Lunar crustal analysis of Mare Orientale from topographic and gravity correlations

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 Clementine satellite and earlier investigations. The analysis considered a 64°-by-64° region of the Moon centered roughly on Mare Orientale at an altitude of 100 km. The topography of the study region, which includes over 11 km 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/cm³. 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 imbalances that may be equilibrated by radial adjustments of the Moho of up to 44 km, assuming Airy-Heiskanen compensation and a mantle density contrast of 0.5 g/cm³ with the crust. These Moho adjustments define mass variations that account for most of the mascon 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 anomalies 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 km. Corresponding terrain-decorrelated free-air gravity anomalies of Mare Orientale may be related to a central cone-shaped body of 0.5 g/cm³ density contrast with apex extending nearly 5 km below the surface, which is surrounded by a ringed-shaped body of -0.5 g/cm³ 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 remelting of the rocks about the impact site may have occurred.