickness may be the single most important factor controlling the compensation of intermediate size craters. The characteristics of the excess mass a nomaly profiles of the eight well-known near side mascon basins are used to identify new mascon-like craters. Ten newly found mascons are confirmed: Humboldtianum, Moscoviense, Mendel–Rydberg, Lorentz, Hertzsprung, Korolev, Schrodinger, Freundlich–Sharonov, Coulomb-Sarton, and Schiller–Zucchius, while two more, Deslandres and Dirichlet–Jackson, are very plausible. These results show that mare flow is not necessarily required to produce mascon-like characteristics.

Tectonics of mascon loading: Resolution of the strike‐slip faulting paradox

Subsidence of lunar mascon maria, impact basins partly filled with mare basalt and sites of prominent positive gravity anomalies, typically led to the formation of concentric graben (arcuate rilles) around the flanks of the basin, while compressive features (mare ridges) formed in interior regions. Although previous numerical models of the response of the lunar lithosphere to mascon loading predict that an annulus of strike‐slip faulting should also have formed around mascon maria, no such faults have been observed. This “strike‐slip faulting paradox,” however, arises from an oversimplification of the earlier models. Viscoelastic finite element models of lunar mascon basins that include the effects of lunar curvature, heterogeneous crustal strength, initial stress conditions, and multistage load histories show that the width of a predicted annulus of strike‐slip faulting may be small. The use of Anderson's criterion for predicting fault styles may also overpredict the width of strike‐slip faulting. A faulting‐style criterion that takes into account transitional faulting, in which both strike‐slip and dip‐slip components are present, predicts zones of pure strike‐slip faulting that are about half of the width predicted by the Anderson criterion. Furthermore, strike‐slip faulting should be observed only in regions in which flexural stresses are sufficient to induce rock failure. However, since stress patterns consistent with strike‐slip faulting around mascon loads represent a transition between compressional and extensional provinces, differential stresses tend to be low in these regions and for at least part of this region are not sufficient to induce rock failure. A mix of concentric and radial thrust faulting is observed in some mascon maria, at odds with previous models that predict only radial orientations away from the basin center. This apparent discrepancy may be partly explained by the multistage emplacement of mare basalt units, a scenario that leads to a stress pattern where concentric and radial orientations of thrust faults are equally preferred. Detailed models of the Serenitatis basin indicate a 25‐km‐thick lunar lithosphere at the time of rille formation and a 75‐km‐thick lithosphere at the time of late‐stage mare ridge formation. The extent of observed mare ridges and the inferred cessation of rille formation around Serenitatis prior to the time of emplacement of the youngest mare basalt units is consistent with the superposition of a global horizontal compressive stress field generated by the cooling and contraction of the lunar interior with the local stresses associated with lithospheric loading.

The lunar mascons revisited

The entire spectra of the surface topography and gravity field of lunar mascons are well presented by the high‐resolution Clementine data. The negative correlation between the topography and gravitational potential of the mascons suggests that they are dynamically supported. Both elastic support and viscous decay models are examined. For the elastic support models, the spectral characteristics and the lateral variations of the thickness of the elastic layer are determined on the basis of the thin spherical shell flexure formulation and using only the antivarying harmonics of the topography and potential. An elastic layer thickness of about 50 km is required to support Imbrium, Serenitatis, and Nectaris mascons, about 35 km for Crisium mascon, and about 30 km for Smythii and Humorum mascons. A layer of about 20 km thickness can support Orientale mascon. The strength envelopes of the upper 100 km of the Moon within 4–3 Gyr ago show that the elastic layer was not thick enough to support the mascons in the early history. They decayed through viscous deformation of the lunar interior. A lower limit of 6×1024 Pa s is estimated for the lunar viscosity within 3.6–3 Gyr ago. We also determine the thicknesses of the crust and mare flows of the mascon basins on the basis of the assumption that the basins were isostatically compensated prior to the mascon formation. The crust beneath the mascon basins is about 30–40 km except for Crisium and Orientale, where the crust is about 20 km. The mare flows of about 3–6 km are obtained for almost all of the mascon basins. The topography and gravitational potential of Aitken basin shows that the basin is compensated at 52–54 km depth. The mantle beneath the basin has rebounded upward by about 20–30 km and is overlain by about 20–25 km of a mixture of the excavated crustal and mantle material. The lack of a mascon and pervasive mare flows in the basin brings into question the current models of the mascon formation.