Abstract. We investigated the use of spectral correlation analysis for modeling the crustal features of Mare Orientale from lunar 70th degree spherical harmonic topographic and gravity field models derived from Clernentine satellite and earlier investigations. The analysis considered a 64ø-by-64 ø region of the Moon centered roughly on Mare Orientale at an altitude of 100 kin. The topography of the study region, which includes over 11 krn of relief, was modeled for its gravity effects in lunar spherical coordinates by Gauss-Legendre quadrature integration assuming a terrain density of 2.8 g/cm 3. We observed substantial positive and negative correlations between terrain gravity effects and free-air gravity anomalies that seriously limit the utility of simple Bouguer gravity anomalies for subsurface studies. Using the wavenumber correlation spectrum between the two data sets, we designed correlation filters to extract the common features. Possible interpretations for the terrain-correlated free-air gravity anomalies include isostatic crustal mass irnbalances that may be equilibrated by radial adjustments of the Moho of up to 44 kin, assuming Airy-Heiskanen compensation and a mantle density contrast of 0.5 g/crn 3 with the crust. These Moho adjustments define mass variations that account for most of the rnascon and flanking negative free-air gravity anomalies. Furthermore, their remarkable correlation with the topographic rings of Mare Orientale points to the possible influence of a strong local stress field of the crust in the development of the ring structures. Subtracting the terrain-correlated free-air ar•ornalies from the free-air gravity anomalies and terrain gravity effects yielded terrain-decorrelated free-air and isostatically compensated terrain gravity anomalies, respectively, that show zero correlation. This lack of correlation may be interpreted for a Moho that involves over 100 km of relief assuming Airy-Heiskanen compensation of the crust. Beneath Mare Orientale, we observed a minimum crustal thickness of about 17 kin. Corresponding terrain-decorrelated free-air gravity anomalies of Mare Orientale may be related to a central cone-shaped body of 0.5 g/cm 3 density contrast with apex extending nearly 5 km below the surface, which is surrounded by a ringed-shaped body of-0.5 g/crn 3 density contrast that may extend about 7 km below the surface. These bodies resulted possibly from meteorite impact that produced a roughly circular region of breccia and highly fractured crust with a higher density core where some rernelting of the rocks about the impact site may have occurred.

[1] A lunar crust-to-core mass model for the region ±64°latitude was developed using gravity components evaluated from available spherical harmonic gravity and topographic field models that include Lunar Prospector, Clementine, and earlier satellite observations. Terrain gravity effects were computed at 100-km altitude in spherical coordinates from 1°topography (GLTM 2) by Gauss-Legendre quadrature integration. Corresponding free-air gravity anomalies (LP75G) were spectrally correlated with the terrain effects to differentiate the terrain-correlated and terrain-decorrelated free-air components. The absence of correlation between these free-air components was interpreted for a Moho that may involve over 120 km of relief, assuming the lunar crust was mainly compensated by its thickness variations. Lacking the strong regional gravity effects of the terrain, the terrain-decorrelated anomalies may be interpreted for their subcrustal components on the basis of their correlation spectrum with the free-air anomalies. Inversions of the subcrustal anomalies inferred boundary undulations for the core-mantle, asthenosphere-lithosphere, and middle-upper mantle that are remarkably correlated with surface impacts. The elevated core topography revealed beneath the Procellarum basin is consistent with the uplifting effects of the Imbrium impact and the development of the great lunar hotspot. Topographic undulations inferred for the lower and middle mantle reflect the dichotomized thermal evolution of the lunar near and far sides during bombardment time. On the nearside the results support the development of relatively thinner and hotter lithosphere by mantle convection that facilitated the diapiric rise of magma and mare flooding of the basins. For the farside the results favor the development of thicker and cooler lithosphere by viscous entrainment of lower density material into the lower mantle that limited basin flooding.

New details on the east Antarctic gravity field from the Gravity Recovery and Climate Experiment (GRACE) mission reveal a prominent positive free‐air gravity anomaly over a roughly 500‐km diameter subglacial basin centered on (70°S, 120°E) in north central Wilkes Land. This regional inverse correlation between topography and gravity is quantitatively consistent with thinned crust from a giant meteorite impact underlain by an isostatically disturbed mantle plug. The inferred impact crater is nearly three times the size of the Chicxulub crater and presumably formed before the Cretaceous formation of the east Antarctic coast that cuts the projected ring faults. It extensively thinned and disrupted the Wilkes Land crust where the Kerguelen hot spot and Gondwana rifting developed but left the adjacent Australian block relatively undisturbed. The micrometeorite and fossil evidence suggests that the impact may have occurred at the beginning of the greatest extinction of life on Earth at ∼260 Ma when the Siberian Traps were effectively antipodal to it. Antipodal volcanism is common to large impact craters of the Moon and Mars and may also account for the antipodal relationships of essentially half of the Earth's large igneous provinces and hot spots. Thus, the impact may have triggered the “Great Dying” at the end of the Permian and contributed to the development of the hot spot that produced the Siberian Traps and now may underlie Iceland. The glacial ice up to a few kilometers thick that has covered the crater for the past 30–40 Ma poses formidable difficulties to sampling the subglacial geology. Thus, the most expedient and viable test of the prospective crater is to survey it for relevant airborne gravity and magnetic anomalies.

[1] For the major lunar basins between ±64°latitude, uncompensated mass components as well as crustal ring and transient cavity attributes were evaluated from the free-air anomaly components of the Lunar Prospector gravity model (LP75G) that are spectrally correlated at 100-km altitude with the gravity effects of the Clementine terrain model (GLTM-2). Inversion of the terrain-correlated anomalies inferred incipient radial adjustments of the Moho that equilibrate the basin topography, assuming the lunar crust is mainly compensated by thickness variations. Nearly all nearside multiring basins are marked by mascon gravity anomalies that reflect mass concentrations from superisostatic mantle plugs plus mare fill. Farside basins, by contrast, are mostly characterized by maslite gravity anomalies from mass deficiencies due to subisostatic mantle plugs with marginal or no mare fill. The preponderance of maslites on the farside is consistent with crust that responded more rigidly to impacts than the nearside crust, which had a higher thermal gradient due to enhanced abundances of radioactive elements in the nearside mantle and crust. However, the extended thermal evolution of the lunar lithosphere also may have promoted the development of interfering mascon and maslite effects for tectonically complicated regions like the farside Freundlich-Sharonov basin. The Moho adjustments also revealed well-defined concentric zones of maxima and minima about the central basins that are strongly correlated with photogeologically inferred basin rings. In addition, the innermost zero-contour of the radial adjustments provides an effective estimate for the diameter of the transient cavity. The inferred transient cavity diameters correlated negatively with crustal thickness for basins with superisostatic mantle plugs and positively with the relative ages of the nearside basins. On the farside, superisostatic mantle plugs apparently developed in thinner crust up to thicknesses of about 30 km, whereas in thicker crust mantle plugs developed only to subisostatic levels. For the nearside, mass balance calculations between the equilibrium mantle plug and excavated basin materials support the proportional scaling relation of roughly 0.1 for the excavation depth-to-diameter ratio of the transient cavity that photogeologic studies have obtained. However, for the Serenitatis and Imbrium basins with possibly atypical nearside crustal properties, this proportionality may overestimate excavation depths by 28% and 37%, respectively. For nearside and farside basins apparently lacking mare flooding, shallow excavation depths were also inferred.