Abstract & Overview
Coupled models of mantle thermal evolution, volcanism, outgassing, weathering, and climate evolution for Earth-like (in terms of size and composition) stagnant lid planets are used to assess their prospects for habitability. The results indicate that planetary CO$_2$ budgets ranging from $\approx 3$ orders of magnitude lower than Earth's to $\approx 1$ order of magnitude larger, and radiogenic heating budgets as large or larger than Earth's, allow for habitable climates lasting 1-5 Gyrs. The ability of stagnant lid planets to recover from potential snowball states is also explored; recovery is found to depend on whether atmosphere-ocean chemical exchange is possible. For a "hard" snowball with no exchange, recovery is unlikely, as most CO$_2$ outgassing takes place via metamorphic decarbonation of the crust, which occurs below the ice layer. However, for a "soft" snowball where there is exchange between atmosphere and ocean, planets can readily recover. For both hard and soft snowball states, there is a minimum CO$_2$ budget needed for recovery; below this limit any snowball state would be permanent. Thus there is the possibility for hysteresis in stagnant lid planet climate evolution, where planets with low CO$_2$ budgets that start off in a snowball climate will be permanently stuck in this state, while otherwise identical planets that start with a temperate climate will be capable of maintaining this climate for 1 Gyrs or more. Finally, the model results have important implications for future exoplanet missions, as they can guide observations to planets most likely to possess habitable climates.