New constraints on the thermal and volatile evolution of Mars

The thermal and volatile evolution of Mars has not been studied from the perspective of consistency with the preservation of the Martian global dichotomy and with estimates of the elastic thickness over time. We use three thermal evolution models for Mars: (1) stagnant lid, (2) early plate tectonics followed by stagnant lid, and (3) mantle overturn, to calculate the amount of relaxation of the dichotomy boundary and elastic thickness values for Noachian- and Hesperian-aged terrains. To explore a wide range of parameters, we evaluate two different initial mantle temperatures, and wet and dry rheologies. Our model results show that the relative water content of the crust has an effect roughly equal to 500 K variations of initial mantle temperature. For all three thermal models, a lower crust viscosity of 1020–1021 Pa s during the first 0.1 Ga after formation of dichotomy would allow for the preservation of the long-wavelength topography of Mars and fitting of the elastic thickness. This viscosity range implies either wet, cold (∼1500 K) lower crust, or dry, hot (∼2000 K) lower crust in Noachian. Additional constraints are necessary to distinguish between the individual thermal models. For the stagnant lid model, neither the cold, wet crust nor the hot, dry crust agree with timing and amount of the crustal production [Hauck II., S.A., Phillips, R.J., 2002. Thermal and crustal evolution of Mars. J. Geophys. Res. 107, 5052]. Moreover, drying of the crust is required for this model in order to match the admittance elastic thickness at the Hesperian/Amazonian boundary implying remelting of the crust. The hot, dry crust in the early plate tectonics model limits a plate tectonic epoch to only 100–200 Myr and implies dry mantle, which is in disagreement with water found in meteorites. The cold, wet crustal rheology implies the formation of crust during the plate tectonics regime because of the low crustal production during the stagnant lid regime. For mantle overturn, the temperature required for wet crust does not fit the original mantle profile while the dry crust does; however, in order to explain the initially hot thermal profile the crust must have been emplaced very fast. Generally, dry crustal rheology does not fit low elastic values in the Hesperian and implies either that rheology may differ between the southern and northern hemispheres: wet in northern hemisphere and dry in southern hemisphere, or that local weakening occurred. Wet crustal rheology fits well all elastic data except S. Hellas rim, which may be anomalous. Mantle rheology is unconstrained by our modeling and can be either dry or wet.