Earth&#8217;s early tectonic modes and implications for habitability
Authors: Cawood Peter , Chowdhury Priyadarshi 
Published: March 2024

Tectonic mode manifests how a planet&#8217;s interior is cooling, and it encompasses all the geological activities (e.g., magmatism, deformation, metamorphism, sedimentation) that characterize the planetary body. Tectonic processes exert first-order control on factors key to planetary habitability (e.g., Southam et al., 2015). For example, tectonic mode controls the long-term prevalence of surface oceans, the sustenance of physicochemical conditions (e.g., temperature) favourable for metabolic activity, fluxing of elements in and out of the planet&#8217;s interior and thereby, the availability of bio-essential nutrients (e.g., C, O, H, N, P, S) (Cockell et al., 2016). However, all tectonic modes do not regulate these processes efficiently. For example, stagnant-lid mode restricts heat and material exchange between a planet&#8217;s interior and surficial reservoirs compared to plate tectonics. Further, certain factors determining a planet&#8217;s tectonic mode &#8211; like internal heat budget, mechanical behaviour of rocks, and volatile content &#8211; can vary with time, leading to the prevalence of different tectonic modes during planetary evolution. Thus, a planet&#8217;s habitability is critically intertwined with its tectonic evolution.Modern Earth is the only known planet with plate tectonics, felsic crust, and life. Plate tectonics has resulted in a Goldilocks environment for long-term habitability via chemical cycling across the Earth system, regulating temperature through the carbonate-silicate cycle, sustaining oceans at the surface, and developing bimodal hypsometry with emergent felsic crust releasing bio-essential minerals through weathering and erosion. This has resulted in diverse habitats facilitating life&#8217;s complex phylogenetic tree. However, life initiated on Earth in the Hadean or early Archean when non-plate-tectonic modes like the stagnant- or squishy-lid modes are inferred to be prevalent (e.g., Cawood et al., 2022). Their potential to promote habitability is unknown, with few studies suggesting that they may lead to habitable conditions (e.g., Tosi et al., 2017). Nevertheless, our terrestrial planetary neighbours&#8217; records suggest that such modes are unlikely to provide the environmental stability necessary to develop a long-term phylogenetic landscape. The geochemical cycling of elements through these modes may occur (e.g., via magmatism and episodic recycling of lithosphere) but is likely to be spatially and temporally discontinuous and limited, thereby limiting the supply of bio-essential nutrients and longevity of oceans on a planetary surface. As such, these modes inhibit a surficial environment in long-term dynamic equilibrium, leading to inhospitable habitats either through the development of a run-away greenhouse (e.g., Venus) or the loss of early atmosphere and oceans to space (e.g., Mars).Thus, the tectonic evolution of Earth and its resultant habitability are a predictable consequence of its position, composition, size, and heat energy within the solar system. These conditions may serve as a template to search for exoplanet habitability; however, a degree of unpredictability will remain in knowing whether a similar set of planetary criteria would produce the same outcome.References:Cawood et al., 2022. Reviews of Geophysics, 60, e2022RG000789Cockell et al., 2016. Astrobiology, 16(1), pp.89-117.Southam et al., 2015. Planets and Moons, 10, pp.473-486.Tosi et al., 2017. Astronomy &amp; Astrophysics, 605, p.A71.