Acknowledgements
The authors wish to thank D. Prelevic, S. Foley, Rob Ellam, K. Bell, F. Sani, G. Bianchini, C. Faccenna, V. Morra, and L. Morbidelli for stirring and focussing discussion and in some cases for suggestions at different stages of the manuscript preparation, L. Fedele for supplying the electronic base of figure 29. Financial support was provided by the Università degli Studi di Firenze (DST – ex 60% funds) issued to Sandro Conticelli, by the Italian Ministry of Education and Research (MIUR) through PRIN_2008 (research grants #: 2008HMHYFP_002; 2008HMHYFP_003; 2008HMHYFP_005).