| A fundamental question in martian geology is whether bolide impact(s) or mantle convection caused the hemispheric dichotomy. Cross-cutting and superposed crustal features interpreted from the MGS topography, magnetic observations, gravity anomalies, and crustal thickness estimates strongly favor the impact origin of the hemispheric dichotomy. Chryse is possibly the earliest giant basin-forming impact recorded in the martian crust. It was followed respectively by the Borealis, Utopia, Arcadia, Hellas, Isidis, and Argyre impacts, whereas the volcanic provinces of the Tharsis Rise and Elysium Planitia formed more recently. Other possible major impacts occurred in the Tharsis province at Sinai-Solis Planum, Syria Planum, and Thaumasia Planum. The giant impacts are superposed on ancient magnetic crust that forms an important temporal marker on the early crustal development of Mars. Impacts predominantly excavated the northern lowlands and covered the southern highlands with ejecta. Subsequent impact gardening, volcanism, and weathering by wind and fluids produced the current martian surface.; The origins of the more recent martian crustal features of the Tharsis province and the Elysium Planitia volcanic provinces are also important problems for martian geology and geodynamics. Tharsis province is antipodal to the Utopia, Hellas, and Isidis basin-forming events, while Elysium Planitia is antipodal to the Argyre, Solis-Sinai, and Thaumasia impact events. Also the Syrtis Major volcanic province is antipodal to the Syria Basin and Hesperia Planum is close to the antipode of the Chryse Basin. Our estimates of the formation energies for the major impact basins and Tharsis province suggest that the impactors may have cumulatively deposited about 3-30 x 10 29 J in the martian mantle if roughly half of the impactor energy propagated into the planet. In the roughly 2 x 1010km 2 Tharsis province ellipse, the estimated excess volume from volcanism and magmatism is roughly 2.1-2.8 x 1017m3, while the estimated energy required to heat and melt the Tharsis province magmatic/volcanic load is roughly 3-8 x 1026 J. Thus the long-lived martian volcanism and magmatism is consistent with substantial contributions of energy transmitted by strong shock and seismic waves from the giant impacts to the underlying mantle and their antipodal axes.; Mars' enigmatic crust also has the strongest remanent magnetism yet discovered in the solar system. (Abstract shortened by UMI.)... |
