Gravity evidence for a heterogeneous crust of Mercury

We modeled gravity data to explore Mercury's internal structure and show the presence of crustal heterogeneities in density. We first evaluated the lithospheric flexure occurring in the spherical harmonic degree range 5-80, according to the flexural isostatic response curve. We thus estimated a mean elastic lithosphere thickness of about 30 [Formula: see text] 10 km and modeled the crust-mantle interface, which varies from 19 to 42 km depth, according to a flexural compensation model. The isostatic gravity anomalies were then obtained as the residual field with respect to the contributions from topography and lithospheric flexure. Isostatic anomalies are mainly related to density variations in the crust: gravity highs mostly correspond to large-impact basins suggesting intra-crustal magmatic intrusions as the main origin of these anomalies. Isostatic gravity lows prevail, instead, above intercrater plains and may represent the signature of a heavily fractured crust.

Mercury's Crustal Thickness and Contractional Strain

The crust of Mercury has experienced contraction on a global scale. Contractional deformation is expressed by a broadly distributed network of lobate thrust fault scarps. The most likely principal source of stress is global contraction from cooling of Mercury's interior. Global contraction alone would be expected to result in a uniformly distributed population of thrust faults. Mercury's fault scarps, however, often occur in long, linear clusters or bands. An analysis of the contractional strain as a function of crustal thickness, estimated in two crustal thickness models (CT1 and CT2) derived from gravity and topography data obtained during the MESSENGER mission, indicates the greatest contractional strain occurs in crust 50-60 km thick. On Earth, mantle downwelling can thicken and compress overlying crust, regionally concentrating thrust faults. Clusters of lobate scarps collocated with regions of thick crust suggest downward mantle flow contributed to the localization of lithosphere-penetrating thrust faults.

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

The Orientale basin is the youngest and best-preserved multiring impact basin on the Moon, having experienced only modest modification by subsequent impacts and volcanism. Orientale is often treated as the type example of a multiring basin, with three prominent rings outside of the inner depression: the Inner Rook Montes, the Outer Rook Montes, and the Cordillera. Here we use gravity data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission to reveal the subsurface structure of Orientale and its ring system. Gradients of the gravity data reveal a continuous ring dike intruded into the Outer Rook along the plane of the fault associated with the ring scarp. The volume of this ring dike is ~18 times greater than the volume of all extrusive mare deposits associated with the basin. The gravity gradient signature of the Cordillera ring indicates an offset along the fault across a shallow density interface, interpreted to be the base of the low-density ejecta blanket. Both gravity gradients and crustal thickness models indicate that the edge of the central cavity is shifted inward relative to the equivalent Inner Rook ring at the surface. Models of the deep basin structure show inflections along the crust-mantle interface at both the Outer Rook and Cordillera rings, indicating that the basin ring faults extend from the surface to at least the base of the crust. Fault dips range from 13-22° for the Cordillera fault in the northeastern quadrant, to 90° for the Outer Rook in the northwestern quadrant. The fault dips for both outer rings are lowest in the northeast, possibly due to the effects of either the direction of projectile motion or regional gradients in pre-impact crustal thickness. Similar ring dikes and ring faults are observed around the majority of lunar basins.