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Mercury’s spin–orbit resonance explained by initial retrograde and subsequent synchronous rotation


The planet Mercury rotates three times about its spin axis for every two orbits about the Sun1,2, in a 3/2 spin–orbit resonance. This unique state has been explained by an initial rapid prograde rotation, which was then decelerated by tidal torques to the present resonance3,4,5,6. When friction at the core–mantle boundary is accounted for, capture into the 3/2 resonance occurs with a probability of only 26%, whereas the most likely outcome is capture into one of the higher-order resonances7. Here we use a numerical model of Mercury’s rotational evolution to investigate the consequences of an initial retrograde rotation of Mercury. We find that in this case, the planet would be captured into synchronous rotation, with one hemisphere always facing the Sun, with a probability of 68%. Strong lateral variations in the impact cratering rate would have existed, consistent with the observed distribution of large impact basins. Escape from this highly stable resonance can be initiated by the momentum imparted by large, basin-forming impact events8,9,10, and subsequent capture into the 3/2 resonance is likely. During synchronous rotation, substantial quantities of volatile deposits would have accumulated on the hemisphere facing away from the Sun, potentially explaining the existence of sublimation hollows on Mercury’s surface11.

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Figure 1: Capture into synchronous rotation from initial retrograde rotation.
Figure 2: Impact crater size and impact velocity required to unlock Mercury from synchronous rotation.
Figure 3: Predicted cratering rates and observed impact basins.

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We acknowledge support from the French Programme National de Planétologie, the French Centre National de la Recherche Scientifique, and from the Portuguese Fundação para a Ciência e a Tecnologia under grant PTDC/CTE-AST/098528/2008.

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M.A.W. developed the scenario of unlocking synchronous rotation by impacts and analysed the distribution of impact basins. A.C.M.C. performed the rotational simulations. M.L.F. calculated the spatial variations in the impact cratering rate. All authors contributed to developing the conclusions and implications in the manuscript.

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Correspondence to Mark A. Wieczorek.

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The authors declare no competing financial interests.

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Wieczorek, M., Correia, A., Le Feuvre, M. et al. Mercury’s spin–orbit resonance explained by initial retrograde and subsequent synchronous rotation. Nature Geosci 5, 18–21 (2012).

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