A. Lenardic, J.W. Crowley, A.M. Jellinek, and M. Weller
Abstract. The presence of life, surface water, and plate tectonics makes Earth unique in our solar system. By contrast,
Venus, the planet closest to Earth in terms of bulk properties, is characterized by significantly higher surface temperature and pressure, a lack of surface water and life, and a different mode of tectonics (Grinspoon, 1997). These observations have motivated decades of studies aimed at the question of what leads to the differences between Earth and Venus. Modeling studies that address this question, as well as similar issues arising with comparisons of Earth and Mars, often assume that the profound differences between the current states of Earth and Venus reflect differences in planetary size, position to the Sun, and material parameters (e.g., strength of near-surface rock) (Fig. 1A). Models of this type are now guiding discussions related to the search for extrasolar planets that could support life. An alternative view is that the current states and tectonic regimes of Earth and Venus represent two equally possible solutions to the dynamical evolution of a planet with the same size, bulk composition, solar proximity, and material characteristics. Rather than being distinct consequences of physical or chemical differences, Earth and Venus may instead represent an inherent ‘‘bistability’’ in the dynamic state of terrestrial planets (Fig. 1B). Viewed this way, the present states of terrestrial planets can be most sensitive to potentially small variations in their geological or climatic histories, as opposed to bulk physical and chemical characteristics (i.e., bistability allows for planetary-scale bifurcations in time such that small, effectively random, fluctuations can cause the evolution paths of two planets with identical bulk properties to diverge over time). The potential for bistability and planetary-scale bifurcations in evolution paths is rarely addressed in studies of extrasolar planets but must ultimately inform the current vigorous search for habitable worlds. Indeed, the growing taxonomy of extrasolar planets may provide a ready means to characterize statistically and understand the potential for similar planetary bodies to evolve in distinct ways.
ASTROBIOLOGY Volume 16, Number 7, 2016ª Mary Ann Liebert, Inc.