Relaxation of the Martian Dichotomy and Elastic Thickness Estimates as Constraints on the Thermal and Volatile Evolution of Mars

To constrain the thermal and volatile evolution of Mars, we have used three different thermal models for Mars, stagnant lid [Hauck and Phillips, 2002], early plate tectonics followed by stagnant lid [Nimmo and Stevenson, 2002; Breuer and Spohn, 2003], and mantle overturn [Elkins-Tanton et al., 2003], to predict the relative amount of relaxation of the global dichotomy and elastic thickness values from the Nochian to the Hesperian/Amazonian boundary [McGovern et al. 2002, 2004]. Both wet and dry rheologies are considered. We model relaxation across the dichotomy boundary using semi-analytical viscoelastic numerical modeling with two density and three viscosity layers. Our results are compared to the topography across the dichotomy and to elastic thickness estimates derived from gravity and topography data. Our goal is to determine the lithospheric conditions that are consistent with both preserving the crustal dichotomy and an increase of elastic thickness from approximately 15 km to over 100 km between the Noachian and the Hesperian/Amazonian. All three thermal models can preserve the long-wavelength topography of Mars while relaxing the short wavelengths if the viscosity in the lower crust is 10e20-10e21 Pas at 4-3.9 Ga. All three thermal models require crustal and mantle rheology in the Noachian and Hesperian to be wet. Constraints on rheology at the Hesperian/Amazonian boundary depend on the crustal thickness and thermal model. Assuming a highlands' crustal thickness of 62 km (an average global crustal thickness of 45 km), crustal rheology has to change from wet to dry during the Hesperian for the stagnant lid model in order to fit the "observed" elastic thickness. This change is consistent with the juvenile water release on the Martian surface associated with volcanism during this period. The other two thermal models fit the observed elastic thickness with either dry or wet crustal rheology. If we assume a larger average crustal thickness of 62 km (77 km under the highlands), all three thermal models require that the rheology transitions from wet to dry in the Hesperian. Mantle rheology at the Hesperian/Amazonian boundary is unconstrained.