Impact-induced crustal dichotomy on Mars: from SPH to long-term mantle convection models

The formation process of the crustal dichotomy of Mars has remained elusive since its discovery more than three decades ago.  Workers put forward different theories including (i) an endogenic origin, where the dichotomy is formed by degree-1 mantle convection [1, 2]; (ii) an exothermic origin, where the northern crust is excavated by an impact [3]; and (iii) a hybrid origin, where an impact generated large amounts of melt, followed by crust production shaping the crustal dichotomy [4]. 

In this study we focus on the last hypothesis. Our previous results using a parameterized impact show that a dichotomy can be formed in this manner.  In order to confirm whether these results still hold when using a realistic impact, and to consider the most probable impact angles and velocities, a SPH code [5] is used to model both the impact itself and the first 24 hours of post-impact evolution. The result is then transferred into mantle convection code StagYY [6] in order to simulate the long-term evolution of both crust and mantle for 4.5 Gyrs.  Due to the different physical nature and assumptions between the SPH impact models and long-term mantle convection models, care in data treatment is required when coupling the two simulations.  In this study, different setups regarding the transfer of data are tested and explored, including the treatment of temperature profiles, the choice of density and viscosity of materials, and the time of transfer.

Preliminary results from coupled SPH-geodynamics evolution models are presented, involving the crust thickness and topography maps after 4.5 Gyrs of evolution.

 

[1] Roberts, J., & Zhong, S. (2006). Degree-1 convection in the Martian mantle and the origin of the hemispheric dichotomy. Journal Of Geophysical Research, 111(E6).

[2] Keller, T., & Tackley, P. (2009). Towards self-consistent modeling of the martian dichotomy: The influence of one- ridge convection on crustal thickness distribution. Icarus, 202(2), 429-443.

[3] Andrews-Hanna, J., Zuber, M., & Banerdt, W. (2008). The Borealis basin and the origin of the martian crustal dichotomy. Nature, 453(7199), 1212-1215.

[4] Golabek, G., Keller, T., Gerya, T., Zhu, G., Tackley, P., & Connolly, J. (2011). Origin of the martian dichotomy and Tharsis from a giant impact causing massive magmatism. Icarus, 215(1), 346-357.

[5] Emsenhuber, A., Jutzi, M., Benz, W. (2018). SPH calculations of Mars-scale collisions: The role of the equation of state, material rheologies, and numerical effects. Icarus, 301, 247-257

[6] Tackley, P. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three- dimensional spherical shell using the yin-yang grid. Physics Of The Earth And Planetary Interiors, 171(1-4), 7-18.