A Research On Sub-surface Strucutres And Evolution Of Mercury Impact Basins Using Gravity Field

Mercury surface resembles the Moon as their both hold large numbers of impact basins.Remote sensing contributes to the investigation about Mercury basins’ spatial distribution,morphology features and spectral characterstics,while gravity field provides constraint on their sub-surface structures.By using the novel gravity field model and crustal thickness model of Mercury,we conducted the researches about Mercury basins’ sub-surface structures.These results are complementary to the study of surface features,which will refine the formation model of impact basins.Moreover,our findings will improve our understanding about the bombardment and modification history of Mercury basins,as well as the evolution of this planet.(1)The sub-surface structures of Mercury basins are inferred from crustal thickness model.We divied them into two group: the heavily relaxed basins and basins with minor relaxation.(2)Based on the sub-surface structures and gravity features of visible basins and basin formation theory,we found three potential buried impact basins under Northern Smooth Plains.Their crustal thickness ratios and approximated sizes are inferred by the inversion results,from which we suggest that these ancient basins formed on hotter crust.(3)The plains thickness is estimated from the diameters of buried impac basins,and it is larger than previous results.A multi-stages volcanic activity model is proposed to interpreated this discrepancy.(4)From crustal thickness ratio and basin formation time,we infer that Mercury was hot during pre-Tolstojan period(4.2-3.7 Ga)and the temperature was significantly lower when Caloris basin formed(3.5 Ga).(5)1-D heat conduction model is performed to verify the temperature evolution scheme and the result suggests that there must be a heating process elevating the temperature of Mercury crust during 4.1-3.7 Ga to ensure the relaxation of preTolsotjan basins.(6)Viscous relaxation process of topography and sub-surface structures of Mercury basins under different compensated states is simulated and we found a critical temperature of 1400 K for uncompensated topography relaxation.The temperatures required for siginificant relaxation for sub-surface structures under Airy isostatic compensation is 1500 K.