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. 2018 Feb 26;11:168-172.
doi: 10.1038/s41561-018-0071-2.

Continuous Reorientation of Synchronous Terrestrial Planets Due to Mantle Convection

Free PMC article

Continuous Reorientation of Synchronous Terrestrial Planets Due to Mantle Convection

Jérémy Leconte. Nat Geosci. .
Free PMC article


Many known rocky exoplanets are thought to have been spun down by tidal interactions to a state of synchronous rotation, in which a planet's period of rotation equals that of its orbit around its host star. Investigations into atmospheric and surface processes occurring on such exoplanets thus commonly assume that day and night sides are fixed with respect to the surface over geological timescales. Here we use an analytical model to show that true polar wander - where a planetary body's spin axis shifts relative to its surface because of changes in mass distribution - can continuously reorient a synchronous rocky exoplanet. As occurs on Earth, we find that even weak mantle convection in a rocky exoplanet can produce density heterogeneities within the mantle sufficient to reorient the planet. Moreover, we show that this reorientation is made very efficient by the slower rotation rate of a synchronous planet compared to Earth, which limits the stabilizing effect of rotational and tidal deformations. Furthermore, the ability of a lithosphere to support remnant loads and stabilize against reorientation is limited. Although uncertainties exist regarding the mantle and lithospheric evolution of these worlds, we suggest that the axes of smallest and largest moment of inertia of synchronous exoplanets with active mantle convection change continuously over time but remain closely aligned with the star-planet and orbital axes, respectively.

Conflict of interest statement

Competing interests The author declares that he has no competing financial interests.


Figure 1
Figure 1. Schematic picture of True Polar Wander driven by mantle convection on a synchronous planet.
A: Hot, low density (lighter shading) upwelling plumes rise and cause surface uplift. The net effect is a positive geoid/mass anomaly (exaggerated here) that coincides with the axis of the smallest moment of inertia. In contrast, cold downwellings (darker shading) are negative anomalies where the axis of largest moment of inertia will lie. B: TPW will tend to align these axes with the star-planet and rotation axes, respectively. If numerous plumes are present instead of the 2-cell pattern shown, the principal axes will be determined by the resulting degree 2 moment. Surface topography follows the reorientation. If plate tectonics occurs, continents will undergo an additional drift with respect to the mantle.
Figure 2
Figure 2. Efficiency of true polar wander (XTPW) on known rocky exoplanets as a function of their orbital period (dots).
The ⊕ symbol shows the value of XTPW for the Earth. The color of the dots refers to the equilibrium blackbody temperature of the planet (TBB) determined assuming a complete redistribution of the incoming stellar energy (See Methods). As expected, cooler planets have longer orbital periods. All synchronous planets with an orbital period above 1-2 days should undergo true polar wander very easily. The right ordinate axis shows the timescale at which the pole is able to follow the axis of largest moment of inertia (τTPW; See Methods).
Figure 3
Figure 3. Minimal inertia anomaly (〈𝒞^min/ℐs) needed to excite significant polar wander as a function of the planetary temperature (TBB).
The size of the dot is proportional to the size of the planet. The ⊕ and ♀ symbols show the convective inertia anomaly for the Earth and Venus. The shaded area illustrates the range of convective contribution predicted by our simple scaling by varying the radius of the planet between 0.5 and 1.6 R and the convective viscosity within two orders of magnitude. For temperate planets, mantle convection would have to be two to three orders of magnitude less vigorous than on Earth to suppress TPW.
Figure 4
Figure 4. Dimensionless contribution of the elastic remnant bulge to the inertia deformation tensor as a function of the planet radius for all known rocky exoplanets
(〈𝒞̂lit/ℐs); See Methods). The color of the dot shows the equilibrium temperature of the planet. For comparison, the dashed black line shows the contribution of convective motions to the inertia tensor expected from our simple scaling (〈𝒞̂conv/ℐs; See text). The shaded area illustrates the uncertainty on this prediction by varying the convective viscosity within two orders of magnitude. For all warm and temperate planets, the deformation is expected to be dominated by convective motions.

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