THE PRESENT AS HISTORY - AN ENSEMBLE APPROACH TO TERRESTRIAL PLANET EVOLUTION

An outline of a general theory of terrestrial planet evolution is presented based on the recognition that, even in principle, only a restricted set of the observed and inferred features familiar to Earth science can be formally explained. The temperature and (volatile) compositional dependence of planetary material's creep resistance on the thermal state of the interior is suggested as crucial to restricting the set of possible planetary states; this subset is marked by a self-regulating character. An important, but subsidiary question of the duration of surface motions on terrestrial planet-like bodies is posed and a further positive role for volatile phases identified. We predict the end of plate tectonics on Earth about 2.5 billion years hence, as a response to increasing solar luminosity. Finally, chemical differentiation by local and transient creation of magma is examined and two mechanisms consistent with self-regulating planetary states are discussed.