INTRODUCTION

In the belief that only unkind gods would arrange two energy sources for planetary dynamos as equally important, this re-exploration of plausible sources seeks to eliminate rotational energy in favor of convection. Recent experiments and theory of the ‘elliptical’ instabilities in a rotating fluid due to precessional and tidal strains provide quantitative results for velocity fields and energy production. The adequacy of these flows to produce a. dynamo on both terrestrial and giant planets is assessed in the context of ‘strong field’ scaling. With little ambiguity it is concluded that Mercury, Venus, and Mars can not have a dynamo of tidal or precessional origin. The case for today's Earth is marginal. Here precessional strains (accidentally comparable to tidal strains) also are potential sources of inertial instabilities. The ancient Earth with its closer Moon, as well as all the giant planets, have tides well in excess of those needed to critically maintain dynamos. Hence the project proposed here proves to be successful only in part – an Earth in the distant future will not be able to sustain the geodynamo with its rotational energy. On the other hand, convection remains a possible dynamo energy source, with such a large number of undetermined processes and parameters that it is unfairly easy to establish conditions for its inadequacy. A large literature explores its adequacy. A brief review of this literature, in both a ‘strong-field’ and ‘weak-field’ context, advances several cautionary restraints to be employed on that day when the limits of validity of a quantitative dynamo-convection theory are to be determined.