Effects of Regional Thermal State on the Crustal Annulus Relaxation of Lunar Large Impact Basins

Abstract

AbstractThe Moon reveals striking asymmetries in the crustal structure and chemical composition between its nearside and farside. With the gravity‐based crustal thickness model, we find that most large impact basins with rim diameters greater than 450 km on the nearside have no crustal annulus, which is instead common for basins on the farside. Previous impact hydrodynamic simulations show that the thermal state has significant influences on the impact cratering process. However, the effects of the thermal state on the long‐term post‐impact relaxation, not well studied, may be equally important for explaining the different crustal annulus structures between the nearside and farside basins. In this work, we export crustal structure and impact‐induced thermal anomaly from impact hydrodynamic simulations to subsequent post‐impact viscoelastic relaxation models, and vary the near‐surface temperature gradient and radiogenic element content (represented by Th content as a proxy) to explore influences of the crustal thermal state on the post‐impact relaxation of the annulus. Our modeling results indicate that as the basin structure starts with a relatively hot state and cools afterwards, a prolonged high‐temperature (>1,200 K) duration of the crustal annulus is required for its effective relaxation, which corresponds to a high near‐surface temperature gradient or Th content of the crust. More specifically, a near‐surface temperature gradient greater than 30 K/km in combination with a Th content of 4 ppm for the bulk crust (or a Th content of 10 ppm for a 10 km‐thick KREEP layer underlying the crust), likely representing the early thermal state of the nearside, can produce the complete annulus relaxation. In contrast, a near‐surface temperature gradient lower than 20 K/km with a crustal Th content of 1 ppm, possibly representing the thermal state of the lunar farside, produces limited relaxation. Our modeling results show that the thermal state of the lunar crust controls the final crustal annulus structure of impact basins, which can explain the different crustal annuli of large impact basins between the nearside and farside of the Moon.