Martian Dichotomy from a Giant Impact: Mantle Convection Models

<p>The Martian crustal dichotomy is one of the most prominent features on the planet, featuring a ≈5.5 km difference in topography and a ≈25 km difference in crustal thickness between the southern highland and northern lowland [1]. It Is thought to have formed within the first 400-500 Myr of Martian history [2]. While its formation process remains unclear, there have been different hypotheses to explain it, including an endothermic degree-1 convection mode [3, 4], and the excavation of the lowland crust by a giant impact [5]. In this study we focus on the hybrid hypothesis, where an early giant impact created a magma pond, and subsequent mantle convection alters the internal mantle structure as well as crustal distribution in the next 4 billion years [6, 7].  By imposing a parametrized giant impact as a thermal anomaly as an initial condition, we simulate the long-term evolution of the crust and mantle using the thermochemical convection code StagYY [8]. In particular, we investigate the effect of physical parameters of both the solid mantle and the impact-induced magma pond, as well as those of the crust production process, on the crystallisation of such pond, its interaction with surrounding mantle and the preservation of impact signature. Diagnostics including topography and crust thickness from these different models will be presented and compared.</p><p> </p><p>[1] Watters, T., McGovern, P., & Irwin III, R. (2007). Hemispheres Apart: The Crustal Dichotomy on Mars. Annual Review of Earth and Planetary Sciences, 35(1), 621-652.</p><p>[2] Taylor, S., & McLennan, S. (2009). Planetary crusts. Cambridge, UK: Cambridge University Press.</p><p>[3] 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).</p><p>[4] 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.</p><p>[5] Andrews-Hanna, J., Zuber, M., & Banerdt, W. (2008). The Borealis basin and the origin of the martian crustal dichotomy. Nature, 453(7199), 1212-1215.</p><p>[6] 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.</p><p>[7] Reese, C., Orth, C., & Solomatov, V. (2011). Impact megadomes and the origin of the martian crustal dichotomy. Icarus, 213(2), 433-442.</p><p>[8] 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</p><p> </p><p> </p>

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

<p>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]. </p><p>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.</p><p>Preliminary results from coupled SPH-geodynamics evolution models are presented, involving the crust thickness and topography maps after 4.5 Gyrs of evolution.</p><p> </p><p>[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).</p><p>[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.</p><p>[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.</p><p>[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.</p><p>[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</p><p>[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.</p>

From a magma ocean to a solid mantle: implications for the thermo-chemical evolution of Mars

<p>Several studies suggest that Mars went through an episode of Magma Ocean (MO) early in its history. When the MO crystallises, solid mantle appears. The crystallisation of this MO starts at the Core-Mantle Boundary (CMB) and continues upwards to the surface of the planet. Assuming that this process occurs by fractional crystallisation, the solid cumulates that form are progressively enriched in incompatible elements, including iron, and an unstable density stratification is developed. This stratification is thought to have resulted in a planetary-scale mantle overturn after MO crystallisation, potentially explaining the early magnetic field, crustal dichotomy and chemical heterogeneities present on martian mantle.</p><p>However, previous studies on the thermo-chemical evolution of Mars consider only fractional crystallisation of the MO, and lack the possibility of re-melting/re-freezing of material at the mantle-MO interface, before the MO is fully crystallised.</p><p>In this study we investigate the effect of re-melting/re-freezing of material at the mantle-MO interface during MO crystallisation, on the dynamics and composition of the solid mantle. We use a numerical method with the convection code StagYY. The solid mantle is represented by a 2D spherical annulus geometry, and the MO by a 0D object at top of the mantle. The boundary condition applied to the solid domain allows the parameterisation of fractional crystallisation/re-melting of material at the mantle-MO interface. We model the growth of the solid mantle from the CMB up to the surface of the planet, and we account for core cooling and the presence of an atmosphere.</p><p>We show that by taking re-melting/re-freezing of material into account, the onset of convection can start earlier in Mars history. These results bring implications for the density stratification and overturn, and to the existence of isotopically distinct reservoirs on the mantle. Moreover, our results show that the mode of convection is preferentially degree-1, which can potentially explain the crustal dichotomy. </p>