Ancient selenophysical structure of the Grimaldi basin: Constraints from GRAIL gravity and LOLA topography

The Grimaldi impact basin is located near the western limb of the moon and lies to the southwest of the Oceanus Procellarum. A clearly visible positive gravity anomaly exists in its low-lying inner wall, implying a subsurface mass concentration beneath the crater. Exploration of this basin could extend our meaningful and fundamental understanding of giant impact processes as well as the structure of mare basins/craters. Limited by the low-resolution of previous gravity field models, it was once impossible to explore the structure beneath Grimaldi. The recent high-resolution gravity data from the Gravity Recovery and Interior Laboratory mission make it possible to break through this barrier. Prior to our investigation of the selenophysical structure around crater Grimaldi, we developed a flexure model that includes surface and subsurface loads. A localized admittance analysis was performed by combining high-resolution gravity data with the high-resolution topography data obtained from Lunar Orbiter Laser Altimeter. Within 2σSTD error constraints, we estimated the best-fit parameters over Grimaldi as well as two other places in its neighborhood. All the predicted admittances show a goodness of fit with their corresponding observations. The large load ratio (∼9.2) found at the Grimaldi is an indirect mirror of the dominant subsurface load, consistent with its large positive gravity anomaly in its low-lying floor. Not only the crustal thickness of 48 km but also the crustal density of 2500 kg m−3 found around Grimaldi shows a great accordance with the recent results of GRAIL. Given the best-fit parameters of f, bc and ρc, the elastic thickness Te is found to be around 28 km over Grimaldi. Taking into account the best-fit values in its neighborhood, a regional elastic thickness of 30 km could be roughly concluded around Grimaldi, which is completely in the previous ranges (20 < Te< 60 km) from Clementine. Our result of the elastic thickness is quite larger than those on the lunar nearside volcanic complexes of the recent GRAIL study. Considering fewer thermal activities occurred on the lunar limb or farside than its nearside, we can then deduce that the lithosphere could be possibly cold and therefore developed a corresponding dense elastic thickness.