The formation of lunar mascon basins from impact to contemporary form

Abstract

Positive 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.

Ring faults and ring dikes around the Orientale basin on the Moon

“…The positive center and negative annulus are both found in the free air gravity as well, indicating the presence of a subisostatic annulus that drove uplift of the super-isostatic center (Andrews-Hanna, 2013;Freed et al, 2014;Melosh et al, 2013).…”

Gravitational search for cryptovolcanism on the Moon: Evidence for large volumes of early igneous activity

“…Mapping of cryptovolcanism with GRAIL data is conceptually similar to the use of gravity observations to map dense igneous intrusions beneath the Marius Hills volcanic province on the Moon ( Kiefer, 2013 ). We identify features in the gravity maps that are attributable to impact basins ( Head et al, 2010;Fassett et al, 2012;Melosh et al, 2013;Freed et al, 2014;Neumann et al, 2015 ), surface mare deposits (e.g., Hiesinger et al, 2011 ) especially including those at the edge of Oceanus Procellarum ( Andrews-Hanna et al, 2014 ), lunar volcanic fields such as Aristarchus Plateau or Marius Hills ( Spudis et al, 2013 ), or igneous vertical tabular intrusions ( Andrews-Hanna et al, 2013 ), and eliminate them as cryptovolcanic candidates. The remaining features that exhibit positive Bouguer gravity anomalies are considered as candidates for hidden igneous (extrusive and intrusive) deposits.…”

Small-scale density variations in the lunar crust revealed by GRAIL

“…The Bouguer gravity (Fig. 3A) shows a strong positive anomaly in the basin center corresponding to the uplifted mantle plug (Neumann et al, 1996;Wieczorek and Phillips, 1998;Wieczorek et al, 2013;Melosh et al, 2013;Freed et al, 2014). …”

“…1). At half wavelengths greater than ~68 km, the correlation between topography and gravity is weak (Wieczorek et al, 2006) due to the effects of crustal thickness variations (Neumann et al, 1996;Wieczorek and Phillips, 1998;Namiki et al, 2009;Huang and Wieczorek, 2012;Wieczorek et al, 2013;Zuber et al, 2013b), mascon loading (AndrewsHanna, 2013; Melosh et al, 2013;Freed et al, 2014), and lithospheric flexure. These longwavelength signals are commonly inverted for global crustal thickness modeling, where the assumption is made that the crust of the Moon is characterized by either a constant density or by long-wavelength variations in regional density (Wieczorek et al, 2006;Wieczorek et al, 2013).…”