The two studies by O'Neill et al. (2017, 2020) are among the first studies to model the role of bolide impacts on planet-wide tectonic behavior applied to Earth. Both studies concluded that, if initiated, subduction would be a transient event, lasting only tens of millions of years, and leaving open the question of when plate tectonics (as the globally connected plate boundary network that we recognize today) took hold. The O'Neill et al. (2017, 2020) results are at odds with the results of Foley et al. (2014). The reason for the discrepancy may be that O'Neill et al. used a yield -stress formulation while Foley et al. (2014) used a formulation where the plate boundaries are weakened by grain-size reduction (Bercovici and Ricard, 2014). Foley (2018) suggested that the difference between the two formulations is related to how they respond to changes in lithospheric stress. In yield-stress formulations, when the lithospheric stress decreases, the stress level at the boundaries may no longer exceed the yield stress, resulting in lithospheric stagnation. In contrast, when lithosphere stress drops with increasing mantle temperature or heat production rate, the deformational work, which drives grain-size reduction, increases. Thus, in grain-size reduction formulations, the ability to form weak plate boundaries is not impeded by early Earth thermal conditions. Both formulations are based on sound physical principles. Yet it is not clear how each method applies to the complex environment of subduction zones. Our understanding of subduction initiation is limited by our understanding of the process by which a stagnant lithosphere begins to deform. Some may wonder whether appealing to a bolide impact as a trigger for subduction initiation is truly necessary. It is clear from the other inner Solar System bodies (Mercury, Venus, Mars, and the Moon) that bolide impacts were a significant process in the inner Solar System during the Hadean and Archean (Bottke et al., 2012). Thus, bolide impacts played a significant role in the Hadean and Archean on Earth. Geodynamicists assumed that because Earth's mantle is convecting well above the critical Rayleigh num ber, over the course of Earth history the initial state of the mantle will be long forgotten. Weller and Lenardic (2012) showed that models with identical parameters starting from different initial conditions may end up in different states (e.g., stagnant-lid versus mobile-lid convection). When and whether a planet exhibits plate tectonics are likely functions of the initial state and history of the planet. When plate tectonics began has a fundamental control on the thermal evolution of Earth, because planets lose heat more effectively with a mobile lid than a stagnant lid. Thus, the earlier plate tectonics began, the further Earth has cooled from its initial state. The hypothesis that plate tectonics is necessary to create the kind of stable surface environment necessary for a habitable planet is of interest to the exoplanet community (Kasting and Catling, 2003).

Kite, E.S., Mischna, M.A., Gao, P., Yung, Y.L., Turbet, M., 2020. Methane release on Early Mars by atmospheric collapse and atmospheric reinflation. Planetary and Space Science 181, 104820.

http://www.sciencedirect.com/science/article/pii/S0032063319302703 A candidate explanation for Early Mars rivers is atmospheric warming due to surface release of H or CH gas. 2 4 However, it remains unknown how much gas could be released in a single event. We model the CH release by 4 one mechanism for rapid release of CH from clathrate. By modeling how CH -clathrate release is affected by 4 4 changes in Mars' obliquity and atmospheric composition, we find that a large fraction of total outgassing from CH clathrate occurs following Mars' first prolonged atmospheric collapse. This atmosphere-collapse-initiated 4 CH - release mechanism has three stages. (1) Rapid collapse of Early Mars' carbon dioxide atmosphere initiates 4 a slower shift of water ice from high ground to the poles. (2) Upon subsequent CO -atmosphere re-inflation and 2 CO -greenhouse warming, low-latitude clathrate decomposes and releases methane gas. (3) Methane can then 2 perturb atmospheric chemistry and surface temperature, until photochemical processes destroy the methane. Within our model, we find that under some circumstances a Titan-like haze layer would be expected to form, consistent with transient deposition of abundant complex abiotic organic matter on the Early Mars surface. We also find that this CH -release mechanism can warm Early Mars, but special circumstances are required in order 4 17 to uncork 10 kg of CH , the minimum needed for strong warming. Specifically, strong warming only occurs 4 when the fraction of the hydrate stability zone that is initially occupied by clathrate exceeds 10%, and when Mars' first prolonged atmospheric collapse occurs for atmospheric pressure >1 bar.