Despinning plus global contraction and the orientation of lobate scarps on Mercury: Predictions for MESSENGER

Observations by the Mariner 10 spacecraft suggest that the lobate scarps on Mercury, which have been interpreted to record at most 1–2 km of radial contraction of the planet after the end of the Late Heavy Bombardment, possess a global, preferred N–S orientation but lack a strong latitudinal dependence on their surface expression. Here, we reexamine the idea that a decrease in the planetary rotation rate (despinning) coupled with global contraction of at least 3–5.5 km prior to the end of Late Heavy Bombardment resulted in global N–S oriented thrust faults. The surface expression of these faults is assumed to have been erased by the end of the Late Heavy Bombardment, and the faults were subsequently reactivated by later global contraction, producing generally N–S oriented thrust faults from an isotropic stress field. We use the estimate of > 3 – 5.5   km contraction prior to ∼ 4   Ga as an additional constraint to thermomechanical simulations of the evolution of Mercury, finding that a wide range of models are consistent with this observation. The fact that a wide range of states are consistent with the contraction of Mercury prior to the end of Late Heavy Bombardment but only a restricted set of states are consistent with the at most 1–2 km of subsequent contraction bolsters the idea that there may be hidden strain on Mercury, features unseen by Mariner 10 but likely visible to the MESSENGER spacecraft.