tokens: We wish to thank T. Fouchet, E. Jaupart, G. Laibe, and D. Stevenson for helpful discussions. G.C. acknowledges the warm hospitality of the OWL Institute at the University of Santa Cruz, where part of this work was accomplished. This work was supported by the Programme National de Planétologie (PNP) of CNRS-INSU cofunded by CNES.
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