EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Asymmetric growth of planetary stagnant lids

Callum Watson1, Jerome Neufeld1,2,3, and Chloé Michaut4,5
Callum Watson et al.
  • 1Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK
  • 2Centre for Environmental and Industrial Flows, University of Cambridge, Cambridge, UK
  • 3Department of Earth Sciences, University of Cambridge, Cambridge, UK
  • 4Laboratoire de Géologie de Lyon, Terre, Planètes, Environnement, École Normale Supériure de Lyon, Université de Lyon, Lyon, France
  • 5Institut Universitaire de France, France

Both the Moon1 and Mars2 are known to have significant degree-1 variations in their crustal thicknesses, with the Moon's far side and Mars's southern hemisphere having far thicker crusts than their respective opposing hemispheres. A number of potential mechanisms have been proposed to explain these dichotomies, including large impacts in both cases3,4, radiant heat from the Earth5 (in the case of the Moon), and large-scale volcanism6 (in the case of Mars). However, the effectiveness of these mechanisms are limited by the difficulty of sustaining a large hemispheric difference during the tens to hundreds of Ma of crustal formation. Both planets' lithospheres are examples of a fluid-dynamical boundary layer known as a stagnant lid, caused by temperature-dependent viscosity in a convecting system. We consider the effect of pressure on the viscosity of magma oceans and mantles, finding that under certain circumstances a spherically-symmetric stagnant lid is linearly unstable to asymmetric perturbations. The fastest-growing wavenumbers of this instability is degree 1, meaning that a small initial asymmetry may grow into a full-scale hemispherical dichotomy. We then numerically examine the stability of these asymmetric states, finding that they may last for hundreds of Ma. We also compare to the case of Mercury, a similarly-sized planet with no such crustal dichotomy, to determine if our analysis matches observations.


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2 Thiriet, M., Michaut, C., Breuer, D. & Plesa, A.-C. 2018 Hemispheric dichotomy in lithosphere thickness on mars caused by differences in crustal structure and composition. Journal of Geophysical Research: Planets 123 (4), 823–848.
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4 Andrews-Hanna, J.C., Zuber, M.T. & Banerdt, W.B. 2008 The borealis basin and the origin of the martian crustal dichotomy. Nature 453 (7199), 1212–1215.

5 Roy, A., Wright, J.T. & Sigurðsson, S. 2014 Earthshine on a young moon: Explaining the lunar farside highlands. The Astrophysical Journal Letters 788 (2), L42.

6 Golabek, G.J., Keller, T., Gerya, T.V., Zhu, G., Tackley, P.J. & Connolly, J.A.D. 2011 Origin of the martian dichotomy and tharsis from a giant impact causing massive magmatism. Icarus 215 (1), 346–357.

How to cite: Watson, C., Neufeld, J., and Michaut, C.: Asymmetric growth of planetary stagnant lids, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4758,, 2022.