A new mechanism for the formation of the Martian dichotomy

The Martian dichotomy is the most conspicuous feature of the surface of the planet. The difference in elevation between the Northern and Southern hemispheres of Mars likely originates from a difference in crustal thickness. Inversion of topography and gravity data constrained by seismic data from the InSight NASA mission suggests that the southern crust is on average thicker by 18 to 28 km than the northern one if one assumes a uniform crustal density of 2900 kg m^-3 (Knapmeyer-Endrun et al., 2021 - Wieczorek et al, 2022).

Several explanations have been proposed for the origin of this crustal dichotomy, involving external processes, such as a large impact (Marinova et al., 2008), or internal ones, such as a degree-one mantle convection (Yoshida and Kageyama, 2006). Here we show that a positive feedback mechanism between crustal growth and partial melting in the mantle could have created this dichotomy. Indeed, because the crust is enriched in heat-producing elements (HPE), the lithosphere of a one-plate planet is thinner where the crust is thicker, inducing a lower pressure at the base of the lithosphere. Because of the pressure-dependence of the mantle solidus, partial melting is more important below a thinner lithosphere, causing a larger rate of melt extraction and crustal growth where the crust is thicker. Larger wavelength perturbations in crustal thickness and extraction, and thus hemispherical perturbations, grow faster because thermal diffusion dampens smaller wavelengths faster.

To model this effect, we use a parametric bi-hemispherical thermal evolution model where a well-mixed convective mantle is topped by two types of lithospheres (North and South) characterized by two potentially different thermal structures (Thiriet et al. 2018). The enrichment in HPE of the crust evolves during crust extraction as the enrichment of the newly formed crustal material depends on mantle melt fraction below the lithosphere, mantle enrichment and partition coefficient. In order to study the growth of a hemispherical perturbation, we impose a small initial difference in lithosphere or crust thickness in between the North and South. We then follow the thermal evolution, mantle melting, crustal growth and crustal enrichment in HPE in both hemispheres over 4.5 Gyr (Fig.1). Our model mainly depends on the mantle reference viscosity, that controls the cooling rate of the convective mantle, on and mantle permeability, that controls crustal extraction from the mantle.

Our results show that this positive feedback mechanism can indeed create a significant crustal dichotomy. The range of North-South crustal thickness differences that we obtain by varying the different model parameters largely encompasses that predicted by inversion of topography and gravity data, assuming different crustal densities. In particular, two types of thermal history allow to reproduce the crustal thickness difference predicted by InSight. The first one is obtained for a rather low viscosity and high mantle permeability; it shows a rapid and early extraction of the crust (Fig1. Solid line) and results in a cold potential temperature at the present-day. The second one is for a higher viscosity and lower mantle permeability; it leads to a late and prolonged extraction of the crust (Fig1. dashed line) and results in a warmer mantle potential temperature and a thicker lid at the present-day. In both cases, the crust is extracted during the first Gyr. The enrichment in HPE of the crust predicted by our model is in agreement with GRS data. 

Figure 1 : Evolution of the (a) lid thickness (b) Crust thickness, (c) average melt fraction in the partially melted zone below the lid. (d) Crustal HPE enrichment relatively to the Bulk Silicate Mars as a function of time for the Northern (blue lines) and Southern (orange lines) hemispheres for 2 different simulations that allow to reproduce the difference in crustal thickness deduced from the InSight mission. One evolution (shown in dashed lines) is for a rather high permeability k₀ = 9.1.10⁻¹⁰ m² and low reference viscosity η₀ = 4.5x10²⁰ Pa.s, while the second evolution (shown in solid lines) has a lower permeability k₀ = 3.72.10⁻¹¹ m² and a higher viscosity η₀ = 2.02x10²¹ Pa.s.

Acknowledgment :

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 101001689) and from the ANR (grant MAGIS, ANR-19-CE31-0008-08).

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