On the evolution of terrestrial planets: Bi-stability, stochastic effects, and the non-uniqueness of tectonic states

The Earth is the only body in the solar system for which significant observational constraints are accessible to such a degree that they can be used to discriminate between competing models of Earth's tectonic evolution. It is a natural tendency to use observations of the Earth to inform more general models of planetary evolution. However, our understating of Earth's evolution is far from complete. In recent years, there has been growing geodynamic and geochemical evidence that suggests that plate tectonics may not have operated on the early Earth, with both the timing of its onset and the length of its activity far from certain. Recently, the potential of tectonic bi-stability (multiple stable, energetically allowed solutions) has been shown to be dynamically viable, both from analytical analysis and through numeric experiments in two and three dimensions. This indicates that multiple tectonic modes may operate on a single planetary body at different times within its temporal evolution. It also allows for the potential that feedback mechanisms between the internal dynamics and surface processes (e.g., surface temperature changes driven by long term climate evolution), acting at different thermal evolution times, can cause terrestrial worlds to alternate between multiple tectonic states over giga-year timescales. The implication within this framework is that terrestrial planets have the potential to migrate through tectonic regimes at similar ‘thermal evolution times’ (e.g., points were they have a similar bulk mantle temperature and energies), but at very different ‘temporal times’ (time since planetary formation). It can be further shown that identical planets at similar stages of their evolution may exhibit different tectonic regimes due to random variations. Here, we will discuss constraints on the tectonic evolution of the Earth and present a novel framework of planetary evolution that moves toward probabilistic arguments based on general physical principals, as opposed to particular rheologies, and incorporates the potential of tectonic regime transitions and multiple tectonics states being viable at equivalent physical and chemical conditions.