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- Title
A Positive Feedback Between Crustal Thickness and Melt Extraction for the Origin of the Martian Dichotomy.
- Authors
Bonnet Gibet, Valentin; Michaut, Chloé; Wieczorek, Mark; Lognonné, Philippe
- Abstract
A North/South difference in crustal thickness is likely at the origin of the Martian dichotomy in topography. Recent crustal thickness maps were obtained by inversion of topography and gravity data seismically anchored at the InSight station. On average, the Martian crust is 51–71 km thick with a southern crust thicker by 18–28 km than the northern one. The origin of this crustal dichotomy is still debated although the hypothesis of a large impact is at present very popular. Here, we propose a new mechanism for the formation of this dichotomy that involves a positive feedback between crustal growth and mantle melting. As the crust is enriched in heat‐producing elements, the lid of a one‐plate planet is hotter and thinner where the crust is thicker, inducing a larger amount of partial melt below the lid and hence a larger rate of melt extraction and crustal growth. We first demonstrate analytically that larger wavelength perturbations, that is, hemispherical perturbations, grow faster because smaller wavelengths are more attenuated by thermal diffusion. We then use a parameterized thermal evolution model with a well‐mixed mantle topped by two different lids characterized by their thermal structures and thicknesses to study the growth of the crust in the two hemispheres. Our results demonstrate that this positive feedback can generate a significant crustal dichotomy. Plain Language Summary: The dichotomy is the most noticeable feature of the Martian surface. The highest elevations in the South are likely due to a larger crustal thickness than in the North, as is the case on Earth below mountain belts. This difference in thickness was recently constrained by the Insight mission. We show here that a positive feedback between crustal thickening and mantle melting could have generated this dichotomy. Indeed, as the crust is enriched in heat‐producing elements, the temperature profile is warmer where the crust is thicker. This results in hotter rocks at lower pressures leading to more melting, and therefore more melt extraction and crustal thickening beneath thicker crustal regions. Since lateral heat diffusion tends to attenuate short wavelength perturbations, the longest wavelengths, that is, hemispherical perturbation, will grow the fastest. In this study, we develop a thermal evolution model that takes into account the extraction of the crust in two different hemispheres. Our results show that small initial perturbations can generate large differences in crustal thickness. This mechanism alone could explain the dichotomy in crustal thickness; it would also act to significantly amplify an initial dichotomy generated by another mechanism such as a large impact or a degree‐one mantle convection. Key Points: On one‐plate planets, regions of thicker crusts are hotter leading to a positive feedback mechanism between crust growth and melt extractionThis mechanism is favored at larger wavelengths because smaller wavelengths are more attenuated by lateral thermal diffusionThis mechanism can explain the hemispheric difference in thickness of the Martian crust
- Subjects
SEISMIC anisotropy; MARTIAN surface; MELTING; OROGENIC belts
- Publication
Journal of Geophysical Research. Planets, 2022, Vol 127, Issue 12, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2022JE007472