Effects of Regional Thermal State on the Crustal Annulus Relaxation of Lunar Large Impact Basins. Issue 3 (7th March 2022)

• Record Type: Journal Article
• Authors: Ding, Min; Zhu, Meng‐Hua
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-07
• Subjects: Moon -- large basins -- viscoelastic relaxation -- thermal state -- crustal annulus -- thermal mechanics; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE007132
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The 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. Plain Language Summary: Significant asymmetries in the crustal structure and chemical composition exist between the nearside and farside of the Moon. These asymmetries are also manifested by the crustal structure of impact basins, including an annulus of thickened crust extending radially from half of to one basin radius. While the impact basins on the farside of the Moon are associated with prominent crustal annuli, the impact basins in the nearside Procellarum KREEP Terrane (PKT, and here KREEP represents for regionally enriched elements, potassium, rare earth elements and phosphorus) are highly relaxed with negligible annulus. We investigate whether this distinction could be explained by different thermal states and radiogenic element contents between the nearside and farside of the Moon. To support this hypothesis, we apply a two‐code modeling method to sequentially simulate the impact dynamics and post‐impact relaxation process, with a focus on the thermal effects at the post‐impact relaxation stage. Our modeling results indicate that 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) can reproduce the complete annulus relaxation in the PKT. In contrast, a near‐surface temperature gradient lower than 20 K/km with a crustal Th content of 1 ppm can produce limited annulus relaxation similar to the farside of the Moon. Our modeling results are consistent with observations, supporting that the thermal state and radiogenic element content indeed controls the degree of annulus relaxation for lunar impact basins. Key Points: We apply viscoelastic relaxation after impact dynamic simulations to model the thermo‐mechanical evolution of lunar impact basins Our results show that the thermal state and radiogenic element content control the post‐impact relaxation of crustal annulus Post‐impact relaxation explains reduction of crustal annulus for largest nearside basins, which does not commonly occur for farside basins